.
RIFT VALLEY INSTITUTE OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC
ENGINEERING
(CEE)
INDUSTRIAL MACHINE CONTROL
(IMC)
Lecture Notes
By
Dr. Cliff Orori Mosiori
©2015
i
.
Contents
INTRODUCTION................................................................................................................................... 1
ELECTRIC MACHINES ........................................................................................................................ 1
Electric Motor ..................................................................................................................................... 1
Principle and Working of Electric Motor .................................................................................................. 2
TOPIC ONE ........................................................................................................................................ 2
D.C MACHINES................................................................................................................................. 2
Principles of D.C machines .................................................................................................................. 2
Construction of D.C machines ............................................................................................................. 2
Frame .................................................................................................................................................. 3
Main poles ........................................................................................................................................... 2
Armature ............................................................................................................................................. 2
Field windings ..................................................................................................................................... 2
Commutator ........................................................................................................................................ 3
Brush and brush holders ...................................................................................................................... 4
Generator E.M.F Equation ................................................................................................................... 1
Methods of Excitation.......................................................................................................................... 1
Torque and power ................................................................................................................................ 2
Voltage and current ............................................................................................................................. 2
Generator Characteristics ..................................................................................................................... 2
Characteristics Series of DC generator ................................................................................................. 6
Characteristics Shunt DC generator...................................................................................................... 7
Characteristics compound generator..................................................................................................... 8
The Efficiency of the DC Motor Increases by: ................................................................................... 10
Motor Characteristics......................................................................................................................... 10
Torque Speed Curves ......................................................................................................................... 10
Direct on line starter ............................................................................................................................ 1
TOPIC TWO: AC MACHINES ............................................................................................................... 2
Induction Motor................................................................................................................................... 2
Principle of operation and comparison to synchronous motors ............................................................. 2
Construction ........................................................................................................................................ 2
Types of rotors .................................................................................................................................... 2
ii
.
Slip ring rotor ...................................................................................................................................... 2
Solid core rotor .................................................................................................................................... 3
Starting of induction motors................................................................................................................. 3
Direct-on-line starting .......................................................................................................................... 3
Wye-Delta starters ............................................................................................................................... 4
Variable-frequency drives .................................................................................................................... 4
Resistance starters ............................................................................................................................... 4
Series Reactor starters.......................................................................................................................... 5
Single Phase induction motor ............................................................................................................... 5
Rotating magnetic field........................................................................................................................ 6
Description of magnetic field ............................................................................................................... 6
Permanent-split capacitor motor........................................................................................................... 1
Capacitor-start induction motor............................................................................................................ 2
Capacitor-run induction motor ............................................................................................................. 2
Resistance split-phase induction motor ................................................................................................ 3
THREE PHASE INDUCTION MOTOR ................................................................................................. 1
Working Principle of Three Phase Induction Motor ................................................................................. 1
Production of Rotating Magnetic Field ............................................................................................. 1
What is the operating principle of a 3ph induction motor? .................................................................... 2
Production of a rotating magnetic field ................................................................................................ 3
Production of magnetic flux ................................................................................................................. 3
SPEED CONTROL OF THREE PHASE INDUCTION MOTOR ............................................................ 5
The Speed of Induction Motor is changed from Both Stator and Rotor Side .............................. 2
Speed Control from Stator Side .................................................................................................... 2
Speed Control from Rotor Side ..................................................................................................... 4
Electric Motor Controls ....................................................................................................................... 2
Motor Starting ..................................................................................................................................... 2
Motor Protection.................................................................................................................................. 5
Other Motor Protection Devices ........................................................................................................... 6
TOPIC THREE: CONTACTORS ............................................................................................................ 2
CONTACTORS .................................................................................................................................. 2
Applications of Contactors................................................................................................................... 2
iii
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1.
Lighting control ....................................................................................................................... 2
2.
Magnetic starter ....................................................................................................................... 3
3.
Vacuum contactor .................................................................................................................... 3
How Contactor Controls an Electric Motor .......................................................................................... 3
Topic four ............................................................................................................................................... 1
Preventive Maintenance........................................................................................................................... 1
Controlling Maintenance Hazards ........................................................................................................ 3
Instrumentation systems ......................................................................... Error! Bookmark not defined.
Assemble a simple instrumentation ......................................................... Error! Bookmark not defined.
Signal processing methods ...................................................................... Error! Bookmark not defined.
Data processing elements ........................................................................ Error! Bookmark not defined.
iv
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INTRODUCTION
ELECTRIC MACHINES
Electric Motor
An electric motor is an electric machine that converts electrical energy into mechanical energy.
In normal motoring mode, most electric motors operate through the interaction between an
electric motor's magnetic field and winding currents to generate force within the motor.
In certain applications, such as in the
from the power grid, inverters or generators.
transportation industry with traction motors,
Small motors may be found in electric
electric motors can operate in both motoring
watches. The largest of electric motors are
and generating or braking modes to also
used
produce electrical energy from mechanical
compression
energy.
applications with ratings reaching 100
for
megawatts.
ship
propulsion,
and
pipeline
pumped-storage
Electric
motors
may
be
classified by electric power source type,
internal construction, application, type of
motion output, and so on
Application
of
electric
motors
revolutionized industry. Industrial processes
were
no
transmission
compressed
Found in applications as diverse as industrial
longer
using
air
or
limited
line
by
power
shafts,
hydraulic
belts,
pressure.
Instead every machine could be equipped
fans, blowers and pumps, machine tools,
with its own electric motor, providing easy
household appliances, power tools, and disk
control at the point of use, and improving
drives, electric motors can be powered by
power
direct current (DC) sources, such as from
transmission
efficiency.
Electric
motors applied in agriculture eliminated
batteries, motor vehicles or rectifiers, or by
human and animal muscle power from such
alternating current (AC) sources, such as
tasks as handling grain or pumping water.
1
.
Household uses of electric motors reduced
common motor works on direct current. So,
heavy labor in the home and made higher
it is also called DC motor. When a
standards of convenience, comfort and
rectangular coil carrying current is placed in
safety possible.
a magnetic field, a torque acts on the coil
which rotates it continuously. When the coil
rotates, the shaft attached to it also rotates
Working Principle of Electric Motor
and thus it is able to do mechanical work.
An electric motor is a device which converts
electrical energy into mechanical energy. A
2
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TOPIC ONE
D.C MACHINES
Principles of D.C machines
D.C machines are the electro mechanical energy converters which work from a D.C source and
generate mechanical power or convert mechanical power into a D.C power. In any electric
motor, operation is based on simple electromagnetism. A current-carrying conductor generates a
magnetic field; when this is then placed in an external magnetic field, it will experience a force
proportional to the current in the conductor, and to the strength of the external magnetic field.
Magnetic field in motor
As you are well aware of from playing with magnets as a kid, opposite (North and South)
polarities attract, while like polarities (North and North, South and South) repel. The internal
2
.
configuration of a DC motor is designed to harness the magnetic interaction between a currentcarrying conductor and an external magnetic field to generate rotational motion.
Direction of rotation
Let's start by looking at a simple 2-pole DC
magnets. The stator is the stationary part of
electric motor (here red represents a magnet
the motor -- this includes the motor casing,
or winding with a "North" polarization,
as well as two or more permanent magnet
while green represents a magnet or winding
pole pieces. The rotor (together with the
with a "South" polarization). Every DC
axle, and attached commutator) rotate with
motor has six basic parts -- axle, rotor
respect to the stator. The rotor consists of
(armature),
windings (generally on a core), the windings
stator,
commutator,
field
magnet(s), and brushes. In most common
being
DC motors, the external magnetic field is
commutator.
produced
by
high-strength
permanent
3
electrically
connected
to
the
.
Torque in motor
The geometry of the brushes, commutator
Given our example two-pole motor, the
contacts, and rotor windings are such that
rotation reverses the direction of current
when power is applied, the polarities of the
through the rotor winding, leading to a "flip"
energized winding and the stator magnet(s)
of the rotor's magnetic field, driving it to
are misaligned, and the rotor will rotate until
continue rotating.DC motors will always
it is almost aligned with the stator's field
have more than two poles. This avoids "dead
magnets. As the rotor reaches alignment, the
spots" in the commutator. With a two-pole
brushes move to the next commutator
motor, there is a moment where the
contacts, and energize the next winding.
commutator shorts out the power supply.
1
.
This would be bad for the power supply,
disadvantage of such a simple motor is that
waste
it would exhibit a high amount of torque
energy,
components
as
and
well.
damage
Yet
motor
"ripple".
another
Current in Motor
Force in Motor
2
.
You'll notice that one pole is fully energized
at a time (but two others are "partially"
energized). As each brush transitions from
one commutator contact to the next, one
coil's field will rapidly collapse, as the next
coil's field will rapidly charge up. You can
see that this is a direct result of the coil
But iron core construction also has several
windings' series wiring. There's probably no
disadvantages. The iron armature has a
better way to see how an average DC motor
relatively high inertia which limits motor
is put together, than by just opening one up.
acceleration. This construction also results
This is a basic 3-pole DC motor, with 2
in high winding inductances which limit
brushes and three commutator contacts. The
brush and commutator life.
use of an iron core armature is quite
common, and has a number of advantages.
In small motors, an alternative design is
First off, the iron core provides a strong,
often used which features a 'coreless'
rigid support
a
armature winding. This design depends upon
consideration for
the coil wire itself for structural integrity. As
high-torque motors. The core also conducts
a result, the armature is hollow, and the
heat away from the rotor windings, allowing
permanent magnet can be mounted inside
the motor to be driven harder than might
the rotor coil. Coreless DC motors have
otherwise be the case. Iron core construction
much lower armature inductance than iron-
is also relatively inexpensive compared with
core motors of comparable size, extending
other construction types.
brush and commutator life. DC motors have
for the windings
particularly important
--
been used in industrial applications for
years. Coupled with a DC drive, DC motors
provide very precise control DC motors can
be used with conveyors, elevators, extruders,
marine
applications,
material
handling,
paper, plastics, rubber, steel, and textile
applications to name a few.
2
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Torque in a DC motor is produced by the
product of the magnetic field created by the
field winding or magnets and the current
flowing in the armature winding. The action
of a mechanical commutator switches the
armature current from one winding to
another to maintain the relative position of
the current to the field, thereby producing
Figure 1 - DC motor in schematic form
torque independent of rotor position.
Standard DC motors are readily available in
one of two main forms:
Wound-field - where the magnetic flux in
the motor is controlled by the current
flowing in a field or excitation winding,
usually located on the stator.
Permanent magnet - where the magnetic
flux in the motor is created by permanent
magnets which have a curved face to create
a constant air-gap to the conventional
armature, located on the rotor. These are
commonly
used
at
powers
up
to
approximately 3 kW.
1
.
VaIa = EaIa + Ia2Ra
The circuit of a shunt-wound DC motor
(3)
(Fig. 2 below) shows the armature M, the
armature resistance
Ra
and the
(or total power supplied = power output +
field
armature losses). The interaction of the field
winding. The armature supply voltage Va is
flux and armature flux produces an armature
supplied typically from a controlled thyristor
torque as given in below equation 4.
system and the field voltage Vf from a
separate bridge rectifier.
Torque M = k2IfIa
(4)
where k2 is a motor constant and If is the
field current. This confirms the straightforward and linear characteristic of the DC
motor and consideration of these simple
equations will show its controllability and
Figure 2 -Shunt wound DC motor
inherent stability. The characteristic of a
motor is represented by curves of speed
As the armature rotates, an electromotive
against input current or torque and its shape
force (emf ) Ea is induced in the armature
can be derived from eqns 1 and 2:
circuit and is called the back-emf since it
opposes the applied voltage Va (according to
k1nφ = Va – (IaRa)
(5)
Lenz’s Law). The Ea is related to armature
speed and main field flux, φ by:
If the flux is held constant by holding the
field
Ea = k1nφ
(1)
current
constant
in
a
properly
compensated motor then:
where n is the speed of rotation, φ is the
n = k2[Va – (IaRa)]
(6)
field flux and k1 is a motor constant. From
Figure 1 it is seen that the terminal armature
From eqns 4 and 6, it follows that full
voltage Va is given by:
control of the DC motor can be achieved
through control of the field current and the
Va = Ea + IaRa
(2)
armature current. In the DC shunt wound
motor shown in Figure 2 these currents can
Multiplying each side of eqn 2 by Ia gives:
be controlled independently. Most industrial
2
.
DC motor controllers or drives are voltage
5 per cent of Va, giving a torque - speed
fed and the current is controlled by
curve shown as a in Figure 6, where speed
measuring the current and adjusting the
remains constant over a wide range of load
voltage to give the desired current. This
torque.
basic arrangement is shown in Figure 3.
The compound-wound DC motor shown in
The series DC motor shown in Figure 4 has
Figure 5 combines both shunt and series
the field and armature windings connected
characteristics. The shape of the torque–
in series. In this case the field current and
speed characteristic is determined by the
armature current are equal and show
resistance values of the shunt and series
characteristically
performance
fields. The slightly drooping characteristic
results, though still defined by eqns. 4 and 6.
(curve b in Figure 6) has the advantage in
In the shunt motor the field flux φ is only
many
slightly affected by armature current, and
mechanical effects of shock loading.
different
applications
the value of IaRa at full load rarely exceeds
Figure 3 - Control structure for a shunt wound DC motor
3
of
reducing
the
.
The series DC motor curve (in Figure 6) shows that the initial flux increases in proportion to
current, falling away due to magnetic saturation. In addition the armature circuit includes the
resistance of the field winding and the speed becomes roughly inversely proportional to the
current. If the load falls to a low value the speed increases dramatically, which may be
hazardous, so, the series motor should not normally be used where there is a possibility of load
loss.
Figure 4 - Schematic of series DC motor
But because it produces high values of torque at low speed and its characteristic is falling speed
with load increase, it is useful in applications such as traction and hoisting, and some mixing
duties where initial stiction is dominant. Under semiconductor converter control with speed
feedback from a tachogenerator, the shape of the speed–load curve is determined within the
controller. It has become standard to use a shunt DC motor with converter control even though
the speed-load curve, when under open-loop control is often slightly drooping. The power-speed
limit for the DC motor is approximately 3 × 106 kW rev/min, due to restrictions imposed by the
commutator.
Construction of D.C machines
A D.C machine consists mainly of two part the stationary part called stator and the rotating part
called rotor. The stator consists of main poles used to produce magnetic flux ,commutating poles
or interpoles in between the main poles to avoid sparking at the commutator but in the case of
small machines sometimes the interpoles are avoided and finally the frame or yoke which forms
the supporting structure of the machine. The rotor consist of an armature a cylindrical metallic
body or core with slots in it to place armature windings or bars, a commutator and brush gears
2
.
The magnetic flux path in a motor or generator is show below and it is called the magnetic
structure of generator or motor. The major parts can be identified as,
1. Frame
5. Commutator and brush gear
2. Yoke
6. Commutating poles
3. Poles Institute of Technology Madras
7. Compensating winding
4. Armature
8. Other mechanical parts
magnetic fluxes from the main poles and
interpoles is called Yoke.
Why we use cast steel instead of cast iron
for the construction of Yoke?
In early days Yoke was made up of cast iron
but now it is replaced by cast steel. This is
because cast iron is saturated by a flux density
of 0.8 Wb/sq.m whereas saturation with cast
iron steel is about 1.5 Wb/sq.m. So for the
same magnetic flux density the cross section
area needed for cast steel is less than cast iron
hence the weight of the machine too. If we use
cast iron there may be chances of blow holes
in it while casting.so now rolled steels are
developed and these have consistent magnetic
Frame
Frame is the stationary part of a machine on
and mechanical properties.
which the main poles and commutator poles
are bolted and it forms the supporting
structure by connecting the frame to the bed
plate. The ring shaped body portion of the
frame which makes the magnetic path for the
3
.
End Shields or Bearings
If the armature diameter does not exceed 35
Armature
The armature
to 45 cm then in addition to poles end shields
conductors are located. The armature is
or frame head with bearing are attached to
constructed by stacking laminated sheets of
the frame. If the armature diameter is greater
silicon steel. Thickness of these lamination is
than 1m pedestals type bearings are mounted
kept low to reduce eddy current losses. As
on the machine bed plate outside the frame.
the laminations carry alternating flux the
These bearings could be ball or roller type
choice of suitable material, insulation coating
but generally plain pedestals bearings are
on the laminations, stacking it etc are to be
employed. If the diameter of the armature is
done more carefully. The core is divided into
large a brush holder yoke is generally fixed
packets to facilitate ventilation. The winding
to the frame.
cannot be placed on the surface of the rotor
is
where
the
moving
due to the mechanical forces coming on the
same. Open parallel sided equally spaced
Main poles
Solid poles
of
fabricated
steel
slots are normally punched in the rotor
with
laminations. These slots house the armature
separate/integral pole shoes are fastened to
winding. Large sized machines employ a
the frame by means of bolts. Pole shoes are
spider on which the laminations are stacked
generally laminated. Sometimes pole body
in segments. End plates are suitably shaped
and pole shoe are formed from the same
so as to serve as ’Winding supporters’.
laminations. The pole shoes are shaped so as
Armature construction process must ensure
to have a slightly increased air gap at the tips.
provision of sufficient axial and radial ducts
Inter-poles are small additional poles located
to facilitate easy removal of heat from the
in between the main poles. These can be
armature winding.
solid, or laminated just as the main poles.
These are also fastened to the yoke by bolts.
Sometimes the yoke may be slotted to receive
these poles. The inter poles could be of
Field windings
In the case of wound field machines (as
tapered section or of uniform cross section.
against permanent magnet excited machines)
These are also called as commutating poles
the field winding takes the form of a
or com poles. The width of the tip of the com
concentric coil wound around the main poles.
pole can be about a rotor slot pitch.
These carry the excitation current and
2
.
produce the main field in the machine. Thus
taped and lowered into the open slots on the
the poles are created electromagnetically.
armature. In the case of small machines, they
Two types of windings are generally
can be hand wound. The coils are prevented
employed. In shunt winding large number of
from flying out due to the centrifugal forces
turns of small section copper conductor isof
by means of bands of steel wire on the
Technology Madras used. The resistance of
surface of the rotor in small groves cut into it.
such winding would
be an order of
In the case of large machines slot wedges are
magnitude larger than the armature winding
additionally used to restrain the coils from
resistance. In the case of series winding a few
flying away. The end portion of the windings
turns of heavy cross section conductor is
are taped at the free end and bound to the
used. The resistance of such windings is low
winding carrier ring of the armature at the
and is comparable to armature resistance.
commutator end. The armature must be
Some machines may have both the windings
dynamically
on the poles. The total ampere turns required
centrifugal forces at the operating speeds.
to establish the necessary flux under the poles
Compensating winding One may find a bar
is calculated from the magnetic circuit
winding housed in the slots on the pole shoes.
calculations. The total mmf required is
This is mostly found in D.C machines of very
divided equally between north and south
large
poles as the poles are produced in pairs. The
compensating winding. In smaller machines,
mmf required to be shared between shunt and
they may be absent.
rating.
balanced
Such
to
reduce
winding
is
the
called
series windings are apportioned as per the
design requirements. As these work on the
concentric coils. Mmf ’per pole’ is normally
Commutator
Commutator is the key element which made
used in these calculations. Armature winding
the D.C machine of the present day possible.
As mentioned earlier, if the armature coils
It consists of copper segments tightly
are wound on the surface ofthe armature,
fastened
such construction becomes mechanically
insulating separators on an insulated base.
weak. The conductors may fly away when
The whole commutator forms a rigid and
the armature starts rotating. Hence the
solid assembly of insulated copper strips and
armature windings are in general pre-formed,
can rotate at high speeds. Each com mutator
same magnetic system they are in the form of
3
together
with
mica/micanite
.
segment is provided with a ’riser’ where the
is taken out by means of flexible pigtail. The
ends of the armature coils get connected. The
brushes are kept pressed on the commutator
surface of the commutator is machined and
with the help of springs. This is to ensure
surface is made concentric with the shaft and
proper contact between the brushes and the
the current collecting brushes rest on the
commutator even under high speeds of
same. Under-cutting the mica insulators that
operation. Jumping of brushes must be
are between these commutator segments has
avoided to ensure arc free current collection
to be done periodi- cally to avoid fouling of
and to keep the brush contact drop low. Other
the surface of the commutator by mica when
mechanical parts End covers, fan and shaft
the commutator gets worn out.
bearings form other important me- chanical
parts. End covers are completely solid or
have opening for ventilation. They support
Brush and brush holders
Brushes rest on the surface
the bearings which are on the shaft. Proper
the
machining is to be ensured for easy
commutator. Normally electro-graphite is
assembly. Fans can be external or internal. In
used
actual
most machines the fan is on the non-
composition of the brush depends on the
commutator end sucking the air from the
peripheral speed of the commutator and the
commutator end and throwing the same out.
working voltage. The hardness of the
Adequate quantity of hot air removal has to
graphite brush is selected to be lower than
be ensured.
as
brush
material.
The
of
that of the commutator. When the brush
wears out the graphite works as a solid
Bearings
lubricant
coefficient.
bearings at both ends. For larger machines
More number of relatively smaller width
roller bearings are used especially at the
brushes is preferred in place of large broad
driving end. The bearings are mounted press-
brushes. The brush holders provide slots for
fit on the shaft. They are housed inside the
the brushes to be placed. The connection
end shield in such a manner that it is not
Brush holder with a Brush and Positioning of
necessary to remove the bearings from the
the brush on the commutator from the brush
shaft for dismantling.
reducing
frictional
4
Small
machines
employ
ball
.
series)
Generator E.M.F Equation
Let Φ = flux/pole in weber Z = total number
in
one
path
=
Z/2
E.M.F.
generated/path is
of armture conductors = No.of slots x No.of
conductors/slot P = No.of generator poles A
= No.of parallel paths in armature N =
For a simplex lap-wound generator
armature rotation in revolutions per minute
No.of parallel paths = P No.of conductors (in
(r.p.m) E = e.m.f induced in any parallel path
series)
in armature Generated e.m.f Eg = e.m.f
in
one
path
=
Z/P
E.M.F.generated/path
generated in any one of the parallel paths i.e
E. Average e.m.f geneated /conductor =
dΦ/dt volt (n=1) Now, flux cut/conductor in
one revolution dΦ = ΦP Wb No. of
In general generated e.m.f
revolutions/second = N/60 Time for one
revolution,
dt
according
to
Electromagnetic
= 60/N
second Hence,
Faraday's
Induction,
Laws
of
E.M.F
generated/conductor is
where A = 2 - for simplex wave-winding A
= P - for simplex lap-winding
For a simplex wave-wound generator No.
of parallel paths = 2 No. of conductors (in
Methods of Excitation
Various methods of excitation of the field windings are shown in Fig.
1
.
Figure shows Field-circuit connections of dc machines:
(a) separate excitation, (b) series, (c) shunt, (d) compound.
Consider first dc generators.
Separately-excited generators and
Self-
excited generators: series generators, shunt
generators, compound generators.
With
self-excited
generators,
residual
magnetism must be present in the machine
iron to get the self-excitation process started.
o N.B.: long- and short-shunt, cumulatively
Figure Volt-ampere characteristics of dc
and differentially compound.
Typical
characteristics
steady-state
volt-ampere
are
in
shown
generators. Any of the methods of excitation
used for generators can also be used for
Fig.7.5,
motors.
constant-speed operation being assumed.
Typical steady-state dc-motor speed-
The relation between the steady-state
torque characteristics are shown in Fig.7.6,
generated emf Ea and the armature terminal
in which it is assumed that the motor
voltage Va is Va =Ea −IaRa (7.10)
terminals are supplied from a constantvoltage source.
2
.
In a motor the relation between the emf
Torque and power
The electromagnetic torque Tmech,
Ea generated in the armature and and the
Tmech =KaΦdIa
armature terminal voltage Va is
The generated voltage, Ea
Ea =KaΦdωm
Va=Ea+IaRa (7.11)
The application advantages of dc
machines lie in the variety of performance
characteristics offered by the possibilities of
Voltage and current
Va: the terminal voltage of the armature
shunt, series, and compound excitation.
winding Vt: the terminal voltage of the dc
machine, including the voltage drop across
the series connected field winding,
Va = Vt
if there is no series field winding Ra: the
resistance of armature, Rs: the resistance of
the series field,
Va =Ea ± IaRa
Vt=Ea ± Ia( Ra+Rs) IL
=Ia±If
Figure Speed-torque characteristics of dc
motors.
Generator Characteristics
The three most important characteristics or curves of a d.c generator are;
1. Open Circuit Characteristic (O.C.C.)
same for all generators whether separately or
This curve shows the relation between the
self-excited. The data for O.C.C. curve are
generated e.m.f. at no-load (E0) and the field
obtained experimentally by operating the
current (If) at constant speed. It is also
generator at no load and constant speed and
known as magnetic characteristic or no-load
recording the change in terminal voltage as
saturation curve. Its shape is practically the
the field current is varied.
2
.
2. Internal or Total characteristic (E/Ia)
1. No-load saturation characteristic
This curve shows the relation between the
(E0/If)
generated e.m.f. on load (E) and the
It
armature current (Ia). The e.m.f. E is less
characteristic
than E0 due to the demagnetizing effect of
Characteristic (O.C.C). It shows the
armature reaction. Therefore, this curve will
relation
lie below the open circuit characteristic
generated e.m.f in armature, E0 and
(O.C.C.). The internal characteristic is of
the field or exciting current If at a
interest chiefly to the designer. It cannot be
given fixed speed. It is just the
obtained directly by experiment. It is
magnetisation curve for the material
because a voltmeter cannot read the e.m.f.
of the electromagnets. Its shape is
generated on load due to the voltage drop in
practically
the
armature
generators
whether
resistance.
The
internal
is
also
know
as
or
Open
between
the
same
Magnetic
circuit
no-load
for
all
separately-
excited or self-excited.
characteristic can be obtained from external
characteristic if winding resistances are
known because armature reaction effect is
included in both characteristics.
3. External characteristic (V/IL)
This curve shows the relation between the
terminal voltage (V) and load current (IL).
The terminal voltage V will be less than E
due to voltage drop in the armature circuit.
Therefore, this curve will lie below the
internal characteristic. This characteristic is
A typical no load saturation curve is shown
very important in determining the suitability
in Figure. It has generator output voltage
of a generator for a given purpose. It can be
plotted against field current. The lower
obtained by making simultaneous;
straight line portion of the curve represents
the air gap because the magnetic parts are
not saturated. When the magnetic parts start
to saturate, the curve bends over until
2
.
complete saturation is reached. Then the
in the poles, some e.m.f (= OA) is generated
curve becomes a straight line again.
even when If =0.Hence, the curve starts a
little way up. The slight curvature at the
lower end is due to magnetic inertia. It is
2. Separately-excited Generator
seen that the first part of the curve is
The No-load saturation curve of a separately
practically straight. This is due to fact that at
excited generator will be as shown in the
low flux densities reluctance of iron path
above figure. It is obvous that when If is
being negligible, total reluctance is given by
increased from its initial small value, the
the air gap reluctance which is constant.
flux and hence generated e.m.f Eg increase
Hence, the flux and consequently, the
directly as current so long as the poles are
generated e.m.f is directly proportional to
unsaturated. This is represented by straight
the exciting current. However, at high flux
portion in figure. But as the flux density
densities, where μ is small, iron path
increases, the poles become saturated, so a
reluctance becomes appreciable and straight
greater increase If is required to produce a
relation between E and If no longer holds
given increase in voltage than on the lower
good. In other words, after point B,
part of the curve. That is why the upper
saturation of pole starts. However, the initial
portion of the curve bends.
slope of the curve is determined by air-gap
width. O.C.C for higher speed would lie
above this curve and for lower speed, would
lie below it.
Separately-excited
consider
a
Generator
separately-excited
Let
us
generator
giving its rated no-load voltage of E0 for a
certain constant field current. If there were
no armature reaction and armature voltage
drop, then this voltage would have remained
constant as shown in figure by the horizontal
The O.C.C curve for self-excited generators
line 1. But when the generator is loaded, the
whether shunt or series wound is shown in
voltage falls due to these two causes,
above figure. Due to the residual magnetism
3
.
thereby
giving
slightly
dropping
and hence generated e.m.f. is also increased
characteristics. If we subtract from E0 the
as shown by the curve. Curve Oa is the
values of voltage drops due to armature
O.C.C. The extra exciting current necessary
reaction for different loads, then we get the
to neutralize the weakening effect of
value of E-the e.m.f actually induced in the
armature reaction at full load is given by the
armature under load conditions. Curve 2 is
horizontal distance ab. Hence, point b is on
plotted in this way and is known as the
the
internal characteristic.
characteristic (V/I) It is also referred to as
internal characteristic.
3.
External
performance characteristic or sometimes
voltage-regulating curve. It gives relation
between the terminal voltage V and the load
current I. This curve lies below the internal
characteristic because it takes in to account
the voltage drop over the armature circuit
resistance.
The values of V are obtained by subtracting
IaRa from corresponding values of E. This
Series Generator
characteristic is of great importance in
judging the suitability of a generator for a
particular purpose. It may be obtained in two
ways
(i)
by
making
simultaneous
measurements with a suitable voltmeter and
an ammeter on a loaded generator or (ii)
graphically from the O.C.C provided the
armature and field resistances are known
and also if the demagnetizing effect or the
armature reaction is known.
In this generator, because field windings are
in series with the armature, they carry full
armature current Ia. As Ia is increased, flux
4
.
separate excitation. This is due to the fact
that, since the output voltage is reduced
because of the armature reaction effect and
armature IR drop, the field voltage is also
reduced which further reduces the flux. It
can also be seen that beyond a certain
critical value, the shunt generator shows a
reversal in trend of current values with
decreasing voltages. This point of maximum
current output is known as the breakdown
point. At the short circuit condition, the only
Figure
above
shows
the
external
flux available to produce current is the
characteristic curves for generators with
residual magnetism of the armature.
various types of excitation. If a generator,
which is separately excited, is driven at
To build up the voltage on a series
constant speed and has a fixed field current,
generator, the external circuit must be
the output voltage will decrease with
connected and its resistance reduced to a
increased load current as shown. This
comparatively low value. Since the armature
decrease is due to the armature resistance
is in series with the field, load current must
and armature reaction effects. If the field
be flowing to obtain flux in the field. As the
flux remained constant, the generated
voltage and current rise the load resistance
voltage would tend to remain constant and
may be increased to its normal value. As the
the output voltage would be equal to the
external characteristic curve shows, the
generated voltage minus the IR drop of the
voltage output starts at zero, reaches a peak,
armature circuit.
and then falls back to zero.
However, the demagnetizing component of
The combination of a shunt field and a series
armature reactions tends to decrease the
field gives the best external characteristic as
flux, thus adding an additional factor, which
illustrated in Figure. The voltage drop,
decreases the output voltage. In a shunt
which occurs in the shunt machine, is
excited generator, it can be seen that the
compensated for by the voltage rise, which
output voltage decreases faster than with
5
.
occurs in the series machine. The addition of
a sufficient number of series turns offsets
the armature IR drop and armature reaction
effect,
resulting
in
a
flat-compound
generator, which has a nearly constant
voltage. If more series turns are added, the
(i) O.C.C. Curve 1 shows the open circuit
voltage may rise with load and the machine
characteristic (O.C.C.) of a series generator.
is known as an over-compound generator.
It can be obtained experimentally by
The speed of a D.C machine operated as a
disconnecting the field winding from the
generator is fixed by the prime mover.
machine and exciting it from a separate D.C
For general-purpose operation, the prime
source as discussed in Sec. (3.2).
mover is equipped with a speed governor so
that the speed of the generator is practically
constant.
Under
such
the
(ii) Internal characteristic Curve 2 shows
generator performance deals primarily with
the total or internal characteristic of a series
the relation between excitation, terminal
generator. It gives the relation between the
voltage and load. These relations can be best
generated e.m.f. E. on load and armature
exhibited graphically by means of curves
current. Due to armature reaction, the flux in
known as generator characteristics. These
the machine will be less than the flux at no
characteristics
the
load. Hence, e.m.f. E generated under load
behaviour of the generator under different
conditions will be less than the e.m.f. EO
load conditions.
generated
show
at
condition,
a
glance
under
no
load
conditions.
Consequently, internal characteristic curve
generated
Characteristics Series of DC generator
Fig. shows the connections of a series
under
no
load
conditions.
Consequently, internal characteristic curve
lies below the O.C.C. curve; the difference
wound generator. Since there is only one
between them representing the effect of
current (that which flows through the whole
armature reaction [See Fig. 3.7 (ii)].
machine), the load current is the same as the
exciting current.
6
.
(iii)External characteristic Curve 3 shows
Now raise a perpendicular from point B and
the external characteristic of a series
mark a point b on this line such that ab =
generator. It gives the relation between
AB. Then point b will lie on the external
terminal voltage and load current IL.
characteristic of the generator. Following
similar procedure, other points of external
V= E-Ia(Ra+Rse)
characteristic can be located. It is easy to see
that we can also plot internal characteristic
from the external characteristic.
Characteristics Shunt DC generator
Fig (3.9) (i) shows the connections of a
shunt
wound generator. The armature
current Ia splits up into two parts; a small
fraction Ish flowing through shunt field
winding while the major part IL goes to the
external load.
Therefore, external characteristic curve will
lie below internal characteristic curve by an
amount
equal
to
ohmic
drop
[i.e.,
Ia(Ra+Rse)] in the machine as shown in Fig.
(3.7)
(ii).
The
internal
and
external
characteristics of a D.C series generator can
be plotted from one another as shown in Fig.
(3.8). Suppose we are given the internal
I. O.C.C. The O.C.C. of a shunt
characteristic of the generator. Let the line
generator is similar in shape to that of a
OC represent the resistance of the whole
series generator as shown in Fig. (3.9)
machine i.e. Ra+Rse.If the load current is
(ii). The line OA represents the shunt
OB, drop in the machine is AB i.e.
field
circuit
resistance.
When
the
generator is run at normal speed, it will
AB = Ohmic drop in the machine =
build up a voltage OM. At no-load, the
OB(Ra+Rse)
terminal voltage of the generator will be
7
.
constant (= OM) represented by the
the increase in load current with the
horizontal dotted line MC.
decrease of load resistance. Any decrease of
II) Internal characteristic
load resistance beyond this point, instead of
When the
increasing the current, ultimately results in
generator is loaded, flux per pole is reduced
reduced current. Consequently, the external
due to armature reaction. Therefore, e.m.f. E
characteristic turns back (dotted curve) as
generated on load is less than the e.m.f.
shown in Fig. (3.10). The tangent OA to the
generated at no load.As a result, the internal
curve represents the minimum external
characteristic (E/Ia) drops down slightly as
resistance required to excite the shunt
shown in Fig.(3.9) (ii).
generator on load and is called critical
(iii)External characteristic Curve 2 shows
external resistance. If the resistance of the
the external characteristic of a shunt
external circuit is less than the critical
generator. It gives the relation between
external resistance (represented by tangent
terminal voltage V and load current IL. V =
OA in Fig. 3.10), the machine will refuse to
E – IaRa = E -(IL +Ish)Ra Therefore,
excite or will de-excite if already running
external characteristic curve will lie below
This means that external resistance is so low
the internal characteristic curve by an
as virtually to short circuit the machine and
amount equal to drop in the armature circuit
so doing away with its excitation.
[i.e., (IL +Ish)Ra ] as shown in Fig. (3.9)
Note. There are two critical resistances for a
(ii). Note. It may be seen from the external
shunt generator viz., (i) critical field
characteristic that change in terminal voltage
resistance (ii) critical external resistance. For
from no-load to full load is small. The
the shunt generator to build up voltage, the
terminal voltage can always be maintained
former should not be exceeded and the latter
constant by adjusting the field rheostat R
must not be gone below
automatically
External Resistance for Shunt Generator
Characteristics compound generator
In a compound generator, both series and
f the load resistance across the terminals of a
shunt excitation are combined as shown in
shunt generator is decreased, then load
Fig. (3.13). The shunt winding can be
current increase? However, there is a limit to
connected either across the armature only
8
.
(short-shunt
across
flux and hence the generated voltage. The
(long-shunt
increase in generated voltage is greater than
connection G). The compound generator can
the IaRa drop so that instead of decreasing,
be
the terminal voltage increases as shown by
armature
connection
plus
series
S)
field
cumulatively
or
compounded
or
curve A in Fig. (3.14).
differentially compounded generator. The
latter is rarely used in practice. Therefore,
(ii) If series winding turns are so
we shall discuss the characteristics of
adjusted that with the increase in load
cumulatively compounded generator. It may
current,
be noted that external characteristics of long
series winding of such a machine has
External characteristic
lesser number of turns than the one in
over-compounded
Fig. (3.14) shows the external characteristics
excitation
aids
the
much
shunt
the
increase
in
and,
for
a
given
load
current.
Consequently, the full-load voltage is
excitation. The degree of compounding
upon
machine
therefore, does not increase the flux as
of a cumulatively compounded generator.
depends
voltage
called flat-compounded generator. The
almost identical.
series
terminal
substantially remains constant, it is
and short shunt compound generators are
The
the
nearly equal to the no-load voltage as
series
indicated by curve B in Fig (3.14).
excitation with the increase in load current.
(iii) If series field winding has lesser
number
of turns than
compounded
machine,
for
the
a flat
terminal
voltage falls with increase in load
current as indicated by curve C m Fig.
(3.14). Such a machine is called undercompounded generator.
(i).If series winding turns are so adjusted
that with the increase in load current the
terminal voltage increases, it is called overcompounded generator. In such a case, as
the load current increases, the series field
m.m.f. increases and tends to increase the
9
.
Voltage Regulation
The change in terminal voltage of a
generator between full and no load (at
constant speed) is called the voltage
regulation, usually expressed as a percentage
of the voltage at full-load. % Voltage
regulation= [(VNL-VFL)/VFL] × 100 where
VNL = Terminal voltage of generator at no
load VFL = Terminal voltage of generator at
full load Note that voltage regulation of a
generator is determined with field circuit
and speed held constant. If the voltage
The graph above shows a torque/speed curve
regulation of a generator is 10%, it means
of a typical D.C motor. Note that torque is
that terminal voltage increases 10% as the
inversely proportional to the speed of the
load is changed from full load to no load.
output shaft. In other words, there is a
tradeoff between how much torque a motor
The Efficiency of the DC Motor Increases
delivers, and how fast the output shaft spins.
by:
Motor characteristics are frequently given as

Increasing number of turns in the coil

Increasing the strength of the current

Increasing X-section area of of the coil
the graph at which the torque is a maximum,

Increasing the strength of the radial
but the shaft is not rotating.
two points on this graph:
The stall torque,, represents the point on
magnetic field
The no load speed,, is the maximum
output speed of the motor (when no torque is
Motor Characteristics
applied to the output shaft).
Torque Speed Curves
In order to effectively design with D.C
The
motors, it is necessary to understand their
characteristic curves. For every motor, there
linear
model
of
a
D.C
motor
torque/speed
curve
is
a
very
good
approximation. The torque/speed curves
is a specific Torque/Speed curve and Power
shown below are actual curves for the green
curve.
maxon motor (pictured at right) used by
10
.
students in 2.007. One is a plot of empirical
Recall that earlier we defined power as the
data, and the other was plotted mechanically
product of torque and angular velocity. This
using a device developed at MIT. Note that
corresponds to the area of a rectangle under
the characteristic torque/speed curve for this
the torque/speed curve with one corner at
motor is quite linear.
the origin and another corner at a point on
the curve. Due to the linear inverse
relationship between torque and speed, the
maximum power occurs at the point where τ
= ½ , and = ½ .
This is generally true as long as the curve
represents the direct output of the motor, or
a simple gear reduced output. If the
specifications are given as two points, it is
safe to assume a linear curve.
11
.
By substituting equations 3. and 4. into equation 2 above, we see that the power curves for a D.C
motor with respect to both speed and torque are quadratics, as shown in equations 5. and 6. From
these equations, we again find that maximum output power occurs at = ½, and = ½ respectively.
12
.
Direct on line starter
In electrical engineering, a direct on line
circuit. The maximum size of a motor
(DOL) or across the line starter starts
limited by the supply utility for this reason.
electric motors by applying the full line
For example, a utility may require rural
voltage to the motor terminals. This is the
customers to use reduced-voltage starters for
simplest type of motor starter. A DOL motor
motors larger than 10 kW.
allowed on a direct on line starter may be
starter also contain protection devices, and
in
some
cases,
condition
monitoring.
DOL starting is sometimes used to start
Smaller sizes of direct on-line starters are
small water pumps, compressors, fans and
manually operated; larger sizes use an
conveyor
electromechanical
to
asynchronous motor, such as the 3-phase
switch the motor circuit. Solid-state direct
squirrel-cage motor, the motor will draw a
on line starters also exist.
high starting current until it has run up to
contactor
(relay)
belts.
In
the
case
of
an
full speed. This starting current is commonly
A direct on line starter can be used if the
around six times the full load current, but
high inrush current of the motor does not
may as high as 12 times the full load current.
cause excessive voltage drop in the supply
1
.
TOPIC TWO
AC MACHINES
Induction Motor
An induction motor or asynchronous motor is a type of alternating current motor where power
is supplied to the rotor by means of electromagnetic induction.
An electric motor converts electrical power
polyphase induction motors, which are
to mechanical power in its rotor (rotating
frequently used in industrial drives.
part). There are several ways to supply
power to the rotor. In a DC motor this power
Induction motors are now the preferred
is supplied to the armature directly from a
choice for industrial motors due to their
DC source, while in an induction motor this
rugged construction, absence of brushes
power is induced in the rotating device. An
(which are required in most DC motors)
induction motor is sometimes called a
and—thanks to modern power electronics—
rotating transformer because the stator
the ability to control the speed of the motor.
(stationary part) is essentially the primary
side of the transformer and the rotor
Principle of operation and comparison to
synchronous motors
3-phase power supply provides a rotating
(rotating part) is the secondary side.
Unlike
the
normal
transformer
which
magnetic field in an induction motor. The
changes the current by using time varying
basic difference between an induction motor
flux, induction motors use rotating magnetic
and a synchronous AC motor is that in the
fields to transform the voltage. The primary
latter a current is supplied into the rotor
side's current creates an electromagnetic
which in turn creates a magnetic field
field which interacts with the secondary
around the rotor. The rotating magnetic field
side's electromagnetic field to produce a
of the stator will impose an electromagnetic
resultant torque, thereby transforming the
torque on the still magnetic field of the rotor
electrical energy into mechanical energy.
causing it to move (about a shaft) and
Induction motors are widely used, especially
rotation of the rotor is produced. It is called
synchronous because at steady state the
2
.
speed of the rotor is the same as the speed of
the rotating magnetic field in the stator.
By way of contrast, the induction motor
currents will be induced. If by some chance
does not have any direct supply onto the
this happens, the rotor typically slows
rotor; instead, a secondary current is induced
slightly until a current is re-induced and then
in the rotor. To achieve this, stator windings
the rotor continues as before. This difference
are arranged around the rotor so that when
between the speed of the rotor and speed of
energized with a polyphase supply they
the rotating magnetic field in the stator is
create a rotating magnetic field pattern
called slip. It is unitless and is the ratio
which sweeps past the rotor. This changing
between the relative speed of the magnetic
magnetic field pattern induces current in the
field as seen by the rotor (the slip speed) to
rotor conductors. These currents interact
the speed of the rotating stator field. Due to
with the rotating magnetic field created by
this an induction motor is sometimes
the stator and in effect cause a rotational
referred to as an asynchronous machine.
motion on the rotor. However, for these
currents to be induced, the speed of the
physical rotor must be less than the speed of
Construction
The stator consists of wound 'poles' that
the rotating magnetic field in the stator, or
carry the supply current to induce a
else the magnetic field will not be moving
magnetic field that penetrates the rotor. In a
relative to the rotor conductors and no
very simple motor, there would be a single
2
.
projecting piece of the stator (a salient pole)
motors have three salient poles per pole
for each pole, with windings around it; in
number, so a four-pole motor would have
fact, to optimize the distribution of the
twelve salient poles. This allows the motor
magnetic field, the windings are distributed
to produce a rotating field, allowing the
in many slots located around the stator, but
motor to start with no extra equipment and
the magnetic field still has the same number
run more efficiently than a similar single-
of north-south alternations. The number of
phase motor.
'poles' can vary between motor types but the
poles are always in pairs (i.e. 2, 4, 6, etc.).
Types of rotors
There are three types of rotor:
Induction motors are most commonly built
Squirrel-cage rotor
to run on single-phase or three-phase power,
The most common rotor is a squirrel-cage
but two-phase motors also exist. In theory,
rotor. It is made up of bars of either solid
two-phase and more than three phase
copper (most common) or aluminum that
induction motors are possible; many single-
span the length of the rotor, and those solid
phase motors having two windings and
copper or aluminum strips can be shorted or
requiring a capacitor can actually be viewed
connected by a ring or sometimes not, i.e.
as two-phase motors, since the capacitor
the rotor can be closed or semi closed type.
generates a second power phase 90 degrees
The rotor bars in squirrel-cage induction
from the single-phase supply and feeds it to
motors are not straight, but have some skew
a separate motor winding. Single-phase
to reduce noise and harmonics.
power is more widely available in residential
buildings, but cannot produce a rotating
Slip ring rotor
field in the motor (the field merely oscillates
A slip ring rotor replaces the bars of the
back and forth), so single-phase induction
squirrel-cage rotor with windings that are
motors must incorporate some kind of
connected to slip rings. When these slip
starting mechanism to produce a rotating
rings are shorted, the rotor behaves similarly
field. They would, using the simplified
to a squirrel-cage rotor; they can also be
analogy of salient poles, have one salient
connected to resistors to produce a high-
pole per pole number; a four-pole motor
resistance rotor circuit, which can be
would have four salient poles. Three-phase
beneficial in starting.
2
.
Solid core rotor
A rotor can be made from solid mild steel.
Starting of induction motors
Direct-on-line starting
The simplest way to start a three-phase
The induced current causes the rotation.
induction motor is to connect its terminals to
Speed control
the line. This method is often called "direct
The synchronous rotational speed of the
on line" and abbreviated DOL.
rotor (i.e. the theoretical unloaded speed
induction motor, the magnitude of the
with no slip) is controlled by the number of
induced
pole pairs (number of windings in the stator)
proportional to the stator field and the slip
and by the frequency of the supply voltage.
speed of the motor, and the rotor current
Under load, the induction motor's speed
depends on this emf. When the motor is
varies according to size of the load. As the
started, the rotor speed is zero. The
load is increased the speed of the motor
synchronous speed is constant, based on the
decreases increasing the slip which increases
frequency of the supplied AC voltage.
emf
in
the
rotor
In an
circuit
is
the rotor's field strength to bear the extra
load.
So the slip speed is equal to the synchronous
speed, the slip ratio is 1, and the induced
Before the development of economical
emf in the rotor is large. As a result, a very
semiconductor power electronics, it was
high current flows through the rotor. This is
difficult to vary the frequency to the motor
similar to a transformer with the secondary
and induction motors were mainly used in
coil short circuited, which causes the
fixed speed applications. As an induction
primary coil to draw a high current from the
motor has no brushes and is easy to control,
mains. When an induction motor starts
many older DC motors are now being
DOL, a very high current is drawn by the
replaced
and
stator, in the order of 5 to 9 times the full
industrial
load current. This high current can, in some
with
accompanying
induction
inverters
motors
in
applications.
motors, damage the windings; in addition,
because it causes heavy line voltage drop,
other appliances connected to the same line
may be affected by the voltage fluctuation.
3
.
To
avoid
such effects,
several other
ii.
strategies are employed for starting motors.
Increased complexity, as more
contactors and some sort of
speed
switch
or
timers
are
needed
Wye-Delta starters
An induction motor's windings can be
iii.
Two shocks to the motor (one for
connected to a 3-phase AC line in two
the initial start and another when
different ways:
the motor switches from wye to
i.
delta)
wye in U.S, star in Europe,
where
the
windings
are
connected from phases of the
Variable-frequency drives
Variable-frequency drives (VFD) can be of
supply to the neutral;
ii.
delta
(sometimes
mesh
in
considerable use in starting as well as
Europe), where the windings are
running motors. A VFD can easily start a
connected between phases of the
motor at a lower frequency than the AC line,
supply.
as well as a lower voltage, so that the motor
starts with full rated torque and with no
A delta connection of the machine winding
inrush of current. The rotor
circuit's
results in a higher voltage at each winding
impedance increases with slip frequency,
compared to a wye connection. A wye-delta
which is equal to supply frequency for a
starter initially connects the motor in wye,
stationary rotor, so running at a lower
which produces a lower starting current than
frequency actually increases torque.
delta, then switches to delta when the motor
has reached a set speed.
Disadvantages of this method over DOL
Resistance starters
This method is used with slip ring motors
starting are:
where the rotor poles can be accessed by
i.
Lower starting torque, which
way of the slip rings. Using brushes,
may be a serious issue with
variable power resistors are connected in
pumps or any devices with
series with the poles. During start-up the
significant breakaway torque
resistance is large and then reduced to zero
at full speed. At start-up the resistance
4
.
directly reduces the rotor current and so
rotation may be commenced by manually
rotor heating is reduced. Another important
giving a slight turn to the rotor. The single
advantage is the start-up torque can be
phase induction motor may rotate in either
controlled. As well, the resistors generate a
direction and it is only the starting circuit
phase shift in the field resulting in the
which determines rotational direction.
magnetic force acting on the rotor having a
favorable angle.
For small motors of a few watts the start
rotation is done by means of a single turn of
heavy copper wire around one corner of the
Series Reactor starters
In series reactor starter technology, an
pole. The current induced in the single turn
impedance in the form of a reactor is
is out of phase with the supply current and
introduced
motor
so causes an out-of-phase component in the
terminals, which as a result reduces the
magnetic field, which imparts to the field
motor terminal voltage resulting in a
sufficient rotational character to start the
reduction of the starting current; the
motor. Starting torque is very low and
impedance of the reactor, a function of the
efficiency is also reduced.
current passing through it, gradually reduces
Such shaded-pole motors are typically used
as the motor accelerates, and at 95 % speed
in low-power applications with low or zero
the reactors are bypassed by a suitable
starting torque requirements, such as desk
bypass method which enables the motor to
fans and record players. Larger motors are
run at full voltage and full speed. Air core
provided with a second stator winding which
series reactor starters or a series reactor soft
is fed with an out-of-phase current to create
starter
and
a rotating magnetic field. The out-of-phase
recommended method for fixed speed motor
current may be derived by feeding the
starting. The applicable standards are [IEC
winding through a capacitor, or it may
289] AND [IS 5553 (PART 3)].
derive from the winding having different
is
in
the
series
most
with
the
common
values of inductance and resistance from the
main winding.
Single Phase induction motor
In a single phase induction motor, it is
In some designs the second winding is
necessary to provide a starting circuit to start
disconnected once the motor is up to speed,
rotation of the rotor. If this is not done,
5
.
usually either by means of a switch operated
by centrifugal force acting on weights on the
motor shaft, or by a positive temperature
coefficient thermistor which after a few
seconds of operation heats up and increases
its resistance to a high value, reducing the
current through the second winding to an
Sine wave current in each of the coils
insignificant level. Other designs keep the
produces sine varying magnetic field on the
second winding continuously energized
rotation axis. Magnetic fields add as vectors.
during running, which improves torque.
Control of speed in induction motor can be
obtained in 3 ways:
1. Scalar control
2. Vector control
3. Direct torque control
Rotating magnetic field
Vector sum of the magnetic field vectors of
Description of magnetic field
A symmetric rotating magnetic field can be
the stator coils produces a single rotating
produced with as few as three coils. The
The result of adding three 120-degrees
three coils will have to be driven by a
phased sine waves on the axis of the motor
symmetric 3-phase AC sine current system,
is a single rotating vector. The rotor has a
thus each phase will be shifted 120 degrees
constant magnetic field. The N pole of the
in phase from the others. For the purpose of
rotor will move toward the S pole of the
this example, the magnetic field is taken to
magnetic field of the stator, and vice versa.
be the linear function of the coil's current
This magneto-mechanical attraction creates
vector of resulting rotating magnetic field.
a force which will drive rotor to follow the
rotating magnetic field in a synchronous
manner.
6
.
A permanent magnet in such a field will
phase system to create the rotating field
rotate so as to maintain its alignment with
utilized in electric motors is one of the main
the external field. This effect was utilized in
reasons why three phase systems dominate
early alternating current electric motors. A
in the world electric power supply systems.
rotating magnetic field can be constructed
Rotating magnetic fields are also used in
using two orthogonal coils with a 90 degree
induction motors. Because magnets degrade
phase difference in their AC currents.
with time, induction motors use short-
However, in practice such a system would
circuited rotors (instead of a magnet) which
be
follow the rotating magnetic field of a multi-
supplied
through
a
three-wire
coiled stator.
arrangement with unequal currents. This
inequality would cause serious problems in
In these motors, the short circuited turns of
the standardization of the conductor size.
the rotor develop eddy currents in the
rotating field of stator which in turn move
In order to overcome this, three-phase
the rotor by Lorentz force. These types of
systems are used where the three currents
motors are not usually synchronous, but
are equal in magnitude and have a 120
instead necessarily involve a degree of 'slip'
degree phase difference. Three similar coils
in order that the current may be produced
having mutual geometrical angles of 120
due to the relative movement of the field and
degrees will create the rotating magnetic
the rotor.
field in this case. The ability of the three
3-φmotor runs from 1-φ power, but does not start
7
.
The single coil of a single phase induction
reverse direction, it will develop a similar
motor does not produce a rotating magnetic
large torque as it nears the speed of the
field,
backward rotating phasor. Single phase
but
maximum
a
pulsating
intensity
at
field
0o
reaching
and
180o
induction motors have a copper or aluminum
electrical. Another view is that the single
squirrel cage embedded in a cylinder of steel
coil excited by a single phase current
laminations, typical of poly-phase induction
produces two counter rotating magnetic field
motors.
phasors, coinciding twice per revolution at
0o and 180o. When the phasors rotate to 90o
and -90o they cancel in figure b. At 45o and
Permanent-split capacitor motor
One way to solve the single phase problem
-45o (figure c) they are partially additive
is to build a 2-phase motor, deriving 2-phase
along the +x axis and cancel along the y
power from single phase. This requires a
axis. An analogous situation exists in figure
motor with two windings spaced apart 90o
d. The sum of these two phasors is a phasor
electrical, fed with two phases of current
stationary in space, but alternating polarity
displaced 90o in time. This is called a
in time. Thus, no starting torque is
permanent-split capacitor motor in Figure
developed.
below,
However, if the rotor is rotated forward at a
bit less than the synchronous speed, it will
develop maximum torque at 10% slip with
respect to the forward rotating phasor. Less
torque will be developed above or below
10% slip. The rotor will see 200% - 10%
Permanent-split capacitor induction motor.
slip with respect to the counter rotating
magnetic field phasor. Little torque (see
This type of motor suffers increased current
torque vs slip curve) other than a double
magnitude and backward time shift as the
freqency ripple is developed from the
motor comes up to speed, with torque
counter rotating phasor. Thus, the single
pulsations at full speed. The solution is to
phase coil will develop torque, once the
keep the capacitor (impedance) small to
rotor is started. If the rotor is started in the
minimize losses. The losses are less than for
1
.
a
shaded
pole
1/4
Capacitor-run induction motor
A variation of the capacitor-start motor
usually
(Figure below) is to start the motor with a
applied to smaller motors. The direction of
relatively large capacitor for high starting
the motor is easily reversed by switching the
torque, but leave a smaller value capacitor in
capacitor in series with the other winding.
place after starting to improve running
configuration
horsepower
motor.
works
(200watt),
well
This
up
motor
to
though,
characteristics while not drawing excessive
current. The additional complexity of the
Capacitor-start induction motor
In Figure below a larger capacitor may be
capacitor-run motor is justified for larger
size motors.
used to start a single phase induction motor
via the auxiliary winding if it is switched out
by a centrifugal switch once the motor is up
to speed. Moreover, the auxiliary winding
may be many more turns of heavier wire
than used in a resistance split-phase motor to
mitigate excessive temperature rise. The
result is that more starting torque is
available
for
conditioning
heavy
loads
compressors.
like
This
air
motor
Capacitor-run motor induction motor
configuration works so well that it is
available
in
multi-horsepower
A motor starting capacitor may be a double-
(multi-
anode non-polar electrolytic capacitor which
kilowatt) sizes.
could be two + to + (or - to -) series
connected polarized electrolytic capacitors.
Such AC rated electrolytic capacitors have
such high losses that they can only be used
for intermittent duty (1 second on, 60
seconds off) like motor starting. A capacitor
for motor running must not be of electrolytic
Capacitor-start induction motor.
construction, but a lower loss polymer type.
2
.
Resistance split-phase induction motor
If an auxiliary winding of much fewer turns
capacitor) arrangement serves well for
of smaller wire is placed at 90o electrical to
driving easily started loads.
motors up to 1/3 horsepower (250 watts)
the main winding, it can start a single phase
induction motor. (Figure below) With lower
This motor has more starting torque than a
inductance and higher resistance, the current
shaded pole motor (next section), but not as
will experience less phase shift than the
much as a two phase motor built from the
main
phase
same parts. The current density in the
difference may be obtained. This coil
auxiliary winding is so high during starting
produces a moderate starting torque, which
that the consequent rapid temperature rise
is disconnected by a centrifugal switch at
precludes frequent restarting or slow starting
winding.
About
30o
of
loads.
3/4 of synchronous speed. This simple (no
Resistance split-phase motor induction motor
3
.
TOPIC FOUR
THREE PHASE INDUCTION MOTOR
Working Principle of Three Phase Induction Motor
An electrical motor is such an electromechanical device which converts electrical energy into a
mechanical energy.
In case of three phase AC operation, most
bars which fits in each slots & they are short
widely used motor is Three phase induction
circuited by the end rings. The slots are not
motor as this type of motor does not require
exactly made parallel to the axis of the shaft
any starting device or we can say they are
but are slotted a little skewed because this
self starting induction motor.
arrangement reduces magnetic humming
For better
understanding the principle of three phase
noise & can avoid stalling of motor.
induction motor, the basic constructional
Production of Rotating Magnetic Field
feature of this motor must be known to us.
The stator of the motor consists of
This Motor consists of two major parts:
overlapping winding offset by an electrical
Stator: Stator of three phase induction
angle of 120°. When the primary winding or
motor is made up of numbers of slots to
the stator is connected to a 3 phase AC
construct a 3 phase winding circuit which is
source, it establishes a rotating magnetic
connected to 3 phase AC source. The three
field which rotates at the synchronous speed.
phase winding are arranged in such a
manner in the slots that they produce a
Secrets behind the rotation:
rotating magnetic field after AC is given to
According to Faraday’s law an e.m.f
them.
induced in any circuit is due to the rate of
change of magnetic flux linkage through the
Rotor: Rotor of three phase induction
circuit. As the rotor winding in an induction
motor consists of cylindrical laminated core
motor are either closed through an external
with parallel slots that can carry conductors.
resistance or directly shorted by end ring,
Conductors are heavy copper or aluminum
1
.
and cut the stator rotating magnetic field, an
• Less armature reaction and brush sparking
e.m.f is induced in the rotor copper bar and
because of the absence of commutators and
due to this e.m.f a current flows through the
brushes that may cause sparks.
rotor conductor.
• Robust in construction.
• Economical.
Here the relative velocity between the
• Easier to maintain.
rotating flux and static rotor conductor is the
cause of electric current generation; hence as
What is the operating principle of a 3ph
per Lenz's law the rotor will rotate in the
induction motor?
same direction to reduce the cause i.e. the
An electric motor converts electrical energy
relative velocity.
into a mechanical energy which is then
supplied to different types of loads. A.C.
Thus from the working principle of three
motors operate on an A.C. supply, and they
phase induction motor it may observed that
are classified into synchronous, single phase
the rotor speed should not reach the
and 3 phase induction, and special purpose
synchronous speed produced by the stator. If
motors. Out of all types, 3 phase induction
the speeds equals, there would be no such
motors are most widely used for industrial
relative velocity, so no emf induction in the
applications mainly because they do not
rotor, & no current would be flowing, and
require a starting device.
therefore no torque would be generated.
Consequently the rotor cannot reach at the
A 3 phase induction motor derives its
synchronous speed. The difference between
name from the fact that the rotor current is
the stator (synchronous speed) and rotor
induced by the magnetic field, instead of
speeds is called the slip. The rotation of the
electrical
magnetic field in an induction motor has the
connections.
The
operating
principle of a 3 phase induction motor is
advantage that no electrical connections
based on the production of rmf.
need to be made to the rotor.
Thus the three phase induction motor is:
• Self-starting.
2
.
speed, a term given to the speed at which the
field produced by primary currents will
rotate, is determined by the following
expression. Synchronous speed of rotation =
(120* supply frequency)/Number of poles
on the stator.
Production of magnetic flux
A rotating magnetic field in the stator is the
first part of operation. To produce a torque
Production of a rotating magnetic field
and thus rotate, the rotors must be carrying
The stator of an induction motor consists of
some current. In induction motors, this
a number of overlapping windings offset by
current comes from the rotor conductors.
an electrical angle of 120°. When the
The revolving magnetic field produced in
primary winding or stator is connected to a
the stator cuts across the conductive bars of
three phase alternating current supply, it
the rotor and induces an emf. The rotor
establishes a rotating magnetic field which
rotates at
a synchronous speed.
windings in an induction motor are either
The
closed through an external resistance or
direction of rotation of the motor depends on
directly shorted. Therefore, the emf induced
the phase sequence of supply lines, and the
in the rotor causes current to flow in a
order in which these lines are connected to
the
stator.
Thus
interchanging
direction opposite to that of the revolving
the
magnetic field in the stator, and leads to a
connection of any two primary terminals to
twisting motion or torque in the rotor.
the supply will reverse the direction of
rotation.
As a consequence, the rotor speed will not
reach the synchronous speed of the rmf in
The number of poles and the frequency of
the
applied
voltage
determine
the stator. If the speeds match, there would
the
be no emf induced in the rotor, no current
synchronous speed of rotation in the motor’s
would be flowing, and therefore no torque
stator. Motors are commonly configured to
would be generated. The difference between
have 2, 4, 6 or 8 poles. The synchronous
the stator (synchronous speed) and rotor
3
.
speeds is called the slip. The rotation of the
delayed in time by one third and two thirds
magnetic field in an induction motor has the
of one cycle of the electric current. This
advantage that no electrical connections
delay between phases causes an effect of
need to be made to the rotor.
giving constant power transfer over each
cycle of the current and also makes it
What results is a motor that is:

Self-starting

Explosion proofed (because of the
possible to produce a rotating magnetic field
in an electric motor.
absence of slip rings or commutators
The sum of the currents is always zero and
and brushes that may cause sparks)
each line returns the current from the other

Robust in construction
two. Thus a three-phase system can operate

Inexpensive
with

Easier to maintain
systems may also have a fourth wire,
only
particularly
three
in
wires.[3]
Three-phase
low-voltage distribution,
which is the neutral wire. The neutral allows
three separate single-phase supplies to be
provided at a constant voltage and is
commonly used for supplying groups of
domestic properties which are each singlephase loads. The connections are arranged
so that as far as possible in each group equal
power is drawn from each phase. Further up
the supply chain in high-voltage distribution
Production of rotating magnetic field in a
the currents are usually well balanced and it
three phase induction motor
is therefore normal to omit the neutral wire.
In a three-phase system, three circuit
Three-phase has properties that make it very
conductors carry three alternating currents
desirable in electric power systems:
(of the same frequency) which reach their

instantaneous peak values at one third of a
The phase currents tend to cancel out
one another, summing to zero in the
cycle from each other. Taking one current as
case of a linear balanced load. This
the reference, the other two currents are
makes it possible to reduce the size
4
.

of the neutral conductor because it
only as single phase. In lower-density areas,
carries little to no current; all the
only a single phase might be used for
phase conductors carry the same
distribution.
current and so can be the same size,
appliances may be powered by three-phase
for a balanced load.
power, such as electric stoves and clothes
Power transfer into a linear balanced
dryers.
Some
large
European
load is constant, which helps to
reduce
generator
and
Wiring for the three phases is typically
motor
identified by color codes which vary by
vibrations.

country. Connection of the phases in the
Three-phase systems can produce a
rotating
specified
magnetic
direction
field
and
with
right order is required to ensure the intended
a
direction of rotation of three-phase motors.
constant
For example, pumps and fans may not work
magnitude, which simplifies the
in reverse. Maintaining the identity of
design of electric motors.
phases is required if there is any possibility
Most household loads are single-phase. In
two sources can be connected at the same
North American residences, three-phase
time; a direct interconnection between two
power might feed a multiple-unit apartment
different phases is a short-circuit.
block, but the household loads are connected
SPEED CONTROL OF THREE PHASE INDUCTION MOTOR
A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to
control its speed.
The speed control of induction motor is
of three phase induction motor as the
done at the cost of decrease in efficiency and
methods of speed control depends upon
low
these formulas. Synchronous speed
electrical
power
factor.
Before
discussing the methods to control the speed
of three phase induction motor one should
know the basic formulas of speed and torque
5
.
Where f = frequency and P is the number of
poles
When rotor is at sand-still slip, s is one. So
• The speed of induction motor is given by,
the equation of torque is,
Where N is the speed of rotor of induction
motor, Ns is the synchronous speed, S is the
Where E2 is the rotor emf
slip.
Ns is the synchronous speed
• The torque produced by three phase
R2 is the rotor resistance
induction motor is given by,
X2 is the rotor inductive
reactance
The Speed of Induction Motor is changed from Both Stator and Rotor Side
The speed control of three phase induction
Speed Control from Stator Side
motor from stator side are further classified
1. V / f control or frequency control -
as:
Whenever three phase supply is given to
1. V / f control or frequency control
three phase induction motor rotating
2. changing the number of stator poles
magnetic field is produced which rotates at
3. controlling supply voltage
synchronous speed given by
4. adding rheostat in the stator circuit
The speed controls of three phase induction
motor from rotor side are further classified
In three phase induction motor emf is
as:
induced by induction similar to that of
1. Adding external resistance on rotor side
transformer which is given by
2. Cascade control method
3. Injecting slip frequency emf into rotor
side
2
.
Where K is the winding constant, T is the
compared to R2 . So, it can be neglected. So
number of turns per phase and f is
torque becomes;
frequency. Now if we change frequency
synchronous
speed
changes
but
with
decrease in frequency flux will increase and
this change in value of flux causes saturation
Since rotor resistance, R2 is constant so the
of rotor and stator cores which will further
equation of torque further reduces to
cause increase in no load current of the
motor . So, its important to maintain flux, φ
constant and it is only possible if we change
voltage i.e if we decrease frequency flux
We know that rotor induced emf E2 ∝ V.
increases but at the same time if we decrease
voltage flux will also decease causing no
So, T ∝ sV2.
change in flux and hence it remains
constant. So, here we are keeping the ratio
From the equation above it is clear that if we
of V/ f as constant. Hence its name is V/ f
decrease supply voltage torque will also
method. For controlling the speed of three
decrease. But for supplying the same load,
phase induction motor by V/ f method we
the torque must remains the same and it is
have
and
only possible if we increase the slip and if
frequency which is easily obtained by using
the slip increases the motor will run at
converter and inverter set.
reduced speed . This method of speed
to
supply
variable
voltage
control is rarely used because small change
2. Controlling supply voltage: The torque
in speed requires large reduction in voltage,
produced by running three phase induction
and hence the current drawn by motor
motor is given by;
increases, which cause over heating of
induction motor.
3. Changing the number of stator poles : The
stator poles can be changed by two methods
In low slip region (sX)2 is very very small as
• Multiple stator winding
method
2
.
• Pole amplitude modulation
wave P1 be the number of poles of induction
method (PAM)
motor whose speed is to be controlled P2 be
the number of poles of modulation wave
• Multiple stator winding method –
In this method of speed control of three
phase induction motor, the stator is provided
by two separate winding . These two stator
After modulation resultant mmf wave;
windings are electrically isolated from each
other and are wound for two different pole
numbers. Using switching arrangement, at a
time, supply is given to one winding only
and hence speed control is possible.
Disadvantages of this method are that the
smooth speed control is not possible. This
method is more costly and less efficient as
So we get, resultant mmf wave
two different stator winding are required.
This method of speed control can only be
applied for squirrel cage motor
• Pole amplitude modulation method
Therefore the resultant mmf wave will have
(PAM) –
two different number of poles
In this method of speed control of three
phase
induction
motor
the
original
sinusoidal mmf wave is modulated by
another
sinusoidal
mmf
wave
Therefore by changing the number of poles
having
we can easily change the speed of three
different number of poles.
phase induction motor
4. Adding rheostat in the stator circuit - In
Let f1(θ) be the original mmf wave of
this method of speed control of three phase
induction motor whose speed is to be
induction motor rheostat is added in the
controlled f2(θ) be the modulation mmf
3
.
stator circuit due to this voltage gets
torque must remains constant. So, we
dropped .In case of three phase induction
increase slip, which will further results in
motor torque produced is given by T ∝ sV22.
decrease in rotor speed. Thus by adding
If we decrease supply voltage torque will
additional resistance in rotor circuit we can
also decrease. But for supplying the same
decrease the speed of three phase induction
load , the torque must remains the same and
motor.
it is only possible if we increase the slip and
The main advantage of this method is that
if the slip increase motor will run reduced
with addition of external resistance starting
speed.
torque increases but this method of speed
control of three phase induction motor also
Speed Control from Rotor Side
suffers from some disadvantages:
1. Adding external resistance on rotor
side – In this method of speed control of
.The speed above the normal value is not
three phase induction motor external
possible
resistance are added on rotor side. The
• Large speed change requires large value of
equation of torque for three phase induction
resistance and if such large value of
motor is
resistance is added in the circuit it will cause
large copper loss and hence reduction in
efficiency
• Presence of resistance causes more losses
The three phase induction motor operates in
• This method cannot be used for squirrel
low slip region .In low slip region term
cage induction motor
(sX)2 becomes very very small as compared
2. Cascade control method – In this
to R2. So, it can be neglected . and also E2 is
method of speed control of three phase
constant. So the equation of torque after
induction motor, the two three phase
simplification becomes,
induction motor are connected on common
shaft and hence called cascaded motor. One
motor is the called the main motor and
another motor is called the auxiliary motor.
Now if we increase rotor resistance, R2
The three phase supply is given to the stator
torque decreases but to supply the same load
4
.
of the main motor while the auxiliary motor
is derived at a slip frequency from the slip
ring of main motor.
;
Let NS1 be the synchronous speed of main
motor; NS2 be the synchronous speed of
Now at no load , the speed of auxiliary rotor
auxiliary motor; P1 be the number of poles
is almost same as its synchronous speed i.e;
of the main motor; P2 be the number of
N = NS2
poles of the auxiliary motor; F is the supply
frequency; F1 is the frequency of rotor
induced
emf
of
main
motor
N is the speed of set and it remains same for
Now rearrange the above equation and find
both the main and auxiliary motor as both
out the value of N, we get,
the motors are mounted on common shaft
S1 is the slip of main motor;
This cascaded set of two motors will now
run at new speed having number of poles (P1
+ P2). In the above method the torque
The auxiliary motor is supplied with same
produced by the main and auxiliary motor
frequency as the main motor i.e
will act in same direction, resulting in
number of poles (P1 + P2). Such type of
cascading is called cumulative cascading.
There is one more type of cascading in
which the torque produced by the main
motor is in opposite direction to that of
auxiliary motor. Such type of is called
Now put the value of;
differential cascading; resulting in speed
corresponds to number of poles (P1 - P2).In
this method of speed control of three phase
5
.
induction motor, four different speeds can
Therefore the efficiency of three phase
be obtained;
induction motor is reduced by this method
of speed control. This slip power loss can be
1. when only main induction motor work,
recovered and supplied back in order to
having speed corresponds to
improve the overall efficiency of three phase
induction
NS1 = 120 F / P1
motor
and
this
scheme
of
recovering the power is called slip power
2. when only auxiliary induction motor
recovery scheme and this is done by
work, having speed corresponds to
connecting an external source of emf of slip
frequency to the rotor circuit. The injected
NS2 = 120 F / P2
3. when cumulative cascading is done, then
emf can either oppose the rotor induced emf
the complete set runs at a speed of
or aids the rotor induced emf. If it oppose
the rotor induced emf, the total rotor
N = 120F / (P1 + P2)
resistance
4. when differential cascading is done, then
increases
and
hence
speed
decreases and if the injected emf aids the
the complete set runs at a speed of
main rotor emf the total resistance decreases
and hence speed increases. Therefore by
N = 120F / (P1 - P2)
injecting induced emf in rotor circuit the
3. Injecting slip frequency emf into rotor
speed can be easily controlled. The main
side - when the speed control of three phase
advantage of this type of speed control of
induction motor is done by adding resistance
three phase induction motor is that wide
in rotor circuit, some part of power called,
range of speed control is possible whether
the slip power is lost as I2R losses.
its above normal or below normal speed.
6
.
TOPIC FIVE
MOTOR ENCLOSURES
The enclosures of electrical motors are standardized by NEMA (National Electrical
Manufacturers Association) as:
1. Open Drip Proof (ODP)
2. Totally
Enclosed
Fan
Cooled
(TEFC)
An open motor in which the ventilating
openings are so constructed that drops of
A motor so enclosed as to prevent the free
liquid or solid particles falling on it, at any
exchange of air between the inside and
angle not greater than 15 degrees for the
outside of the case, but not sufficiently
vertical, cannot enter either directly or by
enclosed to be termed air-tight, and dust
striking and running along a surface of the
does not enter in sufficient quantity to
motor. Designed for reasonably dry, clean,
interfere
and well ventilated (usually indoors) areas.
Cooling is by means of an external fan as an
Outdoor installations require the motor to be
integral part of the motor. The fan provides
protected with a cover that does not restrict
cooling by blowing air on the outside of the
the flow of air to the motor. Ventilation
motor. Suitable where the motor is exposed
openings in shield and/or frame prevent
to dirt or dampness. Not suited in very moist
drops of liquid from falling into motor
humid or hazardous (explosive) locations.
within up to 15 degree angle from vertical.
Same as TENV with an external fan as an
Designed for reasonably dry, clean, and well
integral part of the motor. The fan provides
ventilated (usually indoors) areas. Outdoors
cooling by blowing air on the outside of the
installation requires the
motor.
motor to
be
with
satisfactory
operation.
protected with a cover that does not restrict
3. Totally Enclosed Non Ventilated
the flow of air to the motor.
(TENV)
A motor so enclosed as to prevent the free
exchange of air between the inside and
outside of the case but not sufficiently
2
.
enclosed to be termed air-tight, and dust
5. Totally Enclosed Blower Cooled
does not enter in sufficient quantity to
interfere
with
satisfactory
(TEBC)
operation.
A motor so enclosed as to prevent the free
Cooling is only by convection and radiation
exchange of air between the inside and
from the enclosure. Suitable where the
outside of the case, but not sufficiently
motor is exposed to dirt or dampness. Not
enclosed to be termed air-tight, and dust
suited in very moist humid or hazardous
(explosive)
locations.
No
does not enter in sufficient quantity to
ventilation
interfere with satisfactory operation. Used
openings, enclosed to prevent free exchange
on inverter duty motors. Cooled with
of air (not airtight). No external cooling fan,
external fan on a power supply independent
relies on convection cooling. Suitable where
of the inverter output. Provides full cooling
the motor is exposed to dirt or dampness.
even at lower motor speeds.
Not suited in very moist humid or hazardous
(explosive) air.
6. Explosion Proof
4. Totally Enclosed Air Over (TEAO)
A totally enclosed motor whose enclosure is
A motor so enclosed as to prevent the free
designed and constructed to withstand an
exchange of air between the inside and
explosion of a specified gas or vapor which
outside of the case. A dust-tight enclosure
may occur within it and to prevent the
used on fan and blower motors for shaft
ignition of the specified gas or vapor
mounted fans or belt driven fans. The motor
surrounding the motor by sparks, flashes, or
must be mounted within the airflow of the
explosions of the gas or vapor which may
fan for cooling. Dust-tight fan and blower
occur within the motor housing.
motors for shaft mounted fans or belt driven
fans. The motors mounted within the airflow
7. Explosion-Proof Non Ventilated
of the fan.
(EPNV)
A non-ventilated explosion proof motor.
See TENV and Explosion-Proof above for
more information.
2
.
8. Explosion-Proof
Fan
A fan cooled explosion-proof motor. See
Cooled
TEFC and Explosion- Proof above for more
(EPFC)
information. The motor ambient temperature
shall not exceed +40oC.
3
.
TOPIC SEVEN
ELECTRIC MOTOR CONTROLS
Once the proper motor is selected, understanding the many various control devices available and
their uses and limitations becomes an important part related to reliable operation and protection
of the motor and the personnel using the motor. There are four major motor control topics or
categories to consider. Each of these has several subcategories and sometimes the subcategories
overlap to some extent. Certain pieces of motor control equipment can accomplish multiple
functions from each of the topics or categories.
The four categories include:
An understanding of each of these areas is
1) Starting the Motor
necessary to effectively apply motor control
Disconnecting Means
principles and equipment to effectively
Across the Line Starting
operate and protect a motor.
Reduced Voltage Starting
Motor Starting
All motors must have a control device to
2) Motor Protection
Overcurrent Protection
start and stop the motor called a “motor
Overload Protection
controller”.
Other Protection (voltage, phase, etc)
Environment
3) Stopping the Motor
Coasting
Electrical Braking
Mechanical Braking
4) Motor Operational Control
Speed Control
Reversing
Jogging
Sequence Control
2
.
a. Motor Controller
A motor controller is the actual device that
energizes and de-energizes the circuit to the
motor so that it can start and stop. Motor
controllers may include some or all of the
following
motor
control
functions:
S
starting, stopping, over-current protection,
overload
protection,
changing,
jogging,
reversing,
plugging,
speed
sequence
control, and pilot light indication.
S Controllers range from simple to complex
and can provide control for one motor,
groups of motors, or auxiliary equipment
such as brakes, clutches, solenoids, heaters,
or other signals.
b. Motor Starter
The starting mechanism that energizes the
circuit to an induction motor is called the
“starter” and must supply the motor with
sufficient
current
to
provide adequate
starting torque under worst case line voltage
and load conditions when the motor is
energized. There are several different types
of equipment suitable for use as “motor
starters” but only two types of starting
methods for induction motors:
The above are some motor starters
i) Across the Line Starting
ii). Reduced Voltage Starting
2
.
c. Across the Line Starting of Motors
1. Manual Motor Starters
Across the Line starting connects the motor
A
manual
motor
starter
is
package
windings/terminals directly to the circuit
consisting of a horsepower rated switch with
voltage “across the line” for a “full voltage
one set of contacts for each phase and
start”. This is the simplest method of starting
corresponding thermal overload devices to
a motor. Motors connected across the line
provide motor overload protection. The
are capable of drawing full in-rush current
main advantage of a manual motor starter is
and developing maximum starting torque to
lower cost than a magnetic motor starter
accelerate the load to speed in the shortest
with equivalent motor protection but less
possible time.
motor control capability.
Manual motor starters are often used for
smaller
motors
horsepower
-
motors
typically
but
the
fractional
National
Electrical Code allows their use up to 10
Horsepower. Since the switch contacts
All NEMA induction motors up to 200
remain closed if power is removed from the
horsepower, and many larger ones, can
circuit without operating the switch, the
withstand full voltage starts. (The electric
motor restarts when power is reapplied
distribution system or processing operation
which can be a safety concern. They do not
may not though, even if the motor will).
allow the use of remote control or auxiliary
control equipment like a magnetic starter
does.
d. Across the Line Starters
i)
2. Magnetic Motor Starters
Manual Starter
There are two different types of common
A magnetic motor starter is a package
“across the line” starters including:
consisting of a contactor capable of opening
1. Manual Motor Starters
and closing a set of contacts that energize
2. Magnetic Motor Starters
and de-energize the circuit to the motor
along with additional motor
3
.
overload protection equipment. Magnetic
be required when: The current in-rush form
starters are used with larger motors (required
the motor starting adversely affects the
above 10 horsepower) or where greater
voltage drop on the electrical system;
motor control is desired. The main element
needed to reduce the mechanical “starting
of the magnetic motor starter is the
shock” on drive-lines and equipment when
contactor, a set of contacts operated by an
the motor starts. Reducing the voltage
electromagnetic coil. Energizing the coil
reduces the current in-rush to the motor and
causes the contacts (A) to close allowing
also reduces the starting torque available
large currents to be initiated and interrupted
when the motor starts.
by a smaller voltage control signal.
All NEMA induction motors can accept
The control voltage need not be the same as
reduced voltage starting however it may not
the motor supply voltage and is often low
provide enough starting torque in some
voltage allowing start/stop controls to be
situations to drive certain specific loads. If
located remotely from the power circuit.
the driven load or the power distribution
Closing the Start button contact energizes
system cannot accept a full voltage start,
the contactor coil. An auxiliary contact on
some type of reduced voltage or "soft"
the contactor is wired to seal in the coil
starting scheme must be used. Typical
circuit. The contactor de-energizes if the
reduced voltage starter types include:
control circuit is interrupted, the Stop button
1. Solid State (Electronic) Starters
is operated, or if power is lost. The overload
2. Primary Resistance Starters
contacts are arranged so an overload trip on
3. Autotransformer Starters
any phase will cause the contactor to open
4. Part Winding Starters
and de-energize all phases.
5. Wye-Delta Starters
Reduced Voltage Starting of Motors
Reduced voltage starters can only be used
Reduced Voltage Starting connects the
where low starting torque is acceptable or a
motor windings/terminals at lower than
means exists to remove the load from the
normal line voltage during the initial starting
motor or application before it is stopped.
period to reduce the inrush current when the
motor starts. Reduced voltage starting may
4
.
Motor Protection
Motor protection safeguards the motor, the
fuses or circuit breakers. These devices
supply system and personnel from various
an extremely heavy overload occurs. Most
operating conditions of the driven load, the
overcurrent sources produce extremely large
supply system or the motor itself.
currents very quickly.
operate when a short circuit, ground fault or
ii.
Overload Protection
Overload protection is installed in the motor
circuit and/or motor to protect the motor
from damage from mechanical overload
conditions when it is operating/running. The
effect of an overload is an excessive rise in
Motor protection categories include:
temperature in the motor windings due to
i.
Overcurrent Protection
ii.
Overload Protection
iii.
Other Types of Protection.
current higher than full load current.
Properly
overcurrent
and
protection
supply when the heat generated in the motor
motors and their conductors be protected
both
overload
disconnects the motor from the power
The National Electrical Code requires that
from
sized
circuit or windings approaches a damaging
overload
level for any reason.
conditions.
The larger the overload, the more quickly
i.
Overcurrent Protection
Overcurrent
protection
the temperature will increase to a point that
interrupts
the
is damaging to the insulation and lubrication
electrical circuit to the motor upon excessive
of the motor. Unlike common instantaneous
current demand on the supply system from
type fuses and breakers, overload devices
either short circuits or ground faults.
are designed to allow high currents to flow
Overcurrent protection is required to protect
personnel,
the
motor
branch
briefly in the motor to allow for:
circuit

conductors, control equipment, and motor
Typical motor starting currents
of 6 to 8 times normal running
from these high currents. Overcurrent
current when starting.
protection is usually provided in the form of
5
.


Short duration overloads such
voltage drops below a preset value. The
as a slug of product going
motor must be manually restarted upon
through a system.
resumption of normal supply voltage.
If the motor inlets and outlets
Low Voltage Release - Protection device
are covered by a blanket of lint
interrupts the circuit when the supply
or if a bearing should begin to
voltage drops below a preset value and re-
lock, excessive heating of the
establishes the circuit when the supply
motor windings will “overload”
voltage returns to normal.
the motors insulation which
ii.
could damage the motor.
Phase Failure Protection
Interrupts the power in all phases of a threeThe overcurrent device will not react to this
phase circuit upon failure of any one phase.
low level overload. The motor overload
Normal fusing and overload protection may
device prevents this type of problem from
not adequately protect a polyphase motor
severely damaging the motor and also
from damaging single phase operation.
provides
circuit
Without this protection, the motor will
conductors since it is rated for the same or
continue to operate if one phase is lost.
less current as the conductors. Overload
Large currents can be developed in the
protection trips when an overload exists for
remaining stator circuits which eventually
more than a short time. The time it takes for
burn out. Phase failure protection is the only
an overload to trip depends on the type of
effective way to protect a motor properly
overload device, length of time the overload
from single phasing.
protection
for
the
exists, and the ambient temperature in which
iii.
the overloads are located.
Phase Reversal Protection
Used where running a motor backwards
(opposite direction from normal) would
Other Motor Protection Devices
i.
Low Voltage Protection
cause operational or safety problems. Most
three phase motors will run the opposite
Low Voltage Disconnects - Protection
direction by switching the connections of
device operates to disconnect the motor
any two of the three phases. The device
when the supply
interrupts the power to the motor upon
6
.
detection of a phase reversal in the three-
iv.
Ground Fault Protection
phase supply circuit. This type of protection
Operates when one phase of a motor shorts
is used in applications like elevators where it
to ground preventing high currents from
would be damaging or dangerous for the
damaging the stator windings and the iron
motor to inadvertently run in reverse.
core.
7
.
1
.
TOPIC EIGHT: CONTACTORS
CONTACTORS
A contactor is an electrically controlled switch used for switching a power circuit, similar to a
relay except with higher current ratings. A contactor is controlled by a circuit which has a much
lower power level than the switched circuit
.
interrupt a short circuit current. Contactors
range from those having a breaking current
of several amperes to thousands of amperes
and 24 V DC to many kilovolts. The
physical size of contactors ranges from a
device small enough to pick up with one
hand, to large devices approximately a meter
(yard) on a side. Contactors are used to
control electric motors, lighting, heating,
capacitor banks, thermal evaporators, and
other electrical loads.
A contactor has three components. The
contacts are the current carrying part of the
contactor. This includes power contacts,
auxiliary contacts, and contact springs. The
electromagnet (or "coil") provides the
driving force to close the contacts. The
enclosure is a frame housing the contact and
the electromagnet. Enclosures are made of
Some contactors
insulating materials like Bakelite, Nylon 6,
Contactors come in many forms with
and thermosetting plastics to protect and
varying capacities and features. Unlike a
insulate the contacts and to provide some
circuit breaker, a contactor is not intended to
measure of protection against personnel
2
.
touching
the
contacts.
Open-frame
A basic contactor will have a coil input
contactors may have a further enclosure to
(which may be driven by either an AC or
protect against dust, oil, explosion hazards
DC supply depending on the contactor
and weather.
design). The coil may be energized at the
same voltage as a motor the contactor is
Magnetic blowouts use blowout coils to
controlling, or may be separately controlled
lengthen and move the electric arc. These
with a lower coil voltage better suited to
are especially useful in DC power circuits.
control by programmable controllers and
AC arcs have periods of low current, during
lower-voltage
which the arc can be extinguished with
pilot
devices.
Certain
contactors have series coils connected in the
relative ease, but DC arcs have continuous
motor circuit; these are used, for example,
high current, so blowing them out requires
for automatic acceleration control, where the
the arc to be stretched further than an AC
next stage of resistance is not cut out until
arc of the same current. The magnetic
the motor current has dropped.
blowouts in the pictured Albright contactor
(which is designed for DC currents) more
Applications of Contactors
than double the current it can break,
1. Lighting control
increasing it from 600 A to 1,500 A.
Contactors are often used to provide central
Sometimes an economizer circuit is also
control of large lighting installations, such
installed to reduce the power required to
as an office building or retail building. To
keep a contactor closed; an auxiliary contact
reduce power consumption in the contactor
reduces coil current after the contactor
coils, latching contactors are used, which
closes. A somewhat greater amount of
have
power is required to initially close a
two
operating
coils.
One
coil,
momentarily energized, closes the power
contactor than is required to keep it closed.
circuit
Such a circuit can save a substantial amount
contacts,
which
are
then
mechanically held closed; the second coil
of power and allow the energized coil to stay
opens the contacts.
cooler. Economizer circuits are nearly
always applied on direct-current contactor
coils and on large alternating current
contactor coils.
2
.
How Contactor Controls an Electric
Motor
2. Magnetic starter
A magnetic starter is a device designed to
provide power to electric motors. It includes
a contactor as an essential component, while
also providing power-cutoff, under-voltage,
and overload protection.
3. Vacuum contactor
Vacuum contactors utilize vacuum bottle
Control of electric motor with contactor
encapsulated contacts to suppress the arc.
This arc suppression allows the contacts to
When a relay is used to switch a large
be much smaller and use less space than air
amount of electrical power through its
break contacts at higher currents. As the
contacts, it is designated by a special name:
contacts
contactor.
are
encapsulated,
vacuum
Contactors
typically
have
contactors are used fairly extensively in
multiple contacts, and those contacts are
dirty applications, such as mining. Vacuum
usually (but not always) normally-open, so
contactors are only applicable for use in AC
that power to the load is shut off when the
systems.
coil is de-energized. Perhaps the most
common industrial use for contactors is the
control of electric motors.
The AC arc generated upon opening of the
contacts will self-extinguish at the zero-
The top three contacts switch the respective
crossing of the current waveform, with the
phases of the incoming 3-phase AC power,
vacuum preventing a re-strike of the arc
typically at least 480 Volts for motors 1
across the open contacts. Vacuum contactors
horsepower or greater. The lowest contact is
are therefore very efficient at disrupting the
an “auxiliary” contact which has a current
energy of an electric arc and are used when
rating much lower than that of the large
relatively fast switching is required, as the
motor power contacts, but is actuated by the
maximum break time is determined by the
same armature as the power contacts. The
periodicity of the AC waveform.
auxiliary contact is often used in a relay
logic circuit, or for some other part of the
3
.
motor control scheme, typically switching
and directly break the circuit as a fuse is
120 Volt AC power instead of the motor
designed to do.
voltage.
Rather, overload heaters are designed to
One contactor may have several auxiliary
thermally mimic the heating characteristic of
contacts, either normally-open or normally-
the particular electric motor to be protected.
closed, if required. The three “opposed-
All motors have thermal characteristics,
question-mark” shaped devices in series
including the amount of heat energy
with each phase going to the motor are
generated by resistive dissipation (I2R), the
called overload heaters. Each “heater”
thermal transfer characteristics of heat
element is a low-resistance strip of metal
“conducted” to the cooling medium through
intended to heat up as the motor draws
the metal frame of the motor, the physical
current. If the temperature of any of these
mass and specific heat of the materials
heater elements reaches a critical point
constituting the motor, etc.
(equivalent to a moderate overloading of the
These characteristics are mimicked by the
motor), a normally-closed switch contact
overload heater on a miniature scale: when
(not shown in the diagram) will spring open.
the motor heats up toward its critical
This normally-closed contact is usually
temperature, so will the heater toward its
connected in series with the relay coil, so
that
when
it
opens
the
relay
critical temperature, ideally at the same rate
will
and approach curve. Thus, the overload
automatically de-energize, thereby shutting
contact, in sensing heater temperature with a
off power to the motor.
thermo-mechanical mechanism, will sense
Overload heaters are intended to provide
an analogue of the real motor. If the
overcurrent protection for large electric
overload contact trips due to excessive
motors, unlike circuit breakers and fuses
heater temperature, it will be an indication
which
of
that the real motor has reached its critical
providing overcurrent protection for power
temperature (or, would have done so in a
conductors. Overload heater function is
short while). After tripping, the heaters are
often misunderstood. They are not fuses;
supposed to cool down at the same rate and
that is, it is not their function to burn open
approach curve as the real motor, so that
serve
the
primary purpose
they indicate an accurate proportion of the
4
.
motor’s thermal condition, and will not
provide information on the appropriate
allow power to be re-applied until the motor
heater units to use.
is truly ready for start-up again.
A white pushbutton located between the
Shown here below is a contactor for a three-
“T1″ and “T2″ line heaters serves as a way
phase electric motor, installed on a panel as
to manually re-set the normally-closed
part of an electrical control system at a
switch contact back to its normal state after
municipal water treatment plant. Three-
having been tripped by excessive heater
phase, 480 volt AC power comes in to the
temperature.
three normally-open contacts at the top of
“overload” switch contact may be seen at
the contactor via screw terminals labeled
the lower-right of the photograph, near a
“L1,” “L2,” and “L3″ (The “L2″ terminal is
label reading “NC” (normally-closed). On
hidden behind a square-shaped “snubber”
this particular overload unit, a small
circuit connected across the contactor’s coil
“window” with the label “Tripped” indicates
terminals). Power to the motor exits the
a tripped condition by means of a colored
overload heater assembly at the bottom of
flag. In this photograph, there is no
this device via screw terminals labeled “T1,”
“tripped”
“T2,” and “T3.”
appears clear
Wire
condition,
connections
and
the
to
the
indicator
The overload heater units themselves are
black, square-shaped blocks with the label
“W34,” indicating a particular thermal
response for a certain horsepower and
temperature rating of electric motor. If an
electric motor of differing power and/or
temperature ratings were to be substituted
for the one presently in service, the overload
heater units would have to be replaced with
units having a thermal response suitable for
Contactor for a three-phase electric motor
the new motor. The motor manufacturer can
installed on a panel as part of an electrical
control system
5
.
TOPIC FOUR
PREVENTIVE MAINTENANCE
WHY PREVENTIVE MAINTENANCE?
Preventive maintenance is predetermined work performed to a schedule with the aim of
preventing the wear and tear or sudden failure of equipment components.
Preventive maintenance helps to:
function

following elements:
Protect assets and prolong the
should
incorporate
the
useful life of production equipment

Improve system reliability
1. Planned replacements

Decrease cost of replacement
Planned replacements of components

Decreases system downtime
designed around the following:

Reduce injury

Reliability
of
components
(equipment failure is caused by its
Mechanical,
process
or
least reliable component)
control
equipment failure can have adverse
-
check manufacturer’s information
results in both human and economic
-
check

the costs involved to repair and/or
equipment
parts
industry
best
practices
terms. In addition to down time and
replace
accepted
Maintaining
equipment
service
records
or

components, there is the risk of injury
Scheduling
replacement
of
to operators, and of acute exposures to
components at the end of their
chemical and/ or physical agents.
useful service life

Preventive maintenance, therefore, is a
Acquiring
and
maintaining
inventories of:
very
important ongoing accident prevention
-
east reliable components
activity, which you should integrate
-
critical components
into
-
components
your
manufacturing
operations/
process.
product
To
replacements
be
effective, your preventive maintenance
1
scheduled
for
.

Replacing service-prone equipment
Preventive Maintenance
with more reliable performers
Identifying Maintenance Hazards
By introducing the element of planning
The
into your maintenance function, you
maintenance
are likely to reduce your repair and
classified as follows:
manpower requirements.
Safety Hazards

maintenance
is

plant
can
Mechanical
Electrical
requirements

Pneumatic

Hydraulic

Thermal
specifications of equipment

Combustion

Past experience with components:

Falls
-
inspection records
 slippery floors
-
servicing records
 working at heights
-
replacement frequency

Health Hazards
-
inspected component failures

Chemical Agents

Regularly scheduled
include:

Operating
and
performing
lubrication
 process chemicals
program:
-
identify
 cleaning solvents
lubrication
points
on
 unexpected
equipment
-
-
be
 live equipment
Diagnostic measures to
your
activities
with
 tools
to
anticipate and prevent breakdowns.
analyze
associated
 equipment
2. Exploratory maintenance
Exploratory
hazards
reaction
products
colour code in order to identify
 dusts
lubrication frequency
 other chemical agents
consult manufacturer and accepted

Physical Agents
industry best practices to establish
 noise
schedule
 vibration
 other

2
Ergonomic Hazards
.

Controlling Maintenance Hazards
Ideally, the hazards likely to occur
Biomechanical
 lifting,
pushing,
pulling
during maintenance activities should
(manual handling)
be addressed in the
 stretching, ending (to reach
planning stage.
hard to access areas)

Work/process design
 poorly designed tools
Process Selection
 hard to access work locations
Depending on the nature of the
 ill-fitting personal protective
process, special precautions may be
needed to
equipment
protect
workers when
disassembling and cleaning equipment.
 complex procedures
Consider this factor when you make a
decision to select one process over
Many of these hazards are interrelated.
another. Also consider the following
Examine your process, the layout of
factors which contribute to the level of
your process area, and the process
risk of your maintenance activities:
equipment used, to determine the exact
 How easy temporary structures
nature of the hazards likely to be
are to erect
encountered during your maintenance
 How easy they are to access
activities. For example, maintenance
 Support and reassembly of
work carried out in confined spaces
components
carries a greater risk of critical injuries
associated
agents.
with
These
risks
equipment
large
scale
equipment
and acute exposures to chemical and
physical
of
 Use of hoists and mobile
are
working platforms
and
 Safe use of ladders especially
materials in the space itself and from
near live electrical equipment
nearby operations. Fatalities are quite
 How
common.
much disassembly
is
required to access affected
equipment
 Need for temporary hoisting
equipment
3
.
 Need for personal protective

equipment
A clear, step-by-step procedure, in
checklist
 Housekeeping hazards created
form,
for
controlling
hazardous energy:
at floor level by the presence of
1. Preparing for shutdown
dismantled components
2. Shutting down machine, process or
equipment
Equipment Selection
3. Isolating energy to the machine,
The process you select will determine
process or equipment
the type of equipment you will be
4. Applying lockout devices
using.
5. Controlling stored energy
However,
consider
the
following:
6. Verification of isolation

Reliability:
7. Release from lockout control

manufacturer’s data

Hazards identification

in-plant operating experience

Selection and specification of personal

trade association data

Ease of access to serviceable parts
 appropriate for the hazard

Ease of disassembly
 proper fit

Complexity of repair procedures

Ease of frequency of required
be used:
lubrication
 right tool for the job
Manufacturer/supplier follow-up:
 in good condition
 availability of parts
 appropriate for the environment

protective equipment:

(non-sparking tools in flammable
 availability of service time
atmospheres)
Developing Procedures
 ergonomic design
When servicing equipment, hazards

not related to your process operation

reason, it is important to prepare
servicing
procedures
Step-by-step
procedure
for
disassembly
are likely to be introduced. For this
written
Selection and specification of tools to
Step-by-step checklist for inspection of
components (to establish a baseline for
that
reliability)
include the following:
4
.

Identification of hazards associated
with sub-procedures:
Training
 entering and working in confined
Maintenance
are
often
involved in a complex and changing
spaces
 welding in open and confined
set of problems. Therefore, they need
more thorough training in accident
spaces

personnel
 removing insulation
prevention
 cleaning
Serious consequences to maintenance
 handling and using solvents
and other workers can result from not
 erecting temporary structures
following
 using portable equipment
procedures (e.g., use of work permits,
 using ladders
lockout procedures, confined space
 abrasive blasting
entry procedures). Ensure that your
 painting
maintenance personnel are well trained
Erection
scaffolding
and
disassembly
and
other
than
regular
established
workers.
maintenance
in, and can demonstrate that they
of
understand, all relevant procedures.
temporary
platforms
Also provide training in:

Disassembly of small-scale equipment

Hazard identification

Reassembly of small-scale equipment

Selection,

Support and disassembly of large scale
equipment, machine tools, personal
equipment
protective clothing/equipment, etc.,
use,
and
care
of
required to be used
Examine each procedure thoroughly to

First-aid and life-saving techniques
ensure that the least hazardous method

The
hazards
of
and
control
is selected, and that all precautions
methods for substances which may
necessary to complete the job safely
be encountered in the workplace,
are taken. Keep records of all your
such as:
maintenance activities, indicating the
 irritating, toxic or corrosive dusts
machine(s)
 gases
involved,
the
part(s)
involved, type of maintenance and date
 vapours
on which performed.
 fluids
5
.
How to inspect chains, blocks, fall
needed. Train equipment operators to
protection devices and ropes
recognize the signs of impending

How to secure loads
failure,

Understanding stresses
excessive

such
as
abnormal
vibration,
noise,
declining
or
abnormal output, and to report these
immediately to their supervisor.
It is a good practice to call the
maintenance crew together at the start
of each job, in order to discuss the
Legislation
hazards involved and the method of
The following Regulations made under
doing it safely. In the course of their
the Occupational Health and Safety
daily
Act contain provisions that deal with
work,
members
of
the
maintenance:
maintenance crew travel throughout

the plant, becoming familiar with

selected and trained, they can do much
identify
and
correct
staff
responsible
for
may
also
and
maintaining portable power
tools,
of
Exposure
to
(R.R.O. 833/90)

be
inspecting
Control
Biological or Chemical Agents
unsafe
conditions. In small companies, the
maintenance
Establishments
(R.R.O. 851/90)
every machine and process. If properly
to
Industrial
Workplace
Hazardous
Materials Information System
(R.R.O. 860/90)
extension cords, and the like. If so,

special procedures and training are
6
Designated Substances