UNESCO-NIGERIA TECHNICAL &
VOCATIONAL EDUCATION
REVITALISATION PROJECT-PHASE II
NATIONAL DIPLOMA IN
ELECTRICAL ENGINEERING TECHNOLOGY
ELECTRICAL MACHIENS I
COURSE CODE: EEC 123
YEAR I- SEMESTER II
THEORY
Version 1: December 2008
1
TABLE OF CONTENT
Subject
Electrical Machines I
Year
1
Semester
2
Course Code
EEC123
Credit Hours
6
Theoretical
1
Practical
5
CHAPTER 1 : Magnetism …………………………………………....................WEEK1
1.1 Introduction ............................................................................................... 1
1.2 Concepts of Magnetism .......................................................................... 1
1.3 Types of magnets .................................................................................... 2
1.3.1 Permanent Magnet .................................................................. 2
1.3.2 Temporary Magnet ................................................................. 3
1.4 Electromagnetic Fields .......................................................................... 4
1.5 Magnetic Field Produced by a Coil ...................................................... 5
1.6 Induction ................................................................................................... 6
1.6.1 Induction Meaning ................................................................... 6
1.6.2 Self Inductance ...................................................................... 7
1.6.3 Mutual Inductance.................................................................. 8
CHAPTER 2 : DC Generator ………………………………….…………WEEK2
2.1 Introduction ............................................................................................. 2
2
2.2 The Basic Principle DC generator ....................................................... 2
2.2.1 The simplest AC generator .................................................. 6
2.2.2 The simplest DC generator................................................. 7
2.3 Constructions of DC Generator .......................................................... 2
2.3.1 Magnetic field structure ...................................................... 6
2.3.2 Armature structure ............................................................. 7
2.3.3 Commutator structure .......................................................... 6
2.3.4 Brush structure ..................................................................... 7
2.4 E.M.F Equation ....................................................................... WEEK3 2
2.5 Armature Reaction ................................................................................ 2
2.2.1 Shifting the Brushes .................................................................. 6
2.2.2 Compensating Windings and Interpoles .................................... 7
2.6 Classification Of Generators............................................................... 2
2.7 Voltage Regulation ................................................................ WEEK4 2
2.8 Generator Power Losses ..................................................................... 2
2.8.1 Copper Losses Losses ................................................................. 6
2.8.2 Eddy Current Losses ................................................................ 7
2.8.3 Hysteresis Losses
.................................................................. 7
CHAPTER 3 : DC Motor ………..………..... ............WEEK5
3.1. Introduction .........................................................................................26
3.2. Constructions and Operation Principle of DC Generator...........26
3.2.1 The dc motor torque ..................................................................26
3
3.2.2 The dc motor torque .................................................................29
3.3. Back E. M. F...........................................................................................26
3.4. Types and characteristics of DC Motors ......................................26
3.4.1 Series DC Motor ........................................................................26
3.4.2 Shunt DC Motor ........................................................................29
3.4.3 Compound DC Motor ..................................................................26
3.4. Motor Nameplate ............................................................. WEEK6 37
3.4.1 Nameplate Terms ..................................................................37
3.4.2 Definition Nameplate ..........................................................37
3.5. Power Losses and Efficiency ...........................................................43
3.6. Starting Methods of DC Motor ..................................... WEEK7 45
3.6.1 Face –plate Starter ...........................................................46
3.6.2 Relay Starter .......................................................................48
3.7. Reversing the Rotation of DC Motor ............................ WEEK8 51
3.7.1. Reversing the Rotation of DC Series Motor ........................... 51
3.7.2. Reversing the Rotation of DC Shunt Motor ............................53
3.7.2. Reversing the Rotation of DC Compound Motor ....................54
3.8. Inspection and Maintenance of DC Motors ............... WEEK9 51
CHAPTER 4 : Single Phase Induction Motor…….WEEK10
4.1. Introduction .........................................................................................26
4
4.2. Construction of A.C single-phase induction motor .....................26
4.3. Types and characteristics of DC Motors ......................................26
4.2.1 Rotor ........................................................................................26
4.2.2. Stator ....................................................................................29
4.2.3. Frame enclosure .................................................................26
4.2.4. Fan ..........................................................................................26
4.2..5. Terminal ( connection ) box .............................................29
4.2.6 Centrifugal switch................................................................26
4.3. How Electrical Motor Work ...........................................................62
4.4. Operation Principle..............................................................................64
4.5. Motor Speed ....................................................................... WEEK11 67
4.5.1 Synchronous Speed ............................................................67
4.5.2 Rotor Speed .........................................................................68
4.6. Types of Single Phase Induction Motor ................... WEEK12 69
4.6.1 Split Phase Motors ..............................................................69
4.6.2 Capacitor Motors .................................................................72
4.6.3 Capacitor Run Motors..........................................................73
4.6.4 Capacitor Start Motors ......................................................75
4.6.5 Capacitor Start Capacitor Run Motors ........ WEEK13 77
4.6.6 Shadded Pole Induction Motors .......................................78
5
4.6.7 Repulsion Motors ............................................... WEEK14 80
4.6.8 Universal Motors .................................................................. 81
4.7. Speed-torque characteristics of single-phase
induction motor ....................................................................................83
4.8. power, losses and efficiency
.................................... WEEK15 84
4.8.1 Input power ............................................................................84
4.8.2 Kw to Hp Conversion ............................................................84
4.8.3 Motor Losses .........................................................................84
4.8.3.1 Core or Iron Losses................................................................... 86
4.8.3.2 Rotor Losses ............................................................................... 86
4.8.3.3 Stator Losses ............................................................................. 86
4.8.3.4 Friction and Windage Losses ................................................. 87
4.8.3.5 Stray Losses ............................................................................ 87
4.8.4 Efficiency ...............................................................................88
4.8.5 External speed control drives...........................................89
4.8.5.1 Direct drive ................................................................................. 89
4.8.5.2 Belt and pulley drives ............................................................... 89
4.8.5.3 Gear motors ................................................................................ 90
4.8.5.4 Gear drives .................................................................................. 90
4.8.5.5 Chain and Sprocket ....................................................................91
4.9. Nameplate information ................................................................... 91
4.10. Reversing the direction of rotation .............................................. 91
4.10.1 split-phase induction motor .............................................92
6
4.10.2 capacitor-run induction motor ........................................92
4.10.3 Very small induction motors ..........................................92
4.10.4 shaded-pole induction motors .........................................92
4.11. speed control .......................................................................................93
4.12. Applications .........................................................................................93
7
This Page is Intentionally Left Blank
8
Week 1
Introduction
Now, before we discuss basic electrical machine operation a short review of
magnetism might be helpful to many of us. We all know that a magnet will attract and
hold metal objects when the object is near or in contact with the magnet.
1.2 Concepts of Magnetism
A magnetic field is a change in energy within a volume of space. The magnetic
field surrounding a bar magnet can be seen in the magnetograph shown in fig.(1-1). A
magnetograph can be created by placing a piece of paper over a magnet and sprinkling
the paper with iron filings. The particles align themselves with the lines of magnetic
force produced by the magnet. The magnetic lines of force show where the magnetic
field exits the material at one pole and reenters the material at another pole along the
length of the magnet. It should be noted that the magnetic lines of force exist in threedimensions but are only seen in two dimensions in the image.
Figure(1-1) : The magnetic field surrounding a bar magnet
It can be seen in the magnetograph that there are poles all along the length of the
magnet but that the poles are concentrated at the ends of the magnet. The area where
the exit poles are concentrated is called the magnet's north pole and the area where the
entrance poles are concentrated is called the magnet's south pole.
9
Magnets come in a variety of shapes and one of the
more common is the horseshoe (U) magnet. The
horseshoe magnet has north and south poles just like a bar
magnet but the magnet is curved so the poles lie in the
same plane, the magnetic field is concentrated between
the poles as shown in figure (1-2).
Figure(1-2) : Horseshoe magnet
The number of magnetic lines of force is a known as magnetic flux Φ. The flux
has the weber (wb) as its unit, The number of magnetic lines of force cutting through
a plane of a given area at a right angle is known as the magnetic flux density B. The
flux density or magnetic induction has the tesla as its unit. One tesla is equal to 1
Newton/(A/m). From these units it can be seen that the flux density is a measure of the
force applied to a particle by the magnetic field. T
Types of magnets
There are two kinds of magnets permanent and temporary magnets.
1.3.1 Permanent magnet
Permanent magnet will retain or keep their
magnetic properties for a very long time. Permanent
magnets are made by placing pieces of iron cobalt,
and nickel into strong magnetic fields. Permanent
magnets are mixtures of iron, nickel, or cobalt with
Figure(1-3) : natural magnet
other elements. These are known as hard magnetic materials. The natural form of a
magnet is called a lodestone as shown in fig.(1-3), it contains iron. When man mixed
the pure metals together ( ie. iron, nickel and cobalt ) we created an even stronger
magnet which are the ones we use most today.
1.3.2 Temporary magnets
Temporary magnets will loose all or most of their magnetic properties. Temporary
magnets are made of such materials as iron and nickel. There are two essential
methods for generating a magnetic field. Those two following methods:
10
1- Magnetic material methods
Magnetic material by stroking a
permanent magnet onto a pure metal
in one direction many times, soon it
will become temporarily magnetized
(a)
as shown in fig.(1-4).
Figure(1-4) : Generating magnetic
material
2- Electrical currents methods
Electrical currents can be used to
make a magnet by getting a bar of iron
and wrapping it with wires then run a
current through the wires as shown in
fig.(1-5). This arrangement is called a
(b
Figure(1-6): Magnetic field
around the wire carried current
solenoid and can be used to generate a
nearly uniform magnetic field similar
to that of a bar magnet.
Figure(1-5) :Generating electromagnet
(Solenoid)
Electromagnetic Fields
Magnets are not the only source of magnetic fields. In 1820, Hans Christian
Oersted discovered that the current in the wire was generating a magnetic field. He
found that the magnetic field existed in circular form around the wire and that the
intensity of the field was directly proportional to the amount of current carried by the
wire as shown in fig.(1-6a) . A three-dimensional representation of the magnetic field
is shown in fig.(1-6b).
11
There is a simple rule for remembering the direction of the magnetic field around a
conductor. It is called the right-hand rule. If a person grasps a conductor in ones right
hand with the thumb pointing in the direction of the current, the fingers will circle the
conductor in the direction of the magnetic field as
shown in fig. (1-7).
Magnetic Field Produced by a Coil
A loosely wound coil is illustrated in figure(1-8)
below to show the interaction of the magnetic field. The Figure(1-7): Right-hand rule
magnetic field is essentially uniform down the length of the coil when it is wound
tighter.
Figure(1-8): Magnetic Field Produced by a Coil
The strength of a coil's magnetic field increases not only with increasing current
but also with each loop that is added to the coil. Coiling a current-carrying conductor
around a core material that can be easily magnetized, such as iron, can form an
electromagnet. The magnetic field will be concentrated in the core. This arrangement
is called a solenoid. The more turns we wrap on this core, the stronger the
electromagnet and the stronger the magnetic lines of force become.
12
Inductance
Induction Meaning
Faraday noticed that the rate at which the
magnetic field changed also had an effect on the amount
of current or voltage that was induced. Faraday's Law
for an uncoiled conductor states that the amount of
induced voltage is proportional to the rate of change of
flux lines cutting the conductor. Faraday's Law for a
straight wire is shown below.
Figure(1-9): Induction in wire
Induction is measured in unit of Henries (H) which reflects this dependence on
the rate of change of the magnetic field. One henry is the amount of inductance that is
required to generate one volt of induced voltage when the current is changing at the
rate of one ampere per second. Note that current is used in the definition rather than
magnetic field.
13
Self-inductance
When induction occurs in an electrical circuit
and affects the flow of electricity it is called inductance,
L. Self-inductance, or simply inductance is the property
of a circuit whereby a change in current causes a change
in voltage in the same circuit as shown in fig.(1-10).
The mmf required to produce the changing
magnetic flux (Φ) must be supplied by a changing Figure(1-10): Self inductance
current through the coil. Magnetomotive force generated by an electromagnet coil is
equal to the amount of current through that coil (in amps) multiplied by the number of
turns of that coil around the core (the unit for mmf is the amp-turn). Because the
mathematical relationship between magnetic flux and mmf is directly proportional,
and because the mathematical relationship between mmf and current is also directly
proportional (no rates-of-change present in either equation), the current through the
coil will be in-phase with the flux waveform as shown in fig.(1-11):
Figure(1-11): Current, flux and voltage waveform
14
Mutual-inductance
When one circuit induces current flow in a second nearby circuit, it is known
as mutual-inductance. The image to the right shows an example of mutual-inductance
as shown in fig.(1-12). When an AC current is
flowing through a piece of wire in a circuit, an
i1
electromagnetic field is produced that is constantly
growing and shrinking and changing direction due
to the constantly changing current in the wire. This
changing magnetic field will induce electrical
current in another wire or circuit that is brought
e2
close to the wire in the primary circuit. The current
in the second wire will also be AC and in fact will
Figure(1-12): Mutual inductance
look very similar to the current flowing in the first wire. An electrical transformer uses
inductance to change the voltage of electricity into a more useful level. In
nondestructive testing, inductance is used to generate eddy currents in the test piece.
15
Week 2
2.1
Introduction
A generator does not create energy. It changes mechanical energy into electrical
energy. Every generator must be driven by a turbine, a diesel engine, or some other
machine that produces mechanical energy. For example, the generator (alternator) in
an automobile is driven by the same engine that runs the car.
Engineers often use the term prime mover for the mechanical device that drives a
generator. To obtain more electrical energy from a generator, the prime mover must
supply more mechanical energy. For example, if the prime mover is a steam turbine
more steam must flow through the turbine in order to produce more electricity.
2.2
The Basic Principle DC generator
A generator is a machine that converts mechanical energy into electrical energy by
using the principle of magnetic induction.
This principle is explained as follows: Whenever a conductor is moved within a
magnetic field in such a way that the conductor cuts across magnetic lines of flux,
voltage is generated in the conductor.
• The amount of voltage generated depends on:
1. The strength of the magnetic field
2. The angle at which the conductor cuts the magnetic field
3. The speed at which the conductor is moved
4. The length of the conductor within the magnetic field.
• The polarity of the voltage depends on:
1. The direction of the magnetic lines of flux.
2. The direction of movement of the conductor.
16
To determine the direction of current in a given situation, the left-hand rule for
generators
is
used.
This
rule
is
explained in the following manner.
Extend the thumb, forefinger, and middle
finger of your left hand at right angles to
one another, as shown in fig.(2-1). Point
your
thumb
in
the
direction
the
conductor is being moved. Point your
forefinger in the direction of magnetic
flux (from north to south). Your middle
finger will then point in the direction of
current flow in an external circuit to
Figure(2-1): Left-hand rule for generators.
which the voltage is applied.
2.2.1 The simplest AC generator
The simplest generator that can be built is an ac generator. Basic generating
principles are most easily explained through the use of the elementary ac generator.
For this reason, the ac generator will be discussed first. The dc generator will be
discussed later.
A simplest generator fig.(2-2) consists of a wire loop placed so that it can be
rotated in a stationary magnetic field. This will produce an induced e.m.f
(
electromotive force) in the loop. Sliding contacts (brushes) connect the loop to an
external circuit load in order to pick up or use the induced emf.
17
Figure (2-2): The simplest generator.
The pole pieces (marked N and S) provide the magnetic field. The pole pieces are
shaped and positioned as shown to concentrate the magnetic field as close as possible
to the wire loop. The loop of wire that rotates through the field is called the armature.
The ends of the armature loop are connected to rings called slip rings. They rotate
with the armature. The brushes, usually made of carbon, with wires attached to them,
ride against the rings. The generated voltage appears across these brushes.
The simplest generator produces a voltage as shown in fig.(2-3)
Figure (2-3): Output induced voltage of a simplest generator during one revolution.
18
2.2.2 The simplest DC generator
A single-loop generator with each terminal connected to a segment of a twosegment metal ring is shown in fig.(2-4). The two segments of the split metal ring are
insulated from each other. This forms a simple commutator. The commutator in a dc
generator replaces the slip rings of the ac generator. This is the main difference in
their construction.
The commutator mechanically reverses the armature loop connections to the
external circuit. This occurs at the same instant that the polarity of the voltage in the
armature loop reverses.
Through this process the commutator changes the generated ac voltage to a
pulsating dc voltage as shown in the graph of fig.(2-4). This action is known as
commutation.
Figure (2-4) : Effects of commutation.
For the remainder of this discussion, refer to fig.(2-4), parts A through D. This
will help you in following the step-by-step description of the operation of a dc
generator. When the armature loop rotates clockwise from position A to position B, a
voltage is induced in the armature loop which causes a current in a direction that
deflects the meter to the right. Current flows through loop, out of the negative brush,
through the meter and the load, and back through the positive brush to the loop.
Voltage reaches its maximum value at point B on the graph for reasons explained
earlier. The generated voltage and the current fall to zero at position C. At this instant
19
each brush makes contact with both segments of the commutator. As the armature
loop rotates to position D, a voltage is again induced in the loop. In this case,
however, the voltage is of opposite polarity.
2.3
Constructions of DC Generator
Fig.(2-6), views A through E, shows the main component parts of dc generators.
(1) Magnetic field structure views A, B
(2) Armature structure views C
(3) Commutator structure views D
(4) Brushes structure views E.
\
Figure(2-6) : The main parts of DC generator
20
2.3.1 Magnetic field structure
A magnetic field structure acts like
the simple generator's magnet. It sets
up the magnetic lines of force. It is
electromagnets poles to create the
lines of force in most generators. The
electromagnetic field poles consist of
coils of insulated copper wire wound
on soft iron cores, as shown in fig.(27). The number of field poles
commonly are two or four poles,
some
small
generators
have
permanent magnets.
Figure
(2-7) : Four-pole generator
2.3.2 Armature structure
The armature contains coils of
wire in which the electricity is
induced. It acts like the loop of wire
in the simple generator. The coils
for the armature and field structure
are usually insulated copper wire
wound around iron cores. The iron
Figure (2-8) : Rotor of a dc motor
cores strengthen the magnetic fields
as shown in fig.(2-6) views C and in fig.(2-8)
21
2.3.3 Commutator structure
The commutator converts the
AC
into
a
DC
voltage as
discussed before It also serves as a
means of connecting the brushes
to the rotating coils. In a simple
one-loop generator, the commutator
is made up of two semicylindrical
pieces of a smooth
material,
usually
separated
by
conducting
copper,
mica insulation
Figure (2-9) : Connection of commutation with
the end of armature coils
material, as shown in fig.(2-6) views D and in fig.(2-9). Each half of the
commutator segments is permanently attached to one end of the rotating loop,
and the commutator rotates with the loop. The segments are insulated from each other.
2.3.4 Brush structure
The brushes structure is consist
of brush holder, brush spring and
brush as shown in figs.(2-6) views E
and (2-10).
The brushes usually
made of carbon or graphite, rest
against the commutator and slide
along the commutator as it rotates.
This is the means by which the
brushes make contact with each end
Figure (2-10) : The brushes structure and its
connection with commutation
of the loop. Each brush slides along
one half of the commutator and then along the other half.
The purpose of the brushes is to connect the generated voltage to an
external circuit. In order to do this, each brush must make contact with one of the
ends of the loop. Since the loop or armature rotates, a direct connection is
impractical. Instead, the brushes are connected to the ends of the loop through the
commutator. The brushes are positioned on opposite sides of the commutator; they
22
will pass from one commutator half to the other at the instant the loop reaches the
point of rotation, at which point the voltage that was induced reverses the polarity.
Every time the ends of the loop reverse polarity, the brushes switch from one
commutator segment to the next.
Fig.(2-11) shows the entire DC generator with the component parts installed. The
cross sectional drawing helps you to see the physical relationship of the components
to each other
Figure(2-11) : The cross-sectional of DC generator
23
Week 3
2.4 E.M.F Equation
The principle of DC generator is already been explained in 2.2 section. Whenever a
conductor is moved within a magnetic
field as shown in fig.(2-12) that the
conductor cuts across magnetic lines
of flux, voltage is generated (e.m.f) in
the conductor. The magnitude of
voltage generated (e.m.f in volt)
depends on The strength of the
magnetic field (flux density β in Tesla
or wb/m2), the angle at which the
conductor cuts the magnetic field
(angle of conductor θ relative to
magnetic field), the speed (velocity) at
Figure(2-12): Right-hand rule for e.m.f
which the conductor is moved (V in m/s) , and the length of the conductor within the
magnetic field(the effective length L in m).
e.m.f = β L V sin θ
where,
e.m.f = Induced electromotive force (voltage generated) in V or (volts)
β = Flux density of the magnetic field in Tesla or wb/m2
L = Length of conductor
V
= Velocity of conductor in magnetic field in meter per second(m/s)
θ
= The angle between the magnetic field direction and the conductor
24
Example 2-1
Calculate the e.m.f generated in a conductor of active length 20cm. When
moves with a velocity of 15 m/s in the magnetic field of flux density
300mT at the following cases:
(a) Conductor perpendicular to magnetic field
(b) Conductor at angle of 30o relative to the magnetic field
Solution
(a) e.m.f = β L V sin θ
e.m.f = (300×10-3) × (20×10-2) ×15× (sin 90o) = 0.9 volts
(b) e.m.f = β L V sin θ
e.m.f = (0.3×10-3) × (20×10-2) ×15× (sin 30o) = 0.45 volts
Example 2-2
A conductor of length 50cm, is moved at 10 m/s at right angles to a
magnetic field. If the flux density of the field is 0.3 wb/m2. Find the
induced e.m.f in conductor
Solution
e.m.f = β L V sin θ
e.m.f = (0.3) × (50×10-2) ×10× (sin 90o) = 0.9 volts
= 1.5 V
2.5 Armature Reaction
From previous study, you know that all current-carrying conductors produce
magnetic fields. The magnetic field produced by current in the armature of a dc
generator affects the flux pattern and distorts the main field. This distortion causes a
25
shift in the neutral plane, which affects commutation. This change in the neutral plane
and the reaction of the magnetic field is called armature reaction.
You know that for proper commutation, the coil short-circuited by the brushes
must be in the neutral plane. Consider the operation of a simple two-pole dc generator,
shown in fig.(2-13). View A of the figure shows the field poles and the main magnetic
field.
Figure
(2-13) : Armature reaction.
The armature is shown in a simplified view in views B and C with the cross
section of its coil represented as little circles. The symbols within the circles represent
arrows. The dot represents the point of the arrow coming toward you, and the cross
represents the tail, or feathered end, going away from you. When the armature rotates
clockwise, the sides of the coil to the left will have current flowing toward you, as
indicated by the dot.
The side of the coil to the right will have current flowing away from you, as
indicated by the cross. The field generated around each side of the coil is shown in
view B of fig.(2-13). This field increases in strength for each wire in the armature coil,
and sets up a magnetic field almost perpendicular to the main field.
Now you have two fields - the main field, view A, and the field around the
armature coil, view B. View C of fig.(2-13) shows how the armature field distorts the
main field and how the neutral plane is shifted in the direction of rotation. If the
brushes remain in the old neutral plane, they will be short-circuiting coils that have
voltage induced in them. Consequently, there will be arcing between the brushes and
commutator.
To prevent arcing
26
1) The brushes must be shifted to the new neutral plane.
2) Used compensating windings or interpoles
2.5.1 Shifting the Brushes
In small generators, the effects of armature reaction are reduced by actually
mechanically shifting the position of the brushes. The practice of shifting the brush
position for each current variation is not practiced except in small generators.
2.5.2 Compensating Windings and Interpoles
In larger generators, other means are taken to eliminate armature reaction. for
this purpose fig.(2-14). The compensating windings consist of a series of coils
embedded in slots in the pole faces.
These coils are connected in series with the armature. The series-connected
compensating windings produce a magnetic field, which varies directly with armature
current. Because the compensating windings are wound to produce a field that
opposes the magnetic field of the armature, they tend to cancel the effects of the
armature magnetic field. The neutral plane will remain stationary and in its original
position for all values of armature current. Because of this, once the brushes have been
set correctly, they do not have to be moved again.
Figure (2-14) : Compensating windings and interpoles.
Another way to reduce the effects of armature reaction is to place small
auxiliary poles called "interpoles" between the main field poles. The interpoles have a
few turns of large wire and are connected in series with the armature. Interpoles are
wound and placed so that each interpole has the same magnetic polarity as the main
pole ahead of it, in the direction of rotation. The field generated by the interpoles
produces the same effect as the compensating winding. This field, in effect, cancels
27
the armature reaction for all values of load current by causing a shift in the neutral
plane opposite to the shift caused by armature reaction. The amount of shift caused by
the interpoles will equal the shift caused by armature reaction since both shifts are a
result of armature current.
2.6
Classification Of Generators
When a dc voltage is applied to the field windings of a dc generator, current
flows through the windings and sets up a steady magnetic field. This is called field
excitation. This excitation voltage can be produced by the generator itself (This is
called self-excited generator) or it can be supplied by an outside source, such as a
battery(This is called separately-excited generator).
Self-excitation is possible only if the field pole pieces have retained a slight
amount of permanent magnetism, called residual magnetism. When the generator
starts rotating, the weak residual magnetism causes a small voltage to be generated in
the armature. This small voltage applied to the field coils causes a small field current.
Although small, this field current strengthens the magnetic field and allows the
armature to generate a higher voltage. The higher voltage increases the field strength,
and so on. This process continues until the output voltage reaches the rated output of
the generator.
Self-excited generators are classed according to the type of field connection
they use. There are three general types of field connections series-wound, shuntwound (parallel), and compound-wound. compound-wound generators are further
classified as cumulative-compound and differential-compound. these last two
classifications are not discussed in this book.
28
Classification of DC
Generators
separately-excited DC
generator
Self-excited DC generator
Types of DC
Motors
compoundwound
shunt-wound
series-wound
29
Week 4
2.7 Voltage Regulation
The regulation of a generator refers to the voltage change that takes place
when the load changes. It is usually expressed as the change in voltage from a no-load
condition to a full-load condition, and is expressed as a percentage of full-load. It is
expressed in the following formula:
where EnL is the no-load terminal voltage and EfL is the full-load terminal voltage of
the generator.
Example 2-3 Calculate the percent of regulation of a generator with a no- load
voltage of 462 volts and a full-load voltage of 440 volts ?
Solution:
No-load voltage EnL = 462 V
Full-load voltage EfL= 440 V
NOTE: The lower the percent of regulation, the better the generator. In the above
example, the 5% regulation represented a 22-volt change from no load to
full load. A 1% change would represent a change of 4.4 volts, which, of
course, would be better.
30
2.8 Generator Power Losses
In dc generators, as in most electrical devices, certain forces act to decrease the
efficiency. These forces, as they affect the armature, are considered as losses and may
be defined as follows:
1) Copper loss, or I2R in the winding
2) Eddy current loss in the core
3) Hysteresis loss (a sort of magnetic friction)
2.8.1 Copper Losses
The power lost in the form of heat in the armature winding and field winding
(if its found) is known as copper loss. Heat is generated any time current flows in a
conductor. Copper loss is an I2R loss, which increases as current increases. The
amount of heat generated is also proportional to the resistance of the conductor. The
resistance of the conductor varies directly with its length and inversely with its crosssectional area. Copper loss is minimized in armature and field windings by using large
diameter wire.
2.8.2 Eddy Current Losses
The core of a generator armature is made from soft iron, which is a conducting
material with desirable magnetic characteristics. Any conductor will have currents
induced in it when it is rotated in a magnetic field. These currents that are induced in
the generator armature core are called eddy currents. The power dissipated in the
form of heat, as a result of the eddy currents, is considered a loss.
Eddy currents, just like any other electrical currents, are affected by the
resistance of the material in which the currents flow. The resistance of any material is
inversely proportional to its cross-sectional area. Fig.(2-15), view A, shows the eddy
currents induced in an armature core that is a solid piece of soft iron. Fig.(2-15), view
B, shows a soft iron core of the same size, but made up of several small pieces
insulated from each other. This process is called lamination. The currents in each
piece of the laminated core are considerably less than in the solid core because the
resistance of the pieces is much higher. (Resistance is inversely proportional to crosssectional area.) The currents in the individual pieces of the laminated core are
31
so small that the sum of the
individual currents is much less than
the total of eddy currents in the solid
iron core.
As you can see, eddy current
losses are kept low when the core
material is made up of many thin
sheets of metal. Laminations in a
small generator armature may be as
thin as 1/64 inch. The laminations
are insulated from each other by a
Figure (2-15) : Eddy currents in dc generator
armature cores.
thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces.
Oxidation is caused by contact with the air while the laminations are being annealed.
The insulation value need not be high because the voltages induced are very small.
Most generators use armatures with laminated cores to reduce eddy current losses.
2.8.3 Hysteresis Losses
Hysteresis loss is a heat loss caused by the magnetic properties of the armature.
When an armature core is in a magnetic field, the magnetic particles of the core tend
to line up with the magnetic field. When the armature core is rotating, its magnetic
field keeps changing direction. The continuous movement of the magnetic particles, as
they try to align themselves with the magnetic field, produces molecular friction. This,
in turn, produces heat. This heat is transmitted to the armature windings. The heat
causes armature resistances to increase.
To compensate for hysteresis losses, heat-treated silicon steel laminations are
used in most dc generator armatures. After the steel has been formed to the proper
shape, the laminations are heated and allowed to cool. This annealing process reduces
the hysteresis loss to a low value.
32
Week 5
3.1 Introduction
Motors change electric energy into mechanical energy. Direct current motors
and generators are constructed very similarly as explain in the previous chapter. They
function almost oppositely at first because a generator creates voltage when
conductors cut across the lines of force in a magnetic field, while motors result in
torque-- a turning effort of mechanical rotation. Simple motors have a flat coil that
carries current that rotates in a magnetic field. The motor acts as a generator since
after starting, it produces an opposing current by rotating in a magnetic field, which in
turn results in physical motion.
3.2 Constructions and Operation Principle of DC Generator
Motors change electric energy into mechanical energy. Direct current motors
and generators are constructed very
similarly described earlier in the
previous chapter. They function almost
oppositely at first because a generator
creates voltage when conductors cut
across the lines of force in a magnetic
field, while motors result in torque a
turning effort of mechanical rotation.
Simple motors have a flat coil that
carries
current that rotates
in a
Figure(3-1): Simple motor
magnetic field as shown in fig.(3-1). The motor acts as a generator since after starting,
it produces an opposing current by rotating in a magnetic field, which in turn results in
physical motion.
This is accomplished as a conductor is passed through a magnetic field, then
the opposing fields repel each other to cause physical motion. The left hand rule can
be used to explain the way a simple motor works fig.(3-2). The pointer finger points in
the direction of the magnetic field, the middle finger points in the direction of the
current, and the thumb shows which way the conductor will be forced to move.
33
Figure(3-2): Left hand rules
DC motor has a rotating armature in the form of an electromagnet. A rotary
switch called a commutator reverses the direction of the electric current twice every
cycle, to flow through the armature so that the poles of the electromagnet push and
pull against the permanent magnets on the outside of the motor. As the poles of the
armature electromagnet pass the poles of the permanent magnets, the commutator
reverses the polarity of the armature electromagnet. During that instant of switching
polarity, inertia keeps the DC motor going in the proper direction. See the diagrams
shown in fig.(3-3).
(a)
(b)
(c)
Figure(3-3) : Diagrams that explains the operation of a DC motor.
a) A simple DC electric motor. When the coil is powered, a magnetic field is
generated around the armature. The left side of the armature is pushed away
from the left magnet and drawn toward the right, causing rotation.
b) The armature continues to rotate.
34
c) When the armature becomes horizontally aligned, the commutator reverses the
direction of current through the coil, reversing the magnetic field. The process
then repeats.
3.2.1 The dc motor torque
When the conductor is bent into a coil, the physical motion performs an up and
down cycle. The more bends in a coil, the less pulsating the movement will be. This
physical movement is called torque, and can be measured in the equation:
T = kt Ф Ia
where :
T = Torque in (Newton- meter)
kt = Constant depending on physical dimension of motor
Ф = Total number of lines of flux entering the armature from one N pole
in (wb/m2)
Ia = Armature current in (A)
3.2.2 Back E. M. F.
While a dc motor is running, it acts somewhat like a dc generator. There is a
magnetic field from the field poles, and a loop of wire is turning and cutting this
magnetic field. For the moment, disregard the fact that there is current flowing
through the loop of wire from the battery. As the loop sides cut the magnetic field, a
voltage is induced in them, the same as it was in the loop sides of the dc generator.
This induced voltage causes current to flow in the loop. this current direction opposite
to that of the battery current. Since this generator-action voltage is opposite that of the
battery, it is called "Back emf." (The letters emf stand for electromotive force, which
is another name for voltage.) The two currents are flowing in opposite directions. This
proves that the battery voltage and the back emf are opposite in polarity. At the
beginning of this discussion, we disregarded armature current while explaining how
back emf was generated. Then, we showed that normal armature current flowed
opposite to the current created by the back emf. We talked about two opposite currents
that flow at the same time. However, this is a bit oversimplified, as you may already
suspect. Actually, only one current flows. Because the back emf can never become as
35
large as the applied voltage, and because they are of opposite polarity as we have
seen, the back emf effectively cancels part of the armature voltage. The single current
that flows is armature current, but it is greatly reduced because of the counter emf. In
a dc motor, there is always a counter emf developed. This counter emf cannot be equal
to or greater than the applied battery voltage; if it were, the motor would not run. The
back emf is always a little less. However, the back emf opposes the applied voltage
enough to keep the armature current from the battery to a fairly low value. If there
were no such thing as back emf, much more current would flow through the armature,
and the motor would run much faster. However, there is no way to avoid the back
emf.
3.3 Types and characteristics of DC Motors
There are three basic types of dc motors:
(1) Series motors
(2) shunt motors
(3) compound motors
They differ largely in the method in which their field and armature coils are
connected.
3.3.1 Series DC Motor
In the series motor, the field windings, consisting of a relatively few turns of
heavy wire, are connected in series with the armature winding. Both a diagrammatic
and a schematic illustration of a series motor is shown in fig.(3-4). The same current
flowing through the field winding also flows through the armature winding. Any
increase in current, therefore, strengthens the magnetism of both the field and the
armature.
Figure(3-4) : Series DC motor
Because of the low resistance in the windings, the series motor is able to draw a
large current in starting. This starting current, in passing through both the field and
36
armature windings, produces a high starting torque, which is the series motor's
principal advantage.
The speed of a series motor is dependent upon the load. Any change in load is
accompanied by a substantial change in speed. A series motor will run at high speed
when it has a light load and at low speed with a heavy load. If the load is removed
entirely, the motor may operate at such a high speed that the armature will fly apart. If
high starting torque is needed under heavy load conditions, series motors have many
applications. Series motors are often used in aircraft as engine starters and for raising
and lowering landing gears, cowl flaps, and wing flaps.
3.3.2 Shunt DC Motor
In the shunt motor the field winding is connected in parallel or in shunt with
the armature winding. See fig.(3-5), The resistance in the field winding is high. Since
the field winding is connected directly across the power supply, the current through
the field is constant.
The field current does not vary with motor speed, as in the series motor and,
therefore, the torque of the shunt motor will vary only with the current through the
armature. The torque developed at starting is less than that developed by a series
motor of equal size.
Figure(3-5) : Shunt DC motor
The speed of the shunt motor varies very little with changes in load. When all
load is removed, it assumes a speed slightly higher than the loaded speed. This motor
is particularly suitable for use when constant speed is desired and when high starting
torque is not needed.
37
3.3.3 Compound DC Motor
The compound motor is a combination of the series and shunt motors. There
are two windings in the field: a shunt winding and a series winding. A schematic of a
compound motor is shown in fig.(3-6).
The shunt winding is composed of many turns of fine wire and is connected in
parallel with the armature winding. The series winding consists of a few turns of large
wire and is connected in series with the armature winding. The starting torque is
higher than in the shunt motor but lower than in the series motor. Variation of speed
with load is less than in a series wound motor but greater than in a shunt motor. The
compound motor is used whenever the combined characteristics of the series and
shunt motors are desired.
Figure(3-6) : Compound DC motor
Like the compound generator, the compound motor has both series and shunt
field windings. The series winding may either aid the shunt wind (cumulative
compound) or oppose the shunt winding (differential compound).
The starting and load characteristics of the cumulative compound motor are
somewhere between those of the series and those of the shunt motor.
Because of the series field, the cumulative compound motor has a higher starting
torque than a shunt motor.
Cumulative compound motors are used in driving machines which are subject
to sudden changes in load. They are also used where a high starting torque is desired,
but a series motor cannot be used easily.
38
In the differential compound
motor, an increase in load creates an
increase in current and a decrease in
total flux in this type of motor. These
two tend to offset each other and the
result is a practically constant speed.
However, since an increase in load
tends to decrease the field strength,
the speed characteristic becomes
unstable. Rarely is this type of motor
used in aircraft systems.
Figure(3-7) : Composite of the characteristic
curves for all of the DC motors.
A graph of the variation in speed with
changes of load of the various types of dc motors is shown in fig.(3-7).
39
Week 6
3.4 Motor Nameplate
Motor nameplates are provided by virtually all manufacturers to allow users to
accurately identify the operating and dimensional characteristics of their motors years
after installation.
3.4.1 Definition Nameplate
The nameplate is usually a metal plate, secured by a pair of screws or rivets,
and is generally located on the side of the motor. (Expert maintenance technicians will
tell you that the nameplate is always located on the side of the motor where the
nameplate is most difficult to read!)
The following cryptic information will usually be stamped into the nameplate
(stamping is used because it doesn't wear off as ink tends to do. Unfortunately, the
lack of contrast can make it difficult to read. Sometimes, a little bit of dirty oil or
grease applied to the nameplate and then wiped "smooth" puts the dark substance into
the indentations of the stamped letters and allows for easier reading.).
3.4.2 Nameplate Terms
1) Motor Manufacturer
2) Mod. (Model), Tp. (Type), or Cat. (Catalog)
3) Ser. (Serial Number)
4) HP (Horsepower) or KW (kilowatts)
5) RPM (Revolutions per Minute)
6) V (Volts)
7) ARM. (Armature)
8) FLD. (Field)
9) A (Amps)
10) Fr (Frame)
11) Enc. (Enclosure)
12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)
40
1) Motor Manufacturer
This is the trade name of the company which manufactured the motor. If you
are lucky, the company's home city, and perhaps even an address and/or telephone
number will be on the nameplate.
2) Mod. (Model), Tp. (Type), or Cat. (Catalog)
Some companies distinguish between a Model number and a Type number. (I
don't know why). In any event, this is the key number that you need if you want to
contact the manufacturer.
3) Ser. (Serial Number)
Serial numbers are important because they often contain "date codes". This is
information which helps the manufacturer determine when the motor was
manufactured. Since many motors have multiple revisions through their lifecycle as
the manufacturing process (hopefully) improves, this helps determine which set of
drawings to use and lets the technical people at the manufacturer help you quicker and
more accurately.
4) HP (Horsepower) or KW (kilowatts)
If you are using an American made motor or an older English or Canadian motor, it
will probably be rated in Horsepower. European and Asian motors are usually rated in
kilowatts -- unless they have been designed for export to the American market.
Rule to remember: 1 HP = 3/4 KW (more precisely 746 watts).
Second rule to remember: Volts x Amps = Watts.
5) RPM (Revolutions per Minute)
The number of times each minute that the shaft turns on its axis. This is rated at the
Hertz listed. Typical values are 1750, 1450, 3450, etc. If more than one speed is listed,
this indicates a multi-speed motor. Note that AC inverter drives can change the speed
of a motor from its rated speed.
41
6) V (Volts)
The operating voltage of the motor. DC motors will have numbers such as 24,
48, 90, 180, or other voltage, and will usually say "VDC".
7) ARM. (Armature)
This is the maximum voltage which can be applied to the armature of a DC
motor. Typical values are 90 or 180 VDC. An amperage will often be listed.
8) FLD. (Field)
This is the voltage which should be applied to the field of a DC motor. Typical
values are 100, 150, 200 VDC. An amperage will often be listed.
9) A (Amps)
The amount of current consumed by the motor.
10) Fr (Frame)
The physical dimensional standard to which the motor adheres. This is critical
when it is necessary to locate a mechanical replacement for an old motor. NEMA
motor frames have been established for decades to allow motors from various
manufacturers to replace each other. For example, a foot-mount NEMA 56 frame
motor has the same mounting dimensions no matter which manufacturer has built it.
NEMA refers to the National Electrical Manufacturers Association. NEMA is
part of the IEC. The IEC is the International Electrotechnical Commission. Although
the IEC includes Japan and the United States of America among its members, the IEC
is essentially a European Community standards association. IEC standards are heavily
influenced by VDE - the German electrical standards association.
42
11) Enc. (Enclosure)
This is the degree of protection offered by the enclosure. Common terms are TEFC, TEBC,
TENV, ODP, TEAO, etc.
TEFC
A TEFC enclosure on a motor means "Totally Enclosed, Fan Cooled". This motor is
probably the most commonly used motor in ordinary industrial environments. It costs only a
few dollars more than the open motor, yet offers good protection against common hazards.
The motor is constructed with a small fan on the rear shaft of the motor, usually covered by a
housing. This fan draws air over the motor fins, removing excess heat and cooling the motor.
The enclosure is "Totally Enclosed". This ordinarily means that the motor is
dust tight, and has a moderate water seal as well. Notice that TEFC motors are not
secure against high pressure water. For these applications, consider the "wash down"
motor, which is usually designed to withstand regular washing, such as found in a
food processing facility. In addition, the TEFC motor is not "Explosion-proof", nor is
it capable of operation in "Hazardous Environments".
TEBC
A TEBC enclosure on a motor means "Totally Enclosed, Blower
Cooled". TEBC motors are most commonly used for variable speed motors
combined with variable speed drives of some sort. Sometimes these motors are
rated as "Inverter duty" or "Vector duty". They are considerably more expensive
than similarly rated TEFC motors. The motor is constructed with a dust tight,
moderately sealed enclosure which rejects a degree of water. A constant speed
blower pulls air over the motor fins to keep the motor cool at all operating
speeds. Notice that this motor is not suitable for used in "washdown" or
"Hazardous" environments.
43
TENV
A TENV enclosure on a motor means "Totally Enclosed, Not Ventilated". TENV
motors are used in a wide variety of smaller horsepower variable speed applications. It is
particularly effective in environments where a fan would regularly clog with dust or lint. The
motor is constructed with a dust-tight, moderately sealed enclosure which rejects a degree of
water. The motor radiates its entire excess heat through the body of the motor: Hence, the
TENV motor has extra metal and extra fins to allow radiation of this heat. The TENV motor
is commonly built with special high temperature insulation, since the motor is designed to run
hot. As such, care should be taken to avoid human contact with the body of the motor, as well
as contact between inflammable objects and the motor. Notice that this motor is not suitable
for use in "washdown" or "Hazardous" environments.
ODP
An ODP enclosure on a motor means "Open, Drip Proof". ODP motors are relatively
inexpensive motors used in normal applications. The construction of an ODP motor consists
of a sheet metal enclosure with vent stamped to allow good air flow. The vents are designed
in such a way that water dripping on the motor will not normally flow into the motor. A fan is
mounted on the motor's rear shaft to pull air through the motor to keep the motor cool. The
ODP motor is relatively inexpensive, but care should be taken not to use the motor in
applications where the TEFC motor is required.
TEAO
A TEAO enclosure on a motor means "Totally Enclosed, Air Over". TEAO motors
are designed to be used solely in the airstream of the fan or blower which they are driving. As
such, they are very low cost, but of limited application. TEAO motors are constructed with a
dust-tight cover and an aerodynamic body. They rely upon the strong air flow of the fan or
blower which they are driving to cool them. TEAO motors are not suitable for use in
"Hazardous" environments.
NEMA Enclosure Standard 250
NEMA enclosure standards represent an enclosure's ability to protect against
the external environment. The following represent brief summaries of the NEMA
standard. some examples of NEMA Enclosure Standard 250
44
1- Type 1
Intended for indoor use primarily to provide a degree of protection against
(hand) contact with the enclosed equipment. Sometimes known as a "finger-tight"
enclosure. This is the least costly enclosure, but is suitable only for clean, dry
environments.
2- Type 2
Intended for indoor use primarily to provide a degree of protection against
limited amounts of falling dirt and water.
3- Type 3
Intended for outdoor use primarily to provide a degree of protection against
windblown dust, rain, and sleet; undamaged by ice which forms on the enclosure.
4- Type 3R
Intended for outdoor use primarily to provide a degree of protection against
falling rain and sleet; undamaged by ice which forms on the enclosure. This is the
most common outdoors enclosure.
12) CW (Clockwise Rotation) or CCW (Counter-Clockwise Rotation)
When facing the motor from the shaft end, this is the direction of rotation of the motor
(if the motor is unidirectional).
3.5 Power Losses and Efficiency
Losses occur when electrical energy is converted to mechanical energy (in the
motor), or mechanical energy is converted to electrical energy (in the generator). For
the machine to be efficient, these losses must be kept to a minimum. Some losses are
electrical, others are mechanical. Electrical losses are classified as copper losses and
iron losses; mechanical losses occur in overcoming the friction of various parts of the
machine.
Copper losses occur when electrons are forced through the copper windings of
the armature and the field. These losses are proportional to the square of the current.
They are sometimes called I2R losses, since they are due to the power dissipated in
the form of heat in the resistance of the field and armature windings.
45
Iron losses are subdivided in hysteresis and eddy current losses. Hysteresis
losses are caused by the armature revolving in an alternating magnetic field. It,
therefore, becomes magnetized first in one direction and then in the other. The
residual magnetism of the iron or steel of which the armature is made causes these
losses. Since the field magnets are always magnetized in one direction (dc field), they
have no hysteresis losses.
Eddy current losses occur because the iron core of the armature is a conductor
revolving in a magnetic field. This sets up an e.m.f. across portions of the core,
causing currents to flow within the core. These currents heat the core and, if they
become excessive, may damage the windings. As far as the output is concerned, the
power consumed by eddy currents is a loss. To reduce eddy currents to a minimum, a
laminated core usually is used. A laminated core is made of thin sheets of iron
electrically insulated from each other. The insulation between laminations reduces
eddy currents, because it is "transverse" to the direction in which these currents tend
to flow. However, it has no effect on the magnetic circuit. The thinner the laminations,
the more effectively this method reduces eddy current losses.
46
Week 7
3.6 Starting Methods of DC Motor
If we apply full voltage to a stationary DC motor, the starting current in the
armature will be very high and we run the risk of
a. Burning out the armature;
b. Damaging the commutator and brushes, due to heavy sparking;
c. Overloading the feeder;
d. Snapping off the shaft due to mechanical shock;
e. Damaging the driven equipment because of the sudden mechanical
hammerblow.
All dc motors must, therefore, be provided with a means to limit the starting current
to reasonable values, usually between 1.5 and twice full-load current. One solution is
to connect a rheostat in series with the armature. The resistance is gradually reduced
as the motor accelerates and is eventually eliminated entirely, when the machine has
attained full speed.
3.6.1 Face-plate starter
Fig.(3-8) shows the schematic diagram of a manual face-plate starter for a
shunt motor. Bare copper contacts are connected to current-limiting resistors R1, R2,
R3, and R4. Conducting arm 1 sweeps across the contacts when it is pulled to the right
by means of insulated handle 2. In the position shown, the arm touches dead copper
contact M and the motor circuit is open. As we draw the handle to the right, the
conducting arm first touches fixed contact N.
The supply voltage Es immediately causes full field current Ix to flow, but the
armature current / is limited by the four resistors in the starter box. The motor begins
to turn and, as the emf (Eo) builds up, the armature current gradually falls. When the
motor speed ceases to rise any more, the arm is pulled to the next contact, thereby
removing resistor R1 from the armature circuit. The current immediately jumps to a
higher value and the motor quickly accelerates to the next higher speed. When the
47
speed again levels off, we move to the next contact, and so forth, until the arm finally
touches the last contact. The arm is magnetically held in this position by a small
electromagnet 4, which is in series with the shunt field.
Figure (3-8) : Manual face-plate starter for a shunt motor.
If the supply voltage is suddenly interrupted, or if the field excitation should
accidentally be cut, the electromagnet releases the arm, allowing it to return to its dead
position, under the pull of springing 3. This safety feature prevents the motor from
restarting unexpectedly when the supply voltage is reestablished.
3.6.2 Relay starter
Today, electronic methods are often used to limit the starting current and to provide
speed control as the following.
The most important component of a motor starter is the magnetic relay, or
sometimes called a magnetic contactor (depending the size). The relay is an electromechanical device that contains a coil of wire, a mechanical contactor, and a spring
mechanism. The spring mechanism is used to hold the contactor in its "NORMAL"
state, which is the state of the device when the coil is deenergized. When the coil is
energized, the current flowing through it sets up a magnetic field. The magnetic field
generated by the coil then pulls the contactor to its "ENERGIZED" state. When the
coil is turned off, the spring pulls the contactor back to its normal state again.
The contacts on the contactor can either be open or closed when the coil is
deenergized. If the contacts are closed when the coil is deenergized, they are called
48
normally closed contacts. If they are open when the coil is deenergized, they are
called normally open contacts. When the coil is energized, the contacts change state.
In other words, when the coil is energized, the normally closed contacts open, and the
normally open contacts close.
Overload sensors have normally closed contacts associated with them.
Overload devices can be either magnetic or thermal. Thermal overloads contain two
parts, the heater strip and the contacts. The heater strip senses the armature current,
and when the current becomes excessive, the heater actuates the contacts.
The
contacts in turn secure the motor to prevent damage. Magnetic overloads operate
similarity, except the contacts are actuated magnetically due to an increase in
magnetic flux when the current is excessive.
Timer relays can be one of two types, Time On (TON) or Time Off (TOF). A time on
relay is one where the time delay is associated with the "ON" state, and a time off
relay is one where the time delay is associated with the "OFF" state. For example,
when a TON relay is energized, the timing mechanism starts. After the delay, the
TON contacts change state. When a TON relay is deenergized, the contacts change
state immediately. With a TOF relay, the opposite is true. When the TOF relay is
energized, the contacts change state immediately.
When the TOF relay is
deenergized, the time delay mechanism starts. After the time delay, then the contacts
change state. Most starters are of the TON variety, however, there is one TOF starter
in this laboratory. The difference between TON and TOF are more important when
programmable controllers are studied later in this course. A simple D-C motor starter
is shown below in fig.(3-9).
MOTOR CIRCUIT CONTROL CIRCUIT
Figure (3-9) : Simple D-C Motor Starter and Controller
49
The circuit consists of two major sections, the motor circuit and the control circuit.
The control circuit is usually fused from the motor circuit (not shown) to protect from
shorts. The motor circuit contains the power to the shunt field, and to the armature
circuit. The armature circuits contains the main line contacts (labeled "M"), the
starting resistor (labeled Rs), the overload sensor (labeled OL), and the motor
armature. The motor circuit is the "high current" circuit that handles the current
applied to the motor directly.
The control circuit consists of the start switch, stop switch, overload contacts,
M-coil, and the timer (T-coil). The control circuit is the "low current" circuit that
does not handle any power directly applied to the motor. The operation of the circuit
follows what's called relay logic, or sequential logic.
When the motor is turned off, the four M contacts are open, the start switch
(normally open) is open, the stop switch (normally closed) is closed, the overload
contact is closed, and the T contact is open. With the T contact open, full starting
resistance is inserted in the armature circuit.
To start the motor, the start button is pressed. This completes the circuit to the
M-coil, and the M-coil energizes. When the M-coil energizes, the magnetic field
generated by the coil changes the state of the M-contactor. When this occurs, all four
M-contacts close. The two M-contacts in the armature circuit close which start the
motor with full starting resistance applied. The M-contact across the start switch
closes sealing the start switch, and the last M-contact closes energizing the timer.
The sealing M-contact is necessary to keep the motor running after the start
push button is released. If the sealing contact was not there, as soon as the start push
button was released the M-coil would deenergize, the M-contacts would all open,
which would stop the motor. All motor starters using push buttons will have some
kind of sealing circuit across the start switch.
50
The last M-contact energizes the timer. After the timer is energized, the time
delay starts. After a certain amount of time is allowed for the motor to build up speed,
and CEMF, the timer contacts change state. When this occurs, the T-contact closes,
which shorts the starting resistance. After the T-contact closes, the motor is operating
at base speed.
To stop the motor, the stop switch is pressed. When the stop switch is opened,
the M-Coil deenergizes. When the M-coil deenergizes, all four M-contacts open. The
two M-contacts in the armature circuit open removing power from the armature,
stopping the motor. The M-contact around the start switch opens, resetting the sealing
circuit. The fourth M-contact opens deenergizing the T-coil timer. The timer coil
deenergizes and its contactor immediately changes state, opening the T-contact. Note
there is no time delay associated with the timer when it's turned off. The time delay
applies only when the timer coil is energized. When the T-contact opens, full starting
resistance is reapplied to the armature circuit
51
Week 8
3.7 Reversing the Rotation of DC Motor
3.7.1 Reversing the Rotation of DC Series Motor
The direction of rotation of a series motor can be changed by changing the
polarity of either the armature or field winding. It is important to remember that if you
simply changed the polarity of the applied voltage, you would be changing the
polarity of both field and armature windings and the motor's rotation would remain the
same.
Figure (3-10) : DC series motor connected to forward and reverse motor starter.
Since only one of the windings needs to be reversed, the armature winding is
typically used because its terminals are readily accessible at the brush rigging.
Remember that the armature receives its current through the brushes, so that if their
polarity is changed, the armature's polarity will also be changed. A reversing motor
starter is used to change wiring to cause the direction of the motor's rotation to change
by changing the polarity of the armature windings.
Fig.(3-10) shows a DC series motor that is connected to a reversing motor
starter. In this diagram the armature's terminals are marked Al and A2 and the field
terminals are marked Sl and S2.
52
When the forward motor starter is energized, the top contact identified as F
closes so the Al terminal is connected to the positive terminal of the power supply and
the bottom F contact closes and connects terminals A2 and Sl. Terminal S2 is
connected to the negative terminal of the power supply. When the reverse motor
starter is energized, terminals Al and A2 are reversed. A2 is now connected to the
positive terminal. Notice that S2 remains connected to the negative terminal of the
power supply terminal. This ensures that only the armature's polarity has been
changed and the motor will begin to rotate in the opposite direction.
You will also notice the normally closed (NC) set of R contacts connected in
series with the forward push button, and the NC set of F contacts connected in series
with the reverse push button. These contacts provide an interlock that prevents the
motor from being changed from forward to reverse direction without stopping the
motor. The circuit can be explained as follows: when the forward push button is
depressed, current will flow from the stop push button through the NC R interlock
contacts, and through the forward push button to the forward motor starter (FMS) coil.
When the FMS coil is energized, it will open its NC contacts that are connected in
series with the reverse push button. This means that if someone depresses the reverse
push button, current could not flow to the reverse motor starter (RMS) coil. If the
person depressing the push buttons wants to reverse the direction of the rotation of the
motor, he or she will need to depress the stop push button first to de-energize the FMS
coil, which will allow the NC F contacts to return to their NC position.
You can see that when the RMS coil is energized, its NC R contacts that are
connected in series with the forward push button will open and prevent the current
flow to the FMS coil if the forward push button is depressed. You will see a number
of other ways to control the FMS and RMS starter in later discussions and in the
chapter on motor controls.
3.7.2 Reversing the Rotation DC Shunt Motor
The direction of rotation of a DC shunt motor can be reversed by changing the
polarity of either the armature coil or the field coil. In this application the armature
coil is usually changed, as was the case with the series motor. Fig.(3-11) shows the
53
electrical diagram of a DC shunt motor connected to a forward and reversing motor
starter. You should notice that the Fl and F2 terminals of the shunt field are connected
directly to the power supply, and the Al and A2 terminals of the armature winding are
connected to the reversing starter.
When the FMS is energized, its contacts connect the Al lead to the positive power
supply terminal and the A2 lead to the negative power supply terminal. The Fl motor
lead is connected directly to the positive terminal of the power supply and the F2 lead
is connected to the negative terminal. When the motor is wired in this configuration, it
will begin to run in the forward direction.
When the RMS is energized, its contacts reverse the armature wires so that the
Al lead is connected to the negative power supply terminal and the A2 lead is
connected to the positive power supply terminal. The field leads are connected
directly to the power supply, so their polarity is not changed. Since the field's polarity
has remained the same and the armature's polarity has reversed, the motor will begin
to rotate in the reverse direction. The control part of the diagram shows that when the
FMS coil is energized, the RMS coil is locked out.
Figure (3-11) : Diagram of a shunt motor connected to a reversing motor starter.
54
Notice
that the shunt field is connected across the armature and it is not reversed when the
armature is reversed.
3.7.3 Reversing the Rotation of DC Compound Motor
Each of the compound motors can be reversed by changing the polarity of the
armature winding. If the motor has interpoles, the polarity of the interpole must be
changed when the armature's polarity is changed. Since the interpole is connected in
series with the armature, it should be reversed when the armature is reversed. The
interpoles are not shown in the diagram to keep it simplified. The armature winding is
always marked as A1 and A2 and these terminals should be connected to the contacts
of the reversing motor starter.
55
Week 9
3.8 Inspection and Maintenance of DC Motors
Use the following procedures to make inspection and maintenance checks:
1) Check the operation of the unit driven by the motor in accordance with the
instructions covering the specific installation.
2) Check all wiring, connections, terminals, fuses, and switches for general
condition and security.
3) Keep motors clean and mounting bolts tight.
4) Check brushes for condition, length, and spring tension. Minimum brush
lengths, correct spring tension, and procedures for replacing brushes are given
in the applicable manufacturer's instructions.
5) Inspect commutator for cleanness, pitting, scoring, roughness, corrosion or
burning. Check for high mica (if the copper wears down below the mica, the
mica will insulate the brushes from the commutator). Clean dirty commutators
with a cloth moistened with the recommended cleaning solvent. Polish rough or
corroded commutators with fine sandpaper (000 or finer) and blow out with
compressed air. Never use emery paper since it contains metallic particles
which may cause shorts. Replace the motor if the commutator is burned, badly
pitted, grooved, or worn to the extent that the mica insulation is flush with the
commutator surface.
6) Inspect all exposed wiring for evidence of overheating. Replace the motor if
the insulation on leads or windings is burned, cracked, or brittle.
7) Lubricate only if called for by the manufacturer's instructions covering the
motor. Most motors used in today's airplanes require no lubrication between
overhauls.
8) Adjust and lubricate the gearbox, or unit which the motor drives, in accordance
with the applicable manufacturer's instructions covering the unit.
When trouble develops in a dc motor system, check first to determine the source of
the trouble. Replace the motor only when the trouble is due to a defect in the motor
56
itself. In most cases, the failure of a motor to operate is caused by a defect in the
external electrical circuit, or by mechanical failure in the mechanism driven by the
motor.
Check the external electrical circuit for loose or dirty connections and for
improper connection of wiring. Look for open circuits, grounds, and shorts by
following the applicable manufacturer's circuit testing procedure. If the fuse is not
blown, failure of the motor to operate is usually due to an open circuit. A blown fuse
usually indicates an accidental ground or short circuit.
The chattering of the relay switch which controls the motor is usually caused
by a low battery. When the battery is low, the open circuit voltage of the battery is
sufficient to close the relay, but with the heavy current draw of the motor, the voltage
drops below the level required to hold the relay closed. When the relay opens, the
voltage in the battery increases enough to close the relay again. This cycle repeats and
causes chattering, which is very harmful to the relay switch, due to the heavy current
causing an arc which will burn the contacts.
Check the unit driven by the motor for failure of the unit or drive mechanism.
If the motor has failed as a result of a failure in the driven unit, the fault must be
corrected before installing a new motor.
If it has been determined that the fault is in the motor itself (by checking for
correct voltage at the motor terminals and for failure of the driven unit), inspect the
commutator and brushes. A dirty commutator or defective or binding brushes may
result in poor contact between brushes and commutator. Clean the commutator,
brushes, and brush holders with a cloth moistened with the recommended cleaning
solvent. If brushes are damaged or worn to the specified minimum length, install new
brushes in accordance with the applicable manufacturer's instructions covering the
motor. If the motor still fails to operate, replace it with a serviceable motor.
57
Week 10
4.1 Introduction
Single phase induction motors are used in residential and commercial
applications. Where three-phase power is unavailable or impractical, it's single-phase
motors to the rescue. Though they lack the higher efficiencies of their three-phase
siblings, single-phase motors, correctly sized and rated can last a lifetime with little
maintenance. Single-phase AC motors are as ubiquitous as they are useful, serving as
the prime industry and in the home. Knowing how to apply the various types is the
key to successful design.
Eighty percent of operating motors in the world are AC single phase induction
motors. They are used in applications with power requirements of 10 horsepower or
less. In this chapter, single-phase motors, their constructions, types, principle of
operations and speed control will be detailed. Also the power and efficiency will be
discussed.
4.2
Construction of A.C single-phase induction motor
Figure (1) shows the construction of a single-phase induction motor.
Figure(4-1) : The construction of single-phase induction motor
There are two main parts of a single-phase induction motor are :
a) Rotating part, called the rotor.
b) Stationary part, called the stator.
58
4.2.1 Rotor
The rotor is the rotating
part of the electric
motor.
Motors contain either a squirrel
cage or wound rotor. Like the
stator, rotors are constructed of
a core wound with soft wire, but
with the addition of a shaft and
bearings. The shaft and bearings
Figure(4-2) :rotor
are supported by end caps, which allows the rotor to turn see fig. (4-2 ).
4.2.2 Stator
The stator is the immobile
portion of an electric motor. A stator
is made of pairs of thin sections of
soft iron, called slotted cores. The
cores are wound with insulated copper
wire. Each of these wound cores has
two magnetic poles as shown in fig. (
4-3 ).
When
an
electrical
source
is
Figure(4-3) :stator
connected to the wires, they function as electromagnets. The stator can have several
sets of windings. These include start windings, run windings, and windings for
variable voltage operation.
4.2.3 Frame enclosure
The enclosure is also designed to dissipate heat
from current flow in the windings, friction in the
bearings, and other sources. Without heat dissipation, the
insulation around motor windings deteriorates, causing
short circuits and motor failure. Motor frames differ
according to the size and type of the motor. Motor
enclosures fall primarily into either open or totally
Figure(4-4) : Frame enclosure
59
enclosed categories fig.(4-4).
4.2.4 Fan
Some motors have their own cooling fans to
blow air over the enclosure. Cooling fans can be
located inside or outside the enclosure fig.(4-5).
Without fans, motors cool themselves by conduction
of heat to the surrounding air.
Figure(4-5) : Fan
4.2.5 Terminal ( connection ) box
The
conduit
box
houses
the
electrical
connection points from the motors internal windings
to an electrical power supply. Another important part
in the construction is the centrifugal switch. It used to
disconnect the starting winding after the rotor speed
Figure(4-6) : Terminal
( connection ) box
has reached a predetermined speed see fig.(4-6 ).
4.2.6 Centrifugal switch
A centrifugal switch is an electric switch that operates
using the centrifugal force created from a rotating shaft, most
commonly that of an electric motor or gasoline engine. The
switch is designed to activate or de-activate as a function of the
rotational speed of the shaft fig.( 4-7-a ).
Figure(4-7.a) :
Centrifugal switch
Centrifugal switches typically serve as a means of turning ON or OFF circuit
functions depending on motor speed. The most
widespread use of such switches is as a start
winding cut-out for single-phase
fractional-
horsepower motors. In some clothes driers, the
switches can also be found controlling dryer
heating elements, allowing the dryer to switch on
Figure(4-7.b) : Centrifugal switch
60
only when the drum motor is up to speed.
The basic operating principle of the switch is to use a speed-sensing mechanism that
consists of a conical spring steel disc that has
weights fastened to the outer edge of a circular
base plate. Fingers on the spring attach to an
insulating spool that rides free of the shaft see fig.
(4-7 b,c,d )
Figure(4-7.c) : Centrifugal switch
operation
Figure(4-7.d) : Centrifugal switch
Open &closed position
61
4.3 How electric motor work?
Electric motors function on the principle of magnetism; where like poles repel,
and unlike poles attract. In a simple motor, a freeturning permanent magnet is mounted between the
prongs of an electromagnet fig.( 4-8 ). Since
magnetic forces travel poorly through air, the
electromagnet has metal shoes that fit close to the
poles of the permanent magnet. This creates a
stronger more stable magnetic field. (The
electromagnet functions as the stator, and the freeturning magnet is the rotor.) Fluctuating polarity
Figure(4-8) : a simple- motor
in the electromagnet causes the free-turning magnet to rotate. The poles are changed
by switching the direction of current flow in the electromagnet.
The direction of current flow can be changed in one of two ways. The stator in
an AC motor is a wire coil, called a stator winding as shown in fig. ( 4-8-a ). It's built
into the motor. When this coil is energized by
AC power, a rotating magnetic field is
produced.
Induction motors are equipped with
Figure(4-8a) : stator of A.C motor
squirrel rotors see fig. ( 4-8 –b ), which
resemble the exercise wheels often associated
with pet rodents like gerbils. Several metal bars
are placed within end rings in a cylindrical
pattern. Because the bars are connected to one
another by these end rings, a complete circuit is
formed within the rotor.
Figure(4-8b) : rotor of A.C motor
62
Consider this close-up of a 2-pole stator and one of its rotor bars as shown in
fig. ( 4-8. c ). Alternating current flowing in the
stator causes the poles to change rapidly, from
north to south and back again. If the rotor is
given a spin, the bars cut the stator lines of
Figure(4-8c) : 2-pole stator and
one rotor
force. This causes current flow in the rotor bar.
This current flow sets magnetic lines of force in circular motion around the rotor bars.
The rotor lines of force, moving in the same direction as those of the stator, add to the
magnetic field and the rotor keeps turning see
fig. ( 4-8 d ).
Figure(4-8d) : single-phase motor
4.4 Operation principle
The most common method of starting a
single phase motor combines a capacitor and
auxiliary winding or start circuit. A schematic
view shows an auxiliary starting winding, a
capacitor, and a centrifugal switch. The auxiliary
winding is actually a second winding in the motor
Figure(4-9):induction motor
see fig. (4-9 ).
63
AC single phase induction motors are classified by their start and run
characteristics. An auxiliary starter winding is
placed at right angles to the main stator winding in
order to create a magnetic field. The current
moving through each winding is out of phase by
90 degrees see fig. (4-9 .a ). This is called phase
differential.
After
the
motor
has
reached
approximately 75% of operating speed, the
Figure(4-9a) : phase shift between
winding
auxiliary winding is disconnected from the circuit by a centrifugal switch.
When current is applied to the motor, both the run winding and the start
winding produce magnetic fields. Because the
start winding has a lower resistance, a stronger
magnetic field is created which causes the motor
to begin rotation. Once the motor reaches about 80
percent of its rated speed, a centrifugal switch
disconnects the start winding. From this point on,
the single phase motor can maintain enough
rotating magnetic field to operate on its own. The
Figure(4-9b) : induction motor
characteristics
graph shows a typical torque/speed curve for auxiliary starting on single phase motors
fig.( 4-9 b).
64
There are a variety of starting methods
used in the different single phase motor types.
These are covered in more detail in this chapter
what these starting methods all have in common is
the ability to produce a rotating magnetic field
using the input power that is applied to the motor
fig. (4-9.c ).
Figure(4-9c) : phase-relationship
in split-phase motor
65
Week 11
4.5 Motor speed
4.5.1 Synchronous speed
There are two ways to define motor speed. First is synchronous speed. The
synchronous speed of an AC motor is the speed of the stator's magnetic field rotation.
This is the motor's theoretical speed since the rotor
will always turn at a slightly slower rate. The other
way motor speed is measured is called actual speed
see fig. (4-10 ). This is the speed at which the shaft
rotates. The nameplate of most AC motors lists the
actual motor speed rather than the synchronous
speed. A motor's synchronous speed can be
Figure(4-10) : motor speed
computed using this formula: synchronous speed
equals 120 times the operating frequency, divided by the number of poles.
Where :
Synchronous speed ( Ns ) in
r.p.m
Supply frequency
( f ) in
Hz
Number of poles
( P ) in
poles
Example : (1) A 6-pole, single-phase induction motor is fed from a 50 Hz. Calculate
the Synchronous speed ?
Solution :
Synchronous speed = 120 × f / P
r.p.m
= 120 × 50 / 6
= 1000 r.p.m
4.5.2 rotor speed and slip speed
66
The difference between rotor speed ( nm ) and synchronous speed ( ns ) is
called the ' slip speed '
nslip = nsyn – nm
r.p.m
where :
nslip
= slip speed ( r.p.m )
nsyn
= synchronous speed ( r.p.m )
nm
= motor ( rotor ) speed ( r.p.m )
4.5.3 Slip
The slip speed expressed as a function of n slip is called ' slip '.
Slip (S) = nsyn - nm / nsyn
Example : (2) A 4-pole, single-phase induction motor is fed from a 50 Hz supply
and has a rotor speed of 1425 rpm. Calculate the slip and
percentage slip?
Solution :
Synchronous speed = 120 × f / P
r.p.m
= 120 × 50 / 4
= 1500 rpm
Slip (S) = nsyn - nm / nsyn
= 1500 – 1425 / 1500 = 0.05٪
percentage slip ( S٪ ) =
slip ×100٪
=
0.05 ×100٪
=
5٪
4.6 Types of single-phase induction motors
AC single phase induction motors are
classified by their start and run characteristics. An
auxiliary starter winding is placed at right angles
to the main stator winding in order to create a
magnetic field. The current moving through each
winding is out of phase by 90 degrees fig.(4-11).
This is called phase differential. After the motor
Figure(4-11) :start and run
characteristics
has reached approximately 75% of operating
67
speed, the auxiliary winding is disconnected from the circuit by a centrifugal switch.
The most commonly used types of induction motors are :
4.6.1 Split phase motors
Simply constructed split phase motors are
among the least expensive. They're widely used
on easy starting loads of 1/3 horsepower or less.
Washing machines, tool grinders and small fans
and blowers are among the applications that use
these motors. Split phase start motors are
equipped with a special set of stator windings or
starting purposes fig(4-12 ). They are called start
windings or start pulls. These start windings are
Figure(4-12):split phase motor
made of smaller wire than the run windings.
Because these wires are smaller, they offer less resistance and provide higher current
flow. Accordingly, the start pulls are first to become magnetized when the power is
applied. The current flow through the start winding begins after power is applied to
the motor by 20 degrees or so fig.(4-12. a ).
Current is induced in the rotor as the run
pulls establish a stronger magnetic field. The
interaction of the induced current in the rotor and
the magnetic field causes the rotor to turn one
quarter turn. The current induced in the rotor
perpetuates its motion as speed increases and the
start pulls are no longer needed. At about 75% of
Figure(4-12 a) start and run
operating speed the centrifugal switch opens
winding current
disconnecting current to the start winding see fig. (4-12 b ).
Split phase motors have moderate to
low starting torque. 100 to 125 percent of full
load and high starting current. Sizes range
Figure(4-12b): Split phase motor
winding
68
from 1/20 to 3/4 horsepower as shown fig. ( 4-12.c ).
Split phase motors draw 6 to 8 times normal current when starting. They
usually operate on single voltages. Split phase
motors have lower starting torque and are less
expensive because they use no capacitors in the
start winding circuit.
Figure(4-12c): Starting torque
winding current
The split phase motor is most
widely used, for "medium starting"
applications fig.( 4-12.d ). The split
phase motor has a start and run
winding. Both windings are energized
when the motor is started. When the
motor reaches about 75% of its rated
full load speed, the starting winding is
Figure(4-12 d) speed-torque characteristics
disconnected from the circuit by an
automatic switch.
Applications
This motor is excellent for medium duty applications and where stops and
starts are somewhat frequent. Popular applications of split phase motors include: fans,
blowers, office machines and tools such as small saws or drill presses where the load
is applied after the motor has obtained its operating speed.
69
Week 12
4.6.2 Capacitor motors
Some single phase motors utilize a
capacitor installed in series with one of the stator
windings fig.(4-13). A capacitor is an electrical
device which can rapidly build up an electrical
energy supply that can be used to create more
current flow in the motor's windings. When input
Figure(4-13) :Capacitor motors
power is applied to the motor, the capacitor
becomes charged up almost instantaneously.
The capacitor's energy helps create current flow in the start winding before the
run winding gets any current flow. This difference in timing, called "phase
differential", creates a rotating magnetic field in the stator fig.(4-13a,b). This stronger
magnetic field induces more current into the rotor causing it to rotate quicker. The end
result is a motor with the ability to start
This technique is widely used for motor
applications like air conditioners and compressors.
All the capacitor motors discussed in this section
operate in essentially in the same manner.
The advantages that capacitor motors have
Figure(4-13a):Capacitor motors
over split phase motors are :
•
They produce more starting torque, and
•
They use less current while running at steady speeds
70
Capacitor motors vary in size ranging from
small motors of fractional horsepower to motors
up to 10 horsepower. Torque and voltage ratings
of a motor determine the rating of the capacitor.
The voltage rating of a capacitor must always
Figure(4.13b):Capacitor motors
meet or exceed the voltage requirements
of the motor in which it is used.
4.6.3 Capacitor run motors
One type of capacitor motor is the capacitor run or permanent split capacitor
motor fig.(4-14). These are used in instances where low starting torque is needed as in
air conditioner
Permanent split motors found in sizes up to three horsepower are economical
and easily customized. This type of motor is
similar to the split phase motor with the
exception being that the current to its start
winding is not switched off during motor
operation. In normal split phase motors this
current is turned off after starting. A small
Figure(4.14) :Capacitor run
capacitor within the start circuit of the capacitor
run motor remains functional throughout start and operation of the motor.
Permanent split capacitor motors cost less than those with switching system.
They provide greater starting torque and better running characteristics than split phase
motors.
Capacitor run motors make good replacements for shaded pole motors. In this
role they're more efficient and require lower current levels than shaded pole motors.
For these reasons they're effective in fans with low starting torque requirements.
71
Characteristics
Because of its improved starting ability, the capacitor start motor is
recommended for loads which
are hard to start. The motor
has a capacitor in series with a
starting winding and provides
more than double the starting
torque with one third less
starting current than the split
phase motor see fig.(4-15).
Figure(4.15) :motor characteristics
Applications
It has good efficiency and requires starting currents of approximately five times
full load current. The capacitor and starting windings are disconnected from the circuit
by an automatic switch when the motor reaches about 75% of its rated
full load speed. Special applications include: compressors, pumps, machine
tools, air conditioners, conveyors, blowers, fans and other hard to start applications.
4.6.4 Capacitor start motors
Capacitor start / induction run motors are similar in construction to split phase
motors. The major difference is the use of a
capacitor connected in series to start
windings to maximize starting torque. see
fig.(4-16)
The capacitor is mounted either at
the top or side of the motor. A normally
Figure(4-16) :capacitor start motor
closed centrifugal switch is located
72
between the capacitor and the start winding. This switch opens when the motor has
reached about 75 percent of its operating speed.
Capacitors in induction run motors enable them to handle heavier start loads by
strengthening the magnetic field
of the start windings. These loads
might include refrigerators,
compressors, elevators, and
augers. The size of capacitors
used in these types of
applications ranges from 1/6 to
10 horsepower. High starting
torque designs also require high
Figure(4-16 b) :capacitor start motor –starting
torque
starting currents and high breakdown torque.
Capacitor start / induction run motors typically deliver 250 to 350 percent of
full load torque when starting see fig.(4-16.b ). Motors of this design are used in
compressors and other types of industrial, commercial, and farm equipment.
Capacitor start induction run motors of moderate torque values are used on
applications that require less than 175 percent of the full load. These are used with
lighter loads like fans, blowers, and small pumps.
73
Week 13
4.6.5 Capacitor start capacitor run motors
Capacitor start / capacitor run motors are
more efficient and require less running current
than motors with start capacitors only. These
motors have two capacitors in series with the main
stator winding see fig. (4-17). Start capacitors
have a high capacity while the run capacitors do
not. One optimizes starting torque while another
optimizes running characteristics. Throughout
Figure(4-17) :Capacitor startCapacitor run
Induction motor
both starting and operation all the windings in the motor remain energized.
At operating speed, the switch disconnects the start capacitor and turns on the
run capacitor to maintain the motor's performance. Optimum levels of both starting
torque and running characteristics are achieved with this design.
Capacitor start / capacitor run motors are used over a wide range of single
phase applications primarily starting hard loads. They are available in sizes from 1/2
to 25 horsepower.
4.6.6 Shaded-pole induction motors
The simplest and least expensive type
of single phase motor is the shaded pole
motor. Fig (4-18) shows the construction of
this motor. Due to low starting torque, its use
is limited to applications that require less than
3/4 horsepower, usually ranging from 1/20 to
1/6 horsepower.
Figure(4-18) :shaded-pole induction
motor construction
74
Shaded pole motors use no starting switch. The stator poles, see fig.(4-18a,b)
are equipped with an additional winding in each corner called a shade winding. These
windings have no electrical connection
for starting but use induced current to
make a rotating magnetic field.
Figure(4-18 a ) :main and shaded
winding
The pole structure of the shaded
pole motor enables the development of
a rotating magnetic field by delaying the
buildup
of
magnetic
flux.
A
copper
conductor isolates the shaded portion of the
pole forming a complete turn around it. In
the shaded portion, magnetic flux increases
but is delayed by the current induced in the
copper
shield.
Magnetic
flux
in
the
unshaded portion increases with the winding
current forming a rotating field.
Figure(4-18 b ) :shaded-pole induction
motor structure
Rotor torque initiates as the magnetic
field sweeps across the face of the pole between the unshaded and shaded portions.
The rotor is highly resistant in order to maximize the torque.
Shaded pole motors function best with low torque applications and usually rate
less than 1/10 horsepower. They should never be used to replace single phase motors.
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Shaded pole motors are best suited to low power household application because
the motors have low starting torque and efficiency ratings. Some compatible
applications include hair dryers, humidifiers and timing devices.
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Week 14
4.6.7 Repulsion motors
The repulsion-induction motor is a
combination of a repulsion motor and a squirrelcage induction motor. This motor is always a 2pole configuration. The stator winding is identical
to the run winding of a 2-pole split-phase or
capacitor-start motor. The rotor is nearly identical
to a universal series motor armature, with the
exception of having a greater number of windings
Figure(4-19) :repulsion induction
motor
(in most cases) and no connection to a power source. The brushes are connected to
each other directly, in order that they may complete a circuit through windings within
the rotor see fig.(4-19).
The closed-loop circuits in the rotor are effectively the short-circuited
secondaries of a transformer, where the motor's field windings are the primary coil.
The currents induced in the rotor create a magnetic field which repels that of the field
winding (Lenz's law). This repulsion is what gives the motor it's torque. Rotation
happens because the brushes are offset 15 or so degrees from the field poles, so that
the repulsive forces are pushing on the rotor somewhat tangentially to it's rotation axis
(see the schematics below).
In addition to this repulsion motor setup, the rotor also has buried within it a
squirrel cage winding. As the repulsion-induction motor comes up near synchronous
speed (3600 RPM on 60Hz), the squirrel-cage winding is responsible for most of the
torque, and the repulsion effect diminishes.
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4.6.8 Universal motors
The universal motor is a rotating electric machine similar to a DC motor but
designed to operate either from direct
Fig.(4-20) single-phase universal motor
current or single-phase alternating
current. The stator and rotor windings
of the motor are connected in series
through the rotor commutator.
Therefore the universal motor is also
known as an AC series motor or an
AC commutator motor. The universal motor can be controlled either as a phase-angle
drive or as a chopper drive.
This type of motor is identical in principle to the DC series motor fig(4-20a,b), but
a few modifications have been made to
optimize the motor for AC use: The cores of
the field poles are made from stacks of
laminated sheet metal punchings like you
find in transformers, instead of solid iron.
This is to reduce the eddy-current losses in
the core. In addition, the slots of the
armature are slanted slightly to reduce AC
buzzing and give the motor uniform starting
Figure(4-20 a) : rotor of universal motor
characteristics regardless of the armature's
initial orientation relative to the field coils. Shown here are the armature and field
coils of a typical universal motor. This motor happens to be from a vacuum cleaner,
but the design is common to siren motors as well.
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The name "universal" is derived
from the motor's compatibility with both
AC and DC power. Among the
applications using these motors are
vacuum cleaners, food mixers, portable
drills, portable power saws, and sewing
machines. These motors seldom exceed
Figure(4-20 b) : stator of universal motor
one horsepower.
In most cases, universal motors reach little more than a few hundred rpm under
heavy loads. If the motor is run with no load, speed may approach up to 15,000 rpm.
This can result in serious heat damage to
the motor's components.
Universal series motors differ in
design from true induction motors. They
have series wound rotor circuitry similar
to that of DC motors. The rotor of a
universal series motor is made of a
laminated iron core with coils around it.
The ends of the wire coils connect directly
to the commutator.
Figure(4-20 c) : universal motor diagram
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Electric current in the motor flows
through a complete circuit formed by the
stator winding and rotor winding fig.(420c,d). Brushes ride on the commutator
and conduct current through the rotor
from one stator coil to the other. The rotor
current interacts with the magnetic field of
the stator causing the rotor to turn. As
long as an electrical current is present in
the rotor coils, the motor continues to run.
Figure(4-20d) : universal motor diagram
4.7 Speed-torque characteristics of single-phase induction motor
Figure(4-21) : speed torque characteristics
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Week 15
4.8 power, losses and efficiency
4.8.1 Input power
The electrical power input in kilowatts for a single phase motor is calculated by
multiplying the voltage measured at the motor, by the amperage measured at the motor, then
multiplying this product by the power factor of the motor, and dividing the result by 1,000.
4.8.2 Kw to Hp Conversion
Electric utilities use their meters to measure
the input power of a motor fig.(4-22). Input
power is the power consumed by a motor in
operation. It's typically measured by electric
utilities in terms of kilowatts. Kilowatts can be
converted to horsepower by dividing the
number of kilowatts by a constant of 0.746
Figure(4-22) : power
For example:
To convert an input power of 9 kilowatts to units of horsepower, divide 9
kilowatts by 0.746. The result is 12.06 horsepower.
4.8.3 motor losses
Motor loss refers to the consumption of electrical energy not converted to
useful mechanical energy output. Every AC motor has five aspects of power loss as
shown in fig.(4-23 ). Combined, these five types of energy loss constitute the total
power loss of a motor.
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Power loss comprises energy converted to heat and dissipated from the motor
frame. One of the functions
of a cooling mechanism is
to alleviate power losses.
Motor design alterations
that diminish any of these
losses contribute to the
enhancement of motor
efficiency. Reduction of
energy losses always
improves a motor's
efficiency .
Figure(4-23) :motor losses
4.8.3.1 core or iron losses
Core or iron losses are comprised of the energy required to magnetize the
laminated core and current losses from
magnetically induced circulating currents inside
the laminated core. Core losses make up about 25
percent of the total losses fig.(4-24). Core or iron
losses can be reduced by utilizing higher quality
steels with low core loss characteristics found in
high grade silicon steel, using thinner gauges of
steel, and designing longer cores to reduce
Figure(4-24) : motor core
operating flux density.
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4.8.3.2 rotor losses
Rotor losses are due to the heating effect
of current flow in the rotor fig.(4-25). Rotor
losses are proportional to the current squared and
multiplied by rotor resistance in Ohms. As
current flow in the rotor increases, power loss, as
Figure(4-25) : rotor
well, increases. Rotor losses account for about
25 percent of total motor losses. Rotor losses
diminish with the use of higher grade steel and larger conductor bars with increased
cross sectional area, which lower the resistance of the rotor.
4.8.3.3 stator losses
Stator losses are due to the heating
effect of current flow through resistant stator
windings fig (4-26). Stator losses are
proportional to the current squared and
multiplied by winding resistance in ohms. As
current flow in the stator increases, so does
power loss. Stator losses account for
approximately 35 percent of total motor losses.
Figure(4-26) :stator
Reduction of stator losses is possible with the
use of high grade copper and larger conductors with increased cross sectional area.
This lowers the resistance in the motor windings, reducing stator losses.
4.8.3.4 friction and windage losses
Friction and windage losses comprise bearing friction, wind friction, the
motor's cooling fan load, and any other source of friction or air movement in the
motor. These losses are often appreciable in large and high speed totally enclosed fan
cooled motors. Friction and windage losses typically make up about 5 percent of total
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efficiency loss. Friction and windage losses are less problematic with the use of high
quality bearings and lubricants, and improved fan designs.
4.8.3.5 stray losses
Stray losses are other losses in addition to core, stator, rotor and frictional
losses. They are primarily due to leakage induced by load current, design flaws and
manufacturing variables. Stray losses make up about 10 percent of total motor losses.
Optimizing motor design and enforcing strict quality control largely diminishes the
extent of stray load loss.
4.8.4 Efficiency
Electric motors are not 100% efficient. Upon conversion of input power into
output power, some of the energy consumed is displaced as heat. This amounts to
energy lost in its conversion from electrical to mechanical energy fig.(4-27). The
amount of energy lost in this manner, the difference between the input and output
power, determines the motor's efficiency. Electric motors are not 100% efficient.
Upon conversion of input
power into output power,
some of the energy
consumed is displaced as
heat. This amounts to
energy lost in its conversion
from electrical to
mechanical energy. The
amount of energy lost in
this manner, the difference
between the input and
Figure(4-27) :motor efficiency
output power, determines
the motor's efficiency.
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Also, motor efficiency is a measure of the effectiveness with which a motor
converts electrical energy to mechanical energy output to drive a load. It is defined as
a ratio of motor power output to source power input. The difference between the
power input and power output comprises electrical and mechanical losses.
4.8.5 External speed control drives
4.8.5.1 Direct drive
With the use of direct drive systems, fig.(4-28) very few power losses occur. The
direct drive offers the most efficient transfer of power from motor to load of all drive
types. Direct drive motor and load shafts
connect by a coupling. Where the motor and
load shafts are misaligned, or if a motor's
speed is not controllable, direct drives function
poorly. A flexible coupling, to correct this,
Figure(4-28) :direct drive motor
allows slight misalignment while minimizing
the transmission of adverse thrust to motor bearings. As an added advantage, direct
drives require very little maintenance.
4.8.5.2 belt and pulley drives
Figure(4-29) :belt and pully drives
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A belt drive has at least two
pulleys, fig.(4-29). Connected to the motor
shaft is the drive pulley. The driven pulley
connects to the load shaft. A belt joins
these pulleys, transferring power from the
motor to the load. Belt and pulley drives are low in cost, and capable of speed
variation through alterations in pulley size. This type system is not as efficient as a
direct drive, since wear and loosening of the belts results in wavering efficiency.
Because of this, these systems require heavy and frequent maintenance.
4.8.5.3 gear motors
A gear motor is a combination of a standard motor with a matched gear driven
transmission fig. (4-30). This combination of a constant speed motor with a gear
transmission
functions
to
provide
an
application with the quality of adjustable
speed. A number of application factors must
be considered in properly sizing a gear
motor for a particular application. These
include: the load type, motor type, the
coupling, and other specific requirements.
Figure(4-30) :gear motors
Gear motors are convenient and efficient
since the motor shaft is coupled directly to the gear shaft eliminating belts, chains or
other speed reducers. This provides for an optimally matched system with higher
efficiency than with a motor and gear transmission purchased separately.
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4.8.5..4 gear drives
Certain drives have shafts placed
closely together to transmit large amounts
of power. These drives use gears, fig.(431). Geared drives are more expensive
than others but are nearly as efficient as
direct drives. As long as gears remain well
lubricated, they do not wear out as quickly
as belt-pulley or chain-sprocket systems.
Figure(4-31) :gear drives
Other than regular lubrication, gear drives
require very little maintenance.
4.8.5.5 chain and sprocket
Chain and sprocket drives resemble belt and pulley systems, fig.(4-32). They
are, however, capable of transmitting more
power, since metal chains don't slip as
pulley belts do. Chain and sprocket
systems cost more than belt and pulley
systems, but are more efficient. Drive
efficiency diminishes rapidly as the chain
Figure(4-32) :chain and sprocket
and sprocket components wear-out, so
these systems require a significant amount of maintenance
4.9 Nameplate information's
The motor nameplate contains specific information about the motor, fig.(4-33).
Motors are required to be shipped with a
nameplate. The National Electrical Code
requires specific items:
•
the manufacturer's name, model and
serial number;
•
rated voltage and full load
Figure(4-33) :nameplate information
amperage;
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•
rated frequency;
•
phase;
•
rated full load speed;
•
rated temperature rise or insulation class and rated ambient temperature;
•
duty rating;
•
rated horsepower; and
•
design code letter.
Additional information is sometimes provided on these items:
•
service factor,
•
enclosure type,
•
frame size,
•
connection diagrams,
•
unique or special features.
4.10 Reversing the direction of rotation
4.10.1 split-phase induction motor
In order to reverse the direction of rotation of split-phase motors, we have
interchange the leads of either the auxiliary winding or the main winding.
However, if a single-phase motor is equipped with a centrifugal switch, its rotation
cannot be reversed while the motor is running. If the main winding leads are
interchanged, the motor will continue to run in the same direction.
4.10.3 capacitor-run induction motor
The direction of rotation can be changed while the motor is running because
both windings are in the circuit at all times.
4.10.4 very small induction motors
The direction of rotation can be reversed by using a double throw switch.
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4.10.5 shaded-pole induction motors
for shaded-pole induction motors, there is no easy way to reverse their
direction of rotation. To achieve reversal, it is necessary to install two shading coils on
each pole face and to selectively short one or the other of them.
4.11 speed control
The speed control of a single-phase induction motor may be controlled by using one
of the following techniques :
1.
changing the number of poles.
2.
changing the applied terminal voltages.
3.
varying the stator frequency.
In practical design involving fairly high-slip motors, the usual approach to speed
control is to vary the terminal voltage of the motor. This may be done by on of the
following methods:
• using an autotransformer to adjust the line voltage.
• Inserting a resistor in series with the motor stator circuit.
• Using a solid-state control circuit.
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4.12 Applications
Type of motor
Torque
Applications
-Refrigeration compressors
-loaded conveyor belts
-reciprocating pumps
Split-phase induction motor
Capacitor motors
High starting torque
Universal motors
-portable hand drills
-power saws
-rowters
-portable hand jointers
-planers
-centrifugal pumps
shaded-pole motors
normal starting torque
-machine tools
low starting torque
-fans
-tape-recorder
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