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 three dimensions 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.
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
1
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:
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 as shown
in fig.(1-4).
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 solenoid and can be used to generate a
(a)
Figure(1-4) : Generating magnetic
material
(b
Figure(1-6): Magnetic field
around the wire carried current
2
nearly uniform magnetic field similar
Figure(1-5) :Generating electromagnet
(Solenoid)
to that of a bar magnet.
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 threedimensional representation of the magnetic field is shown in fig.(1-6b).
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
3
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.
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.
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):
4
Figure(1-11): Current, flux and voltage waveform
Mutual-inductance
When one circuit induces current flow in a second nearby circuit, it is known as mutualinductance. The image to the right shows an example of mutual-inductance as shown in fig.(112). When an AC current is flowing through a piece of wire in a circuit, an electromagnetic field
is produced that is constantly growing and shrinking and
i1
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
close to the wire in the primary circuit. The current
Figure(1-12): Mutual inductance
in the second wire will also be AC and in fact will
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.
2.1
e2
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
5
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.
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.
6
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.
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)
7
Figure (2-3): Output induced voltage of a simplest generator during one revolution.
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.
8
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 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
9
Figure
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.
(2-7) : Fourpole
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 cores strengthen the magnetic
fields
Figure (2-8) : Rotor of a dc motor
as shown in fig.(2-6) views C and in fig.(2-8)
10
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 semi
cylindrical pieces of a
Figure (2-9): Connection of commutation with
the end of armature coils
smooth conducting material, usually copper, separated by mica insulation 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.(26) 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 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.
11
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
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(212): 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,
12
e.m.f = Induced electromotive force (voltage generated) in V or (volts)
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)
The angle between the magnetic field direction and the conductor
β
=
θ
=
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
2
field. If the flux density of the field is 0.3 wb/m . 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
13
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 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
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.
14
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 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
15
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.
Classification of DC
Generators
separately-excited DC
generator
Self-excited DC generator
Types of DC
Motors
compoundwound
shunt-wound
series-wound
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-
16
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 E nL = 462 V
Full-load voltage E fL= 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.
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)
17
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
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.
18
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.
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
Figure(3-1): Simple motor carries
current that rotates in
a
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,
19
the middle finger points in the direction of the current, and the thumb shows which way the
conductor will be forced to move.
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.
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.
20
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)
3.3 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 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
21
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.
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.
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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.
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
Figure(3-7) : Composite of the characteristic
used in aircraft systems.
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.(37).
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
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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)
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)
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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 multispeed motor. Note that AC inverter drives can change the speed of a motor from its rated speed.
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.
11) Enc. (Enclosure)
This is the degree of protection offered by the enclosure. Common terms are TEFC, TEBC,
TENV, ODP, TEAO, etc.
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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.
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
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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
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.
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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.
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.
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The current immediately jumps to a higher value and the motor quickly accelerates to the next
higher speed. When the 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 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
29
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).
Figure (3-9) : Simple D-C Motor Starter and Controller
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-
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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.
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
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.
31
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.
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
32
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 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.
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Figure (3-11) : Diagram of a shunt motor connected to a reversing motor starter.
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.
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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
5)
6)
7)
8)
spring tension, and procedures for replacing brushes are given in the applicable
manufacturer's instructions.
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.
Inspect all exposed wiring for evidence of overheating. Replace the motor if the insulation
on leads or windings is burned, cracked, or brittle.
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.
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 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.
35
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.
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.
36
b) Stationary part, called the stator.
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 ).
Figure(4-3) :stator
When
an
electrical
source
is
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
enclosed categories fig.(4-4).
4.2.4 Fan
Some
motors
have their own cooling
37
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 has reached a predetermined
speed see fig.(4-6 ).
Figure(4-6) : Terminal
( connection ) box
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 cutout for single-phase fractionalhorsepower 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
only when the drum motor is up to speed.
38
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
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
39
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 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(4-8a) : stator of A.C motor
Figure(4-8b) : rotor of A.C motor
Consider this close-up of a 2-pole stator and one of its rotor bars as shown in fig. ( 48. c ). Alternating current flowing in the stator causes the
poles to change rapidly, from
north to south and back again. If the rotor is
Figure(4-8c) : 2-pole stator and
given a spin, the bars cut the stator lines of
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 ).
40
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
Figure(4-9):induction motor
winding is actually a second winding in the motor see fig. (49 ).
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 Figure(4-9a) : phase shift between
approximately 75% of operating speed, the
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 magneticFigure(4-9b) : induction motor
field to operate on its own. The
characteristics
graph shows a typical torque/speed curve for auxiliary starting on single phase motors fig.( 4-9 b).
41
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
42
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
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?
43
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).
Figure(4-11) :start and run
This is called phase differential. After the motor
characteristics
has reached approximately 75% of operating
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 ).
44
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
winding current
operating speed the centrifugal switch opens 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
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.
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 disconnected from the
circuit by an automatic switch.
Applications
45
Figure(4-12c): Starting
torque winding
current
Figure(4-12 d) speed-torque characteristics
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.
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
46
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.
Figure(4-13a):Capacitor motors
The advantages that capacitor motors have
over split phase motors are :
•
They produce more starting torque, and
•
They use less current while running at steady speeds
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 capacitor
within the start circuit of the capacitor
Figure(4.14) :Capacitor run
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.
47
For these reasons they're effective in fans with low starting torque requirements.
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
between the capacitor and the start winding. This
switch opens when the motor has reached about 75
percent of its operating speed.
48
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
Figure(4-16 b) :capacitor start motor –
starting
torque designs also require high 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.
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
Figure(4-17) :Capacitor start-
not. One optimizes starting torque while another
optimizes running characteristics. Throughout Induction
operation all the windings in the motor remain energized.
motor
Capacitor run
both starting and
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.
49
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
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
Figure(4-18 b ) :shaded-pole induction
current forming a rotating field.
motor structure
50
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.
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.
51
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
Figure(4-19) :repulsion induction
motor exception of having a greater number of windings
(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.(419).
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.
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
52
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
be controlled either as a phase-angle drive or as
chopper drive.
can
a
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.
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.
53
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
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
54
4.7 Speed-torque characteristics of single-phase induction motor
Figure(4-21) : speed torque characteristics
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.
55
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.
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
Figure(4-23) :motor losses efficiency .
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
Figure(4-24) : motor core
steel, and designing longer cores to reduce
operating flux density.
56
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
well, increases. Rotor losses account for about
Figure(4-25) : rotor
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 (426). 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(426) :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 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.
57
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.
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
58
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 allows slight misalignment while minimizing the transmission of
adverse thrust to motor bearings. As an added advantage, direct drives require very little
maintenance.
Figure(4-28) :direct drive motor
59