7.1. ELECTROMAGNETIC INDUCTION
Out of three principal effects of
electric current, one is its magnetic effect. A current carrying conductor is
surrounded by a magnetic field. Conversely, a conductor is placed in a magnetic
field
and the magnetic flux passing through the condutor is made to change, an (Lila is induced in the conductor. 1 he principle of producing e.m.f. by induction is called electro-magnetic induction.
and the magnetic flux passing through the condutor is made to change, an (Lila is induced in the conductor. 1 he principle of producing e.m.f. by induction is called electro-magnetic induction.
An e.m.f. may be induced in a
conductor in the following two manners:
1. Dynamically Induced e.m.f.
When a conductor is moved in a magnetic field in such a way that its movement
produces a change in the magnetic field, an e.m.f. is induced in it which is
called dynamically induced e.m.f. Following formula is used for its
calculation:
where, e = induced e.m.f.,
volts
B =
magnetic flux density, Wb/m2
I =
length of conductor, metres
v =
velocity of conductor, m/s
sin 0 = sine of the
angle between the direction of magnetic flux and the motion of the conductor.
2. Statically Induced e.m.f. When
a conductor is placed in an alternating magnetic field and the presence of
conductor produces a hindrance in the change in magnetic flux, an e.m.f. is
induced in it which is called statically induced e.m.f Following formula is
used for its calculation: 
7.2. FARADAY'S LAWS OF ELECTRO-MAGNETIC INDUCTION
First Law. A change in the
magnetic flux passing through a conductor induces an e.m.f. in that conductor.
The existence of induced e.m.f. lasts so long as the magnetic flux is changing.
Second Law. The magnitude of
induced e.m.f. is directly proportional to the rate of change of the magnetic
flux.
7.3. FLEMING'S RIGHT HAND RULE
If the first and second fingers
and the thumb of right hand are stretched in such a way that they remain
mutually perendicular, and the first finger points the directions of magnetic
field and the thumb points the direction of motion then the second finger will
point the direction of induced e.m.f. This rule is used for the determination
of direction of the induced e.m.f. in alternators and generators. 
7.4. SIMPLE ALTERNATOR
(a) Definition. An alternator is
a machine which converts mechanical energy into electrical energy.
(b) Principle. It works on
Faraday's laws of electromagnetic induction.
(c) Construction. An alternator
has following three main parts:
(i) Magnetic Field. Two powerful
permanent magnets or electromegnets are used for producing magnetic field or
'field'. For making electromagnets, insulated laminations are used for the two
pole pieces and coils of enamelled copper wire are wound on the pole pieces.
The current to the field coils is supplied from an external battery or from the
induced e.m.f in the armature coils.
(ii) Armature. The central part
of the machine which consists of a shaft, armature coils and armature drum is
called an armature. The shaft of the armature is mounted on the body with the
help of two ball bearings in such a way that it can rotate freely in the field.
(iii) Slip rings and Brushes.
Armature coils are connected to two slip rings for supplying the induced e.m.f.
to the external circuit. These rings are mounted on the shaft with the help of
insulator cylinders. Two brushes are used for establishing contact with the two
slip rings continuously with the help of springs.
(d) Working. When the armature is
moved by the mechanical energy, the magnetic flux passing through the armature
coils begins to change. As a result an e.m.f. is induced in the armature coil.
This e.m.f. is of alternating (A.C.) nature.
Let the conductor loop (coil) to
start from a position which is perpendicular to the magnetic field. In this
position, the magnetic flux intersected by the loop is minimum, hence the
magnitude of induced e.m.f is also minimum. As the loop forwards to horizental
position (0° to 90), the amount of flux intersected by the loop increases and
the magnitude of induced e.m.f. attains its maximum value. Now the loop has
completed one fourth cycle of rotation. After it, when the loop forwards to
vertical position (90° to 180°), the amount of flux intersected by the loop
decreases and at 180° position the magnitude of induced e.m.f. attains its
minimum value again. Similarly, during loop's movement from 180° to 270°
position the magnitude of induced e.m.f. attains its maximum value in negative
direction at 270° position and its minimum value at 360° or 0° position.
The magnitude of e.m.f. induced
at any instance in the loop is calculated in the following manner:
where, V = instantaneous
induced e.m.f
Vmax = maximum
induced e.m.f.
Sin
θ = sine of the angle between the
conductor and the direction of the field.
7.5. SINE CURVE AND CYCLE
The graph plotted for various
values of instantaneous induced e.m.fs. against time is called a sine-curve.
A complete set of e.m.f.
variations from zero to maximum in positive direction and from positive maximum
to maximum in negative direction through zero, and back to zero again is termed
as one cycle, see Fig. 7.3.
 |
| Fig. 7.3. Various loop positions |
7.6. FREQUENCY
The number of cycles completed
per second is termed as frequency. Its symbol is f and its unit is cycles per
second or hertz (c/s or 11z). The number of revolutions completed by the
armature of a two pole alternator is equal to its frequency while for an
alternator having P poles.
where, f = frequency, Hz
P ---
no. of poles
N =
R.P.M. of the armature.
The unit of frequency is hertz (Hz).
Its multiples are:
1 Kilo hertz, 1 kHz = 103 Hz
1 Mega hertz, 1 MHz =- Hz
1 Giga hertz, 1 Gliz = 109 Hz
1 Tega hertz, 1 'FHz = 1012 Hz
Note. (i) Frequencies of the
order of kilohertz and above are generated by oscillator circuits.
(ii) 1 cis = 1 Hz
7.7. TIME PERIOD
The time taken by a cycle for its
completion is termed as time period. Its symbol is T and its unit is second.
where, T = time period, seconds
f =
frequency, hertz.
7.8. DIFFERENT VALUES OF A.C.
1. Peak Value. The maximum value
of voltage or current in the positive or negative direction is called peak
value of A.C. It is expressed by E., and Imp respectively. The maximum value is
also expressed as amplitude.
2. Average Value. The average
value of A.C. of a sine-wave form is always zero. Therefore, the average of
instantaneous values of voltage or current in half cycle of A.C. is called its
average value.
3. R.M.S. (Root Mean Square)
Value. If the heat produced by a certain amount of D.C. in a given time is H
calories then the amount of A.C. required to produce H calories in the same
time is called R.M.S. value of A.C.
The above stated value is
determined by a sinusoidal wave-graph where i1, i2 etc. are the instantaneous
values of current and el, e2 etc. are the instantaneous values of e.m.f.
Ordinary measuring instruments
indicate R.M.S. values.
4. Form Factor. The ratio of
R.M.S. value to the average value of A.C. is called form factor. Its value is
1.11.
5. Amplitude Factor or Peak
Factor. The ratio of peak value to the R.M.S. value of A.C. is called peak
factor. Its value is 1.414.
Peak factor is also known as
crest factor.
Example 7.1. Calculate the frequency
of a 12 pole, 500 R.P.M. alternator.
7.9. PHASE
1. Vector. A physical quantity
having magnitude and direction as well is called a vector. A vector is
represented by a straight line having an arrow mark on its one end, e.g.,
velocity, force, acceleration, etc. are vector quantities.
2. Phase. A comparative
representation of two voltage vectors or two current vectors or voltage and
current vectors is known as phase.
3. Phase Difference. If two quantities
reach their peak or zero at the same time they are said to be in phase. If
there exists a time interval between the peaks or zeros of the two quntities,
they are said to be out of phase or have a phase difference. It is represented
by angle 0 when the time period T is represented by 3600.
4. Single Phase. If the armature
of an alternator has only one winding or only one set of winding then the
e.m.f. generated by the machine is of single phase type. It means that there is
only one e.m.f./ current cycle.
5. Three Phase. A three phase
alternator has three armature windings placed 120° apart from each other. Hence,
three e.m.f. / current cycles are formed. A 3-phase motor has a higher torque
which is three times higher than that of a single phase motor. The transmission
of 3-phase supply is economical as it requires fine cables in comparison to
those required for the transmission of single phase supply.
In three phase system, the
windings can be connected in the following two ways :
(i) Star Connection, Y. In this
connection, the A, B and C terminals of the windings are
where, Line current = the current
flowing through any two windings
Phase current
= the current flowing through any one winding
Line voltage
= the voltage existing between any two phases
Phase voltage = the
voltage existing between
one phase and the neutral.
(ii) Delta Connection, A. In this
connection, the terminals A'-B, B'-C and C'-A are connected
The formula for calculating the
power in three phase circuits is the same whether the windings are connected in
'Star' or in 'Delta'
7.10. POWER FACTOR
In A.C. circuits, the ratio of
actual power to the apparent power is called power factor.
where, V =
circuit voltage
I = circuit current
cos
(5 = cosine of the phase angle between voltage and current
R
= resistance of the circuit, ohms
Z
= impedance of the circuit, ohms.
1. Value of Power Factor. The
maximum value of power factor can be unity, i.e., 1 and it has no unit.
Inductive circuits such as circuits having motors, tube lights, fans etc. have
a power factor of less than unity. P.F. is of the following three types:
(i) Unity P.F. If the impedance
of an A.C. circuit is equal to its resistance then the P.F. of the circuit is
unity. Therefore —
(ii) Leading P.F. If the
capacitive reactance of an A.C. circuit is greater than its inductive reactance
then the current leads the voltage and the P.F. is said to be leading P.F.
Therefore —
(iii) Lagging P.F. If the
capacitive reactance of an A.C. circuit is lesser than its inductive reactance
then the current lags behind the voltage and the P.F. is said to be lagging
P.F. Therefore —
2. Demerits of Lagging P.F. :
(i) On lagging P.F., a
machine/equipment draws more current and its actual power consumption will be
more.
(ii) The efficiency of a
machine/equipment will be reduced.
(iii) The voltage regulation of
an alternator, transformer and line will increase.
(iv) In order to compensate the
rate of actual power consumption, per unit charge is kept higher for industries
than for common use.
3. Reactive Power. There are two
components of the current in A.C. circuits:
(i) Active Component or Wattful
Component:
(ii)
Reactive Component or Wattless Component:
This component of current does
not cause any power consumption.
7.11. WAVE-LENGTH AND VELOCITY
1. Wavelength. The distance
travelled by a wave in one cycle time period is called its wavelength. Its
symbol is X (lambda) and its unit is metre.
2. Velocity. The distance
travelled by a wave in one second time is called its velocity. Its symbol is v
and its unit is m/s.
Velocity = frequency x wavelength
where, v = velocity, m/s
f =
frequency, Hertz
λ = wavelength,
m.
The velocity of sound in air at
0°C is 332 m/s, whereas the velocity of radio waves, electric current, light
rays and heat waves is 3 x 108 m/s.
7.12. DYNAMO OR GENERATOR
As an alternator converts
mechanical energy into electrical energy, in the same way, a dynamo also
converts mechanical energy into electrical energy. The little difference in
both the machines is this that dynamo generates D.C. while an alternator
generates A.C. A dynamo employs split-rings or a commutator in place of slip
rings for supplying D.C. to the external circuit.
Commutator. It consists of copper
segments mounted on an insulator cylinder. The number of segments is kept double
of the number of armature coils. All coil terminals are soldered at the
segments of the commutator. Two carbon brushes are fitted on either side of the
commutator with the help of springs in such a way that they maintain a sliding
contact with the commutator.
For half cycle of rotation a
positive e.m.f. and for the rest half cycle a negative e.m.f. is induced in
each armature winding. In this way, one carbon brush remains positive and the
other negative. The amount of e.m.f. between the two brushes is equal to half
of the sum of average e.m.fs. induced in the armature coils.
Commutation. When a brush comes
in contact with two armature coils then it short-circuits the two coils. On the
next moment, the short-circuit gets open due to movement of the commutator and
it causes sparking between the commutator and the brush. The action is called
commutation and it can be reduced by using interpoles.
7.13. DYNAMO EFFICIENCY
The ratio of electrical power
generated to the mechanical power applied is called dynamo efficiency. It is
expressed as a percentage.
The dynamo efficiency can never
be 100% because a dynamo has some losses also. Therefore, the efficiency can
have a maximum value of 95%.
7.14. MOTOR
(1) Definition. A motor is a
machine which converts electrical energy into mechanical energy.
(2) Principle. If a current
carrying conductor is placed in a magnetic field then it experience', a for
acting on it. The direction of the force so produced is determined by using
Fleming's left-hand rule.
(3) Consturction. The construction of a motor
is almost similar to that of an alternator. It consists magnetic field,
armature and an arrangement for supplying current to the armature. In case of
d.c. motor, a commutator and brushes are used while in case of an a.c. motor
slip rings and brushes are used.
(4) Working. On supplying current
to the 'armature' and field coils, they both produce their individual magnetic
fields. Since two magnetic fields are working at the same place, therefore, an
attraction or repulsion will take place and will result in the production of a torque
which will rotate the armature or the field as the case may be. The power
obtained by rotation of the shaft can be utilised for driving various types of
machines.
7.15. FLEMING'S LEFT HAND RULE
If the first and second fingers and the thumb of left hand
are stretched in such a way that they remain mutually perpendicular, and the
first finger points the direction of magnetic field and the second finger
points the direction of applied e.m.f., then the thumb will point the direction
of motion of the conductor. This rule is used for the determination of
direction of rotation of an armature of a motor.
7.16. ARMATURE REACTION
As the armature of a motor starts
rotation, it begins to intersect the magnetic field of the field coil. As in
case of a generator, an e.m.f. is induced in the armature coils of the motor.
According to Lenz's law', the direction of induced e.m.f. so induced is known
as back e.mf Its magnitude can be determined by using the formula e=
—N dÇ¿/ dt.
It is evident that the magnitude of back e.m.f. is directly proportional to the
rotational speed of the armature. If the armature's speed is zero, the back
e.m.f. will also be zero. Hence, the back e.m.f. opposes its cause i.e., the
rotation of the armature and is known as armature reaction,
If the magnitude of applied and
back e.m.f. would have become equal then the armature's rotation would have
stopped. But the magnitude of back e.m.f. is always lesser than applied e.m.f.
and thus armature continues to rotate at the difference of applied and back
e.m.f.
7.17. LENZ'S LAW
The direction of induced e.m.f.
and current is such that it always opposes the cause producing them. It means
that the direction of induced current is opposite to the direction of applied
current.
7.18. STARTER
1. Starting Resistance. The
resistance of armature coils of a motor is kept to a minimum so as to minimise
the consumption of electric energy in them. The armature coils are designed in
such a way that only necessary amount of current flows through the armature
coils at full speed of the armature. At such speed the difference between the
applied and back e,m.f. is quite small and thus the armature current too has a
low value.
where, V= applied
e.m.f., volts
back e.m.f.,
volts
R =
armature resistance, ohms.
2. Starter. In the start there is
no back e.m.f., hence, a high amount of current will flow through the armature
coils which will burn them. In order to protect the coils, a high resistance is
connected in series with the coils. As the motor speeds up, the external
resistance is reduced. The device employed for performing the above stated
function is called a motor starter or starter.
Starters are mainly of following
two types:
(1) Hand starter (ii) Automatic starter.
(i) Hand Starter. A simple hand
starter is shown in Fig. 7.6. When the starter arm is put to stud no. I by
rotating it in a clockwise direction, the armature coil's circuit is completed
and the flow of current is started. Armature begins to rotate and the back
e.m.f. starts to develop. The starter arm is brought to 'on' position in steps
and in this position total external resistance is cut out of the circuit.
 |
| Fig. 7.8. Hand starter |
A no volt coil is used to hold
the arm in 'on' position. This coil is connected in series with the field coil.
The electromagnet of the no-volt coil holds the arm against a spring and as the
supply gets 'off ', the electromagnet releases the arm. The spring brings the
arm back to 'off' position and the motor is stopped.
There is an overload release coil
also in the starter. The OLC gets energised at high input voltage and short
circuits the NVC, which stops the motor. In this way, the OLC provides safety
to the motor against overload.
(ii) Automatic Starter. These
starters are used with 3-phase induction motors. In this starter, the Operator
has to give a slight push to the start button and the motor itself attains its
full speed. It consists of magnetic contactors, thermal overload release relay,
time relay etc. The device operates the motor in STAR first. The speed of the
motor (connected in star) is low because the starting resistance of the coils
is more. After passing the preset time, the time relay connects to motor in
DELTA with the help of contactors and the motor then runs at full speed.
There is another type of matter
too which is called a semi-automatic starter. There is no time relay in it and
the starting handle changes the motor connections from star to delta. There are
overload release and no-volte-relays in its also.
7.19. TYPES OF ALTERNATORS
Alternators mainly of following
two types :
1.
Rotating armature type
2.
Rotating field type.
1. Rotating Armature Type
Alternator. Its construction is similar to that or a.b.c. generator. Its stator
consists of field poles and the motor consists of armature. A.C. is obtained
through slip rings brushes. These alternators are small in size and have a low
output capacity.
2. Rotating Field Type
Alternator. Its consists of a fixed armature and rotating field poles. Its
rotor being light in weight can rotate at a high speed
and is capable to generate more output voltage. These alternators are large in
size.
7.20. TYPES OF GENERATORS
Generators
may be classified in the following two ways
I. On the Basis of Magnet
(i) Permanent magnet type.
(ii) Separately excited.
(iii) Self-excited.
2. On the Basis of Winding
Connections
(i) Series wound.
(ii) Shunt wound.
(iii) Compound wound.
1. On the Basis of Magnet. On the
basis of magnet used for making the field, generators are classifies as follows:
(i) Permanent Magnet Type
Generator. In this type of generator, permanent magnets are used for the
'field'. These generators are small in size and are commercially known as
'magneto'. These are used in cycles, scooters etc. for generating e.m.f. They
are made for generating A.C. or D.C. as required, see Fig. 7.9.
(ii) Separately-excited Generator. In this type of generator, electromagnets are used for the 'field' Electromagnets are excited by a separate D.C. source or a battery, that is why it is called a separately-excited generator. This type of machine can be made for generating A.C. and in that case it is called an a.c. generate or an alternator, see Fig. 7.10.
(iii) Self-excited Generator. In
this type of generator, electromagnets are used for the 'field', which are
excited by the e.m.f. generated by the machine itself. In the start, the
residual magnetism of the field poles induces a small amount of e.m.f. in the
armature. This induced e.m.f. is then applied to tln field culls which in turn
generates more e.m.f. Slowly and slowly the generator's armature starts to
generate full e.m.f.
2. On the Basis of Winding
Connections. On the basis of winding connections, the sell-excited generators
are classified as follows:
(i) Series Wound Generator, in
this type of generator, armature winding is connected in series with field
winding. Field is made of a few turns of thick copper wire. This type of generator
will not generate any e.m.f. without a load, hence it is used only for the
purposes where the load remains connected all the times such as in a booster.
The terminal voltage of the generator increases with a rise in load current,
see Fig.7.11
(ii) Shunt Wound Generator. In
this type of generator the armature current is divided into two parts, one
through the 'field' and the other through the load (external circuit). The
magnitude of field current is kept low so that more current is available for
the external circuit, therefore, the armature and field are connected in
series. The field winding is made of large number of turns of fine copper wire.
This generator is capable to generate e.m.f. even without a load. The terminal
voltage of the generator decreases with a rise in load current. It is useful
for battery charging purposes, see Fig. 7.12.
(iii) Compound Wound Generator.
This is a combination of series and shunt wound generators. The field winding
of the generator is divided into two parts, one part is connected in series
with the armature winding and the other is connected across the armature
winding. Consequently, the load current variations do not affect the terminal
voltage. It is a very useful generator and it is used extensively for various
purposes, see Fig. 7.13.
 |
| Fig 7.13. Compound wound generator |
7.21. TYPES OF D.C. MOTORS
D.C. motors are of following
three types:
1. Series motor.
2. Shunt motor.
3. Compound motor.
1. Series Motor. In this motor,
the armature and the field windings are connected in series with the source of
supply. The field winding is made of a few turns of thick copper wire. The
motor must not be operated without a load because it may acquire a tremendous
speed at no load. The speed and the magnitude of current of the motor depends
on the load. It is useful for traction purposes e.g., trams, see Fig. 7.14.
2. Shunt Motor. In this motor the armature winding is connected across the field winding. Field winding is made of a large number of turns of fine copper wire. The motor runs at a constant speed at load and no-load. It is used in workshops for driving different types of machines, see Fig. 7.15
3. Compound Motor. In this motor
the field winding is divided into two parts, one part is connected in series
with the armature winding and the other part across the armature winding. It
comprises the properties of both the series and shunt motors. Hence, it is a
very useful motor which is utilised in lifts, elevators etc., see Fig. 7.16.
 |
| Fig. 7.16 D.C. compound motor |
Note. The construction of above
stated three types of d.c. motors is similar to three types of self-excited
generators. Hence, their circuits are quite similar.
7.22. SPEED CONTROL OF D.C. MOTORS
(A) SPEED CONTROL METHOD OF D.C.
SHUNT MOTORS
1. Field control method.
2. Armature control method.
3. Supply voltage variation
method.
1. Field Control Method. In this
method a rheostat is connected in series with the field winding for controlling
the field flux ; see Fig. 7.17(i). By varying the rheostat resistance, the
field flux varies and it varies the motor speed. The speed of a motor can be
increased by 15% to 30% than its normal speed by this method. The method is
simple and economical.
2. Armature Control Method. In
this method a rheostat is connected in series with the armature winding, see
Fig. 7.17 (ii). By reducing the rheostat resistance, the armature e.m.f.
reduces and the motor speed is increased. This method causes more electric consumption
and the motor speed remains lower than its normal speed.
 |
| Fig 7.17 Speed control of d.c. shunt motors |
3. Supply Voltage Variation
Method. In this method the rheostat is connected in series with the source of
supply, see Fig. 7 .17 (iii). By reducing the rheostat resistance, the field
and the armature get more voltage and as a result the motor speed is increased.
This method causes more electric consumption and hence it is used rarely.
(B) SPEED CONTROL OF D.C. SERIES
MOTORS
The speed control of d.c. series
motors is achieved by connecting a rheostat in parallel to the field or the
armature and is called a diverter. By increasing the diverter's resistance the
speed of the motor increases and conversely by decreasing the diverter's
resistance the speed of the motor decreases.
7.23. TYPES OF A.C. MOTORS
In d.c. motors both the field and
armature windings are connected to the source of supply but in a.c. motors,
only stator winding is connected to the source of supply. A.C. motors may be
classified in the following two main groups:
1. Three phase motors.
2. Single phase motors.
1. Three Phase Motor. In this
type of motor, the three phase supply is given to the stator windings which
produces a rotating magnetic field. Since, the rotor winding intersects the
rotating magnetic field hence an e.m.f. is developed in it. The rotor winding
sets its own magnetic field due to e.m.f. induced in it. Now, the interaction
of two magnetic fields acting at one place sets the rotor into continuous
rotation. All a.c. motors employ the induction principle and hence they are
called induction motors. The types of 3-phase motors are as follows:
(a) Squirrel cage motor
(i) Single cage motor
(ii) Double cage motor
(b) Slip ring motor
(c) Commutator motor
(d) Synchronous motor
(e) Auto-synchronous motor
(a) Squirrel Cage Motor. This
type of motor consists of a winding wound on the stator. The rotor is made of
laminated iron cores and copper bars are driven into the closed slots cut near
the periphery of the rotor. The ends of copper bars are riveted to copper rings
on either side. The arrangement of bars resembles to the cage of a squirrel and
hence it is called a squirrel cage rotor. The short-circuited bars act as rotor
winding.
A single cage type motor has one
cage of copper bars while a double cage motor has two such cages.
(b) Slip Ring Motor. This type of
motor consists of a rotor winding also in addition to the stator winding. The
three phase rotor winding is connected to a three phase external rheostat. In
the starting, the external resistance remains connected in series with the
rotor winding. A small amount of current flows in the rotor winding which
develops a high starting torque. As the motor speeds up, the rheostat is cut
out of the circuit. This motor is also known as wound rotor motor.
(c) Commutator Motor. This type
of motor consists of a stator, a rotor and a d.c. armature type windings.
3-phase rotor windings are connected to 3-phase supply and armature winding is
connected to D.C. supply. It is a variable speed motor and is made upto 1000
H.P.
(d) Synchronous Motor. This type
of motor is not of a self-start type. It requires a rotating force in the
start. When the rotor attains a speed equal to the speed of rotating magnetic
field (which is equal to the supply frequency), it sets into continuous
rotation. It consists of a 3-phase stator winding and d.c. rotor winding. The
motor has a constant speed characteristic and is used for the purpose where
constant speed is required.
(e) Auto-synchronous Motor. This
type of motor is constructed in such a way that it can start by itself like an
induction motor. As the motor attains its full speed it begins to work as a
synchronous motor. That is why it is called an auto-synchronous motor.
2. Single Phase Motor. A rotating
magnetic field can not be produced by a single phase supply. Therefore, one
phase is split into two parts, which act at a phase difference of 90°
(electrical) and hence produce a rotating type magnetic field. The types of
single phase motors are as follows :
(a) Split phase motor
(b) Capacitor motor
(i) Capacitor start motor
(ii) Permanent capacitor motor
(iii) Capacitor-start capacitor-run
motor
(c) Shaded pole motor
(d) Universal motor
(e) Repulsion motor
(i) Repulsion-start motor
(ii) Repulsion induction motor
(iii) Repulsion-start-run motor
(f) Slip ring motor
 |
| Fig. 7.18 Split phase a.c. motor |
(b) Capacitor Motor. A capacitor
is used for phase splitting purpose in this motor and hence it is called a
capacitor motor.
(i) Capacitor Start Motor. In
this type of motor a centrifugal switch and a capacitor connected in parallel,
arc connected in series with the starting winding. A 90' leading current is
produced by the capacitor which produces more starting torque in it. The motor
is useful for lathe machine etc., see Fig. 7.19.
(ii) Permanent Capacitor Motor.
In this type of motor the capacitor is permanently connected in series with the
starting winding and no centrifugal switch is used in it. Both the running and
starting windings are made with a single type of wire. The motor has a low
starting torque and it is useful for electric fans, see Fig. 7.20.
(iii) Capacitor-start
Capacitor-run Motor. In this type of motor two capacitors are used, one is
permanently connected in series with the starting winding and the other is
connected across the first capacitor through a centrifugal switch. The second
capacitor gets switched OFF when the motor attains its full speed. The motor is
useful for air-conditioner, pump, blower etc., see Fig. 7.21.
 |
| Fig. 7.21. Capacitor start capacitor rum motor |
(c) Shaded Pole Motor. The poles
of this type of motor are constructed in such a way that their 2/3rd part is
wound with running winding and their 1/3rd part is wound with starting winding.
The starting windings are short-circuited and are called shaded pole windings.
The rotor of the motor is of squirrel cage type. The shaded poles are made
after the main poles in the direction of rotation of the rotor. The production
of magnetic flux in the shaded pole is delayed and hence a phase difference is
developed in it with respect to main pole. The motor then works as a two phase
motor. The motor is useful for small fans, electric clocks, hair driers, tape
recorders etc. see Fig. 7.22.
 |
| Fig. 7.22. Shaded pole motor |
(d) Universal Motor. The
construction and working of a universal motor is identical to that of a d.c.
series motor. The motor is capable to work on either A.C. or D.C. The armature
drum is made of iron laminations and the motor is operated at a low frequency.
Compensating winding and interpoles are also used in it. The motor is useful
for sewing machine, blower, mixer-grinder and railway engine. For railway
engine a 200-600 volts, 15-25 Hz, 2200 H.P. universal motor is suitable, see
Fig. 7.23.
 |
| Fig. 7.23. Universal motor |
(e) Repulsion Motor. A repulsion
motor works on the principle of repulsion. It comprises two windings — one
stator winding and the other rotor winding. The rotor winding works as a d.c.
armature. The e.m.f. is supplied to the stator winding only and it induces an
e.m.f. in the rotor winding. The flow of current is started in the rotor
winding on short-circuiting the same, and it establishes its own magnetic
field. The repulsion force produced, by the two magnetic fields runs the motor.
(i) Repulsion-start Motor. In
this type of motor, the repulsion winding is used only for starting the motor.
The motor's speed and the direction of rotation can be changed by shifting the
brush positions. The motor is useful for cranes, hoists etc.
(ii) Repulsion Induction Motor.
This type of motor comprises of a cage winding also together with the rotor
winding and it works as a repulsion motor. The motor does not require much
maintenance and hence it is useful for industrial purposes.
(iii) Repulsion-start-run Motor.
The construction of this type of motor is similar to that of a repulsion start
motor. In addition, there is a copper ring which short-circuits all commutator
segments with the help of a centrifugal plunger at the event when the motor
attains its full speed. This action increases motor's torque. The motor is
useful for dunes etc.
(f) Slip-Ring Motor. The
construction and working of a single phase slip-ring motor is almost similar to
that of a three phase slip-ring motor. It consists of a running winding wound
on the stator and a three phase winding wound on the rotor. The later one is
connected to a three phase rheostat. When the motor attains its full, speed,
the starting winding is short-circuited so as to increase the torque of the
motor. The motor is made upto one H.P. and is useful for refrigerator, grinder,
lathe machine etc.
7.24. L.P. MOTOR
The motor used in record players
and tape recorders is called a L.P. motor, means low potential motor. It is a 6
or 9 volts d.c. shunt motor. It has a constant speed characteristic. A governor
is used in it for speed controlling which works on the air damping principle.
When the arms of the governor are spread its speed is lowered and vice-versa.
Record player's motor working on
A.C. mains is of shaded pole type. The speed control of the motor is achieved
by connecting a voltage regulator in series with the field coil.
7.25. STEPPER MOTOR
1. Introduction. A motor which
can be operated in 'steps' by dividing its one rotation into 4, 8, 12, upto 500
steps (in degrees) is called a stepping motor or stepper motor. This type of
motor can be rotated in 'forward' or 'reverse' directions for a pre-decided
fraction of a rotation (in degrees). The motor is operated by applying digital
input signals through a control circuit.
2. Construction. A stepper motor
consists of the following two main parts:
(i) Stator,
(ii) Rotor.
(i) Stator. A stepper motor
contains two or three windings which are wound on the stator. Since, the
windings are wound only on the stator portion of the motor hence, there is no
necessity of a commutator etc. with this motor.
(ii) Rotor. The rotor of a stepper
motor is made of either a magnetic metal in the form of a toothed-wheel or
permanent magnets.
3. Types. On the basis of rotor
construction, stepper motors may be classified into the following two main
classes:
(i) Variable reluctance stepper
motor,
(ii) Permanent magnet stepper
motor.
(i) Variable Reluctance Stepper
Motor. Its stator generally contains' 6 poles and each winding is wound on two
opposite poles. The rotor contains four teeth, see Fig. 7.24.
 |
| Fig. 7.25. Permanet magnet stepper motor |
One terminal '0' of the three
windings is kept common and the same is connected to the positive terminal of
the supply. The remaining three terminals A, B and C are connected to the
negative terminal of the supply in the requisite sequence for exciting the
windings.
When the winding A is exicited then
the rotor teeth X and X' come in front of stator poles Rotor A and A' and rest
there. On switching the supply to the winding A to OFF and switching the supply
to winding B to ON, the rotor teeth Y and Y' will rotate and reach in front of
stator poles B and B' and thus the rotor will rotate through 30°.
For rotating the motor
continuously, the windings A, B and C will have to be excited in a sequential
order. In the positive logic system 1 stands for ON and 0 stands for OFF.
Therefore, on exciting the windings in the following sequence, the motor will
rotate through 2 rotations in 24 'steps'.
In this way, the degrees through which the
rotor rotates in 
Therefore, the 24 motor consisting 6 stator
poles and 4 rotor teeth will rotate through 30° per 'step'. For obtaining a
'step' of less than 30°, the number of stator poles and rotor teeth will have
to be increased.
Therefore, the 24 motor consisting 6 stator
poles and 4 rotor teeth will rotate through 30° per 'step'. For obtaining a
'step' of less than 30°, the number of stator poles and rotor teeth will have
to be increased.
(ii) Permanent Magnet Stepper
Motor, In this type of motor, permanent magnets are used in the rotor in place
of toothed wheel. As shown in Fig. 7.25, three bar magnets are arranged in such
a manner that they form 3 South poles and 3 North poles on the periphey of the
rotor.
 |
| Fig. 7.25. Permanet magnet stepper motor |
Two center tapped windings are
used in the stator of the motor. The winding 1 is wound on the top and bottom
poles while the winding 2 is wound on the left and right poles.
When the center tap of the
winding 1 is connected to positive supply and its terminals a and b to the
negative supply in an alternate order then upper pole becomes North pole while
the lower pole becomes South pole. The N-S poles attract the S-N poles of the
rotor. Now, on switching the supply to the winding 1 to OFF and the supply to
the winding 2 to ON, the magnetic poles of the rotor will rotate through 30°.
It is one 'step' rotation of the motor.
For rotating the motor
continuously, the, two windings will have to be excited alternately. In the
positive logic system 1 stands for ON and 0 stands for OFF. Therefore, on
exciting the windings in the following sequence, the motor will rotate through
4 rotations in '24 steps'.
Observe that both the terminals
of a winding are not excited at a time.
Since, the rotor of the motor is
not a toothed rotor but it is round like a drum hence, it is also known as
Unipolar Motor.
4. Control Circuit. For the
operation of a stepper motor, a control circuit is required which can connect
the winding in a required sequence on applying digital pulses to the same which
means that switching can be done between supply time and the windings. For this
purpose, IC based circuits are employed. Such type of I.Cs. are — ULN 2003, ULN
2803, UDN 2547B, SN7541, SN7542, SN 7543 etc.
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