EE 6504 Electrical Machines II

SPEED CONTROL OF INDUCTION MOTOR

1. Control from stator side
a. By changing the applied voltage
b. By changing the applied frequency
c. By changing the number of stator poles
2. Control from rotor side
d. Rotor rheostat control
e. By operating two motors in concatenation or cascade
f. By injecting an emf in the rotor circuit.
(a) By changing the applied voltage
Tα
Rotor induced emf at standstill, E2 depends on the supply voltage V
∴ E2 α V
For low slip region, sX2 2 R2, hence
Tα α s V2 for constant R2
If supply voltage is reduced below rated value, as per above equation, torque produced also decreases. But to
supply the same load it is necessary to develop same torque hence value of slip increases so that torque produced
remains same.
Slip increases means motor reacts by running at lower speed, to decrease in supply voltage. So motor produces
the required load torque at a lower speed.
This method, though the cheapest and the easiest, is rarely used because
(i) A large change in voltage is required for a relatively small change in speed
(ii) Due to reduction in voltage, current drawn by the motor increases. Due to increased current, the
motor may get overheated.
(iii) This large change in voltage will result in a large change in the flux density thereby seriously
disturbing the magnetic conditions of the motor.
(b) By changing the applied frequency or supply frequency control or V/f control
Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which
rotates at synchronous speed given by Ns =
In three phase induction motor, emf is induced by induction similar to that of transformer which is given by

E or V = 4.44 Φ K T f
Φ=
.
Where, K is the winding constant, T is the number of turns per phase and f is frequency. If we change frequency,
synchronous speed changes. But with decrease in frequency, flux will increase and this change in value of flux causes
saturation of rotor and stator cores which will further cause increase in no load current of the motor .
To maintain flux, φ constant, it is only possible if we change voltage. i.e if we decrease frequency, flux increases
but at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains
constant. So, here we are keeping the ratio of V/f as constant. Hence its name is V/ f method. For controlling the speed

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of three phase induction motor by V/f method we have to supply variable voltage and frequency which is easily
obtained by using converter and inverter set.
The normal supply available is constant voltage constant frequency a.c. supply. The converter converts this supply
into a d.c. supply. This d.c. supply is then given to the
inverter. The inverter is a device which converts d.c.
supply, to variable voltage variable frequency a.c.
supply which is required to keep V/f ratio constant. By
selecting the proper frequency and maintaining V/f
constant, smooth speed control of the induction motor is possible.
Disadvantages
 Used where the induction motor is the only load on the generators.
 Range over which the motor speed may be varied is limited.
 The supply cannot be used to supply other devices which require constant voltage.
(c) By changing the number of stator poles
The method is called Pole Changing method of controlling the speed. In this method, it is possible to have one, two
or four speeds in steps, by changing the number of stator poles. A continuous smooth speed control is not possible by
this method.
The stator poles can be changed by following methods
1. Consequent poles method
2. Multiple stator winding method
3. Pole amplitude modulation method.
Consequent Poles Method
In this method, connections of the stator winding are changed with the help of simple switching. Due to this, the
number of stator poles get changed in the ratio 2 1. Hence either of the two synchronous speeds can be selected.
Consider the pole formation due to single phase of a three phase winding, as shown in the Fig. There are three
tapping points to the stator winding. The supply is given to two of them and third is kept open.
The current in all the parts of
stator coil is flowing in one
direction only. Due to this, 8 poles
get formed as shown in the Fig. So
synchronous speed possible with
this arrangement with 50 Hz
frequency is N = 750 r.p.m.
If the two terminals to which
supply was given earlier are joined
together and supply is given
between this common point and the open third terminal, the poles are formed as shown in the Fig.
The direction of current through
two coils is different than the
direction of current through
remaining two. Thus upward
direction is forming say S pole and
downward say N. In this case only 4
poles are formed. So the
synchronous speed possible is 1500
r.p.m. for 50 Hz frequency.

Disadvantage
 The speed change is in step and
smooth speed control is not
possible.
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 The method can be used only for the squirrel cage type motors as squirrel cage rotor adjusts itself to same number
of poles as stator which is not the case in slip ring induction motor.
Applications
Elevators, traction motors and small motors to drive machine tools.
Multiple stator winding method
In this method instead of one winding, two separate stator windings are placed in the stator core. The windings
are placed in the stator slots only but are electrically isolated from each other. Each winding is divided into coils to
which, pole changing with consequent poles, facility is provided.
Thus giving supply to one of the two windings and using switching arrangement, two speeds can be achieved. Same is
true for other stator winding. So in all four different speeds can be obtained.
Limitations
1. Can be applied to only squirrel cage motor.
2. Smooth speed control is not possible. Only step changes in speed are possible.
3. Two different stator windings are required to be wound which increases the cost of the motor.
4. Complicated from the design point of view.
Pole amplitude modulation method
In this method of speed control of three phase induction motor the original sinusoidal mmf wave is modulated by
another sinusoidal mmf wave having different number of poles.
Let
f1(θ) be the original mmf wave of induction motor whose speed is to be controlled.
f2(θ) be the modulation mmf wave.
P1 be the number of poles of induction motor whose speed is to be controlled.
P2 be the number of poles of modulation wave.

After modulation resultant mmf wave

So we get, resultant mmf wave

Therefore the resultant mmf wave will have two different number of poles
P11 = P1 – P2 and P12 = P1 + P2

Therefore by changing the number of poles we can easily change the speed of three phase induction motor.
(d) Rotor rheostat control
Tα

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 For low slip region, sX2 2 R2, and can be neglected and for constant supply voltage E2 is
also constant.
∴ Tα α

Thus if the rotor resistance is increased,

the torque produced decreases. But when the load
on the motor is same, motor has to supply same
torque as load demands. So motor reacts by
increasing its slip to compensate decrease in T
due to R2 and maintains the load torque constant.
So due to additional rotor resistance R2,
motor slip increases i.e. the speed of the motor decreases.
Advantage
 By increasing the rotor resistance R2 speeds below normal value can be achieved.
 The starting torque of the motor increases proportional to rotor resistance.
Disadvantage
 The large speed changes are not possible. This is because for large speed change, large resistance is required to be
introduced in rotor which causes large rotor copper loss to reduce the efficiency.
 The method cannot be used for the squirrel cage induction motors.
 The speeds above the normal values cannot be obtained.
 Large power losses occur due to large 12R loss.
 Sufficient cooling arrangements are required which make the external rheostats bulky and expensive.
 Due to large power losses, efficiency is low.

(e) By operating two motors in concatenation or cascade or Tandem operation

In this method, two induction motors are mounted on the same shaft. One of the two motors must be of slip ring
type which is called main motor. The second motor is called auxiliary motor. The arrangement is shown in the Fig.

The auxiliary motor can be slip ring type

or squirrel cage type.
The stator of the main motor is connected
to the three phase supply while the supply of
the auxiliary motor is derived at a slip
frequency from the slip rings of the main
motor. This is called cascading of the motors.
If the torques produced by both act in the
same direction, cascading is called cumulative
cascading.
If torques produced are in opposite direction, cascading is called differential cascading.
PA = Number of poles of main motor
PB = Number of poles of auxiliary motor
f = Supply frequency
NSA =
Let N = actual speed of the concatenated set
sA =
fA = frequency of rotor induced emf of motor A
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∴ f A sA f as fr sf
The supply to motor B is at frequency fA, ie. fB fA
∴ NSB
On no load, the speed of the rotor B i.e. N is almost equal to its synchronous speed NSB.
∴ NSB N
∴ N x 1‐ x 1‐

N x 1‐

N 1

∴ N
If by interchanging any two terminals of motor B, the reversal of direction of rotating magnetic field of B is
achieved then the set runs as differentially cascaded set. And in such a case effective number of poles are PA – PB .
Thus in cascade control, four different speeds are possible as,
a. With respect to synchronous speed of A independently,
NS =
b. With respect to synchronous speed of B independently with main motor is disconnected and B is directly connected
to supply,
NS =
c. Running set as cumulatively cascaded with,
N
d. Running set as differentially cascaded with,
N
Disadvantages
1. It requires two motors which makes the set expensive.
2. Smooth speed control is not possible.
3. Operation is complicated.
4. The starting torque is not sufficient to start the set.
5. Set cannot be operated if PA = PB.
(f) By injecting an emf in the rotor circuit.
In this method, a voltage is injected in the rotor circuit. The frequency of rotor circuit is a slip frequency and hence
the voltage to be injected must be at a slip frequency.
The injected voltage may oppose the rotor induced e.m.f. or may assist the rotor induced e.m.f.
 If it is in the phase opposition, effective rotor resistance increases.
 If it is in the phase of rotor induced e.m.f., effective rotor resistance decreases.
Thus by controlling the magnitude of the injected e.m.f., rotor resistance and effectively speed can be
controlled.
Practically two methods are available which use this principle. These methods are,
1. Kramer system 2. Scherbius system
Kramer system
 It consists of main induction motor M, the speed of which is to be controlled. The two additional equipments are,
d.c. motor and a rotary converter.
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 The slip rings of the main motor are connected to the a.c. side of a rotary converter.
 The d.c. side of rotary converter feeds a d.c. shunt motor commutator, which is directly connected to the shaft of
the main motor.
 A separate d.c. supply is required to excite the field winding of d.c. motor and exciting winding of a rotary
converter.
 The variable resistance is introduced in the field circuit of a d.c. motor which acts as a field regulator.

 The speed of the set is controlled by varying the field of the d.c. motor with the rheostat R.
 When the field resistance is changed, the back e.m.f. of motor changes. Thus the d.c. voltage at the commutator
changes. This changes the d.c. voltage on the d.c. side of a rotary converter.
 Now rotary converter has a fixed ratio between its a.c. side and d.c. side voltages. Thus voltage on its a.c. side also
changes.
 This a.c. voltage is given to the slip rings of the main motor.
 So the voltage injected in the rotor of main motor changes which produces the required speed control.
Advantages
 smooth speed control is possible
 wide range of speed control is possible
 the design of a rotary converter is practically independent of the speed control required
 if rotary converter is overexcited, it draws leading current and thus power factor improvement is also possible
along with the necessary speed control
Very large motors above 4000 kW such as steel rolling mills use such type of speed control.
Scherbius system
 This method requires an auxiliary 3 phase or 6 phase a.c. commutator machine which is called Scherbius machine.
 The difference between
Kramer system and this
system is that the Scherbius
machine is not directly
connected to the main motor,
whose speed is to be
controlled.
 The Scherbius machine is
excited at slip frequency from
the rotor of a main motor
through a regulating
transformer.
 The taps on the regulating
transformer can be varied, this
changes the voltage developed
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 EE 6504 Electrical Machines II

in the rotor of Scherbius machine, which is injected into the rotor of main motor. This controls the speed of the
main motor.
 The Scherbius machine is connected directly to the induction motor supplied from main line so that its speed
deviates from a fixed value only to the extent of the slip of the auxiliary induction motor.
 For any given setting of the regulating transformer, the speed of the main motor remains substantially constant
irrespective of the load variations.

Similar to the Kramer system, this method is also used to control speed of large induction motors.
The only disadvantage is that these methods can be used only for slip ring induction motors.
SLIP POWER RECOVERY SCHEME
Static Kramer Drive
The static Kramer-drive is the method of controlling the speed of an induction motor by injecting the opposite-
phase voltage in the rotor circuit. The injected voltage increases the resistance of the rotor, thus controlled the speed of
the motor. By changing the injected voltage, the
resistance and speed of an induction motor are
controlled.
The static Kramer-drive converts the slip
power of an induction motor into AC power and
supply back to the line.
The slip power is the air gap power between
the stator and the rotor of an induction motor which
is not converted into mechanical power.
Thus, the power is getting wasted. The
static Kramer drives fed back the wasted power into
the main supply. This method is only applicable
when the speed of the drive is less than the
synchronous speed.

Static Scherbius System

 Both the converters are controlled converter. Because of this the power flow in the rotor circuit becomes
bidirectional and the induction motor can be operated in sub synchronous as well as super synchronous region
of operation.
Sub synchronous mode
 Power must be extracted from the rotor
 Therefore Bridge 1 act as rectifier ( α1 <
90° )
 Bridge 2 act as inverter (α2 > 90°) to feed
the slip power (extracted from the rotor)
back to A.C. supply.
 The slip power flows from the rotor
circuit to bridge1, bridge2 and
transformer and to the supply.
 At subsynchronous speeds the slip power
sPm is supplied to the rotor by the exciter
and so the remaining output power (1-s)
Pm is supplied to the shaft.
Super- synchronous mode
 Power must be supplied to the rotor
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