The speed of a three phase induction motor is given by,
N = Ns(1-s)
and Ns = 120f/P
This shows the speed of three phase induction motor depends upon synchronous speed (Ns) and slip(s). The synchronous speed of induction motor depends upon supply frequency and the number of stator poles.
So by changing the supply frequency, the number of stator poles and slip we can change the speed of 3 phase induction motor. Therefore, speed control of 3 phase induction motor can be achieved by following methods:

Stator frequency — 3 phase motor speed control methods

The synchronous speed can be changed by changing the stator supply frequency (f). But only change in supply frequency, keeping supply voltage constant has an adverse effect on the air gap flux. Because air gap flux is proportional to the ratio of supply voltage and supply frequency.
Hence the ratio of supply voltage and supply frequency is kept constant by changing the stator voltage and frequency simultaneously. This is necessary to keep the air gap flux constant. So this method is also known as constant (V/f) control.
In this 3 phase motor speed control method, the AC input of constant voltage and constant frequency is given to a rectifier which converts AC into DC. The output of the rectifier is filtered by a capacitor bank and given to an inverter circuit. The inverter converts the DC voltage into a 3 phase variable voltage variable frequency supply.
This supply is applied to the stator winding of the motor. Thus we get the constant V/f speed control of induction motor.
In this method, air gap flux remains constant. Therefore smooth speed control can be obtained. By this method, it is possible to get maximum torque at all speeds.
Because of large capital outlay, this system is justified only for drives wherein the rugged, maintenance-free characteristics of the induction motor are essential. Otherwise, a DC motor with converter control is the logical and cheap alternative.

Stator voltage — 3 phase motor speed control methods

In this 3 phase motor speed control method, only stator voltage is varied to vary the speed of the motor, the stator frequency is kept constant. We can vary the motor speed by varying the stator voltage. With the increase in stator voltage, the motor speed will increase.
In this speed control method of 3 phase induction motor when the stator voltage is reduced, the air gap flux and the motor torque both will reduce. The speed control is obtained at the cost of reduction of motor torque.
In this method, very low starting torque is produced. Therefore, this type of control is not suitable for constant torque loads. This type of control is preferred in the applications such as fans, centrifugal pumps and blowers where low starting torque is required.
This method is only applicable for small motors and for fan type loads where the load torque increases with the speed. The motor tends to get overheated with other loads. It is a commonly used method for ceiling fans driven by single-phase induction motors which have large standstill impedance limiting the current drawn by the motor.

Stator pole — 3 phase motor speed control methods

By means of a suitable switch, the stator connection may be changed in such a manner that the number of stator poles is changed. This changes the actual speed of motor since the actual speed of the motor is approximately inversely proportional to the number of poles.
By suitable connections, one winding can give two different speeds. If more than two speeds are required, two separate windings are placed in the same slot. Now each winding can give two speeds and two windings can give four different speeds. In a slpring motor, it is necessary to change the rotor poles accordingly. Therefore it is difficult to apply this method to a slipring motor.

Rotor resistance control

As the name indicates, this 3 phase motor speed control method is only applicable to the slip-ring induction motor.
As we know that sub-synchronous speed (speeds below synchronous speed) control of the slip-ring induction is possible by introducing a variable resistance in the rotor circuit of the motor. This method is based upon the general principle that the induction motor slip increases (i.e. speed decreases) as more power loss occurs on the rotor circuit.

3 phase motor speed control methods

This method provides a wide range of speed and good starting torque. The maximum torque in this case remains constant. Good regulation of speed can be achieved for both constant-torque and fan-type loads.
The conventional method of rotor resistance control demands simultaneous and precise variation of all the three balanced resistors in each phase. Often this is difficult to achieve.
To overcome this problem, a high-frequency thyristor chopper which enables external resistance to be varied simultaneously and sleeplessly is employed.

Slip-ring induction motor speed control by chopper

Figure shows a circuit in which the rotor slip power is rectified in a three-phase bridge rectifier and fed through a filtering choke to an external resistance.
The thyristor (shown by the switch symbol in Figure) in the chopper connected across the resistor is switched on and off at a high frequency. The ratio of on-time to off-time determines the effective value of rotor circuit resistance and thus controls the motor speed by changing its speed-torque characteristics.
The greatest drawback of this method is its poor efficiency due to power wasted in the external rotor resistance. Therefore, this method is adopted for a narrow speed range and usually for a short time duration.
Rather than wasting the slip energy in external rotor resistance, it can be feedback to supply to increase the efficiency of this speed control scheme. This is achieved by using a converter and inverter in the rotor circuit as shown in Figure.

slip-ring induction motor speed control

By feeding the electric power into the rotor circuit (negative rotor power loss), super synchronous speed operation (i.e. speeds above synchronous speed) becomes possible.

Speed Control of Induction Motor by Cascade Method

In this method, two machines are mechanically coupled. Mostly both motors are of the slipring type. In this case, supply is connected to the stator of one of the induction motors and induced EMF of the rotor is fed to the stator of another motor.

Cascade method of speed control

If P1 and P2 are the numbers of poles of the two machines and f is the supply frequency, then the set can give following different speeds:

  • When machine 1 works alone,
  • the synchronous speed = 120f/P1

  • When machine 2 works alone,
  • the synchronous speed = 120f/P2

  • When machine 1 and 2 is running so that the torque of the two motors is in the same direction,
  • the synchronous speed of the set = 120f/(P1 + P2).

  • When machine 1 and 2 is running so that the torque of the two motors is in opposite direction,
  • the synchronous speed of the set = 120f/(P1 – P2).

Thanks for reading about 3 phase motor speed control methods.

Electrical Drives – 4 | Objective Type Question Answers


#1 …………. method can bring the locomotive to dead stop.

Plugging braking

#2 The value of co-efficient of adhesion will be high when rails are

cleaned with sand

#3 For three-phase induction motors which of the following is the least efficient method of speed control ?

Rheostatic control

#4 Specific energy consumption becomes

all of the above

#5 In main line service as compared to urban and suburban service

all of the above

#6 Locomotive having monomotor bogies

has all above qualities

#7 Energy consumption in propelling the train is required for which of the following

All of the above

#8 An ideal traction system should have

all of the above

#9 …………have maximum unbalanced forces.

Steam locomotives

#10 Specific energy consumption is affected by which of the following factors ?

All of the above

#11 In case of ………………free running and coasting periods are generally long.

main-line service

#12 Specific energy consumption is least in ………….. service.

main line

#13 Locomotives with monomotor bogies have

suitability for passenger as well as freight service

#14 …………..was the first city in India to adopt electric traction.


#15 Power for lighting in passenger coach, in a long distance electric train, is provided

through individual generator of bogie and batteries

#16 Which of the following happens in Kando system?

Single phase supply is converted into three phase system

#17 For which of the following locomotives the maintenance requirements are the least ?

Electric locomotives

#18 Which of the following methods is used to control speed of 25 kV, 50 Hz single phase traction ?

Tap changing control of transformer

#19 If the co-efficient of adhesion on dry rails is 0.26, which of the following could be the value for wet rails?


#20 The braking retardation is usually in the range of

3 to 5 km phps

#21 The rate of acceleration on suburban or urban service is in the range

1.6 to 4.0 km phps

#22 The coasting retardation is around

0.16 km phps

#23 ……………is the method of braking in which motor armature remains connected to the supply and draws power from it producing torque opposite to the direction of motion.




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