Speed Control of AC Motors

Speed Control Methods Of Induction Motor

An induction motor is practically a constant speed motor that means, for the entire loading range, change in speed of the motor is quite small. Speed of a DC shunt motor can be varied very easily with good efficiency, but in case of Induction motors, speed reduction is accompanied by a corresponding loss of efficiency and poor power factor. As induction motors are widely being used, their speed control may be required in many applications. Different speed control methods of induction motor are explained below.

Induction Motor Speed Control from Stator Side

1. By Changing the Applied Voltage:

From the torque equation of induction motor,

Rotor resistance R₂ is constant and if slip s is small then (sX₂)² is so small that it can be neglected. Therefore, T ∝ sE₂² where E₂ is rotor induced emf and E₂ ∝ V
Thus, T ∝ sV², which means, if supplied voltage is decreased, the developed torque decreases. Hence, for providing the same load torque, the slip increases with decrease in voltage, and consequently, the speed decreases. This method is the easiest and cheapest, still rarely used, because

1. Large change in supply voltage is required for relatively small change in speed.
2. Large change in supply voltage will result in a large change in flux density, hence, this will disturb the magnetic conditions of the motor.

2. By Changing The Applied Frequency

Synchronous speed of the rotating magnetic field of an induction motor is given by,

Where, f = frequency of the supply and P = number of stator poles.
Hence, the synchronous speed changes with change in supply frequency. Actual speed of an induction motor is given as N = Ns (1 - s). However, this method is not widely used. It may be used where, the induction motor is supplied by a dedicated generator (so that frequency can be easily varied by changing the speed of prime mover). Also, at lower frequency, the motor current may become too high due to decreased reactance. And if the frequency is increased beyond the rated value, the maximum torque developed falls while the speed rises.

3. Constant V/F Control of Induction Motor

This is the most popular method for controlling the speed of an induction motor. As in above method, if the supply frequency is reduced keeping the rated supply voltage, the air gap flux will tend to saturate. This will cause excessive stator current and distortion of the stator flux wave. Therefore, the stator voltage should also be reduced in proportional to the frequency so as to maintain the air-gap flux constant. The magnitude of the stator flux is proportional to the ratio of the stator voltage and the frequency. Hence, if the ratio of voltage to frequency is kept constant, the flux remains constant. Also, by keeping V/F constant, the developed torque remains approximately constant. This method gives higher run-time efficiency. Therefore, majority of AC speed drives employ constant V/F method (or variable voltage, variable frequency method) for the speed control. Along with wide range of speed control, this method also offers 'soft start' capability.

4. Changing the Number of Stator Poles

From the above equation of synchronous speed, it can be seen that synchronous speed (and hence, running speed) can be changed by changing the number of stator poles. This method is generally used for squirrel cage induction motors, as squirrel cage rotor adapts itself for any number of stator poles. Change in stator poles is achieved by two or more independent stator windings wound for different number of poles in same slots.
For example, a stator is wound with two 3phase windings, one for 4 poles and other for 6 poles.
for supply frequency of 50 Hz
i) synchronous speed when 4 pole winding is connected, Ns = 120*50/4 = 1500 RPM
ii) synchronous speed when 6 pole winding is connected, Ns = 120*50/6 = 1000 RPM

Speed Control from Rotor Side:

1. Rotor Rheostat Control

This method is similar to that of armature rheostat control of DC shunt motor. But this method is only applicable to slip ring motors, as addition of external resistance in the rotor of squirrel cage motors is not possible.

2. Cascade Operation

In this method of speed control, two motors are used. Both are mounted on a same shaft so that both run at same speed. One motor is fed from a 3phase supply and the other motor is fed from the induced emf in first motor via slip-rings. The arrangement is as shown in following figure.

Motor A is called the main motor and motor B is called the auxiliary motor.
Let, N_s1 = frequency of motor A
       N_s2 = frequency of motor B
       P₁ = number of poles stator of motor A
       P₂ = number of stator poles of motor B
       N = speed of the set and same for both motors
       f = frequency of the supply

Now, slip of motor A, S₁ = (N_s1 - N) / N_s1.
frequency of the rotor induced emf in motor A,   f₁ = S₁f
Now, auxiliary motor B is supplied with the rotor induce emf

therefore,  N_s2 = (120f₁) / P₂  =  (120S₁f) / P₂.

Now putting the value of  S₁ = (N_s1 - N) / N_s1

At no load, speed of the auxiliary rotor is almost same as its synchronous speed.

i.e. N = N_s2.
from the above equations, it can be obtained that

With this method, four different speeds can be obtained
1. When only motor A works, corresponding speed = .Ns1 = 120f / P₁
2. When only motor B works, corresponding speed = Ns2 = 120f / P₂
3. If commulative cascading is done, speed of the set = N = 120f / (P₁ + P₂)
4. If differential cascading is done, speed of the set = N = 120f (P₁ - P₂)

3. By Injecting EMF In Rotor Circuit

In this method, speed of an induction motor is controlled by injecting a voltage in rotor circuit. It is necessary that voltage (emf) being injected must have same frequency as of the slip frequency. However, there is no restriction to the phase of injected emf. If we inject emf which is in opposite phase with the rotor induced emf, rotor resistance will be increased. If we inject emf which is in phase with the rotor induced emf, rotor resistance will decrease. Thus, by changing the phase of injected emf, speed can be controlled. The main advantage of this method is a wide rage of speed control (above normal as well as below normal) can be achieved. The emf can be injected by various methods such as Kramer system, Scherbius system etc.