Speed Control of Three Phase Induction Motor

     A 3-phase induction motor is a constant speed motor just like a D.C shunt motor. However, speed of induction can be varied but speed variation will be accompanied by a corresponding loss of frequency.

The speed of an induction motor is given by,

N  =  Ns ( 1 - s )   

 =  120f / p  ( 1 - s )

     From the above equation, it is evident that speed of an induction motor can be regulated by varying any of the above three quantities i.e., frequency, number of poles and slip. The various methods of speed control of induction motors are classified as follows.

From the Stator Side :

1.  By changing the supply voltage 

2. By changing the supply frequency

3. By changing the number of stator poles

From the Rotor Side : 

4. By changing rotor circuit resistance  

5. By injecting voltage in to the rotor circuit 

6. By cascade control

Form the stator side :

i. By changing supply voltage :

     The torque developed by an induction motor is proportional to square of supply voltage ( T  α V2 ). 

     If supply voltage is reduced below rated value, torque also decreases. To produce the same torque for supplying the same load, slip increases. i.e., motor running at low speed to decrease in supply voltage. The maximum torque developed is reduced considerably with the slight reduction in speed.

     This method is simple and cheapest method but it is rarely used because a large change  in voltage is required for small change in speed. This method is used on small motors driving fans whose torque is proportional to the square of speed.

ii. By Changing Supply Frequency :

     The speed of an induction motor is proportional to supply frequency. By gradually changing the supply frequency. speed can be decreased or increased smoothly. If speed control is to be achieved by this method, the supply voltage should also be changed simultaneously. This is because if the supply frequency is decreased by keeping the applied voltage constant, then the flux is increased ( E = 4.4 ⍉m f  ). If the flux is increased, core losses will reduce the efficiency. 

     On the other hand, if the frequency is increased, flux will decrease, hence torque developed will be reduced. Therefore, it is very important that the frequency changing device should change frequency and voltage simultaneously as direct ratio. That means the ratio V/F should be kept constant in order to keep the flux constant.

iii. By Changing Number of stator Poles :

     This method of speed control is suitable for squirrel cage type of induction motors. The synchronous speed of an induction motor is a function of the number of stator poles. The change in the number of stator poles can be achieved by having two or more number of independent stator windings in the same slots. Each stator winding is wound for different number of poles resulting in different synchronous speed. This method is used in the case of fraction motors, elevator motors and small motors used for driving machine tools.

From the rotor side :

i. By Changing rotor resistance :

      This method of speed control is similar to the speed control of D.C shunt motor by armature control. In this method, the speed of the motor is varied by inserting an external resistance in the rotor circuit of the motor. This method is suitable for slip ring induction motors only.

     In slip ring induction motor, the speed can be continuously reduced, when the supply voltage and frequency is kept constant, and by adding external resistance in the rotor circuit. In this method, the speed of motor reduces with addition of resistances in the rotor circuit because part of energy available in the rotating magnetic field is wasted in the external resistance and thus, the part of energy available at the shaft gets reduced.

ii. By injecting voltage into rotor circuit :

     The speed of a induction motor can be controlled by injecting a voltage of slip frequency into the rotor circuit ( i.e., across the slip rings ). If the injected voltage is in phase with the rotor induced emf is equivalent to decreasing the rotor resistance, hence slip decreases or speed increases.

      If injected voltage is in phase opposition to the rotor induced emf is equivalent to increasing the rotor resistance, hence slip increases or speed decreases. A large and smooth speed control is possible by this method.

      However, it requires an auxiliary machine ( Schrage motor or brush shifting motor ) capable of injecting the correct slip frequency voltage into the rotor circuit. Therefore, it is costlier and used with motors of very large rating.

iii. By cascade control :

This is also called as Concatenation or Tandem Operation of Induction Motors.

      In this method, two motors are required, at least one of which must have a slip ring type called as Main Motor while the second motor is called as Auxiliary Motor ( may be either slip ring or cage motor ). The two motors are mechanically coupled as shown in Figure so that both run at the same speed.

Cascade Connection

     The stator of main motor ( Motor-l ) is connected to 3-⍉ supply while the auxiliary motor ( Motor-2 ) is fed from the rotor circuit of main motor. This is called as 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 as 'Differential Cascading'.

Disadvantages of Cascade Connection :

     This method is rarely used due to the following disadvantages    

(i) It requires two motors which makes the set expensive.

(ii) Smooth speed control is not possible.

(iii) Operation is complicated.

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