How can control Speed of three Phase Induction Motor Detailed explanation motor control

Speed control methods of three phase Induction Motor Detailed explanation:

A three phase induction motor is  a constant speed motor as dc  shunt motor.  But the speed of d.c.  shunt motor can be changed smoothly just by using rheostats. This maintains the speed regulation and efficiency of dc. shunt motor.  But in case of 3 phase  induction motors very difficult to achieve smooth speed control.  if the speed control is achieved by using some methods , the performance of the induction motor in term of  its power factor,  efficiency etc.  gets badly  affected. 

speed equation of induction motor is,
 

figure: induction motor speed control

From above equation it observed that speed of the induction motor can be changed either the synchronous speed Ns or by changing value of  slip s.

Torque produced in  three phase induction motor is ,

Fig: Torque equation of induction motor

if the value of  R and E  are changed then to keep the torque constant for constant load condition,  motor reacts by change in its slip.  Effectively its speed changes. 

Thus speed of the induction motor can be controlled by methods: 

it includes following methods

a)  Supply frequency control  I. e v/f control. 
b)  Supply voltage control.

c)  By Pole changing.  

d)  Adding rheostats in stator circuit
 

 

Let discus in detail  Speed control methods of three phase Induction Motor:

a ) Supply Frequency control or v/f control 

The synchronous speed is given by,

fig:speed equation of induction motor

 

Thus by controlling the supply frequency ( f )smoothly,  the synchronous speed can be controlled smoothly 

b) speed control by supply voltage:

Torque equation 

For low slip region which is operating region of induction motor (sX)^2 <<R and hence neglected

Now

From above expression if supply voltage reduced below rated value, torque produced also decrease 

But due to reduction in voltage, current drawn by motor increases. Large  change in voltage for small change in speed required is the biggest disadvantage of this method. Due to increased in current motor may get overheated hence this method rarely used in practice. 

c) By Pole changing: 

 
This method called pole changing method of controlling speed of induction motor. In this method it is possible to have various speed  in steps by changing stater pole.  Continues smooth speed control not possible by this method  

d) Adding rheostat  circuit: 

fig:Rotor resistance starter

 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 torque  due to resistance  R and maintains the load torque R constant So due to additional rotor resistance R
motor slip increases ie the speed of the motor decreases Thus by increasing the rotor resistance R.  speeds below normal value can be achieved Another advantage of this method is that the starting torque of the motor increases Fi proportional to rotor resistance  But this method has following disadvantages 1.  The large speed changes are not possible This is because for large speed change.required  large in rotar which causes large rotor losses, which reduce the efficiency of motor . 

 disadvantages of speed control of induction motor by adding external resistances:

1)  this method can not be used for the squirrel cage induction motors 
2)  The speed above the normal values can not be obtained
3)  Large power looses occur hence sufficient cooling arrangements required
4)  Due to large power losses,  efficiency is low Thus the method is rarely used


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