Induction Motor Speed Control Methods
Speed Control Methods of Induction Motor:
In this post let us see what are the various methods to control the speed of Induction Motor...We know that the speed of the induction motor is given by,
From the above equation, the speed of the motor can be controlled
by changing supply frequency and
by changing number of poles in the stator.
Control From Stator Side:
- By changing the supply frequency
- By changing number of stator poles
- By changing the supply voltage
Control From Rotor Side:
- By inserting resistance in rotor circuit
- By various ways of cascade connection
- By injecting EMFs in the rotor circuit.
Speed Control by frequency variation:
- By varying supply frequency (on small amount), we can vary the speed.
- But a decrease in supply frequency decreases the speed and increases the flux, core losses which leads heating and low efficiency.
- Increase in frequency increases the speed and reduces the torque.
- A separate costlier auxiliary equipment is required to provide a variable frequency.
- So this method is not used in practical.
Speed Control By Pole Changing:
- The change of number of poles is done by having two or more entirely independent stator windings in the same slots.
- Each winding gives a different number of poles, so we will get different speeds.
- Due to cost and complex switching arrangements, it is not practical to provide more than two arrangements of poles (ie, two normal speeds).
- This method is applicable to squirrel cage induction motor only.
- It is not practically applicable with wound rotor.
Speed control by varying Supply voltage:
- The speed of induction motor can be varied by changing supply voltage.
- The torque developed in this method is proportional to the square of the supply voltage.
T ∝ V2
- This is the cheapest and easiest method, but it is rarely used because of the below reasons.
- A small change in speed requires a large change in voltage.
- This large change in voltage will result in a large change in the flux density.
Speed control by varying Rotor Resistance:
- This method is applicable to three-phase slip-ring induction motor only.
- By introducing external resistance in the rotor circuit, the speed of the motor can be reduced.
- The change in speed depends upon both rotor circuit resistance and load.
- Due to power loss in the resistance ,this method is used where speed changes are required for short period only.
- This method is similar to armature rheostat control method of DC shunt motors.
Speed control by injected EMF:
- Instead of applying the resistance into the rotor circuit of the motor, the speed can be varied by applying EMFs into the circuit.
- These EMFs are applied at the rotor by a suitable source whose frequency should be same as slip frequency.
- Inserting the EMF in phase with the rotor induced EMF is equivalent to decreasing the rotor resistance .
- Inserting the EMF in phase opposition to the induced rotor EMF is equivalent to increasing its resistance.
- Thus by injecting EMF into the rotor the speed can be controlled.
Speed control by Cascade Connection:
- This method needs two motor, one of them is wound motor.
- The two motors are mechanically coupled together to drive a common load.
- The starter of slip ring induction motor is connected to three-phase supply and its rotor is connected to stator of the other machine.
- There are four possible speeds can be obtained by this arrangement.
Where f = supply frequency
P1 = No of poles in slip ring motor
P2 = No of poles in other motor
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