Speed Control of 3 phase Induction Motor

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Speed Control of 3 phase Induction Motor


The speed control of 3 phase induction motor depends on various factors like supplied voltage, frequency, number of poles. Speed of 3 phase induction motor needs to varied according to requirement.  Speed controls from stator side as well as rotor side are also.

induction motor is practically a constant speed motor, that means, for under loading condition, change in speed of the motor is quite small. 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.

the basic formula of speed control is synchronous speed

Where, f = frequency

P = the number of poles

actual speed of induction motor is Rotor speed

where, s = slip

so, the speed is depending on supply frequency,  stator poles and slip.

Different speed control methods of 3 phase induction motor


  • V / f (variable frequency) control or frequency control
  • changing the number of stator poles
  • By Changing The Applied Voltage
  • Rotor resistance control
  • Cascade control of two motors
  • EMF Injecting in rotor circuit

  • V / f (variable frequency) control or frequency control

This is the most popular method for controlling the speed of an induction motor. If we change frequency, the synchronous speed of induction motor changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor. 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 φ constant.

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.

  • Changing the number of stator poles

From the above equation of synchronous speed, it can be seen that synchronous speed can be changed by changing the number of stator poles. Thus by changing in number of stator poles we can change the speed of induction motor.

This method is easily applicable for squirrel cage type induction motors, Change in stator poles is achieved by two or more independent stator winding wound for different number of poles in same slots.

According to the above equation in one motor for 2 poles and other for 4 poles.
for supply frequency of 50 Hz

  1. Ns = 120 * 50 / 2 = 3000 rpm  (for p = 2)
  2. Ns = 120 * 50 / 4 = 1500 rpm  (for p = 4)
  • By Changing The Applied Voltage

This method is most easiest and cheapest.In this method speed of the motor is controlled by changing the applied voltage across the motor terminals.

The torque produced by running three phase induction motor is given by

torque produced by running three phase induction motor

Rotor resistance R2 is constant and if slip s is small then (sX2)2 is so small that it can be neglected.

T ∝ sE22

 E2 V

where, E2 = rotor induced emf

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 not used widely for following two reasons.

  1. Large change in voltage is required for relatively small change in motor speed
  2. This large change in voltage may disturb the magnetic conditions of the motor, as it changes the flux density.
  • Rotor resistance control

This method is applicable for slip ring motors. This method is similar to that of armature rheostat control of DC motor. 3 phase rheostat is joined in series with the rotor circuit via slip rings. Here slip rings are not short circuited as they are when rheostat is only used for starting of a induction motor. Slip for a given torque can be varied by varying the rotor resistance.Main disadvantage of this method is I^2.R (Cu) losses are also increased with increase in rotor resistance.

Addition of external resistance in the rotor of squirrel cage motors is not possible.

 

  • Cascade control of two motors

In this method, two 3 phase induction motors are mounted on a same shaft. One motor is fed from a 3 phase supply and the other motor is fed from the induced emf in first motor via slip-rings. One motor is the called the main motor and another motor is called the auxiliary motor. main motor is called A and auxiliary motor called B.

cascade type speed control method

where, Ns1 = frequency of main motor A

Ns2 = frequency of auxillary motor B

P1 = number of poles of motor A

P2 = number of poles of motor

f = supply frequency

N = speed of the set and same for both motors

four speeds can be obtained in following cases.

  • Motor A may be run separately from the supply giving synchronous speed

Ns = 120f / Pa (Pa is no. of stator poles for motor A).

  • Motor B may be run separately giving synchronous speed

Ns = 120f / Pb (Pb is no. of stator poles for motor B).

  • Two motors may be connected in cumulative cascade giving

Ns = 120f / (Pa + Pb).

  • Two motors may be connected in differential cascade giving

Ns = 120f / (Pa – Pb)

 

  • EMF Injecting in rotor circuit

In this method, speed of an induction motor is controlled by injecting a voltage in rotor circuit. When we insert voltage which is in phase with induced rotor emf, it is equivalent to decreasing resistance of rotor. speed is increase.

Whereas when we insert voltage which is opposite in phase with induced emf in rotor, its like increasing resistance of rotor circuit. speed is decrease. Thus, by changing the phase of injected emf, speed can be controlled.

The main advantage of this method is a wide range of speed control can be achieved.

The emf can be injected by various methods such as Kramer system and Scherbius system.

 

If any query or suggestion about speed control of induction motor please comment below or Email on Dipak@electricalidea.com.

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