Slip and Rotor Speed of Induction Motor

Slip and Effect of Slip on Rotor Parameters

The rotating field revolves with the speed of synchronism, and if the rotor conductors wereto revolve at the same speed there would not be any torque. Thus, there is a differencebetween rotor and rotating field speeds. The rotor speed is less than the rotating field speedand the difference in speed is known as the slip of motor. Generally, slip lies between zerosto five percent. So the difference between the synchronous speed of the magnetic field and the shaft rotating speed is called slip that is:Valou 001

Where Nsis synchronous speed of rotating field in stator and Nris speed of rotor.valou 02

Three phase induction motor secondarywinding rotating and in induction motor the frequency of emf induced in rotor (secondary winding)depends on slip that is:

Rotor emf frequency = s× frequency of applied voltage to stator = sf

The induced emf in rotor also depends on slip. When rotor is stationary the emf induced inrotor is:

valou 03

Where n= turns ration = n1/n2

Where E1 is applied voltagein statorand n2 and n1 are number of turns in rotor and stator windings. Hereboth the E1 and E2 remain fixed by above relation even at different slip or load values.

The rotor resistance (R2) remains constant with change in slip until external resistance is addedor there is a change in temperature.

Rotor reactance at slip one is X2that is:

X2 = fL2

Where fis frequency of rotor emf and L2 is inductance of rotor winding.

Now moving rotor frequency becomes sf, so

Rotor reactance = 2πsfL2 = sX2

­So, rotor independence at motion is:Valou 004

Effect of Slip

  • When the motor starts rotating the slip is 100 %and the motor current is at maximum. The slip and motor current are reduced when the rotor starts to turn.
  • Frequency decrease when slip decrease.
  • Inductive reactance depends on the frequency and the slip. When the rotor is not turning, the slip frequency is at maximum and so is the inductive reactance.
  • When the rotor is turning, the inductive reactance is low and the power factor approaches to one.
  • When the motor starts rotating the inductive reactance is high and impedance is mostly inductive. When the speed increases the inductive reactance goes down equaling the resistance.

The slip increases with an increasing load, thus providing a greater torque as showing in following figure:pic 0022



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