What Is Induction Motor Slip?
An AC induction motor has two assemblies: a stator and a rotor. The current in both bars attached to the rotor and the magnetic field of the stator generate a torque. But in case of actual operation, the rotor speed is always less than the speed of the magnetic field. Useful torque is produced when the rotation causes the rotor bars to dissect the magnetic field that is developed. (For a well-done illustration of the induction slip, please click here.)
It is a fact that slip in the motor is dependent to a great extent on the parameters associated with motor performance. Slip of an induction motor varies in the same proportion as the load torque, rotor resistance, and the voltage frequency of stator. The induction speed of the wounded motor can be controlled by increasing the slip. The traditional way of increasing slip is by subjecting the rotor circuit to increased resistance. Lower horsepower motors have higher slip as compared to high horsepower motors since small motors have the tendency to generate greater resistance in the rotor winding.
In the condition of induction motor slip, the voltage remains the maximum and slip is 100% as the motor starts rotating but both slip as well as voltage reduce as soon as the rotor starts to turn. Frequency is directly proportional to the slip, i.e. frequency decreases with decrease in slip. Inductive reactance of an induction motor depends on both frequency as well as slip. When the rotor is stationary, the frequency, slip, and inductive reactance are at the maximum level. When the rotor turns, the inductive reactance remains low and power factor reaches to 1. The inductive reactance changes with slip because the rotor is the summation of constant resistance and changeable inductive reactance.
As the motor starts rotating, the inductive reactance remains high and the impedance is inductive, however, as soon as the speed of the motor increases, the inductive reactance decreases and becomes equal to the resistance. As discussed earlier, the slip generates torque. Since the slip never becomes 0, it drives the rotor in induction motors. If the slip becomes zero, then the rotor field catches the stator field and the force between the rotor and stator becomes 0 and finally the rotor stops rotating. This creates attraction between rotor and stator and the slip becomes effective.