Induction motor is also known as asynchronous motor since the actual speed of motor is always less than the synchronous speed. For proper working of induction motor, rotor has to rotate slower than the rotating stator field speed. The amount by which rotor lag (runs slower) with respect to the rotating synchronous stator field speed is known as slip.
Hence motor slip can be defined as the difference between the synchronous rotating stator field speed and rotor speed.
Mathematically, slip (S) can be expressed by the following formula:
Ns=synchronous speed of stator field in rpm (revolution per minute.)
N= Actual speed of rotor in rpm.
Slip is usually expressed as a percentage of synchronous speed.
When rotor is stationary (N=0), and slip S= 1 or 100%.
Importance of Slip in Induction Motor:
For induction motor to make continuous torque, there must be some difference between the stator field speed and rotor speed. This difference is known as slip.
If the slip of motor is zero, torque will be zero because in that case there will no relative speed between rotor and synchronous rotating stator field (i.e. N=Ns). Hence no emf will be induce in the rotor conductors and current in the rotor circuit is also zero. If rotor current is zero, no torque will be produced by the motor and motor stops running.
Hence slip plays an important role in the operation of induction motor. In the absence of slip, operation of induction motor is not possible. Since the torque produced by induction motor depend on the slip. (i.e. torque is directly proportional to the slip of induction motor). Now, let me explain that how slip in induction motor makes the motor to work properly.
Slip At No Load:
When motor is running on no load, slip of the induction motor is less. The small value of slip produces less torque and motor also required small torque to overcome friction and windage losses at no load. Hence slip of the induction motor adjusts itself according to the torque required for the motor.
Slip At Load:
When the load on motor (rotor shaft) increases, high torque is required for driving the load. Since synchronous (stator field) speed is constant, hence rotor speed decreases as the load on motor increases. Decrease in rotor speed will result in the increase value of slip.
Increase in motor slip means that there will be more emf induced in the rotor and hence more current in the rotor circuit. This simply result in the increase of motor torque. Thus, motor delivers greater torque required to drive the load.
From the above discussion, we can say that motor slip adjusts itself to such a value so as to meet the driving torque required for the load.
Effect of Slip on Rotor:
At slip S=1,
The rotor is stationery and voltage induced in the rotor conductors is maximum.
At slip S=0,
The rotor rotates at synchronous speed equal to synchronous rotating stator field and voltage induced in the rotor conductors is zero.
Frequency of rotor current in induction motor is slip times the supply frequency and is stated as:
Rotor current frequency = Fractional slip Supply Frequency.
Fr’ = S F.
When rotor is stationary (s=1), frequency of the rotor current is equal to that of supply frequency.
Fr’ = S F (s=1,when rotor is standstill)
Fr’ = 1 F = F
Torque of the motor depend upon the slip. Greater the value of motor slip, more will be the value of rotor current (since large current is induced in the rotor conductors) hence more will be the torque induced and vice versa. Moreover, if there is no slip, no torque is produced and motor cannot run. Slip is expressed in percentage and its value is 4-6% in large motors and 8 to 9% in small motors.
That’s all. Hope this will helps you.