What is Slip?
The linked flux to the rotor conductors induces voltage in the rotor, and as the rotor conductors are short circuited, the current starts flowing through the rotor conductors.Due to an interaction between the magnetic field and the rotor current, the torque is produced and the rotor starts rotating. Let the rotating speed of the rotor is N.
In an induction motor the rotor speed always lags the synchronous speed of the rotating magnetic field. The induction motor is called asynchronous motor because the actual speed of the motor ia always less than the synchronous speed of the motor.
s = Ns – N
The slip is in RPM.
The slip is usually expressed as percentage of the synchronous speed.
For example :
A 4 poles,50Hz induction motor having 1480 RPM at full load.
Synchronous Speed of the motor
Ns = 120f/P
Ns = 120 x 50/4
Ns = 1500 RPM
s = Ns -N
s = 20 RPM
% slip = [(Ns-N) /Ns] x 100
= [(1500-1480) /1500] x 100
= [20 /1500] x 100
% slip =1.33 %
Why Slip is must for operation of an Induction Motor?
The torque is produced when the current flows in the rotor conductor. If the slip is zero, no EMF will be induced in the rotor conductor and hence there will be no flow of the current in the rotor circuit. The torque is produced due to an interaction of the main flux and the rotor current. If the rotor current is zero, motor will produce no torque. In absence of the slip, the operation of motor is not possible. Th torque produced in an induction motor is proportional to the slip.The torque equation of the induction motor is as given below.
From above torque equation of an induction motor, it is clear that if the slip is zero the torque will be zero. When the load on the motor increase the slip gets increased and speed of the motor decreases slightly, thus the motor delivers higher torque for driving the load.
The slip plays very vital role in the operation of an induction motor. At no load , the slip of the induction motor is less and the slip gets increased with increased loading on the motor. The slip of the motor gets self adjusted according to the torque demands from the load side.
The other parameters of an induction motor is governed according to the value of the slip of the motor.
Rotor Induced EMF
The EMF induced in the rotor is directly proportional to the slip. At standstill condition the slip is unity and the maximum voltage is induced in the rotor circuit.
E2(r) ∝ (Ns-N)
E2(r) ∝ s
E2(r) = s E2
E2 is the rotor induced voltage / Phase when motor is at standstill after applying the stator voltage.
E2 is the rotor induced EMF/ Phase in running condition
At standstill condition
s= (Ns – N)/Ns = (Ns -0)/Ns = 1
so, E2(r) = s E2
E2(r) = 1 x E2 = E2
E2(r) = E2
The EMF induced in the rotor at standstill is equal to the maximum rotor voltage (OCV) or equal to the open circuit voltage of the rotor.
ω = 2πfr
L2 = Rotor inductance
X2 = 2πfs L2 [ As fr =fs, at standstill condition]
The rotor induced frequency in running condition depends on the slip of the motor.
In running condition,
The rotor reactance in running condition will be;