Slip in an induction motor is the difference between the speed of the rotating magnetic field (synchronous speed) and the actual speed of the rotor. It is a crucial concept because torque is produced only due to slip; without slip, the motor cannot operate.
The mathematical expression for slip is:
$$ s = \frac{N_s – N_r}{N_s} $$
where:
- $N_s$ = Synchronous speed (RPM)
- $N_r$ = Rotor speed (RPM)
- $s$ = Slip
Table of Contents
🔹 Key Points about Slip
- Slip is expressed as a fraction or percentage:
$$ \text{Slip (\%)} = s \times 100 $$
- The rotor never reaches synchronous speed.
- If $N_r = N_s$, then slip = 0:
- No relative motion
- No induced EMF
- No torque is produced
- Slip provides the necessary relative motion between:
- Rotating magnetic field (stator)
- Rotor conductors
🔹 Physical Interpretation
The stator produces a magnetic field rotating at synchronous speed, and the rotor tries to follow it. However, the rotor always lags behind slightly. This lag is called slip.
In simple terms:
- The rotor is continuously trying to catch up with the rotating field
- But it always stays slightly behind, which keeps the motor running
🔹 Typical Values of Slip
- No load: Slip ≈ 0.5% to 1%
- Full load: Slip ≈ 2% to 6%
🔹 Important Conditions
- Slip = 0:
- $N_r = N_s$
- No torque → Motor cannot operate
- Slip = 1:
- Rotor is stationary (starting condition)
- Maximum relative motion
Conclusion: Slip is a small but essential difference in speed that enables electromagnetic induction and torque production in an induction motor.