📘 Induction Motor – Working Principle & Slip
Induction motor works on the principle of electromagnetic induction. It is the most widely used motor in industry.
🔹 1. Basic Construction
Two main parts:- Stator – 3-phase winding
- Rotor – Squirrel cage or wound rotor
🔹 2. Rotating Magnetic Field (RMF)
When 3-phase supply applied: A rotating magnetic field is produced. Synchronous speed:N_s = 120 f / P
Where: f = frequency (Hz) P = number of poles🔹 3. Slip (Most Important Concept)
Rotor speed (N_r) is always less than synchronous speed. Slip:s = (N_s − N_r) / N_s
In percentage:% Slip = s × 100
At standstill: s = 1 At synchronous speed: s = 0 Motor never reaches synchronous speed.🔹 4. Rotor Frequency
Rotor current frequency:f_r = s f
At starting: f_r = f At normal operation: f_r small.🔹 5. Example 1
Given: f = 50 Hz P = 4 Find synchronous speed.N_s = (120 × 50) / 4 = 1500 rpm
🔹 6. Example 2
If rotor speed = 1440 rpm Slip:s = (1500 − 1440) / 1500 = 60 / 1500 = 0.04
Slip = 4%
Rotor frequency:f_r = 0.04 × 50 = 2 Hz
🔹 7. Important Observations
- Slip small at full load (2–5%)
- High slip at starting
- Rotor frequency reduces as speed increases
- Torque depends on slip
🎯 GATE Important Points
- Remember N_s formula clearly
- Slip concept frequently asked
- Rotor frequency question common
- Motor cannot run at synchronous speed
Slip = Heart of Induction Motor Analysis
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