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Motor slip is the difference between the synchronous speed of the rotating magnetic field and the actual rotor speed in an induction motor, expressed as a percentage:

s = (N_s − N) / N_s × 100%

Slip is essential for torque production. Without relative motion between the rotor and the magnetic field, no voltage would be induced in the rotor conductors, and no torque would be generated.

Typical full-load slip values:

• 1–3% — Large motors (>100 kW)
• 2–5% — Standard motors (NEMA Design B)
• 3–7% — Small motors (fractional HP)
• 5–13% — High-slip motors (NEMA Design D)

At no load, slip is typically 0.5–1%. Slip increases proportionally with load up to the breakdown torque point.

Slip exists because an induction motor works on the principle of electromagnetic induction:

• The stator creates a rotating magnetic field at synchronous speed N_s
• This field must "cut" the rotor conductors to induce a voltage (Faraday's Law)
• The induced voltage drives current through the rotor bars, creating rotor magnetic field
• Interaction of stator and rotor fields produces torque

If the rotor reached synchronous speed, there would be zero relative motion, zero induced EMF, zero rotor current, and zero torque. Therefore, the rotor always runs slightly slower than the field.

Load and slip are directly related:

• No load: Very low slip (0.5–1%), motor runs near synchronous speed
• Increasing load: Motor slows down → slip increases → more rotor current → more torque
• Full load: Slip reaches nameplate value (typically 2–5%)
• Overload: Slip continues to increase up to breakdown torque (typically 2–3× rated torque)
• Beyond breakdown: Motor stalls — torque decreases rapidly with increasing slip

The slip-torque curve is approximately linear from no-load to full-load for normal operation.

Motor slip can be measured using several methods:

• Tachometer: Measure actual RPM with contact or optical tachometer and compare to synchronous speed
• Stroboscope: Flash at line frequency — if rotor appears to slowly rotate backward, the slow rotation rate equals slip speed
• Vibration analysis: Detect slip frequency sidebands around line frequency in the vibration spectrum (2× slip frequency sidebands indicate rotor issues)
• Nameplate method: Use nameplate RPM as full-load speed and calculate: s = (N_s − N_nameplate) / N_s

For vibration analysts, monitoring slip frequency is critical for detecting broken rotor bars and rotor eccentricity.