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Motor

Induction Motor Slip

Calculate induction motor slip, synchronous speed, slip frequency, and rotor speed for AC induction motors.

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Formula

n_s = 120f/p, s = (n_s − n_r)/n_s

n_s— Synchronous speed (RPM)
n_r— Rotor speed (RPM)
f— Supply frequency (Hz)
p— Number of poles

How It Works

An AC induction motor's rotor always rotates slightly slower than the rotating magnetic field (synchronous speed) because it needs relative motion between the field and rotor conductors to induce current and produce torque. This speed difference, expressed as a percentage of synchronous speed, is called slip (s). Synchronous speed N_s = 120 × f / P (where f is supply frequency in Hz and P is the number of poles). Slip increases with load torque; at rated load, slip is typically 2–8% for standard induction motors.

Worked Example

A 4-pole, 60 Hz induction motor runs at 1746 RPM under rated load. Step 1 — Synchronous speed: N_s = 120 × 60 / 4 = 1800 RPM Step 2 — Slip: s = (N_s − N_r) / N_s × 100 s = (1800 − 1746) / 1800 × 100 = 54/1800 × 100 = 3.0% Step 3 — Rotor frequency (frequency of currents induced in rotor): f_r = s × f = 0.03 × 60 = 1.8 Hz Step 4 — Effect of load increase: if load torque doubles and slip increases to 6%: N_r = N_s × (1 − s) = 1800 × 0.94 = 1692 RPM Result: At rated load, the motor runs 3% below synchronous speed. Doubling the load reduces speed to 1692 RPM — still acceptable for most applications.

Practical Tips

  • ✓For variable-frequency drive (VFD) applications, remember that synchronous speed changes proportionally with output frequency, so rated slip speed in RPM stays approximately constant across the speed range
  • ✓High-efficiency motors (IE3/IE4) have lower slip (1–2%) than standard motors (IE1 at 5–8%) because they have lower rotor resistance — this also means they are harder to start with reduced-voltage starters
  • ✓Measure actual shaft speed with a tachometer to determine operating slip; this quickly reveals overloading or increased mechanical friction before thermal damage occurs

Common Mistakes

  • ✗Expecting an induction motor to run at exactly synchronous speed — it cannot, as zero slip means zero induced rotor current and zero torque
  • ✗Ignoring slip when calculating motor speed from pole count and frequency alone — a 4-pole 60 Hz motor runs at ~1750 RPM, not 1800 RPM
  • ✗Confusing slip frequency with supply frequency — rotor currents are at the much lower slip frequency (1–5 Hz typically), not 50/60 Hz

Frequently Asked Questions

The slip at breakdown (maximum) torque is the pull-out slip, typically 10–25% for standard motors. Beyond this point, any additional load causes the motor to decelerate rapidly and stall. The ratio of breakdown torque to rated torque is typically 2–3× for Design B motors.
Torque is proportional to V². If voltage drops 10%, torque capacity drops ~19%. To maintain the same load torque, slip must increase substantially, raising rotor current and losses. Sustained low voltage causes overheating and is a leading cause of premature motor failure.
Yes — when the rotor turns faster than the synchronous field (e.g., driven by an external load or during regenerative braking), slip is negative and the machine acts as an induction generator, feeding power back into the supply.

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