Induction motor slip calculator
Work out the slip of a three-phase induction motor from synchronous and actual speed (or poles and frequency) — the figure that tells you if the motor is loaded or on the way to stalling.
Slip from speed, poles & frequency
Poles
Supply frequency
Formula: N (RPM) = 120 × f (Hz) / poles, then slip % = (Ns − N_actual) / Ns × 100.
Reverse mode — speed at a target slip
Formula: N = Ns × (1 − slip % / 100), using the same poles and frequency from above.
Slightly above typical full-load slip — the motor may be working harder than normal.
Reference: full-load slip for common 4-pole 50 Hz motors — 1.5 kW ≈ 4%, 11 kW ≈ 3%, 45 kW ≈ 2%. If your motor runs at 5%+ slip under load it is likely overloaded; near-synchronous speed at no-load is normal. AO Ctrl stocks the motors and drive control for industrial motion:
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Frequently asked questions
How do I calculate the synchronous speed of an induction motor?
Synchronous speed is N (RPM) = 120 × f (Hz) / poles. A 4-pole motor on a 50 Hz supply runs at 120 × 50 / 4 = 1500 RPM synchronous, so with the calculator default of 1450 RPM actual speed you get slip = (1500 − 1450) / 1500 × 100 = 3.3%. On 60 Hz the same 4-pole motor runs at 1800 RPM synchronous.
What is normal slip for an induction motor?
Most motors run 0–3% slip at full load and near synchronous speed at no-load. Typical full-load slip for common 4-pole 50 Hz motors is about 4% at 1.5 kW, 3% at 11 kW and 2% at 45 kW. The calculator flags anything at or above 5% as heavily loaded or overloaded — a sign to check for stalls or an undersized drive.
What is the formula for motor slip?
Slip % = (Ns − N_actual) / Ns × 100, where Ns is synchronous speed from poles and frequency. The calculator also works in reverse: given a target slip it finds the shaft speed as N = Ns × (1 − slip % / 100). For example, 3% slip on a 1500 RPM synchronous motor gives about 1455 RPM.
AO Ctrl provides this tool for reference only.