Enter values and press Calculate.
The synchronous speed Ns is the speed at which the rotating magnetic field inside an AC motor's stator turns, set entirely by the supply frequency and the number of magnetic poles wound into the stator. An induction motor's rotor can never quite reach this speed (it needs relative motion between the field and rotor to induce torque-producing current), so it always runs slightly slower — the fractional difference is called slip.
| Quantity | Formula |
|---|---|
| Synchronous speed | Ns = 120×f/P (RPM) |
| Slip | s = (Ns−Nr)/Ns × 100% |
| Rotor speed from slip | Nr = Ns×(1−s) |
| Slip (rotor) frequency | fslip = s×f |
Typical induction motor slip at full load is small, usually 2–5%. A synchronous motor, by contrast, locks exactly to Ns (zero slip) once running, which is the key operating difference between the two machine types.
The speed of the rotating magnetic field produced by the stator windings of an AC motor, determined solely by supply frequency and pole count: Ns=120f/P.
The fractional difference between synchronous speed and actual rotor speed, s=(Ns−Nr)/Ns. It exists because an induction motor needs relative motion between the field and rotor to induce current and produce torque.
If the rotor ever caught up to the rotating field exactly, there would be no relative motion, no induced rotor current, and therefore no torque — the motor would simply coast down again. A small slip is required to sustain torque production.
Small induction motors typically run at 3–6% slip at full load; larger, more efficient motors often run at 1–3% slip. Slip is near zero at no load and increases with mechanical load.
More poles produce a slower synchronous speed for the same frequency (Ns=120f/P) — a 2-pole motor at 50Hz runs at 3000 RPM, while an 8-pole motor at the same frequency runs at only 750 RPM.
Poles always come in north/south pairs in a rotating field winding, so the total pole count P is always even (2, 4, 6, 8, ...).
fslip=s×f is the frequency of the current actually induced in the rotor bars; it matters for rotor design, heating, and for diagnosing motor condition via slip-frequency analysis.
No — a synchronous motor (once started and pulled into step) runs at exactly Ns with zero slip, unlike an induction motor which always needs some slip to produce torque.
By varying the output frequency f (and proportionally the voltage, to maintain flux), a VFD shifts the synchronous speed Ns=120f/P, and the motor's actual speed follows proportionally, minus its normal small slip.
A slip that increases over time under the same load can indicate broken rotor bars, bearing wear, or increased mechanical friction; monitoring slip trends is a common predictive-maintenance technique.
Because Ns=120f/P scales directly with frequency; a 4-pole motor is 1500 RPM synchronous on a 50Hz grid but 1800 RPM synchronous on a 60Hz grid.
The rotor slows slightly, increasing slip, which increases the induced rotor EMF and current, producing more torque to match the higher load — slip is the motor's natural self-regulating mechanism.
Yes, if an external force drives the rotor faster than synchronous speed (e.g. an induction generator), slip becomes negative and the machine feeds power back into the electrical supply instead of drawing it.
The nameplate RPM is the rated (full-load) rotor speed Nr, always slightly below the theoretical synchronous speed Ns for that pole count and frequency — the gap is the motor's rated slip.
Advanced Motor Calculator • Frequency Calculator • Three-Phase Power Calculator • All Calculators