Synchronous speed calculator
Convert the synchronous speed of an induction motor between supply frequency (Hz) and shaft speed (RPM) from the pole count — then read the actual running speed from nameplate RPM and slip.
Synchronous speed — Hz ↔ RPM
Mode
Poles
Quick set
Formula: n (RPM) = 120 × f (Hz) / P, and in reverse f (Hz) = n × P / 120, where P is the number of poles (2, 4, 6, 8, 10 or 12).
Slip from actual nameplate speed
Formula: slip % = (n_s − n) / n_s × 100 and slip (RPM) = n_s − n. The synchronous speed n_s is taken from the mode and settings above — the frequency/poles in “Frequency → RPM” mode, or the target speed in “Speed → frequency” mode.
Synchronous speeds at 50 Hz
Full pole-count reference grid for the frequency entered in “Frequency → RPM” mode.
2 poles
3,000 RPM
4 poles
1,500 RPM
6 poles
1,000 RPM
8 poles
750 RPM
10 poles
600 RPM
12 poles
500 RPM
AO Ctrl stocks the motors and drive control for industrial motion:
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Frequently asked questions
How do I calculate synchronous speed from poles and frequency?
Synchronous speed is n (RPM) = 120 × f (Hz) / P, where P is the number of poles. A 4-pole motor on 50 Hz runs at 120 × 50 / 4 = 1500 RPM synchronous; on 60 Hz the same motor runs at 120 × 60 / 4 = 1800 RPM. A 2-pole 50 Hz motor runs at 3000 RPM, a 6-pole at 1000 RPM (120 × 50 / 6) and a 12-pole at 500 RPM (120 × 50 / 12).
What is the synchronous speed of a 4-pole motor at 60 Hz?
A 4-pole motor on a 60 Hz supply has a synchronous speed of 120 × 60 / 4 = 1800 RPM. The same motor at 50 Hz is 1500 RPM — the 20% difference in supply frequency scales directly to speed, which is why a 60 Hz machine derates when run on 50 Hz and why VFD control changes motor speed by varying output frequency.
How do I find slip from the nameplate speed?
Slip % = (n_s − n) / n_s × 100, where n_s is the synchronous speed and n the actual nameplate speed. For a 4-pole 50 Hz motor the synchronous speed is 1500 RPM; a nameplate of 1455 RPM gives slip = (1500 − 1455) / 1500 × 100 = 3.0% and a slip speed of 45 RPM. Most motors run 0–3% slip at full load.
What supply frequency does a motor need for a target synchronous speed?
Reverse the formula: f (Hz) = n × P / 120. For a 4-pole motor to run synchronously at 1500 RPM you need f = 1500 × 4 / 120 = 50 Hz; for 1800 RPM you need 1800 × 4 / 120 = 60 Hz. This is the calculation a VFD uses to command speed from frequency.
Worked example
A 4-pole motor on a 50 Hz supply has a synchronous speed of 1500 RPM. Its nameplate says 1455 RPM, so slip = (1500 − 1455) / 1500 × 100 = 3.0%, a slip speed of 45 RPM. At 60 Hz the same motor runs at 1800 RPM synchronous. A 6-pole motor at 60 Hz runs at 1200 RPM, and a 12-pole motor at 50 Hz runs at 500 RPM.
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AO Ctrl provides this tool for reference only.