Speed
Poles to r/min (50 Hz and 60 Hz)
Synchronous speed of an AC induction motor is n = 120 × frequency ÷ pole count. A 4-pole motor is 1500 r/min at 50 Hz and 1800 r/min at 60 Hz. The shaft runs a little slower because of slip (typically a few percent on a loaded S1 machine). Match poles, not only kW, or a pump or fan will change speed.
About this tool
Synchronous speed of an AC induction motor is n = 120 × frequency ÷ pole count. A 4-pole motor is 1500 r/min at 50 Hz and 1800 r/min at 60 Hz. The shaft runs a little slower because of slip (typically a few percent on a loaded S1 machine). Match poles, not only kW, or a pump or fan will change speed.
Who this is for
Use it to read a nameplate (2 / 4 / 6 / 8 pole) into r/min before you quote a replacement.
Calculator
What the numbers mean
- Poles
- How many magnetic poles the stator winding is built with (2, 4, 6, 8 on general-purpose machines). More poles → slower synchronous speed at the same frequency. Poles are a winding fact, not a “size”.
- Synchronous speed nₛ
- nₛ = 120 × f / p, in r/min. f is supply frequency in Hz; p is pole count. A 4-pole motor is 1500 r/min at 50 Hz and 1800 r/min at 60 Hz.
- r/min (rpm)
- Revolutions of the shaft per minute. IEC writes r/min or min⁻¹. The nameplate speed is usually slightly below nₛ because of slip.
- Slip
- s = (nₛ − n) / nₛ. A loaded S1 induction motor typically slips a few percent. Example: 4-pole 50 Hz, 3% slip → about 1455 r/min. Always prefer the printed nameplate r/min when it is there.
- Why poles must match
- Pumps and fans are speed machines. Same kW at 2-pole vs 4-pole is a different hydraulic duty. Convert frequency as well if the plant is 60 Hz and the old motor is 50 Hz.
Synchronous speed table
| Poles | 50 Hz nₛ (r/min) | 60 Hz nₛ (r/min) | Typical loaded 50 Hz (≈3% slip) | Typical loaded 60 Hz (≈3% slip) |
|---|---|---|---|---|
| 2 | 3000 | 3600 | 2910 | 3492 |
| 4 | 1500 | 1800 | 1455 | 1746 |
| 6 | 1000 | 1200 | 970 | 1164 |
| 8 | 750 | 900 | 728 | 873 |
Formula reminder: nₛ = 120 × Hz ÷ poles. Slip estimate: n ≈ nₛ × (1 − slip/100).
Need a JW motor?
Send kW or hp, volts, frequency, poles, mounting, and a nameplate photo. Stock IEC three-phase (YE3 / Y2) or custom windings — we quote from the plate, not from the logo.
Contact JWMotor