Motor RPM Calculator — Poles, Synchronous Speed, Slip and 50/60 Hz
An AC induction motor's speed is set by the supply frequency and the number of poles: n = 120 × f / poles, minus a few percent of slip. Enter the frequency and poles (or the nameplate rpm) to get synchronous speed, actual speed and slip — and, through the fan laws, how a fan's airflow, pressure and power change on a 60 Hz or 50 Hz supply or a VFD.
No signup · Works on mobile · Formulas shown below · Updated 2026-11-02
Motor Speed & Frequency
ns = 120 × f / P (P = number of poles); n = ns × (1 − s). EC and permanent-magnet synchronous motors have no slip; their speed is set by the drive.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
Motor rpm formula
Synchronous speed ns = 60 × f / p = 120 × f / P // p = pole pairs, P = poles Slip s = (ns − n) / ns Actual speed n = ns × (1 − s) On a VFD n ≈ nrated × fVFD / frated // EC and permanent-magnet synchronous motors: no slip; speed is set by the drive and does not depend on grid frequency
An induction motor needs some slip to produce torque, and slip grows with load. That is why a 4-pole 60 Hz motor is stamped around 1,725–1,760 rpm rather than its 1,800 rpm synchronous speed. Small permanent-split-capacitor (PSC) and shaded-pole motors have much larger slip, which fans use for speed control by reducing voltage.
Synchronous speed chart (60 Hz and 50 Hz)
Poles
60 Hz synchronous rpm
60 Hz typical nameplate rpm
50 Hz synchronous rpm
50 Hz typical nameplate rpm
2
3,600
approx. 3,450–3,550
3,000
approx. 2,850–2,950
4
1,800
approx. 1,700–1,780
1,500
approx. 1,400–1,480
6
1,200
approx. 1,100–1,180
1,000
approx. 920–980
8
900
approx. 830–880
750
approx. 690–730
10
720
approx. 670–710
600
approx. 560–590
12
600
approx. 550–590
500
approx. 460–490
Nameplate ranges are typical for general-purpose three-phase induction motors. Single-phase and small fractional-horsepower motors slip more — always go by the actual nameplate.
Worked example: a 50 Hz fan on a 60 Hz supply
Given: a 2-pole 50 Hz fan, nameplate 2,880 rpm, rated at 1,770 CFM and 2.41 in. w.g. with 1.21 BHP (0.9 kW) shaft power.
Shaft power × 1.728 ≈ 2.09 BHP (1.56 kW) — the original motor is badly overloaded
The right approach: select a motor and impeller designed for 60 Hz, or a 50/60 Hz EC fan whose controller holds the speed.
Load the preset “50 Hz fan connected to 60 Hz” to see the numbers.
Frequently Asked Questions
What is the formula for motor rpm?
Synchronous speed n = 120 × f ÷ number of poles (or 60 × f ÷ pole pairs). An induction motor's actual speed is that times (1 − slip). For a 4-pole motor at 60 Hz: 120 × 60 ÷ 4 = 1,800 rpm synchronous, about 1,750 rpm actual.
How do I tell how many poles a motor has from its rpm?
Find the synchronous speed just above the nameplate rpm. At 60 Hz, about 3,500 rpm is 2-pole, 1,750 is 4-pole, 1,150 is 6-pole and 870 is 8-pole. At 50 Hz, 2,900, 1,450, 960 and 720 rpm correspond to 2, 4, 6 and 8 poles.
Can a 50 Hz fan run on 60 Hz?
Usually not as-is. Speed rises 20 % and shaft power rises about 73 % by the cube law, so the motor overloads and the impeller may exceed its permitted speed. Use a motor and impeller designed for 60 Hz, or a 50/60 Hz EC fan.
How does fan power change when I lower the VFD frequency?
With an unchanged system curve, airflow is proportional to frequency, pressure to frequency squared and power to frequency cubed. At 48 Hz on a 60 Hz motor (80 % speed), theoretical shaft power is about 51 % of full speed.
Do EC motors have poles and slip?
EC motors are electronically commutated permanent-magnet (brushless DC) motors. They do have poles, but speed is set by the drive's commutation frequency, with no induction-motor slip and no dependence on a 50 or 60 Hz grid — so one model can serve both.
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