Synchronous speed calculator: motor rpm, slip and load
This synchronous speed calculator uses the formula from the U.S. Department of Energy motor sourcebook: synchronous speed = 120 × frequency ÷ number of poles. A 4-pole motor on 60 Hz turns its field at 1800 rpm; running at 1750 rpm it has 2.78 % slip.
Example: Supply frequency 60 Hz · Number of poles: 4 poles · Actual (measured) speed (optional) 1,750 rpm → Motor speed: Synchronous speed 1800 rpm, slip 2.78 %. Source: U.S. Department of Energy (AMO), “Improving Motor and Drive System Performance: A Sourcebook for Industry” (2014), p. 6: Synchronous Speed = 120 × frequency (Hz) ÷ number of poles; p. vii glossary “slip”; p. 10: most induction motors operate within 1 % to 3 % of synchronous speed (read 09.10.2026). Updated: .
How it is calculated
Synchronous speed (rpm) = 120 × frequency (Hz) ÷ number of poles. It is the speed of the rotating magnetic field in the stator. The DOE motor sourcebook gives the examples: on 60 Hz a 2-pole field turns at 3600 rpm, a 4-pole field at 1800 rpm and a 6-pole field at 1200 rpm. An induction motor always runs a little slower – the difference is the slip.
Synchronous speeds
| Poles | 60 Hz | 50 Hz |
|---|---|---|
| 2 | 3600 rpm | 3000 rpm |
| 4 | 1800 rpm | 1500 rpm |
| 6 | 1200 rpm | 1000 rpm |
| 8 | 900 rpm | 750 rpm |
| 10 | 720 rpm | 600 rpm |
| 12 | 600 rpm | 500 rpm |
Slip
Slip = (synchronous speed − actual speed) ÷ synchronous speed. A 4-pole motor on 60 Hz running at 1750 rpm has 50 rpm or 2.78 % slip; at 1770 rpm it is 1.67 %. According to the DOE, most induction motors run within 1 % to 3 % of synchronous speed. A nameplate reading such as 3500 rpm therefore points to a 2-pole motor (3600 rpm, 2.78 % slip).
Estimating motor load from speed
The DOE fact sheet “Determining Electric Motor Load and Efficiency” uses the slip to estimate load: load = (synchronous speed − measured speed) ÷ (synchronous speed − nameplate full-load speed). Its example: 1800 rpm synchronous, 1750 rpm nameplate, 1770 rpm measured – 30 ÷ 50 = 60 % load, so a 25 hp motor delivers about 15 hp. The DOE warns that nameplate speeds are rounded to 5 rpm and that slip changes with the square of the voltage. The power of a three-phase motor comes from the three phase power calculator; for pulleys and gears use the gear ratio calculator.
Frequently asked questions
What is the formula for synchronous speed?
Synchronous speed = 120 × f ÷ p. With 60 Hz and 4 poles: 120 × 60 ÷ 4 = 1800 rpm.
How do I calculate motor slip?
Slip = (Ns − N) ÷ Ns. 1800 rpm synchronous and 1750 rpm actual give 50 ÷ 1800 = 2.78 %.
Does every AC motor have slip?
No. According to the DOE, induction motors run somewhat below synchronous speed – most within 1 % to 3 % – while synchronous motors run at synchronous speed without slip.
How many poles does my motor have?
Compare the nameplate speed with the table: 1750 rpm on 60 Hz is a 4-pole motor (1800 rpm), 1450 rpm on 50 Hz also (1500 rpm).
Can I estimate the load from the speed?
Yes, roughly: (Ns − measured) ÷ (Ns − nameplate). 1800, 1770 and 1750 rpm give 60 % load. Rounded nameplate values and voltage make this an estimate only.
What changes on 50 Hz?
All synchronous speeds drop by one sixth: a 4-pole motor turns at 1500 rpm instead of 1800 rpm.
Sources and legal basis
- U.S. Department of Energy (AMO), “Improving Motor and Drive System Performance: A Sourcebook for Industry” (2014), p. 6: Synchronous Speed = 120 × frequency (Hz) ÷ number of poles; p. vii glossary “slip”; p. 10: most induction motors operate within 1 % to 3 % of synchronous speed (read 09.10.2026)
- U.S. Department of Energy, Motor Challenge Fact Sheet “Determining Electric Motor Load and Efficiency”: Table 1 synchronous speeds at 60 Hz, Equation 5 slip method (example 1800/1750/1770 rpm → 60 % load), Equation 6 voltage correction (read 09.10.2026)
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