Electrical · Free Calculator
Synchronous Speed and Slip Calculator
Find the synchronous speed of an AC motor from supply frequency and number of poles, and the slip at the measured rotor speed.
Ns = 120f/PSlip %50 and 60 HzPole count
Hz
rpm
Synchronous speed1,500 rpm
Slip2.67 %
Slip speed40 rpm
Rotor frequency1.333 Hz
How this result was calculated
- Ns = 120 × f ÷ P = 120 × 50 ÷ 4 = 1,500 rpm
- Slip = (Ns − N) ÷ Ns × 100 = 2.67 %
Synchronous speeds at 50 Hz
| Poles | Synchronous speed |
|---|---|
| 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 |
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Synchronous Speed and Slip
Ns = 120 × f ÷ P
Slip = (Ns − N) ÷ Ns × 100 %
Slip = (Ns − N) ÷ Ns × 100 %
Worked example
A 4 pole motor on 50 Hz has Ns = 1500 rpm. Running at 1460 rpm, the slip is (1500 − 1460) ÷ 1500 = 2.67 %.
Key insight: slip rises roughly in proportion to load, so measuring speed with a tachometer gives a quick estimate of how heavily a motor is loaded.
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Frequently Asked Questions
What is the synchronous speed of a 4 pole motor?
1500 rpm at 50 Hz and 1800 rpm at 60 Hz.
What is normal slip for an induction motor?
Typically 1 to 5 % at full load; small motors have higher slip than large ones.
How do I find the number of poles from the nameplate?
Divide 120 × frequency by the rated speed and round down to the nearest even number. 1440 rpm at 50 Hz gives 4.17, so it is a 4 pole motor.
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Slip in an induction motor explained
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