Synchronous speed and slip calculator
Enter the supply frequency, the number of poles and, for an induction motor, its rated or measured speed. The calculator gives the synchronous speed of the rotating field and the slip, both as a percentage and in rpm, plus the frequency of the rotor currents.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the supply frequency: 50 Hz in most of the world, 60 Hz in North America, or the output frequency of a variable-frequency drive.
- Enter the number of poles (not pole pairs): 2, 4, 6, 8…
- Enter the motor’s actual speed from the nameplate or a tachometer to get the slip.
- Read the synchronous speed in rpm and rad/s, the slip and the rotor frequency.
- Synchronous speed Ns = 120 × f / p rpm (= 60 × f / pole pairs)
- Slip s = (Ns − N) / Ns × 100 %
- Slip speed = Ns − N rpm
- Rotor current frequency f_r = s × f
- Angular speed ω = Ns × 2π / 60 rad/s
Worked example
A 4-pole motor on 50 Hz with a nameplate speed of 1,450 rpm: Ns = 120 × 50 / 4 = 1,500 rpm (157.08 rad/s). The slip is (1,500 − 1,450) / 1,500 = 3.333 %, or 50 rpm, and the rotor currents alternate at 0.0333 × 50 = 1.667 Hz.
The same motor type for 60 Hz rated 1,750 rpm: Ns = 1,800 rpm and the slip is 50 / 1,800 = 2.778 %.
Tips
- Frequency drives: the synchronous speed scales with frequency, so a 4-pole motor on a drive at 25 Hz has Ns = 750 rpm.
- Slip grows with load. At no load an induction motor runs very close to Ns; the nameplate speed is the speed at rated load.
- Count poles, not pairs. A formula with 60 × f uses pole pairs; this calculator asks for poles.
Frequently asked questions
What is the formula for synchronous speed?
Ns = 120 × f / p, with f the supply frequency in Hz and p the number of poles. At 50 Hz a 4-pole motor's field turns at 1,500 rpm; at 60 Hz it is 1,800 rpm.
What is slip in an induction motor?
The rotor of an induction motor turns slightly slower than the field so that the field cuts the rotor bars and induces current. Slip s = (Ns − N) / Ns; a motor rated 1,450 rpm on 50 Hz has 3.33 % slip at full load.
How do I tell the number of poles from the nameplate speed?
Find the synchronous speed just above the rated speed. On 50 Hz: 2,900 rpm means 2 poles (3,000), 1,450 rpm means 4 poles (1,500), 960 rpm means 6 poles (1,000). On 60 Hz the steps are 3,600, 1,800, 1,200 and 900 rpm.
Does a synchronous motor have slip?
No. A synchronous motor runs exactly at Ns; enter Ns as the actual speed and the slip is zero. Slip only applies to induction (asynchronous) motors.