sparkytools

Third year Motors · tool

Motor synchronous speed and slip

Pick the supply frequency and enter the nameplate speed. The tool finds the pole count (or use your own), the synchronous speed, and the slip in rpm and percent — plus the frequency of the current in the rotor.

Supply frequency
  • Synchronous speed—
  • Poles—
  • Slip—
  • Slip, percent—
  • Rotor current frequency—
Show the working

    Physics from frequency and poles. Full-load slip changes with load; the nameplate speed is the full-load figure.

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    How it works

    The stator’s magnetic field rotates at synchronous speed, set only by the supply frequency and the number of poles the winding is built with. The rotor chases it and always lags a little — that lag is slip. The rotor’s bars see the field sweeping past at the slip speed, so the current induced in them runs at slip × supply frequency: only a hertz or two at full load, and the full 60 Hz at the instant of starting, which is why starting current is so high.

    • Ns = 120 × f ÷ Psynchronous speed, rpm
    • slip = (Ns − N) ÷ Ns× 100 for percent
    • f rotor = slip × frotor current frequency

    Worked: a 1750 rpm motor on 60 Hz

    1. The synchronous speed just above 1750 on 60 Hz is 1800 rpm: 120 × 60 ÷ P = 1800 gives P = 4.
    2. Slip = 1800 − 1750 = 50 rpm, or 50 ÷ 1800 = 2.78%.
    3. Rotor frequency = 0.0278 × 60 = 1.67 Hz.

    Questions people ask

    How do I find the number of poles from the nameplate?

    Take the nameplate speed and find the synchronous speed just above it. On 60 Hz the synchronous speeds are 3600 (2-pole), 1800 (4-pole), 1200 (6-pole) and 900 (8-pole); on 50 Hz they are 3000, 1500, 1000 and 750. A 1750 rpm motor on 60 Hz sits just under 1800, so it is a 4-pole motor. The tool does this for you when poles is set to auto.

    Why does an induction motor never reach synchronous speed?

    The rotor only makes torque when the rotating field is sweeping past its bars and inducing current in them. At synchronous speed the rotor would be turning with the field, nothing would be cut, and there would be no current and no torque — so it always runs a little slower. That difference is slip, and it grows with load.

    What is a normal slip?

    For general-purpose motors at full load it is commonly a few percent — roughly 1 to 5% — with larger motors at the low end. High-slip designs exist for loads like punch presses. Your own motor’s full-load slip is on its nameplate: synchronous speed minus the nameplate speed.

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