Motor Synchronous Speed, Slip, and Rotor Frequency
The synchronous speed Ns = 120 x frequency / poles, the slip relative to the nameplate full-load speed, and the rotor (slip) frequency, for commissioning a motor, reading a VFD parameter set, or diagnosing a wrong-speed complaint. The nameplate full-load speed governs.
Formula and source
Ns = 120 x f / P (the 120 carries the 60 s/min x 2 poles-per-pole-pair bridge); slip = (Ns - rated_rpm) / Ns; slip_pct = slip x 100; rotor (slip) frequency = slip x f.
First-principles AC-machine speed relation; classical induction-machine theory.
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