Motor Pole Count from Nameplate RPM
The inverse of the synchronous-speed tile, for a tech who only reads a speed off the plate: from the nameplate full-load rpm and line frequency, the pole count is the nearest even integer to 120 x f / rpm - pole-pairs = round(60 x f / rpm), poles = 2 x pole-pairs, then Ns = 120 x f / poles and the slip. A 1750 rpm 60 Hz motor is 4-pole (Ns 1800, 2.78% slip); 1150 rpm is 6-pole, 3450 rpm is 2-pole. A speed at/above the identified Ns (zero or negative slip) never appears on an induction nameplate - recheck the rpm. The nameplate governs.
Formula and source
pole-pairs = round(60 x f / rpm); poles = 2 x pole-pairs; Ns = 120 x f / poles; slip = (Ns - rpm) / Ns.
First-principles AC-machine speed relation Ns = 120 x f / P inverted to identify the poles; classical induction-machine theory.
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