Motor Pole Count from Nameplate RPM
For a tech who only reads a speed off the plate.
Example
You enter
- Nameplate full-load speed (rpm) 1750
- Line frequency (Hz) 60
You get
- Poles 4
- Sync (RPM) 1800
- Slip (%) 2.78
Checked against published worked examples from: WEG Electric Corp.
Details, formula, and sources
ScopeFrom the nameplate full-load rpm and line frequency, the pole count is the even number at or below 120 x f / rpm - pole-pairs = floor(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 or tablepole-pairs = floor(60 x f / rpm) (a speed up to 5% above a synchronous speed keeps that pole count and is flagged); poles = 2 x pole-pairs; Ns = 120 x f / poles; slip = (Ns - rpm) / Ns.
Source and editionFirst-principles AC-machine speed relation Ns = 120 x f / P inverted to identify the poles; classical induction-machine theory.
Free accessFirst-principles physics; no licensed source required.
What governsEstimate. AHJ and licensed professional govern.
API field namesrated_rpm, line_freq_hz, poles, sync_rpm, slip_pct
- Poles at or below the ratio poles = 2 x floor(60 x f / rpm); the synchronous speed sits just above the running speedAC-machine theory
- Induction machine the nameplate speed is below synchronous; a zero/negative slip flags a bad rpm or frequencyAC-machine construction