Motor Slip Calculator

Method, example and boundaries

Field speed is n_s = 120 × f / p. Per-unit slip is s = (n_s − n_r) / n_s, and slip frequency is s × f. At 60 Hz and 4 poles, n_s = 1,800 rpm. A rotor at 1,765 rpm has s = 35 / 1,800 = 0.019444…, shown as 0.02, or 1.94%. At 50 Hz and 4 poles, a rotor at 1,440 rpm has s = 60 / 1,500 = 0.04, shown as 0.04 and 4.00%.

The English page starts at the NEC 60 Hz preset. IEC and GB presets only change the frequency field. Zero slip is the synchronous boundary. Rotor speed above field speed is reported as negative slip and is not approval to run as a generator. Rotor speed above twice synchronous speed is rejected. This is not an overload, broken-bar, or installation decision.

Primary source: NEMA, Motor Terminology and Electrical Performance Characteristics (SRC-062). That PDF is a training handout, not the text of NEMA MG 1. It states synchronous speed = 120 × frequency / poles and gives the %Slip formula for the rotor lag behind that speed. It does not print an MG 1 clause number for the formula; an MG 1 clause number remains UNVERIFIED. Secondary source only: Engineering Toolbox, Electrical Induction Motors — Slip (SRC-063), which writes s = (n_s − n_a) × 100% / n_s. Engineering Toolbox is a copyrighted website, not a standard, and its license for reuse is review-required. The NEMA handout is also review-required and is not copied here.