Motor Synchronous Speed & Slip Calculator

How it works

n_sync = 120 × f ÷ poles; slip s = (n_sync − n) ÷ n_sync; rotor frequency = s × f; torque (lb·ft) = hp × 5,252 ÷ rpm; VFD speed ≈ 120 × f_out ÷ poles × (1 − s)

The rotating magnetic field in an AC motor’s stator turns at the synchronous speed, fixed by line frequency and the number of magnetic poles — 3,600, 1,800, 1,200 and 900 rpm for 2, 4, 6 and 8 poles at 60 Hz (3,000/1,500/1,000/750 at 50 Hz). An induction rotor must turn a little slower so the field can induce current in it; that difference, slip, is 1–3% at full load for standard motors and shrinks toward zero when unloaded, which is why a “1,750 rpm” nameplate belongs to a 1,800 rpm four-pole machine. Slip also tells you the rotor’s electrical frequency and lets you estimate speed at other loads. A variable-frequency drive changes the synchronous speed itself, so the motor tracks any frequency with roughly the same slip, keeping torque constant up to base frequency.

Worked example

A 20 hp four-pole motor on 60 Hz has a synchronous speed of 1,800 rpm; a 1,750 rpm nameplate means 2.78% slip and a rotor frequency of 1.67 Hz. Full-load torque is 60 lb·ft (81 N·m); on a drive at 45 Hz it would run about 1,313 rpm.

Frequently asked questions