How the motor RPM formula works
The synchronous speed of an AC induction or synchronous motor depends only on supply frequency and pole count:
RPM = 120 × f ÷ p
where f is the line frequency in hertz and p is the number of stator poles (always an even number: 2, 4, 6, 8…). The 120 folds together two factors of 60 — seconds per minute and the two electrical half-cycles per pole pair. A 4-pole motor on 60 Hz turns at 120 × 60 ÷ 4 = 1800 RPM synchronously; the same motor on 50 Hz runs 1500 RPM.
An induction rotor always slips below synchronous speed — typically 2–5% at full load, so that 1800 RPM motor may nameplate at 1725 or 1750 RPM. With a variable-frequency drive, speed scales linearly with commanded frequency, which is exactly how VFDs control motors.
Angular units
Control engineering and physics prefer radians: ω (rad/s) = RPM × 2π ÷ 60. A handy identity: 1 RPM = 6 degrees per second exactly, so the deg/s figure is just RPM × 6. The calculator shows all three units at once.
Worked example
A 6-pole motor on a 50 Hz supply: RPM = 120 × 50 ÷ 6 = 1000 RPM synchronous, ω ≈ 104.72 rad/s. Expect roughly 950–980 RPM at the shaft under load, depending on slip.