Synchronous Speed from Frequency and Poles
Also known as motor RPM from frequency · 1800 RPM motor
Worked example: 60 Hz, 4 poles → 1800 rpm — press Try an example to run it live, then adjust anything.
Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Try different units for next level excitement!
Synchronous speed and slip →
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Synchronous Speed from Frequency and Poles explained
A pair of poles takes one full electrical cycle to sweep the field once around, so the field turns f/(p/2) = 2f/p revolutions per second. Multiply by 60 and you get the version every electrician knows: Ns = 120f/p in rpm. At 60 Hz that gives 3600, 1800, 1200 and 900 rpm for 2, 4, 6 and 8 poles; at 50 Hz, 3000, 1500, 1000 and 750. Enter f in hertz and read Ns in rpm and the two forms agree exactly.
Pole count is fixed by the winding, so before variable-frequency drives the only way to change a motor's speed was to change machines or use pole-changing windings. A VFD attacks the f instead, which is why a 4-pole motor on a 30 Hz drive turns near 900 rpm. Two traps: p counts poles, not pole pairs, and the actual shaft speed is always a percent or two below Ns because of slip — a true synchronous machine is a different animal with a DC-excited or permanent-magnet rotor.
Synchronous Speed from Frequency and Poles formula
- = Synchronous speed (rpm)
- = Supply frequency (Hz)
- = Number of poles
Missing one of these? Work it out first, then come back
- Synchronous speed — Induction Motor Slip, Motor Torque from Power and Speed
- Supply frequency — Power-Factor Correction Capacitance, 555 Astable Frequency
- Number of poles — Conduit Fill, Number of Elementary Charges (N = Q/e)