How fast does an AC motor turn?
An AC induction motor’s speed is set mainly by the supply frequency and the number of magnetic poles in its stator winding. The rotating magnetic field turns at the synchronous speed:
Ns (rpm) = 120 × f ÷ p
where f is the frequency in hertz and p the number of poles.
| Poles | 60 Hz | 50 Hz |
|---|---|---|
| 2 | 3,600 rpm | 3,000 rpm |
| 4 | 1,800 rpm | 1,500 rpm |
| 6 | 1,200 rpm | 1,000 rpm |
| 8 | 900 rpm | 750 rpm |
Slip
The rotor of an induction motor always turns a little slower than the field; that difference, the slip, is what induces current in the rotor and produces torque:
Slip (%) = (Ns − N) ÷ Ns × 100
Typical full-load slip is 1 to 6%: lower for large, efficient motors and higher for small ones. The rotor current alternates at the slip frequency, s × f.
Example
A 4-pole, 5 hp motor on 60 Hz with a nameplate speed of 1,750 rpm:
- synchronous speed: 120 × 60 ÷ 4 = 1,800 rpm;
- slip: 50 rpm, or 2.8%;
- rotor frequency: 1.67 Hz;
- rated torque: 5 × 5,252 ÷ 1,750 ≈ 15 lb·ft.
Torque from power
- In US units: torque (lb·ft) = hp × 5,252 ÷ rpm.
- In SI units: torque (N·m) = kW × 9,549 ÷ rpm.
At the same power, a slower motor delivers more torque, which is why low-speed motors are larger and heavier.
Speed control with a VFD
A variable frequency drive (VFD) changes the supply frequency to change the speed. Below base frequency, the slip in rpm stays roughly constant, so the speed is about 120 × f_VFD ÷ p minus the rated slip. At 45 Hz, the example motor turns at about 1,300 rpm.
Points to watch:
- Below about 30 to 50% of base speed, the motor’s own fan cools poorly; long low-speed operation may need a separate blower.
- Above base frequency, available torque falls as the drive cannot raise the voltage further.
- Centrifugal pumps and fans save a lot of energy on a VFD: their power falls roughly with the cube of speed.
Synchronous and DC motors
Synchronous motors, including many permanent-magnet motors, turn at exactly Ns with no slip. Universal and DC motors do not follow the formula at all; their speed depends on voltage and load.