Motor speed and poles calculator
Read a worn nameplate, or work out how fast a motor will turn at a different drive frequency. Enter the nameplate rpm to find the pole count, or choose poles and frequency to find the speed.
50 Hz on the South African grid. Use the drive output frequency when running from a drive.
Number of poles
4poles
Synchronous speed 1500 rpm at 50 Hz.
Slip at this speed
3.3%
Typical full-load slip is a few per cent, so this is consistent with a 4-pole motor.
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How it works
Synchronous speed in rpm is 120 × frequency ÷ poles. On the 50 Hz South African supply that gives 3000 rpm for two poles, 1500 for four, 1000 for six and 750 for eight.
A loaded induction motor runs a few per cent below synchronous speed. A four-pole 50 Hz motor typically shows 1400 to 1485 rpm on its nameplate: lower for small motors, closer to 1500 for large ones.
On a variable frequency drive the speed scales with frequency. Running above the base frequency reduces torque, and the motor and its load must be suitable for the higher speed.
Common questions
My nameplate says 1450 rpm. How many poles is that?
Four. The next synchronous speed above 1450 rpm at 50 Hz is 1500 rpm, which is a four-pole motor.
Can I run a motor above 50 Hz from a drive?
Often yes within limits, but available torque falls above base frequency and bearings, balance and the driven machine limit the safe speed. Check with the motor manufacturer first.
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