Generator Frequency Calculator (Poles & RPM, f = PN/120)

How to use the generator frequency calculator

Enter the number of poles and the rotor speed in rpm to get the output frequency. To find the speed for a target frequency, enter the frequency and clear the speed field; to find the pole count for a turbine speed, clear the poles field.

Generator frequency formula

f = P × N ÷ 120  ·  N = 120 f ÷ P

Example

A 4-pole diesel generator set running at 1,500 rpm:

• f = 4 × 1500 ÷ 120 = 50 Hz
• At 1,800 rpm the same machine gives 4 × 1800 ÷ 120 = 60 Hz

Synchronous speed by number of poles

Notes

• The governor holds speed — and therefore frequency — constant as load changes; a 2% droop governor lets a 50 Hz set fall to 49 Hz at full load.
• Induction motors run slightly slower than synchronous speed; see the induction motor slip calculator.
• Before paralleling with the grid, a generator must match its frequency, voltage and phase angle.

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