Motor RPM Calculator — Synchronous & Actual Speed

Motor Speed Fundamentals

The speed of an AC induction motor depends on:

1. Frequency of the power supply (50 Hz in India, 60 Hz in USA)
2. Number of poles in the motor winding
3. Slip — the difference between synchronous and actual speed

Synchronous Speed (RPM) = 120 × Frequency / Number of Poles
Actual Speed (RPM) = Synchronous Speed × (1 − Slip/100)

Standard Motor Speeds in India (50 Hz)

Slip

Slip is the difference between synchronous and actual speed expressed as a percentage. For standard induction motors:

• At full load: 3–5% typical
• At no load: <1%
• At overload: 5–8%

High slip indicates high current draw, poor efficiency, and potential motor overloading.

Variable Frequency Drive (VFD)

A VFD changes the motor speed by varying the supply frequency. With a VFD:

Motor Speed = (120 × VFD Output Frequency / Poles) × (1 − Slip/100)

VFDs are common on dairy plant pumps, agitators, and fans for energy savings and process control.

Common Applications by Motor Speed