VFD Frequency and Speed Calculator
Find the motor speed at any VFD output frequency, or the frequency needed for a target speed, based on the motor nameplate.
- Poles = 4, rated slip = 40 rpm
- N = 120 × f ÷ P − slip = 120 × 40 ÷ 4 − 40 = 1,160 rpm
VFD Speed Formula
f = (N + slip) × P ÷ 120
P is the number of poles (found from the nameplate) and slip is the rated slip in rpm, assumed constant at constant torque. This gives a good estimate for V/f and sensorless vector drives.
Worked example
A 1460 rpm, 50 Hz motor (4 poles, 40 rpm slip) on 40 Hz runs at 120 × 40 ÷ 4 − 40 = 1160 rpm.
Frequently Asked Questions
How do I convert Hz to rpm for a motor?
Synchronous rpm = 120 × Hz ÷ poles. Subtract the slip for an induction motor.
Can a motor run above its nameplate frequency on a VFD?
Yes, usually up to about 1.5 to 2 times, but above base frequency the voltage is limited so available torque falls (field weakening).
Why is the speed slightly below the calculated synchronous speed?
Induction motors need slip to produce torque. Slip in rpm stays roughly constant for a given load torque.
Learn more on the blog
VFD working principle explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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