VFD Frequency & Speed Calculator
Enter motor data and the target frequency to get shaft rpm, V/Hz voltage and the torque you can hold continuously.
Step-by-step
- Enter valid values to begin.
Speed uses the constant-slip-fraction form N = 120 f (1 − S) / P. Under a constant-torque load the slip in rpm stays nearly constant instead, and that estimate is shown as well. The low-speed torque limit is a typical estimate for a self-cooled (TEFC) standard-efficiency motor; inverter-duty and forced-ventilation motors do better, so use the manufacturer’s curve for a final decision.
VFD Frequency and Speed: Volts per Hertz and the Low-Speed Trap
Core Engineering Principles
A drive changes motor speed by changing the frequency it feeds the stator, and it changes the voltage along with it. The reason is flux. Motor flux is roughly voltage divided by frequency, so if you halve the frequency and leave the voltage alone, the iron saturates, magnetising current balloons and the motor overheats. Hold V/f constant and the flux, and therefore the torque capability, stays the same. At 460 V and 60 Hz that’s 7.67 V per hertz; at 30 Hz the drive delivers about 230 V.
Above base frequency the drive runs out of voltage and the ratio falls. Flux weakens, torque drops as 1/speed, and you are in constant-power operation. Below base the trouble is cooling. A TEFC motor’s shaft-mounted fan turns slower as the speed falls, so a motor loaded to full torque at 20 Hz gets far less airflow than it was designed for. Fans and pumps are forgiving, because their load torque falls as speed squared. Conveyors, extruders and hoists are not.
Above base: torque ∝ fbase / f (constant power)
NEC & Standard References
NEMA MG-1 Part 30 covers motors on non-sinusoidal supplies and Part 31 defines definite-purpose inverter-fed motors with the insulation and winding requirements. IEC 60034-25 is the matching guide for converter-fed machines, and IEC TS 60034-17 gives the cage motor guidance. NEC Part X (430.120 onward) covers adjustable-speed drive systems, and IEEE 519 sets the harmonic limits the drive’s input side has to meet. Ask the manufacturer for the real speed-torque curve, because the derating shown here is a typical self-cooled estimate.1. Rated slip: Ns = 1800 rpm, S = 50 / 1800 = 2.78%.
2. N = 120 × 30 × (1 − 0.0278) / 4 = 875 rpm, which is 50% speed.
3. Voltage at 30 Hz = 460 × 30 / 60 = 230 V.
4. Real torque holds slip rpm roughly constant, so the true speed is closer to 900 − 50 = 850 rpm.
5. A self-cooled motor can only give about 92% of rated torque continuously here. A 100% load needs a blower or a bigger frame.
- Low speed, full torque, shaft fan: that’s the classic burn-up. Put an independent blower on the motor, or oversize it, whenever a constant-torque load runs below about 50% speed for long periods.
- Don’t run a standard motor past base frequency for long. Bearings, balance and rotor stress all get worse with speed; check the mechanical limit.
- Voltage boost matters at very low frequency. At a few hertz the stator resistance drop dominates, so set boost or use vector control.
- Never assume the display RPM is shaft RPM. The drive shows commanded speed; slip makes the real shaft a little lower.