sparkytools

Fourth year Drives · tool

VFD frequency and speed calculator

Enter the motor’s nameplate base frequency, speed and voltage. Then give a drive frequency to get the shaft speed, or a target speed to get the frequency to set — with the V/Hz and the voltage the drive will put out.

Work out…
  • Estimated shaft speed—
  • Frequency to set—
  • Synchronous speed at that frequency—
  • Poles / nameplate slip—
  • Volts per hertz—
  • Approximate output voltage—

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    How it works

    • Ns = 120 × f ÷ Psynchronous speed at the drive frequency
    • N ≈ Ns − nameplate slipconstant-torque estimate
    • f = (N + slip) × P ÷ 120frequency for a target speed
    • V/Hz = V rated ÷ f baseV out ≈ V/Hz × f below base

    Worked: 60 Hz, 1750 rpm, 460 V motor run at 45 Hz

    1. 1750 rpm on 60 Hz is a 4-pole motor (1800 rpm synchronous); nameplate slip = 50 rpm.
    2. At 45 Hz: Ns = 120 × 45 ÷ 4 = 1350 rpm; shaft ≈ 1350 − 50 = 1300 rpm.
    3. V/Hz = 460 ÷ 60 = 7.67; output ≈ 7.67 × 45 = 345 V.

    Questions people ask

    Why does the speed drop a little more than the frequency suggests?

    An induction motor always runs below synchronous speed by its slip, and at a given load torque the slip in rpm stays roughly the same as the frequency changes. So at 30 Hz a 1750 rpm motor runs about 900 − 50 = 850 rpm, not half of 1750. This tool uses the nameplate slip as that estimate; a drive in sensorless-vector mode compensates for much of it.

    What is volts per hertz?

    The ratio of rated voltage to rated frequency — 460 ÷ 60 ≈ 7.67 V/Hz for a 460 V, 60 Hz motor. Keeping that ratio constant keeps the magnetic flux in the motor constant, so it can make full torque at lower speeds without saturating. A drive in V/Hz mode lowers its output voltage in step with frequency below base speed.

    What happens above base frequency?

    The drive cannot give more than its supply voltage, so above base frequency the voltage stays at rated while the frequency rises. Volts per hertz falls, the flux weakens, and the available torque drops roughly in proportion — the motor is in its constant-horsepower region. Whether the motor and the load are safe at that speed is a question for the motor and machine makers.

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