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25 — Calculators

Calculate the current of a single-phase motor

On a single-phase supply the entire current flows through one line conductor. At the same power it is therefore a multiple of what the same motor draws on a three-phase supply — exactly what gets overlooked when a machine is to be plugged into a socket "just for now".

Input

Single-phase motors are markedly below comparable three-phase motors.

Empirical value in µF per kW at 230 V and 50 Hz, for running a three-phase motor on a single-phase supply. The manufacturer figure takes precedence.

Result

Current drawn
—A
Active power drawn
—kW
Apparent power
—kVA
Run capacitor, rough estimateAn indication only — the capacitor belongs to the motor, not to a formula.
—µF
Current on a 400 V three-phase supplyFor comparison: the same motor on a three-phase supply.
—A

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

Shaft power delivered
1.5 kW
System voltage
230 V
Efficiency
75 %
Power factor cos φ
0.9
Frequency
50 Hz
Coefficient for the run capacitor
70 µF/kW

Result

Current drawn
9.66 A
Active power drawn
2 kW
Apparent power
2.22 kVA
Run capacitor, rough estimate
105 µF
Current on a 400 V three-phase supply
3.21 A

Formula

  • P_in = P_out / η
  • I = P_in / (U · cos φ)
  • S = U · I
  • Three-phase comparison: I = P_in / (√3 · U · cos φ)

Assumptions and standards

  • The capacitor value is an empirical figure for running a three-phase motor on a single-phase supply and is expressly an indication only. The motor manufacturer figure governs.
  • A three-phase motor run on a single-phase supply delivers markedly less power than on a three-phase supply and runs less smoothly.
  • Starting capacitor and run capacitor are different components with different values. Only the run capacitor is given here.

Frequently asked

Why is the current so much higher than on three phases?

Because the whole power flows through one line conductor instead of three, and the voltage is 230 instead of 400 volts. Together that gives roughly three times the current. A 1.5 kW machine drawing a good three amperes on three phases reaches nine on a single phase.

Can I run any three-phase motor on a single-phase supply?

Technically often yes, sensibly rarely. The power delivered falls markedly, the starting torque too, and the running becomes rougher. At small ratings it is a workable stopgap; beyond a few kilowatts, install the three-phase supply.

Why is the capacitor value only an indication?

Because it depends on the winding, the speed and the load case and cannot be cleanly derived from the power. Too large a capacitor causes over-excitation and heating, too small a one a weak start. Use the manufacturer figure where it exists.

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

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