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Motor Shaft Power, Efficiency and Line Current

Estimate AC motor line current from mechanical shaft output, efficiency, power factor and RMS voltage, or solve a supported missing quantity.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A balanced three-phase motor delivering 0.255 kW at 85% efficiency, power factor 0.5 and 100 V line-to-neutral draws 2 A in each line.

Pshaft = η PF VI (single-phase); Pshaft = η PF √3 VLL I = η PF 3 VLN I (balanced three-phase), with η as a fraction.

Worked example

Enter these known values and leave the other values blank.

Three-phase voltage convention
Line to Neutral Voltage
Mechanical shaft output P
0.255 kW
RMS motor voltage V
100 V
Power factor PF
0.5
Motor efficiency η
85 %
RMS line current I
2 A

Assumptions and limitations

  • Power is mechanical shaft output, not measured electrical input. Motor efficiency converts real electrical input to shaft output; divide shaft power by efficiency before finding electrical current. The efficiency control uses percent.
  • Voltage and current are RMS magnitudes. Three-phase supply is balanced and sinusoidal; choose line-to-line or line-to-neutral voltage, and use line current rather than winding current.
  • The retained finite loaded-motor model requires positive power, voltage and current, 0 < PF ≤ 1, and 0% < efficiency < 100%. The defaults PF=1 and efficiency=85% are examples, not measured properties of your motor.
  • All five numeric quantities have supported algebraic inverses. Keep phase/voltage choices selected, unlock a locked numeric default where needed, and clear the quantity to solve.
  • Mechanical horsepower uses exactly 550 ft·lbf/s. Metric horsepower uses 75 kgf·m/s. Electrical horsepower is the defined 746 W convention. These selectors are distinct.
  • Starting/inrush current, overloads, drive input, voltage unbalance, harmonics, conductor/protection sizing and manufacturer ratings require separate data. The estimate does not replace nameplate measurements.

Common questions

Which voltage should I enter?

Select its convention first. Use RMS line-to-line voltage for that choice or RMS line-to-neutral voltage for the other. In balanced three-phase operation the two voltages differ by √3.

Is the current total across all three phases?

No. The result is RMS current in each line for the balanced model. Total three-phase power already includes the three phases.

Can I solve a different missing quantity?

Yes. Keep the categorical phase and voltage choices selected, clear the missing numeric value and enter enough independent known values. Unlock a numeric default before solving for it.

References

The line-to-line balanced three-phase factor uses √3 rounded once to 64 significant decimal digits instead of the captured 12-digit approximation. Mode choices remain explicit; positive apparent or shaft power requires positive voltage and line current. Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor.

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