Motor kW, current, efficiency and power factor: keep the quantities straight

Motor nameplates and energy bills describe related but different quantities. Shaft kW is mechanical output. Electrical input kW is real power entering the motor. Current also depends on supply voltage and power factor. Mixing these boundaries can understate an energy budget or misinterpret a measured current.

FIG. 05Power at the shaft

Work through one operating point

For an illustrative 15 kW shaft output at 90% efficiency, real electrical input is 15 / 0.90 = 16.6667 kW. At a balanced 400 V line-to-line supply and power factor 0.85, the sinusoidal three-phase estimate is:

I = shaft_kW × 1000 / (sqrt(3) × line_voltage × power_factor × efficiency)

The result is approximately 28.30 A. Use the motor running-current calculator to change the operating point. For single-phase AC, the analogous real-power equation omits √3.

Efficiency and power factor do different jobs

Efficiency converts electrical real input to mechanical output after losses. Power factor relates real input to apparent power. Improving power factor can reduce current for the same real power; it does not automatically reduce the shaft's required energy or the motor's internal losses by the same proportion.

Efficiency and power factor vary with operating point. Use values applicable to the actual load rather than combining a full-load efficiency with an unrelated partial-load power factor. A current measurement alone is a poor precision estimate of shaft load, especially at light load.

Convert input power into an energy budget

At an average measured electrical input of 12 kW for 16 hours/day and 250 days/year, energy is 48,000 kWh/year. At 0.15 currency/kWh, the energy charge is 7,200/year. The energy cost calculator reproduces this by entering 12 kW and a 100% average power ratio, or 15 kW electrical full-load input and an 80% power ratio.

That ratio describes electrical power. It is not a direct mechanical load percentage. For variable schedules, measure or sum representative operating modes and inspect demand charges separately.

Know what the equation leaves out

Starting current, acceleration, supply imbalance, harmonics and VFD input behavior are outside this simple model. Do not select cable size, protective devices, overload settings or a drive from the web estimate. Use the applicable manufacturer data and electrical design requirements.

When an energy project looks promising, check the driven process too. A more efficient motor still wastes energy if it drives an unnecessary load. Connect the power evidence to plant energy management and document realizable benefits in the ROI guide.

Source

U.S. Department of Energy: determining electric motor load and efficiency provides background on motor input, load and efficiency. The operating-point examples here are illustrative calculations under the stated AC assumptions.