Electrical Motor Efficiency

Calculating electric motor efficiency


Electrical motor efficiency is the ratio between the shaft output power - and the electrical input power.
Electrical Motor Efficiency when Shaft Output is measured in Watt


If power output is measured in Watt (W), efficiency can be expressed as:
ηm = Pout / Pin? (1)
where
ηm = motor efficiency
Pout = shaft power out (Watt, W)
Pin = electric power in to the motor (Watt, W)

Electrical Motor Efficiency when Shaft Output is measured in Horsepower
If power output is measured in horsepower (hp), efficiency can be expressed as:
ηm = Pout 746 / Pin (2)
where
Pout = shaft power out (horsepower, hp)
Pin = electric power in to the motor (Watt, W)
rimary and Secondary Resistance Losses


The electrical power lost in the primary rotor and secondary stator winding resistance are also called copper losses. The copper loss varies with the load in proportion to the current squared - and can be expressed as


Pcl = R I2 ? (3)
where
Pcl = stator winding - copper loss (W)
R = resistance (Ω)
I = current (Amp)
Iron Losses


These losses are the result of magnetic energy dissipated when when the motors magnetic field is applied to the stator core.
Stray Losses


Stray losses are the losses that remains after primary copper and secondary losses, iron losses and mechanical losses. The largest contribution to the stray losses is harmonic energies generated when the motor operates under load. These energies are dissipated as currents in the copper windings, harmonic flux components in the iron parts, leakage in the laminate core.
Mechanical Losses
Mechanical losses includes friction in the motor bearings and the fan for air cooling.
NEMA Design B Electrical Motors


Electrical motors constructed according NEMA Design B must meet the efficiencies below:
Power
(hp) Minimum Nominal Efficiency1)
1 - 4 78.8
5 - 9 84.0
10 - 19 85.5
20 - 49 88.5
50 - 99 90.2
100 - 124 91.7
> 125 92.4