Answer / venu

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)

Primary 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.

Answer / venu

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
as per NEMA

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