The efficiency of an electrical motor is electric power input - mechanical power output rating.

NEMA has established an efficiency index:

NEMA Letter Index Nominal Efficiency Minimum Efficiency

A   95.0

B 95.0 94.1

C 94.1 93.0

D 93.0 91.7

E 91.7 90.2

F 90.2 88.5

G 88.5 86.5

H 86.5 84.0

K 84.0 81.5

L 81.5 78.5

M 78.5 75.5

N 75.5 72.0

P 72.0 68.0

R 68.0 64.0

S 64.0 59.5

T 59.5 55.0

U 55.0 50.5

V 50.5 46.0

W   46.0

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.

NEMA Design B Electrical Motors

Electrical motors constructed according NEMA Design B must meet the efficiencies below:

Power(hp)

Minimum Nominal Efficiency1)

1 - 4 78.85 - 9 84.010 - 19 85.520 - 49 88.550 - 99 90.2100 - 124 91.7> 125 92.4

1) NEMA Design B, Single Speed 1200, 1800, 3600 RPM. Open Drip Proof (ODP) or Totally Enclosed Fan Cooled (TEFC) motors 1 hp and larger that operate more than 500 hours per yea