Electric Motors - Efficiency
Electrical motor efficiency is the ratio between shaft output power - and electrical input power.
Electrical Motor Efficiency when Shaft Output is measured in Watt
If power output is measured in Watt (W) then 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, watts)
R = resistance (Ω)
I = current (A, amps)
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 winding, 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 |
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 year.
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