Electric Motors  Efficiency
Calculate electric motor 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
η_{m} = P_{out} / P_{in} (1)
where
η_{m} = motor efficiency
P_{out} = shaft power out (Watt, W)
P_{in} = 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
η_{m} = P_{out} 746 / P_{in} (2)
where
P_{out} = shaft power out (horsepower, hp)
P_{in} = 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
P_{cl} = R I^{2} (3)
where
P_{cl} = 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 Efficiency^{1)} 

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.
Related Topics

Electrical
Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors.
Related Documents

Asynchronous Induction Motors  Electrical Properties
Typical electrical motor data like nominal current, fuse, start ampere, size of contactor and circuit breaker  for asynchronous induction motors. 
Efficiency
The measure of usefulness. 
Efficiency
Efficiency is the ratio useful energy output to energy input. 
Efficiency of Small Machine Elements
Friction and efficiency in bearings and roller chains. 
Electric Motor Calculator
Calculate amps, hp and kVA for electrical motors. 
Electric Motors  480 Volt Wiring
480V electrical motor wiring data  NEMA amps, starter size, HMCP size for motors ranging 1/2 to 500 hp. 
Electric Motors  Efficiency
Calculate electric motor efficiency. 
Electric Motors  Efficiency
Calculate electric motor efficiency. 
Electrical Induction Motors  Synchronous Speed
Operating speed of an induction motor depends on the input power frequency and the number of magnetic poles in the motor. 
Electrical Motors  Frame Dimensions
Electrical motors NEMA frame dimensions. 
Electrical Motors  Heat Loss
Heat loss from an electrical motor to the surroundings. 
Electrical Motors  Horsepower vs. Voltage and Amps
Electrical motors horsepower rating vs. their ampere rating. 
Electrical Motors  Insulation Classes
Electrical motors NEMA temperature and insulation classes. 
Electrical Motors  Locked Rotor Design Code Letters
NEMA locked rotor indicating code letters for electrical motors. 
Electrical Motors  Service Factors
Service factor  SF  is a measure of periodically overload capacity at which a motor can operate without beeing damaged. 
Electrical Motors  Speed at Operating vs. Synchronous Load
Speed of an operating electrical motor with load is lower than the synchronous speed (no load) of the motor. 
Electrical Motors  Speed vs. No. of Poles and Frequency
The speed of electrical motors with 2, 4, 6 or 8 poles at 50 Hz and 60 Hz. 
Electrical Motors  Starting Devices
Directonline starters, stardelta starters, frequency drives and soft starters. 
Heat Gain from Electrical Motors in Continuous Operation
Amount of heat transferred from electrical motor to ambient room vs. locations of fan and motor. 
NEMA  National Electrical Manufacturers Association
National Electrical Manufacturers Association. 
NEMA A, B, C and D Electrical Motor Design
NEMA has established the four different designs A, B, C and D for electrical induction motors. 
Polyphase Motors  Voltage Imbalance vs. Derating Factor
Increased voltage imbalance and decreased efficiency. 
Single Phase Power Equations
Power equations for single phased electrical systems. 
ThreePhase Electrical Motors  Power vs. Amps and Voltage
Full load amps, wire and conduit sizes for three phase electrical motors. 
ThreePhase Power  Equations
Electrical 3phase equations.