Electric motors efficiency

Electric motor principle is convert the energy from electrical to mechanical process.The mechanical process is a load such as pump,blower,grinding and etc.Energy will lost from this process.

We can determine the efficiency of a electric motor by intrinsic losses.To reduced it we only can changes in motor design and operating condition. From lab test,a losses can vary from 2% to 20 %.

The simple definition of efficiency motor is “the ratio of a motor’s useful power output to its total power output.”


Electric motor efficiency factor

I list a several factors that influence motor efficiency include:

  • Age / Running hour : New motors are more efficient than old motor.
  • Type of loads : pump,compressor,blower and etc
  • Capacity : As with most equipment, motor efficiency increases with the rated capacity
  • Temperature / Ambient : Totally-enclosed fan-cooled (TEFC) motors are more efficient than screen protected drip-proof (SPDP) motors
  • Rewinding of motors can result in reduced efficiency
  • Speed : Higher speed motors are usually more efficient
  • Type of motor : Squirrel cage motors are normally more efficient than slip-ring motors


The electric motor’s efficiency and the load is always affected. Motor designer normally design it to operate at a 50-100% load and to be most efficient at a 75% load.If the load drops below 50%,the motor efficiency will decreases.

Normally motors running in 50% of rated loads, but if running less,it can impact on the power factor value.In the market now,we can selected the efficiencies motor  and improved power factor close to 1 for an efficient operation and for costs down of the electrical usage.

When we design a system,we must to determine the load and efficiency of motor’s performance. Normally some country required manufacture to display the full-load efficiency on the motor’s nameplate.

How we can measure the motor’s efficiency?First we must disconnected motor from the load.Setting the motor to the test bench for a series of tests.From the test,we can get the result and compare with the standard motor performance curves.This standard usually provided by the motor manufacturer.


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