Efficiency of Motors

The efficiency of a motor is determined by intrinsic losses that can be reduced only by changes in motor design and operating condition. Losses can vary from approximately 2 to 20 %.

The below table shows the types of losses and their typical shares for an induction motor.

Type of losses and shares for induction motors

The efficiency of a motor can be defined as “the ratio of a motor’s useful power output to its total power output”.

Factors that influence motor efficiency include

• Age - New motors are more efficient

• Capacity - As with most equipment, motor efficiency increases with the rated capacity

• Speed - Higher speed motors are usually more efficient

• Type - For example, squirrel cage motors are normally more efficient than slip-ring motors

• Temperature - Totally-enclosed fan-cooled (TEFC) motors are more efficient than screen protected drip-proof (SPDP) motors

• Rewinding of motors can result in reduced efficiency

• Load, as described below

There is a clear link between the motor’s efficiency and the load. Manufacturers design motors to operate at a 50–100% load and to be most efficient at a 75% load.

But, once the load drops below 50% the efficiency decreases rapidly as shown in below Figure overleaf.

Operating motors below 50% of rated loads has a similar, but less significant, impact on the power factor. High motor efficiencies and power factor close to 1 are desirable for efficient operation and for reducing costs down of the entire plant and not just the motor.

Motor Load

Because the efficiency of a motor is difficult to assess under normal operating conditions, the motor load can be measured as an indicator of the motor’s efficiency.

As loading increases, the power factor and the motor efficiency increase to an optimum value at around full load.

It is necessary to see the percentage loading of the motor. If the motor runs at more than 70% load, then the power factor and efficiency will be good.