Induction motors are a prime cause of low power factor for many customers. Poor power factor is an issue especially for customers with large numbers of small fractional horsepower motors, those who purchase cheap or poorly made motors, and those having oversized, under-loaded motors.
Most small fractional horsepower motors have low power factors in the 50 to 60 percent range. As the rated horsepower of the motor increases, in general, the power factor of the motor at full load increases. Therefore, larger horsepower motors have better power factors than small horsepower motors if each motor is properly loaded.
Power factor may vary significantly between two motors of the same size made by different manufacturers. It is important to select motors from reputable manufacturers and consider the power factor ratings of the individual motors. Generally, if a motor has a high power factor, it is constructed of better materials, will last longer, and may have a longer warranty.
Under loaded or extremely lightly loaded motors are a prime cause of low power factor in many installations. Power factor varies significantly with motor load. This figure illustrates how power factor is affected by the total loading of a typical motor. The power factor is near its maximum at the motor's rated load. The power factor falls significantly if the motor is only partially loaded. Installations may still exhibit low power factors even though they have high power factor motors when the motors are only partially loaded.
