Electric Motors and Horsepower

Compressors are often rated by Horsepower (HP). As simple as this sounds, there are different variations of HP. Some compressor manufacturers rate their air compressors by peak horsepower, also known as brake horsepower. Peak horsepower is the maximum output that a motor can produce while the motor has the start windings engaged. Peak HP can be as much as 5-7 times the rated or running HP. Under normal operating conditions, the start windings are only engaged for a small fraction of a second. Therefore, using peak horsepower as a comparison tool can be somewhat misleading since under normal operating conditions the motor only develops this horsepower during startup. If a motor drive system causes the start winding to remain engaged for a long period of time, the motor will either overheat if it has thermal protection or fail prematurely.

Most electric motor manufacturers rate their motors by the horsepower developed after the motor has come up to its designed operating rpm’s and disengaged the start windings. This is often referred to as running or rated horsepower and is a true indication of the HP a motor can sustain over a long period of time.

Some other factors to consider when looking at motors is Duty Cycle and Service Factor. Duty cycle is normally rated as either intermittent or continuous and is defined as the time rating under full load. In other words can the motor run at full load horsepower continuously or only intermittently. The other of the two noteworthy factors is the Service Factor (S.F.) rating of the motor. It is defined as the percentage of rated horsepower at which the motor can safely operate (i.e. 1.15 SF = 115% of rated HP). Higher service factors allow motors to handle more varied conditions without causing motor overheating or premature motor failure. Examples that could cause a motor to run within its Service Factor could be caused by low voltage, higher ambient temperatures, higher startup load .etc.

Note: Many companies are now eliminating the peak HP reference and instead using SCFM as a more accurate performance indicator.