UFC 3 -520-01
June 10, 2002
POWER DISTRIBUTION AND UTILIZATION --MOTORS AND MOTOR CONTROL
BASIC MOTOR CRITERIA.
Ensure motors have mechanical and electrical characteristics suitable for the
application. Three-phase motors have better starting torque, run more quietly, have
better efficiency, and are smaller than single -phase motors of the same horsepower
rating. In ratings of 5 horsepower (3,730 watts) or more, they are also less expensive.
For these reasons, use three-phase motors if more than 0.5 horsepower (373 watts)
rating when such service is available. If three-phase service is not available, operate
motors 0.5 horsepower (373 watts) and larger at phase-to-phase voltage rather than
phase-to -line voltage. Motors smaller than 0.5 horsepower (373 watts) should be single
phase, with phase-to-phase voltage preferred over phase-to-ground voltage.
The kilowatt horsepower rating of motors should be limited to no more than
125 percent of the maximum load being served unless a standard size does not fall
within this range. In this case, select the next larger standard size.
Use motor voltage ratings suitable for the voltage supplied. Do not use 230
volt motors on 208 volt systems because the utilization voltage will commonly be below
the -10 percent tolerance on the voltage rating for which the motor is designed (a 230
volt motor is intended for use on a nominal 240 volt system).
Ensure three-phase motors of 1 horsepower (746 watts) or more meet the
minimum full-load efficiencies as indicated in Table 7 -1. New motors should be rated as
high efficiency. Replacement motors should also be of the high efficiency type provided
that the upstream protective devices can continue to provide adequate electrical
protection. For more information, refer to Air Force Pamphlet (AFPAM) 32-1192,
Energy Efficient Motors and Adjustable Speed Drives.