UFC 3 -520-01
June 10, 2002
When making connections to the above selector switches, ensure that only
the normal automatic control devices are bypassed when the switch is in the manual
position. Confirm that all safety control devices, such as low- or high-pressure cutouts,
high-temperature cutouts, and motor overload protective devices, remain connected in
the motor control circuit in both the manual and automatic positions.
Provide all motors with overcurrent protection in accordance with the NEC to
automatically disconnect the motor from the supply source in the event of an internal
short circuit or sustained overload in the motor. Provide each motor of 0.125
horsepower (93.25 watts) or larger with thermal overload protection. Provide three-
phase motors with overload protection in each ungrounded conductor. Overload
protection can be provided either integral with the motor or controller, or can b e
mounted in a separate enclosure. Provide ambient temperature-compensated overload
protection if the ambient temperature varies by more than 10 C (18 F).
ADJUSTABLE SPEED DRIVES .
Adjustable speed drives (also referred to as variable speed d rives or variable
frequency drives) are electronic devices that control a motor's speed to match an actual
load demand signal.
Refer to NEMA ICS 7, Industrial Control and Systems: Adjustable Speed
Drives, for design criteria related to the selecti on and design of ASDs. For additional
information, refer to AFPAM 32-1192. For the Navy, guidance is provided in Appendix
D of MIL -HDBK-1003/3, Heating, Ventilating, Air Conditioning, and Dehumidifying
Systems. The following provides additional design criteria.
At the rated full load of the driven equipment, the output voltage and
frequency of the ASD should be the same as the motor's rating. Note that this design
recommendation also places limits on the motor design; the motor should not have a
significantly higher full load horsepower or speed rating than the driven load.
Mismatches can easily cause operational problems, including efficiency losses and
increased ASD input current. In extreme cases, a mismatch can cause the ASD to trip
on overcurrent during motor starting or cause the ASD input current to be substantially
higher than the design without the ASD.
The ASD short term current rating should be adequate to produce the
required motor starting torque, including loads with high sta rting torque. Inappropriate
designs, such as operating an 1,800 rpm motor at reduced speed to drive an 870 rpm
load can cause the ASD to exceed its short term current rating.
Motors can overheat at the lower operating speed set by an ASD and, in
some cases, they can overheat even at full load/full speed operation because of the
ASD's non-sinusoidal output. On fan-cooled motors, decreasing the motor's shaft
speed by 50 percent decreases the fan's cooling effects proportionately. If the motor is
fully loaded and speed decreases, the motor must supply full torque with less than
intended cooling. In extreme cases, this can cause the motor insulation to fail or can