June 10, 2002
reduce the motor life. For many applications, the load will be well less than full load and
the motor will be able to operate at reduced speed without overheating.
The motor should have a minimum 1.15 service factor or be rated well above
the actual load it will be carry. Verify with the manufacturer that the motor is capable of
acceptable operation with an ASD. Standard motors can often operate down to 50
percent of rated speed, high efficiency motors can often operate down to 20 percent of
rated speed, and "inverter duty" motors can operate below 20 percent of rated speed
without problems in a variable load application. Motors designed specifically for ASD
operation usually incorporate special cooling provisions and might use a higher class
Ensure that the final installation does not create voltage or current harmonic
distortion beyond acceptable limits. The amount of harmonic current distortion
generated depends on the ASD design and the ASD filter design. Some ASD
manufacturers provide software to assist with a harmonic distortion evaluation; AFPAM
32-1192 describes one commercially-available software program designed for ASD
analysis. Power quality field measurements should be taken after the installation is
complete to confirm that the system total harmonic distortion is not degraded beyond
acceptable levels. If the ASD can be provided power from a standby generator upon
loss of normal commercial power, the harmonic distortion evaluation must include the
system effects when powered from the standby generator.
Voltage sags can cause nuisance tripping. Ensure that the ASD either has a
minimum of 3-cycle ride-through capability or automatic reset circuitry.
Nearby capacitor switching can cause transient overvoltages, resulting in
nuisance tripping. In this case, ensure the ASD either has input filtering to reduce the
overvoltage or automatic reset circuitry.
Important applications should include bypass operation capability to allow
motor operation independent of the ASD. Refer to AFPAM 32-1192 for additional
Refer to Appendix B for additional information regarding an energy efficiency
economic evaluation associated with ASDs.
Standard motors can often operate down to 50 percent of rated speed, high efficiency motors can often operate down to 20 percent of rated speed, and "inverter duty" motors can operate below 20 percent of rated speed without problems in a variable load application. The amount of harmonic current distortion generated depends on the ASD design and the ASD filter design.