AC Induction Motor Design (8) - FINISH

10. Inverter Duty Motors

Inverter duty motors are specially designed to withstand the new challenges presented by the use of inverters. There are a number of ways to designate motors "inverter duty," however, several things must exist as a minimum:

• Class F insulation - to withstand the higher heat generated by non-sinusoidal current from the drive.
• Phase insulation - Insulation between phases is a must to avoid "flashover" between phases from current surges.
• Layered Conductors - To reduce turn to turn potential between conductors.
• Solid varnish system - to reduce partial discharge and corona damage.
• Tight machine tolerances and good air gap concentricity - to reduce shaft currents and resulting bearing damage.

A proper inverter duty motor will have special rotor bar construction designed o withstand variations in airgap flux densities and rotor harmonics. Additionally, the first few turns of wire may be insulated to better withstand standing waves which occur due to the faster rise times in modern inverter technology.
Caution: Some manufacturers may only de-rate motors. This is done by reducing the motor by (about) 25%. Therefore, a 10 hp motor may be rated as a 7.5 hp motor.
It should be noted, also, that an inverter application does not always require an inverter duty motor. The old motor or an energy efficient motor may be sufficient for the application.