Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 181)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 181
Diagnostics and Protection Chapter 3
When the voltage at the motor terminals exceeds the insulation rating of the
motor, corona begins to appear. This corona deteriorates the insulation system,
eventually leading to a fault to ground. Such a failure is shown below.
The level of the DC bus voltage has a direct effect on the peak level of the ringing
surge voltage. If the drive operates at 230V AC, the DC bus voltage is about
310V DC and two times peak only reaches 620Vpk. This peak will not damage
most motors. However, a 460V AC drive operates at 620V DC bus voltage and
1240Vpk and a 575V AC drive operates at 775V DC and 1550Vpk.
Non-inverter grade motors have insulation systems rated to 1000V and 1200V
depending on their construction.
• 1000V motors are assembled without phase paper.
• 1200V motors are assembled with phase paper and slot insulation.
Non-inverter grade motors will fail if operated from a 460V or 575V drive.
There are three ways to eliminate the effects of reflected waves on motors.
1. Match the motor surge impedance to the cable surge impedance.
2. Reduce the dv/dt.
These methods reduce or eliminate reflected wave and surge voltage at the
motor.
3. Better insulate the motor so the effects of the surge voltage will not
damage the motor.
For inverter applications, NEMA updated the standard MG 1-1998, section 31
regarding motor insulation systems. An inverter duty motor needs to withstand
surge voltages that are 3.1 times the rated motor voltage and rise times greater
than 0.1 μs. This is 1488V for a 460V motor. to provide better protection, some
motor manufacturers have started producing 1600V rated insulation for inverter
grade motors. However, even if a motor can withstand 1600V surges, it can still
fail if the insulation cannot hold up under rated motor temperatures.