Disconnect at motor when VFD driven

[mshields]

In reference to 430.102 (B), (2)

Per this section, the requirement for a disconnect at the motor is not required if including it "introduces additional or increased hazards to persons or property"

Under the FPN it sites VFD driven motors as an example of such a hazardous condition.

What is the hazard associated with having a disconnect at a motor which is driven by a VFD?

Thanks,

Mike

 

[Jraef]

Because opening a disconnect under load downstream from a VFD can cause the transistors to fry. Remember that the PWM output is not really AC, it is "pseudo AC" made up of pulses of DC at the peak voltage level. So on a 480V VFD, the DC pulses are all 678VDC, more than any 3P disconnect switch is rated for. The dielectric on the switch may be exceeded, causing a flash over.