Is a change-over-switch with a VFD dangerous?

Question

We are using a variable frequency drive (VFD) with a compressor. We added a rotary cam changeover switch (600 V/50 Hz, picture is attached) in between the compressor and the motor. The purpose of adding this switch that if the VFD is not working, we can switch to direct grid mode.

The number 1 is for compressor running on VFD and number 2 is for compressor running directly on grid.

When we run the compressor through this switch on VFD, the VFD is damaged after some time (may be 10 days.)

What could damage the VFD with this setup?

Answer 1

There are a number of possible situations:

1. Switching connecting a motor to a VFD while the VFD is operating could damage the VFD.
2. Switching a motor from a VFD to the grid while the motor is running could damage the motor.
3. Switching a motor from the grid to the VFD while the motor is running could damage the VFD.
4. Switching a motor to the VFD while the motor is coasting could damage the VFD.
5. There may be a few VFD models that could be damaged by disconnecting the motor while it is running.

Most VFDs have protections built in that will protect the VFD in most situations. Even with extensive protection built in, VFD manufacturers often recommend against 1, and 3.