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Is it possible to check a motor ON/OFF state by connecting a phase control relay to the inverter and the motor 3 phase terminals?

I have tested a phase monitoring relay to a main 3 phase supply from distribution and the relay works fine but when I connect it to the inverter/motor, it doesn't work. also in one of 6 different possible connection ways, relay get too hot and smoke came out of it! I want to know why? where is my mistake?

*inverter model is: Schneider ATV12H018M2 *phase control relay is: Schneider RM17TG20

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  • \$\begingroup\$ You check the motor on/off state with an output from the drive, or an external rotation sensor (assuming that it does not sit at zero speed when energized). Oh, and you probably damaged your phase control relay with your experiment. \$\endgroup\$
    – R Drast
    Oct 31, 2017 at 12:35
  • \$\begingroup\$ @R Drast I was trying to check it by drive output and yes sadly it damged the phase control relay :-( the best way is to use the VFD I/Os, I read the manual and found a working discrete +24V output to detect motor ON/OFF. \$\endgroup\$
    – rocknroll
    Nov 7, 2017 at 14:22

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No, this is not recommended. The output of a VFD is not suitable of any electronics except the motor, you can't even measure it with your multimeter without putting it in a special low-z (low pass filter) mode.

The output looks like this:
vfd output

If you'd want to do this, you'd need a harmonics filter in front of the three phase monitoring relay. And then there is no guarantee the device would survive the other features of the drive. (eg: braking and dc hold)

The best way to detect if a VFD is active is to use it's IO functionality. Almost all VFD's will have a one or more relays, analog (0-10V/0-20mA) outputs or digital (24V) transistor outputs that are freely programmable in the software.

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