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125 hp 480 vac HMax VFD. Controlling a pump motor. Motor is not spinning and line to the VFD is disconnected. I was putting the output wires back on the VFD, when moving the second one into place, the cable lug brushed against the DC bus terminal and sparked. I thought that was strange. I went ahead and measured the voltage and there was 10VDC on the bus terminals. When the motor leads are connected, the DC bus will charge. the most voltage I read was 20VDC. When I disconnect the output wires the bus bleeds off. I checked for AC and DC voltage on the wires and there was none. The VFD is installed in an MCC. There is another VFD beside it same size controlling another pump. When it (the other vfd) is running that is when the DC bus gets a charge going to it. When it (the other vfd) is not running it does not charge the bus. The motor wires both exit the MCC on the bottom into their own conduit, and run parallel underground to their respective motors. Any thoughts on why the DC bus is charging, and how I could stop it from happening? Would a filter on the output of the other VFD help. Would transients or noise from the other VFD be causing it?

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  • \$\begingroup\$ Sounds like electromagnetic induction. Is the motor cable specially shielded for VFD application? Metal conduit does not preclude this. \$\endgroup\$ – Kartman Mar 24 at 3:33
  • \$\begingroup\$ Dielectric relaxation in the DC link capacitor. \$\endgroup\$ – user_1818839 Mar 24 at 17:11
  • \$\begingroup\$ Thanks for responding, Is there a way to correct this? \$\endgroup\$ – Mike Hayden Mar 25 at 3:56
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If the wires are long, not shielded and there are power signals near them, you´ll get some voltage to ground. The output IGBT diodes, looking from the motor side, is a 3 phase rectifier and will charge the DC bus capacitors. Disconnect wires and measure voltage to ground when the other VFD is running or stopped. Shielded cables will reduce this voltage. Also a resistive load (with power losses when running) in output terminals can help, but be careful with a filter, as you can overload the own filter or the VFD output.

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  • \$\begingroup\$ thanks for your response. The pump is only about 30 feet from the MCC so probably no more than 50' depending on burial depth. They aren't shielded. The strange thing is I don't read any voltage, I might of picked up less than 1 volt. I double check tomorrow to see exactly what I got. \$\endgroup\$ – Mike Hayden Mar 25 at 3:48

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