Inverter Grade Wiring

I was called upon to diagnose interesting problem that occurred earlier this week that is inverter related.
A 20yr old Mazak Lathe displayed a over current fault on an Inverter drive, if the motor is disconnected the fault is usually the IGBT transistors.
Upon opening the cabinet containing the controller and inverter, I saw right away that one of the ground conductors to the large brass ground plate had burned all the insulation off the wire for a distance of around 20" from the ground plate, continuing from there to its destination, it was in perfect shape.
The Ground wire was loosely bundled with 3ph hyd pump and chiller and the Inverter motor wires, no flexing or moving of the cables would normally occur, the heat had burnt some of the Hyd pump and chiller cables and inverter cables, these are heavily insulated wires.
The excess current in the ground wire had obviously originated 20" from the Gnd point.
The copper wire composition had changed so much that the 14g wire could be snapped easily, obviously some heavy current had been passed.
The 3ph motors were megger'd to Gnd and were OK.
One of the inverter wires was burnt through to the conductor in around a 3" length.
All conductors were repaired and replaced and the machine came on with no fault.
The only explanation I have is somehow there was punch through from the VFD wiring to the Ground wire. It appeared to have occurred over a period of time.
See this ref: http://www.belden.com/pdfs/Techpprs/Evaluating VFD.pdf
M.

You could lower the carrier frequency.

Another thing that would help with some of the problems that crop up with VFDs is to lower the carrier frequency. You can drop the carrier frequency to get certain harmonics to go away, and help with EMI/RFI. This will also help with the impedence problems with high frequency signals in the AC motors, and degrading the insulation. It's a LOT more complicated than simply too much voltage. The high frequency can make problems with the insulation too. For the average VFD application where the VFD is less than 10' away from the motor, the wiring impedence won't have much to do with it. So long as you don't have a crazy ammount of wire between the drive and the motor, your wire won't matter much as long as it's thick enough. Raising the carrier frequency of a VFD can quiet it down in terms of that audible "squeal" you hear, but it makes more problems with the motor, and reduces the lifespan of the drive.

conductor overheat

I remember getting burned -literally, it raised a blister- when a wire connection was poorly made on something or another I was testing. From that point on I learned to use a tool to test for loose spot welds on buss wires.

Point is through the burning-up process the connector problem may be hidden, and the rebuilding of the wiring would correct the root fault.

The detail about the areas where the deterioration was passed between my ears without full digestion, though.

Regards
JD2

JayDee2 said:

From that point on I learned to use a tool to test for loose spot welds on buss wires.

Click to expand...

Yes, Terminal points I can see it happening, but part way along a conductors length was the strange part, the ground wire in question was also insulated.
M.

It would be interesting to know the physical relationship of these various wires. Also the VFD frequency, HP, and general voltage (probably 230V??)

I am going to assume that the ground wire was next to the burnt VFD wire, is that correct?

Mitsubishi Frequol drive, Guess around 7HP, 230v.
The 10 wires, 2x 3ph motors, VFD Motor & Ground conductor, loosely run together in Electrical cabinet, not particularly twisted together, Ground wire in contact with some of other conductors, including VFD.
Ground wire was actually ground conductor for 3ph motors.
Age of machine ~20yrs. Probably thousands built by Mazak.
M.