VFD short circuit/ground fault

4 replies [Last post]
houst2's picture
Joined: 2014-01-06

We have an ABB DC drive. When we start it and it runs with little to no load on it (100 amps) there's no problem. However, when we get up over that 100A, the drive goes to ground fault or short circuit. Was wondering what everyone's input would be on what could possibly be wrong. Any help is appreciated.

mc5w's picture
Joined: 2013-10-09

THWN wire, particularly when wet, has more than its fair share of insulation capacitance and can act as a virtual short circuit for the carrier frequency of a pulse width modulation drive. One way around this is to install an inductance in EACH wire immediately downstream of the drive on the DC side. This will minimize the effects of wiring capacitance. Changing the wires to XHHW-2 or better yet USE-2/RHH/RHW-2 will further lower wiring capacitance.

Thermal expansion of something could be causing an intermittent short circuit.

A dirty or corroded fuse clip or copper oxide in a wiring connection on the supply side of the drive could be causing it to operate partially single phase. I have encountered this with an old 3-phase motor that after rewinding would not get up to speed and when idling would trip the overload relays. Amp and voltage checks revealed the partial single phase conduction. Cleaning all 3 knife fuse and the all 6 knife fuse clips with #220 silicon carbide paper and then greasing the fuse blades with Ilsco Deox(R) fixed the problem.

There could be the DC equivalent of a neutral to ground fault inside of a 3-phase wye connected motor. The carrier of a VFD creates a strong zero sequence voltage at the carrier frequency. I have seen this happen with a 480V VFD and a motor that is delta low voltage wye high voltage and the junction box has a terminal block in it.The motor would start and accelerate for a few seconds and then the drive tripped out on overload or current limit. When one of the other electricians moved the liquidtight flexible metal conduit to a different threaded opening of the motor junction box (because he moved the drive cabinet to the other side of the machine) he screwed the liquidtight box connector right into the uninsulated crimp barrel of one of the neutral wires of the motor. A little bit of work with a socket wrench and some electrical tape to insulated the crimp barrels of the ring tongues fixed the problem.

Mike Cole, Ohio elec. contractor license number EL45,008
Cellular: two one six - eight three two - seven zero two four
mc5w at earthlink dot net

Eddi's picture
Joined: 2014-07-07

The title is regarding a VFD (AC drive) and question is about ABB DC drive. Knowing the type of drive will help get better answers and possible solutions to the problem.

OilandGas's picture
Joined: 2014-05-08

Did you verify the drive is compatible with the installation?
Did you input the nameplate data?
Go to the Fault History used in conjunction with the equipment manual.
What is the Model #?

rayp's picture
Joined: 2014-07-02

I recently had a similar problem with a Eaton drive. Checked the motor and cabling and all checked out to be good then noticed that when the drive would kick out it was reading a different amperage load than my amp meter. When they read the same, the drive would not kick out. Wound up changing out the drive. One of the internal amp meters was failing on the drive causing this problem.

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