An interesting thing happened yesterday. I had been talking to a customer that had purchased a new VFC to replace one that had gone bad in a Motor Control Center. He told me that the 4-20mA signal was acting very erratic and that the drive would not follow the signal.
I went to the site and sure enough, the signal was all over the place from zero to maximum and everywhere in between. We looked at everything from cable routing to the shield ground connection but still didn’t see anything.
I replaced the control board at this time thinking that maybe something was wrong with it. I asked him to power the unit back up and when he did he received a small jolt. I quickly measured the potential between the VFC chassis and the ground in the panel and there was 67 volts…..No ground strap on the chassis to the panel.
Here’s the lesson, the paint on the steel chassis of the drive is a thick powder coating and it will not ground just by the mounting bolts. US Drives provides two ground terminals near the input/output connection points for not only connecting wires for incoming and outgoing but also for a connection to the equipment. There must be a solid ground for the drive and this one was floating.
Right after we ran a ground the signal worked just as it should. Lesson learned.
Just thought you should make note of this.
Scott
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