VFD controls

5 replies [Last post]
jomamma1's picture
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Joined: 2013-10-08

We have a problem in my plant with VFDs using an ice cube type relay for the start circuit. These relays are 120vac coils with a PLC output energizing them. Quite ofter when the output is off the relay will remain energized. I am assuming that the leakage voltage from the transistor output of the PLC is just enough to hold the relay. If the relay is pulled and put back it will not energize until the PLC output is high again. I am hoping that adding a bleeder resistor accross the relay coil will cure the problem.
I am interested in hearing if others have had this problem and what your cure was.
Thanks in advance,
Joe

Joe Cleary's picture
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Joined: 2013-08-22

Are you sure coil is still energized and the relay is not stuck? Is the PLC supplying the 120VAC to the coil or is there another relay?

werner m's picture
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Joined: 2013-12-23

I had this problem a long time ago. I used a Christmas tree style bulb as the bleeder for a temporary fix. Turned out to be the PLC output card had a bad channel, and of course our stock room had no spare! None of the other channels exhibited this problem and were wired the same way with the ice cube relays. The new card had MOVs across the output terminals, perhaps this was the factory's answer to the problem.

Larry Dingethal's picture
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Joined: 2014-02-18

A couple of things:
1. Make sure that the VFD installation has been properly engineered. I've seen cases where VFDs installed without an isolation transformer or a line reactor have caused some very strange control issues.
2. Is the control voltage floating? Ungrounded control legs will also cause similar problems that are difficult to pinpoint.
3. Most likely the type of output card is the problem - I'd suggest using a true relay output card instead of a solid state one.

Jim Alberton's picture
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Joined: 2014-05-07

Let us assume that your PLC card is good and the relay is good.
First, look up the PLC card and find the leakage current of the output.
Second, look up the coil current of the relay. There will be both a pull in current and a holding current.
Then, compare the two.
You probity need to increase the current load for the PLC output. This can be done by adding a resistor is parallel to the coil or adding a second relay, light or other load.
A surge suppressor would not hurt across the coil. The relay manufacturer makes them for your coil both in AC and DC.

cdv5002's picture
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Joined: 2013-12-10

For your application I would recommend you use a relay output from the PLC not a transistor output. Don't try to fix the symptom, fix the problem.

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