VFD controls

11 replies [Last post]
jomamma1's picture
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 Cleary's picture
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
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
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
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
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.

Naveed shahnawaz's picture
Joined: 2015-06-07

Dear All,

We have Allen Bradley VFD CAT1336F-B050-AN-EN. We are facing problem that when we give RUN command from HMI, then drive card 1336F-MCB-SP1K shows that drive is running by turning ON RUN (GREEN) LED, but on HMI drive remains on stopped condition and not goes on “at speed” position and also drive not run physically.

We have this same issue on other Allen Bradley Drives: 1336F-BP400-AN-EN-NCM & 1336F-B200-AN-EN.
Please give valuable suggestions to solve the problem.

With best regards,

Mumtaz Ahmed
AGM- Electrical

Shorty's picture
Joined: 2016-03-01

I'm new to this forum and I have gotten a lot of insight recently by just viewing comments, so maybe someone can help me.

I work in the recycling industry and have been having an issue with a DC bus overvoltage fault (F022) on a WEG 20 hp VFD controlling a 15 hp, 3-phase belt driven motor for a shaker table. I just changed from a Dura pulse brand VFD to a WEG brand hoping that a better brand would solve my issue.Tech support with WEG has gotten me past the fault on the stopping and I can start up with no fault until I put material and/or a load in my feeder pan; during ramp up it's faulting on DC overvoltage. I've checked distance of wire to motor and AC input is good. I have a 75 ohm 1,000W braking resistor and VFD has a dynamic braking unit from factory. I set my parameters according to resistor size and so far all settings are correct. I'm stumped why it starts empty with no problem but when material is dumped in I can't start up, even if I speed up ramp time from 10 sec to 5 sec. Any help will be awesome as to what may cause this or a parameter setting I can change.

highvoltpower's picture
Joined: 2016-04-08

It's an electric device to change convenience control source to variable frequency. Normally, VFD components include a rectifier and inverter. Where the modifier changes the voltage and separates input and output circuit, the battery pays power loss in the VFD drive circuit. You make sure the installation of VFD is correctly planned because in many cases wherever VFD installation is placed without an isolation transformer it creates switching problems.

user-1004642's picture
Joined: 2016-08-15

Maybe your coil is broken. Check it.

jcbvet's picture
Joined: 2017-02-27

Speed pot on VFD

We have a very old Dodge 300A VFD on one of our production machines. The drive is from the early 70s. We have to continually replace the speed pot (1K, 2W pots) on this drive and I cannot figure out why they keep failing after 6 months or so. The original schematic shows a 10 ohm resistor in series with the wiper. The resistor is an adjustable ceramic ohmite panel-mounted resistor.

Any ideas on why these keep burning out?

jcbvet's picture
Joined: 2017-02-27

You may need to put an MOV across the coils on the relays. It's called bounce. PLCs can cause this on some relays.

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