AB drive contactor


Thread Starter

Adrian S. Wilson

hello, i am currently having problems with burning up contacts on a Allen bradley size 4 contactor. I have a 75hp Baldor SWEO drive powering a 75 hp ac motor. I have been looking into the fact that the contactor is opening up under load, either running or in brake regeneration. But my biggest question has any one else had similer problems using a contactor on the motor side of a ac drive. The way it's currently set up is the contactor is energized when the enable on the drive is set, and opens up when the enable is lost. When the press is e-stopped we have a 5 sec. delay until the contactor opens up. Any information would be


adrian wilson

Daniel Boudreault

Hello Adrian,

Think of the motor as an inductor.
When you are braking the motor, you
pretty much have full load current flowing.
When you open the contactor at this time,
you will have voltage spikes that will
burn up your contacts.

I would move the contactor to the front of the drive.

Another thing that I just thought of, If you have the
current limit set over the motor F.L.A., you
should upgrade the contactor to 100 HP.

Dan B.
Hi Adrian,

I agree with Dan about installing a larger contactor. You didn't mention the drive voltage. If it's a 200V drive, the contactor is undersized. I disagree with moving the contactor ahead of the drive though (I'm guessing there's probably one there already?). I think the purpose of the contactor between the drive and the motor is to disconnect the motor from the drive in case of a drive problem. It takes some time for the drive to discharge, and during this time, a drive failure could cause the motor to run. I have no
experience with motors/drives larger than about 5HP, so maybe someone else has some other ideas??

Hope this helps,

- Eric Nelson
[email protected]
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA

Mark Sevelis

I think you have problems with turning off the drive. NEMA size 4 is rated for 135 Amps continuous, so you should be ok with your 75 HP drive (even a 100 HP could use this contactor).
The contacts are burning due to current--not voltage. Are you disabling the drive simultaneously with opening the contactor? The resulting inductive spark can be causing the damage to the contacts because the current will be in excess of 135 Amps for very short periods of time. How long does a contactor last? How often do you stop the drive/hr?

Mark Sevelis
Allen-Bradley Std Drives Technical Support
Mequon, WI
Excellent points, Anthony! After re-reading Adrian's original post, I now see why he's having problems with the contactor. Opening up under
load?!...I missed that part! I assumed that the contactor was only being used as an emergency stop disconnect. I had mentioned that it may be
undersized, but most likely that's NOT the case. I think if he follows your advice he'll be able to solve the problem.

Thanks for prompting me to examine his original post more thoroughly ;o)

- Eric
[email protected]
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA

John G. Boland

Tom Gianni wrote:

<< Not a good idea to put contactor on AFD output unless need for E-stop only purpose. Only alternative is to ensure output current zero before opening contactor. >>

Agreed about the contactor location. IS the drive applying braking current to the motor during the contactor opening? What stops the motor?

This just triggered a remaining neuron. The electric motor, depending on the type, may be generating a pretty hefty back EMF even if the drive output is disabled (high-impedance state - no braking current).

I saw memorable *problems* from that once upon a time, when dropping an already-full-speed-rotating electric motor onto a VFD to with a contactor (don't ask why).

Back to lurk mode.

John G. Boland, president
Strateg!c Method$ Corporation

Adrian S. Wilson

I first would like to thank everyone who has replied already. The drive is feed from a 480v system, so I believe the size 4 rating is ok. The
company I work for requires the a physical set of contacts on the motor side for safety reasons. I understand if you open up the contractor while the load is still driven from the VFD your asking, to have premature contact burn up. The contactor opens up after 5 seconds after an e-stop, most cases the press is stopped in about 2.5 to 3 seconds, with regen braking.

I been looking at our plc program, and can't find out any where the contactor should open up, while the VFD is still driving the motor. My question now is there a holding current on some drives? and if so is this to prevent motor shaft rotation while the motor is not turning? We have similar setups of ac drives with contactors on the motor side, and haven't had this much trouble.

thanks for any more help,

Adrian Wilson

Johan Bengtsson

------- Forwarded message follows -------
From: [email protected]
To: [email protected]
Subject: Re: MOTION, PLCS: AB drive contactor

Some VFD:s hold the motor with a current after it is stopped, this current should not be that high, but it is of course not alternating. The lack of zero crossing may be what burns your contactor, not the current really being that high.

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/

Anthony Kerstens

First of all, what is the purpose of having a contactor on the load side of the VFD? Is this for emergency stop purposes, or for overload
purposes? I have seen both and they are a source of grief in terms of the maintenance of VFD's.

Also, why 5 seconds? This introduces a hesitation in what is presumably supposed to be an emergency stopping mechanism. This is very bad.

Assuming the purpose is for emergency stopping, the e-stop circuit should immediately drop out the contactor coil, not the drive enable. The drive enable should be dropped out by series-wired early-break/late-make contacts in the local motor disconnect switch, the e-stop circuit, and the contactor O/L. The early break contact should kill the drive before the contactor opens. Also, the drive should be set for coast-to-stop: NO regen braking.

It is seriously unhealthy for VFD to be opened-up under load in the manner you describe. Conversely, it is just as unhealthy for the line
side of the drive to have power cycled faster than what's specified in the manual.

As for burning-up the contactor, you should also consider the duty-cycle of your process. How often is the e-stop used for non-emergency purposes, in lieu of a PROPER machine shut-down??

I stongly suggest you read through the drive manuals, paying particularly close attention scenarios where AB says the warranty is void.

Anthony Kerstens P.Eng.
I agree with Dan B. Also, a contactor on the AFD output can cause erroneous current faults in the AFD due to pole-pole transient currents caused by the arcing/dc bus voltage/dV/dT with stray capacitance. Not a good idea to put contactor on AFD output unless need for E-stop only purpose. Only alternative is to ensure output current zero before opening contactor.

Tom G

Adrian S. Wilson

Problem solved!

In a nutshell, here's what we found. The press that it's on has a Main drive motor(the 75 h.p. i mentioned) and also has a smaller pony motor
and drive assembley. During special set-up's of the press we use this pony motor, and unlike our other press there is no clutch to clutch out
the main motor and tach. The press programing was still allowing the main dirve to have a enable signal, but it had a zero speed from the analog output card for the dirve speed ref. There for the drive sensing the motor was not at zero speed, tried to correct this action. While trying to correct the action, if the press was e-stopped while the pony motor was inching the press, the main drive contactor would open up, causing the contact failure.