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Contactor Not Staying Closed?

mesa

Plastic
Joined
Apr 5, 2017
I'm trying to use a VFD to control an older lathe I purchased, be re-using the only motor control contactors are the fwd/reverse inputs to the VFD. The control circuit uses one contactor (A Taian Electric C-11G) for logic related to the foot brake, and when that contactor is closed, operator of the forward or reverse switches open and close the main motor contractors as appropriate.

The problem I'm having is that the forward/reverse control circuits will randomly fail to operate, even though all of the switches are in the correct location. The brake contactor will be closed (as it should be) and when I operate the forward control lever, the contactor will snap open (even though it shouldnt). Other times I'll repeat this exact same process at the brake contactor will remain closed and the main motor control contactor will close as well.

Is it possible for a contactor to fail in a way that makes it intermittent like this? I had assumed it would either weld itself closed or fail to operate at all. I also noticed that the brake contactor seems to have a very loud electrical hum when it's operating, more so than anything else in the control box.
 
If you are using a VFD, what are these contactors being used for?

The VFD should handle forward and reverse without any contactors at all.

If you are trying to operate the machine with it's original controls by just connecting a VFD to the power inputs, don't do that. It's not how the system should work, and it will only cause trouble. The VFD will not allow speed changes without the probability of having trouble with the machine controls.

You want the VFD output to go direct to the motor it powers, and you want the control signals to go directly to the VFD to tell it what to do with the motor. Most of the control box should not be used.
 
These contactors are being used as over sized relays, only to control the low voltage control signals on the VFD. The VFD forward and reverse terminals can be controlled by a relay, by connecting it between the FWD/REV terminal and the VFD Common terminal.

I'm trying to re-use the contactors (now disconnected from any high voltage, outside of their coils), since I'm trying to maintain the Forward/Reverse switch, foot brake, power switch, e-stop and jog buttons. The way it's wired this will all work, except for that fact that this brake contactor (I think) is flakey.
 
All of the functions that you describe can be handled with wiring direct to the VFD and a little parameter setting. There is no need to use relays at all. Some VFDs go further and implement simple ladder logic like a PLC. This gives you even greater control over logic, well beyond what you currently need. So don't use the relays.

I'm not sure how you are powering the relays(coils) that you are using. You should not be powering them from the VFD supply - that could well cause problems.
 
These contactors are being used as over sized relays, only to control the low voltage control signals on the VFD. The VFD forward and reverse terminals can be controlled by a relay, by connecting it between the FWD/REV terminal and the VFD Common terminal.

I'm trying to re-use the contactors (now disconnected from any high voltage, outside of their coils), since I'm trying to maintain the Forward/Reverse switch, foot brake, power switch, e-stop and jog buttons. The way it's wired this will all work, except for that fact that this brake contactor (I think) is flakey.

More inclined to suspect that your control transformer is 'flakey'. Especially if non-existent.

VFD nor DC Drives signal voltage supplies do NOT tolerate loads the size of heavy contactor coils. They spell that out right in their manuals.

If you wish to use the former 'power' contactors to control LOW power 'signal' control inputs to the VFD (or a DC Drive), whatever switches those contactors in, out, or sideways must have its own, isolated, control-voltage supply, and it must be in good health.

If you are trying to preserve the machine-tool's original control locations, look, feel, and general type?

Use - or update and replace, same location - the lower-powered 'signal' switches that formerly controlled the contactors as direct connects to the VFD (or DC Drive).

Strip the power contactors out of the circuit, if not also out of the machine.

They are but fossils, now. Mere baggage. Sources of confusion, now and going forward.

Already, even.

QED
 
All of the functions that you describe can be handled with wiring direct to the VFD and a little parameter setting. There is no need to use relays at all. Some VFDs go further and implement simple ladder logic like a PLC. This gives you even greater control over logic, well beyond what you currently need. So don't use the relays.

I'm not sure how you are powering the relays(coils) that you are using. You should not be powering them from the VFD supply - that could well cause problems.


Sorry, I should have stated up front, I'm not using the VFD output power AT ALL other than directly connecting it to the lathe motor. All of these contactors are being powered through the machine's original transformer, which is stepping down 240 single phase to 110 for the control circuit. The reason I didn't duplicate all of this functionality through the VFD is because I'd have to completely re-wire the machine since the jog, power, e-stop, etc.. buttons are exactly long enough to reach the control cabinet underneath the machine, not the VFD on the wall.

Since the relays/contactors are being controlled by their original switches/voltages, I didn't see a need to muck with that. Since the VFD manual states you can use a relay to open/close the FWD/REV inputs, I also didn't see a problem with re-purposing the contactors (again, disconnected from any old wiring, save the coils) for this task.


If you wish to use the former 'power' contactors to control LOW power 'signal' control inputs to the VFD (or a DC Drive), whatever switches those contactors in, out, or sideways must have its own, isolated, control-voltage supply, and it must be in good health.

I'd have a hard time being confident that the transformer is in good health, considering it's as old as the rest of the machine (mid 80's?). Is there a way to diagnose it as a bad transformer without simply replacing it? Voltage drop when switching?
 
Another significant issue is that power contactors are NOT good for very low power control.

The low voltages and currents are not enough to keep the contacts clean, or break through the nonconductive layer that builds up on them.. The controls may not operate the VFD effectively.

If you insist on doing it that way, you may need to find much lower power relays that use the same control voltage
 
I concur with JST, your problems may be(probably) are related to using power switches for logic duty. If you insist on using old logic system, I would replace the power contactors with logic relays. All the US manufacturers had low current/voltage multipole control relays just for this application. GE 120C was one, AB, CH and SqD also had theirs.

Tom
 
A noisy mag can be caused by a plunger that is sticky or dirty and doesn't pull in squarely. Make sure the magnet faces are clean and not rusty so the plunger seats all the way. If you just purchased the lathe and transported to your shop there could be junk that has fallen into one of the contactors or a jumper or wire in the control circuit that is loose or almost broken off.

Using a meter, make sure you have the required full voltage present for the coil rating. 240 volt coils won't work with 120 volts. It sounds like something is sticking partially open . If a plunger is hanging a little open then the latching contacts could be marginally seated and a bit of a jolt will signal the mag to drop out.

Stuart
 
If all original wiring appears at one place (where the contactors are) simply add terminal strips as necessary and run new wire from there to the VFD and program it as needed.
 
If all original wiring appears at one place (where the contactors are) simply add terminal strips as necessary and run new wire from there to the VFD and program it as needed.

+1 Those low-power signal & control inputs to VFD and DC drives are set up to manage the odd spike or common-mode rudeness easily as well and sometimes even for as great a distance as the output power. They have always HAD to be.

They ordinarily do not even need shielded cable, though it is still a good idea to supply such if one can do. Smacking powerful contactor coils around actually makes their job HARDER, not easier.
 
+1 Those low-power signal & control inputs to VFD and DC drives are set up to manage the odd spike or common-mode rudeness easily as well and sometimes even for as great a distance as the output power. They have always HAD to be.

They ordinarily do not even need shielded cable, though it is still a good idea to supply such if one can do. Smacking powerful contactor coils around actually makes their job HARDER, not easier.

Your insightful comments add so much to the subject discussed.
 








 
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