Bridgeport mill keeps c**pping out

[FulmenParticipant @fulmen]

My mill (A&S built) died on me yesterday in the middle of a cut. Figured it was a blown fuse and called it a night, but today it started up just fine before dying again after a minute or so. The contactors won’t engage, but there is power if I push them manually. After a while it wakes up only to die again after a minute or less.

I checked the transformer, all the windings seems to be at the proper voltage. I’ve checked the fuses and switches, everything seems fine. Are there any thermal/overcurrent protectors in the contactors?

 

[Mark RandParticipant @markrand96270]

Assuming that it’s the original 3 phase wiring, there probably are thermal overloads in there somewhere. You’d need to have a look in all the boxes to find where. You will still need to track down the source of the problem though.

[FulmenParticipant @fulmen]

Yes, the wiring is original as far as I can tell. The motor runs on a VFD but that’s connected between panel and motor.

[Clive FosterParticipant @clivefoster55965]

+1 to Marks suggestion about thermal overloads.

The old style automatic reset overloads are known to get crotchety in old age cutting things out randomly because they feel like it. I imagine many years of just sitting there doing nothing leads to build up of “stuff” and, perhaps, a loosening of the device that does the auto reset.

Mine had a small spate of such happenings about a decade back. Cured “for now” by decent spray of switch cleaner lubricant and operating the auto manual selector a few times. Whether it was a real fix or whether the problem just randomly went away I know not.

But if it ever happens again the old contactors and overloads are coming out and a new modern, DIN rail mount, set are going in. Truth to tell if I could have afforded the downtime to obtain new contractors and a DIN rail they would have been replaced there and then. But I needed the mill up and running to finish a paying job.

Clive

[FulmenParticipant @fulmen]

The contactors are of this type:

But I don’t see how they could be the cause since they all loose power. And I can’t find any thermal switches or other resettable components.

[noel shelleyParticipant @noelshelley55608]

A loose or faulty connection in the holding circuit or coils ? The thermal overloads are in this circuit and often part of the contactor unit. Noel.

[Clive FosterParticipant @clivefoster55965]

Later style of contactor to mine. I think your originals have already been replaced.

The thermal overload units are on the right hand side of your picture, dial adjusts the currents setting.

The Bridgeport control circuit basically puts all the overloads in series so if any one pops out the machine stops. Sensible safety precaution really. But it does mean they all have to work.

Tightening up all the connections isn’t a bad idea. I have seen screw connectors on solid wire drift loose over the years. Presumably the copper wire flows a touch under long therm pressure.

Do you have the wiring diagram? If not I could send you a PDF of the wiring for a typical UK built one.

Clive

[FulmenParticipant @fulmen]

I can only see a current setting on the contactors, and these have not tripped. Besides, if one tripped they shouldn’t all loose power should they? I think I’ll focus on the holding circuit next, after giving the contactors a once-over with some switch cleaner.

[SillyOldDufferModerator @sillyoldduffer]

[Clive FosterParticipant @clivefoster55965]

[FulmenParticipant @fulmen]

[Les Jones 1Participant @lesjones1]

If the motor is fed by a VFD there should be no conractors between the VFD and the motor. Overcurrent protection should be provided by the VFD. (The VFD parameters should be set to suit the motor ratings.) Is the VFD supplied with single phase or three phase ? Even if you can’t provide a schematic can you at least provide a block diagram showing the path from the incoming supply to the motor.

Les.

[FulmenParticipant @fulmen]

The VFD is single phase, fed by the original contactor. This has worked for many years so that’s not the problem.

 

@SOD: No interrupts.

@ Clive: I’m pretty sure the contactors are original, I see no signs of fiddling or repairs.

[Les Jones 1Participant @lesjones1]

I suggest that you monitor the single phase input to the VFD to see if the fault is before or after the VFD. You could connect a small mains voltage lamp directly to the VFD input terminals.

Les

[FulmenParticipant @fulmen]

The fault is located before the VFD, this I know. Simply put all the contactors loose power, but they will deliver power if I push them manually.

So a buggy overload sensor seems like a reasonable hypothesis. Not that any of them have tripped mechanically, I’ve checked the reset button multiple times. I’ll have a look later this week.

Are these contactors serviceable?

[Les Jones 1Participant @lesjones1]

I assume that the contactor coil is fed with a relativly low voltage from a winding on the tansformer. (Probably 25, 50 or 110 volts.) There will be a number of normaly closed contacts between one side of the transformer secondary and the contactot coil. These will be the stop button, the three contacts on the thermal overloads and any safety cutout switches. these cutout switches would be on machine guards and possibly on covers on electrical circuits. Check these contacts with your meter.

Does the macine run if the start button is held pressed ? If it does then the fault will be the maintainer contact on the contactor. This is a normally open contact  on the contactor that is connected in parallel with the start button.

A good clear picture of YOUR would be helpfull.

Les.

[duncan webster 1Participant @duncanwebster1]

I’ve had an mcb which had corroded contacts repeatedly tripping, the resistance of the contacts heated the bimetallic thing which made it trip, which made the contacts worse and so the heating was worse and so on on a vicious cycle. Fortunately I managed to find a replacement (now obsolete).

[FulmenParticipant @fulmen]

[Mark RandParticipant @markrand96270]

Couple of things:-

1. Contactors downstream of the VFD haven’t been an issue for 20-30 years. They were an issue with GTO SCRs, but those are ’70s-early ’80s technology. Current VFDs use IGBTs, which do not suffer the problems.
2. If you are feeding the single phase input VFD through the original 3 phase overload, with the current going through one phase’s connections, you’ll be using up to 1.73 times the expected 3 phase current (√3), so if that’s the case, you may get an over current trip if you haven’t adjusted the limits.

[FulmenParticipant @fulmen]

I just realized this might be a punishment for this recent setup:

 

Don’t worry, I didn’t turn the switch. But I’m running out of options, cutting it with an angle grinder will take forever.

[FulmenParticipant @fulmen]

Had another try today. Gave the contactors a dash of switch cleaner and checked all the screw terminals I could see. I also gave the adjustment wheels a spin, but the result was the same. All contactor magnets loose power

Here’s the panel:

I think it’s safe to assume it’s original and unaltered.

Bottom contactor is for the Erskine PSU and doesn’t seem to have any overload protection. Second is for the mill head, third for the optional slotting head and the top one is for the coolant pump. And yes, it has dawned on me that I should swap over to the slotting head circuit, I’ll try that next time (it seems to reset eventually).

[Les Jones 1Participant @lesjones1]

Your picture does not show the wires comming out of the transformer connector so I can’t trace the wires from there. The botom sets of contacts on the contactors will be the maintainer contacts that hold the power to the contactor coils on.
I SUSSPECT that the single fuse at the bottom right could be a fuse for the low voltage supply to the contactor coils. Make sure that is screwd in properly.
Also the resolution of your picture is not good enough to read the lables on the wires.
(The picture pixelates when enlarged.) You will have to trace the wiring to the contactor coils via interlock contacts and stop buttons.

Les.

[SillyOldDufferModerator @sillyoldduffer]

[FulmenParticipant @fulmen]

Well, it’s working again.

I suspect it was the switch cleaner that needed some time to soak in, after some fiddling today it started behaving like it should again. It was the overload protector for the motor that wouldn’t reset properly (I was running some fairly heavy cuts). It’s also possible it’s become a bit more sensitive over the years, if it happens again I’ll probably bypass it and let the VFD limit the maximum current.

Thanks for all the help and support guys.

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