Bridgeport Contactor Problem

After the cock up on silver soldering last night I decided to hog the gear change levers for my traction engines out of some square hot rolled steel. So I set to on the Bridgeport, progress is good. Then I have problems with the main spindle motor not starting properly. A fiddle around in the control box and all is well for a while. Then the spindle stops while running and the contactor no longers pulls in.

I've seen contactors fail to hold in before due to dirt on the core preventing said core closing properly, but they normally buzz before complete failure. After fudging the lock out, so I can work on the live control box while it is open, I decide to take the contactor out and dismantle it. The contacts are quite pitted so I clean everything up, re-assemble, and re-fit in the control box.

When I hit the start button on the box at the front the main spindle starts, but stops as soon as I let go of the button. Basically the contactor pulls in, but fails to hold in. After a lot of checking I am sure that the auxiliary contacts are working and that the wiring in the control box agrees with circuit diagram in the manual, so the hold in should work.

The contactor is 415V three phase AC, and the coil is 110V AC. Having been forced to read up a bit on t'internet it seems that AC contactors have a shaded section on the core to prevent buzzing at twice the supply frequency.

So my question is thus: what happens if I have put the coil and/or half the core back in the wrong way round during re-assembly? Would this cause the symptoms I see?

Andrew

PS: I went to the supermarket last night, but I feel another trip coming on as I am in dire need of a beer or several and some comfort food.

No it would not, providing the armature is free to clunk in and out , should be fine. The contactor would be held in by the aux contacts AND in series, the thermal overload - which are notoriously unreliable.

Frank

The contactors used on UK built Bridgeports for many years feature switched selection of manual or automatic reset and are known to suffer hold in failure gremlins after many years service giving the symptoms you describe. Had jobs on the table when it happened to me (twice!) so a quick'n dirty fix was indicated. A couple of heavy sprays with switch cleaner, attempting to get the nostrum well inside the device, with several actuations of the selector switch after each spray got things going again pending "proper investigation when I have time" (AKA never). The second fail occurred only a month or so after the first one but its been good for a couple of years since so it may well be properly fixed. Wouldn't surprise me if the root cause is that the thing never operates on a machine in good order so the creeping dust endemic to mechanical parts close to respectable voltage electrics eventually builds up enough to cause problems.

If it ever goes again the whole set of contactors is coming out and being replaced with a modern DIN rail set arranged for easy access to the wiring. Standard box is impressive but a bit short on servicing space when in situ. Getting too old to futz around!

Clive.

Edited By Clive Foster on 13/06/2015 21:48:40

Frank: Thanks, I think you are correct about the contactor, if it pulls in when the push button is operated then it can't be anything to do with the coil and core. I should have thought about it a bit more.

The control box has four contactors, main spindle motor, auxiliary spindle motor, coolant pump and table feed. Each has a thermal overload between the contactor and appropriate motor. Also, as Frank says, each thermal overload has a cutout in series with the circuit holding the contactors in. But there's an odd thing here. When I push the button with the spindle motor selected, the contactor pulls in, but drops out as soon as I let go. However, if I select both spindle and auxiliary motors then both contactors pull in, but only the spindle motor contactor drops out when I release the button. Since the thermal overloads are in series with all the contactors I don't think they can be the problem. After the thermal overloads, in series, is the stop button. When I push that all the auxiliary motor contactor drops out as one would expect. As far as I can see from the schematic once one contactor is energised it should hold all the other selected contactors in, and voltage measurements seem to confirm this. But clearly something is causing the spindle motor contactor to drop out.

I'll have to have another poke about tomorrow. I'm too tired to look at it now. Last time I was faffing around with contactors, on another machine, I gave it a good clean, refitted it, and turned on. It was behaving better, but not perfect, so I took it apart again. It was at that point I realised I hadn't disconnected the three phase. Lesson learnt, don't mess about in control boxes when you're tired!

Andrew

Have you checked that the aux contact is making OK when the contactor is operated? Would have thought it has to be either a problem with the aux contact not making or the stop button (which usually just breaks the aux circuit when pressed) being permanently open circuit.

Yippee! Fixed it, or at least fudged it temporarily!

I spent half the night puzzling over the schematics, and the measurements made, and came to the conclusion that they didn't add up. So the schematics must be wrong. I couldn't understand how the main spindle contactor was closing when the start button was pressed, but dropped out when the button was let go, even if the auxiliary spindle motor contactor was engaged.

Each overload relay has a fourth auxiliary set of contacts. These are shown all in series, along with the stop button in the schematics. However, the results of measurements indicated that there must be another switch in series with each set of auxiliary contacts on the contactors, exact as Frank said. That explains why the contactor operates when the start button is pushed, and the auxiliary contacts on the contactor close, but the coil is unpowered as soon as the start button is released. Sure enough on the main spindle motor thermal overload relay the auxiliary contacts are open, not closed as they should be. So I bypassed the overload relay aux contacts and away we went! For those that are interested here are the internals of the contactor during cleaning:

And making up for lost time this evening here's the mill in full flight; 1500rpm, 7.5mm depth of cut, 3mm width of cut and 500mm/min feedrate. Loads of blue swarf.

So hearty congratulations are in order to Frank who correctly identified the problem.

If I could find a way of manually tripping the thermal overload relay I'd have a go at giving the contacts a clean, but I can't find a way to do so?

Andrew