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

On 1 July 2024 at 09:44 Clive Foster Said:

…
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. …

Without a circuit diagram I can only guess, but has the machine any safety interlocks?   Switches on doors, polycarbonate screens, covers, and those protecting against over-travel etc.   Last time my lathe misbehaved, it was because a safety switch had loosened just  enough to make intermittent content.  Had me confused for over an hour.

In addition to checking connections, it’s not unusual to find fatigue breaks inside the wiring of older equipment.   It occurs where vibration is able to waggle a wire in one spot, eventually causing an intermittent break. They can be difficult to find.   Try flexing the wires, especially any not firmly supported in a wiring loom, and have a good look at them wherever they pass through a bulkhead or bridge a large gap.

Fulmen seems to be taking the right general approach.  Isolate the fault by either working back from the motor until power disappears, or do the same by working forward from the power input.   That the motor runs when the contactor is worked manually suggests a problem in that area, and the locking circuit is my chief suspect!   Locking can be quite elaborate – a chain of switches all of which have to be right before the machine will run:  check everything.

In older equipment current and thermal trips are much the same technology – a bimetallic strip bends as it heats up, and if it gets hot enough, it releases a spring loaded contact.  After the bimetallic strip has cooled down, the contact can be reset.  Easily cleaned dirt often causes problems, but more seriously so does old age.   Over time the mechanicals wear out – insulating supports, springs, strip, contacts etc,  in which case the cure is a new contactor.

Dave

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).

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.

On 2 July 2024 at 19:29 Mark Rand Said:

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.

When one of the phases between a VFD and the motor lifts off, by design or accident, there’s a high risk the winding will react to the pulse input like a car ignition or Tesla coil,  resulting in the coil outputting several tens of kilovolts.   Whilst the ordinary IGBTs in a modern VFD might well survive such an event,  voltage spikes of that size won’t do the motor any good!   Magnet wire insulation is likely to have tiny holes punched in it.  Might cause instant magic smoke, more likely each incident will cause the winding to slowly degrade due to shorted turns etc.   Older motors are more vulnerable than new ones.   If a contactor is in circuit that too will get the kilovolt treatment, also likely to damage it in some way.

Probably other risks too: mechanically damaging the motor, say by the voltage spike earthing through and pitting a bearing surface; or zapping the operator by jumping into the low voltage system.

I’ve read manuals that explicitly said the VFD must always be directly wired to the motor, and a few that don’t.   But the manuals that are silent on the need for fixed direct connection all have circuit diagrams showing only that, and don’t discuss alternatives.   An optimist might see that silence as confirmation that switching phases after the VFD is OK;  I don’t, perhaps because I’m a cowardly lion!

Also possible that I’m out of date!   Mark highlights the massive improvement in IGBTs, and it’s true the current generation of these devices has impressive high-voltage and high-power performance.   It’s also possible that the VFD circuitry that exploits IGBTs has improved whilst I was dozing such that a loose phase is detected and automatically managed without risk to the electronics, motor or operator.     Easy to forget how quickly electronic engineering develops compared with mechanical, and how rapidly features that cost a fortune a few years ago are quietly bundled into the economy model, at no extra cost.   Unwise though I think to assume that such improvements apply to all VFDs on the market and that amateur electricians can therefore do what they like without consequences.   That a circuit apparently works does not mean all is well: any damage being done may take months to appear.   A simple example is what happens when the capacitor across the centrifugal switch in a single-phase motor loses capacity.   Although the motor apparently runs normally, the contacts are wearing rapidly due to sparking, and one day the motor won’t start.   Replacing the capacitor is cheap and easy, but fixing a spark eroded centrifugal switch could be nasty, perhaps to the point of having to replace the motor.

Dave