TCC Plessey Box Capacitor Question

[Trevanion]

So I recently bought a single phase Wadkin BRA 10” radial arm saw that has a 20uF TCC Plessey box capacitor between the control gear and the motor.

The saw takes about ten seconds to get to speed and it makes a bit of gravelly noise on startup, so I’m unsure whether the capacitor has gone weak with time or if it’s just normal for these, but it doesn’t seem right.

I tried a modern 50uF capacitor (I know, too big but I thought it would be fine for one start) in its stead and it did start the motor in same sort of time and noise, then it popped.

Curious if anyone else has experience with these capacitors, or the Brook switchgear, which I’m not entirely certain is wired correctly.

As an aside, I love this old switchgear, very heavy duty and much more serviceable than the new stuff.

 

[Guineafowl]

I’ve not seen a single phase 10”. Pic?

20 uF is either a very small start cap, or a start/run unswitched type. Is there a centrifugal switch in the motor? Can you post a pic of the motor connection box and plate, and ohm out the connections, see what’s there?

I’d use a capacitance meter to check it’s within 17-23 uF spec, then perhaps a 1000V DC leakage test. In the meantime, perhaps best to order a decent modern run-type motor cap.

Top left in the diagram is just a standard contactor/overload arrangement, with one side looped round again to balance the three paths, as mentioned in the text below, no mention of the cap. (The C in the circle is the contactor coil). It looks like only L and N go towards the motor, so any cap connections must be made after that. I assume the motor will have three connections, L, cap-shifted L, and common N.

Maybe just unwire it and start again, if you think someone’s been fiddling.

I’m not sure why your 50 uF popped. Maybe it’s not a run type, or just faulty. You should, in theory, be able to connect a good run type one straight across the mains, and it’ll just sit there passing current. Maybe by overspeccing, you brought the motor coil/cap combination close to LC resonance, causing voltage spikes. The motor may not have enjoyed that.

 

[Trevanion]

Looking at the diagram in the motor case it looks like it’s not wired correctly there either? On the contactor, the brown live wire goes in but underneath it comes out blue and goes to blue on the motor through the capacitor, and the blue neutral wire goes in and comes out as red, then to brown from the capacitor to the motor.

Am I right in saying that the diagram indicates that the live should be connected to A1, not A2? And the neutral connected to A2 and not Z2?

 

[Guineafowl]

Am I right in saying that the diagram indicates that the live should be connected to A1, not A2? And the neutral connected to A2 and not Z2?

Yes, that’s how I’d read it, although reversing L and N shouldn’t make any difference. A1 and Z1 are strapped together, so at least that part’s right.

So it’s a permanent split capacitor type (PSC), and will need a run-rated 230V rms/450V peak 20 uF motor cap. No centrifugal switch, in that case.

 

[Trevanion]

Yes, that’s how I’d read it, although reversing L and N shouldn’t make any difference. A1 and Z1 are strapped together, so at least that part’s right.

So it’s a permanent split capacitor type (PSC), and will need a run-rated 230V rms/450V peak 20 uF motor cap. No centrifugal switch, in that case.

I just tried swapping the wire from Z2 to A1 and the motor is now spinning backwards.

This was the capacitor I tried, unfortunately a lot of the info has worn off but you can just about make out 50uF and 280v. Possibly incompatible with what I was trying to do with it.

You’ll probably laugh at this setup, but I think it’s the same as the existing capacitor, which may be wrong anyway, I really don’t know.

 

[Guineafowl]

You’ve just got the cap in parallel with the mains there. Diagram incoming.

 

[Guineafowl]

Keeping those black and red wires you already have, coming from the bottom of the contactor/overload:

 

[Trevanion]

So just to be clear, because I am in fact a simpleton, the black wire (which in this instance is neutral) directly to A1, and red (Live) to the capacitor going to Z2, with a second wire going to A2?

Potentially the existing capacitor is wired incorrectly then? Because that has live going to one terminal and coming off the same terminal to the motor, and neutral going to the other terminal and also coming off that terminal to the motor.

I did actually notice in the motor connections that there were three old-type crimp connections that had been cut off but the crimps left behind. So perhaps the last person who messed with it last missed a wire out?

 

[Guineafowl]

So just to be clear, because I am in fact a simpleton, the black wire (which in this instance is neutral) directly to A1, and red (Live) to the capacitor going to Z2, with a second wire going to A2?

Yes, so if you arbitrarily number the cap terminals 1 and 2:
1 connects to the red wire AND to A2.
2 connects to Z2.

I’d prefer Live to be on the left, as it should be the switched one for the start circuit, but never mind for now.

I can’t quite see how it was wired originally, but at the cap there should have been 2 wires coming from the contactor, and three wires out to the motor (plus earths).

 

[carbon]

This was the capacitor I tried, unfortunately a lot of the info has worn off but you can just about make out 50uF and 280v. Possibly incompatible with what I was trying to do with it.

Does this capacitor have intermittent marked on it? The voltage to me indicates that it would be a start capacitor. Leaving that in circuit wont be good.

 

[Trevanion]

Does this capacitor have intermittent marked on it? The voltage to me indicates that it would be a start capacitor. Leaving that in circuit wont be good.

Yes, it did. It blew up on startup anyway, so not to worry!

Yes, so if you arbitrarily number the cap terminals 1 and 2:
1 connects to the red wire AND to A2.
2 connects to Z2.

I’d prefer Live to be on the left, as it should be the switched one for the start circuit, but never mind for now.

I can’t quite see how it was wired originally, but at the cap there should have been 2 wires coming from the contactor, and three wires out to the motor (plus earths).

Ah, I think I get it now. So the red wire comes from the contactor to one of the capacitor terminals and goes off to A2, then the second terminal connects to Z2, black goes directly to A1. I'll give it a go tomorrow. I'm going to rewire the whole thing anyway because as you say, it doesn't seem correct, and its a bit of a mish-mash of different wire thicknesses with quite thin cable going from the plug to the contactor and to the capacitor, then much thicker three-core cable going to the motor itself, but I just want it running properly first. I'll probably get some four-core SY cable to go from the capacitor to the motor, I'm not sure if the plastic tubing is original or not or whether it would've been metal sheathing as goes from the contactor box to the capacitor.

Also, as requested:

 

[Guineafowl]

Yes, it did. It blew up on startup anyway, so not to worry!



Ah, I think I get it now. So the red wire comes from the contactor to one of the capacitor terminals and goes off to A2, then the second terminal connects to Z2, black goes directly to A1. I'll give it a go tomorrow. I'm going to rewire the whole thing anyway because as you say, it doesn't seem correct, and its a bit of a mish-mash of different wire thicknesses with quite thin cable going from the plug to the contactor and to the capacitor, then much thicker three-core cable going to the motor itself, but I just want it running properly first. I'll probably get some four-core SY cable to go from the capacitor to the motor, I'm not sure if the plastic tubing is original or not or whether it would've been metal sheathing as goes from the contactor box to the capacitor.

Also, as requested:

Nice! Looks all seized up. That tilt clamp is open but it hasn’t dropped. Is it missing the stop at the end of the track?

Mine had the original motor cable sheath - metal flex hose (?), a bit like shower hose anyway. I replaced it with SY, which should be fairly robust, although H05 or H07 RNF would be even tougher, but maybe too thick.

 

[Trevanion]

Nice! Looks all seized up. That tilt clamp is open but it hasn’t dropped. Is it missing the stop at the end of the track?

Yes, it's very crusty, the back legs particularly so. The 10" BRA is a little bit different from the ubiquitous 14" BRA, that cast piece that's screwed on the front of the arm is the "stop". Fortunately, it seems the runners and concave bearings seem to be in relatively decent condition and should clean up OK.

 

[Guineafowl]

Yes, it's very crusty, the back legs particularly so. The 10" BRA is a little bit different from the ubiquitous 14" BRA, that cast piece that's screwed on the front of the arm is the "stop". Fortunately, it seems the runners and concave bearings seem to be in relatively decent condition and should clean up OK.

Those concave bearings are expensive from Dalton’s, to say the least. I made the mistake of using compressed air to clean them, and drove the crap in deeper.

Seeing as you’re a Wadkin and Hardinge man, what about a Fluke meter for diagnostics (capacitance check), say a 115? Maybe a Megger for insulation testing all those motors?

 

[ukracer]

I just tried swapping the wire from Z2 to A1 and the motor is now spinning backwards.

I wish mine would.. Lol

 

[Trevanion]

Seeing as you’re a Wadkin and Hardinge man, what about a Fluke meter for diagnostics (capacitance check), say a 115? Maybe a Megger for insulation testing all those motors?

So I wired it as per your expert instructions and the motor fires up with complete ease now, though the start box isn’t working correctly now and I’m pushing the contactor in with an insulated screwdriver, I’m guessing it’s probably more incorrect wiring, or it doesn’t like being sat flat on its back as I have the motor and box on the bench now as I’ve taken the frame to be sandblasted.

 

[Guineafowl]

So I wired it as per your expert instructions and the motor fires up with complete ease now, though the start box isn’t working correctly now and I’m pushing the contactor in with an insulated screwdriver, I’m guessing it’s probably more incorrect wiring, or it doesn’t like being sat flat on its back as I have the motor and box on the bench now as I’ve taken the frame to be sandblasted.

If you’ve ordered a new run cap, it might be small enough to fit in that huge start box, which would be neater. What’s the contactor doing, not pulling in, not staying in?

Might be a bad contact or loose wire. Again, a Fluke would help, failing that, the UNI-T offerings aren’t too bad. With dirty contacts, I fold a piece of paper up, spray it with contact cleaner and work it back and forth while pinching it between them. Sandpaper will embed grit in them and could make things worse.

Now, how do you think I got some experience with old motors?

 

[Trevanion]

If you’ve ordered a new run cap, it might be small enough to fit in that huge start box, which would be neater. What’s the contactor doing, not pulling in, not staying in?

Might be a bad contact or loose wire. Again, a Fluke would help, failing that, the UNI-T offerings aren’t too bad. With dirty contacts, I fold a piece of paper up, spray it with contact cleaner and work it back and forth while pinching it between them. Sandpaper will embed grit in them and could make things worse.

Now, how do you think I got some experience with old motors?

I didn’t buy another capacitor yet as I did have a small hunch the wiring was wrong. From what I’ve seen online these big capacitors rarely fail, when they do they seep oil out apparently.

It’s just not latching at all when you press the “go” button, so I assume the control wiring is also wrong somewhere, when you push the contactor in it actually latches in place so there can’t be much wrong with it. I already cleaned the contacts with a bit of grey scotchbrite, which may or may not have been a good idea!

 

[Trevanion]

I figured it out, there was a neutral wire missing that should have come out of the control circuit. I haven’t got a clue how the control circuit was even operating before as it’s not something I’ve even removed or messed with, everything I’ve done has been below it!

 

[Guineafowl]

I figured it out, there was a neutral wire missing that should have come out of the control circuit. I haven’t got a clue how the control circuit was even operating before as it’s not something I’ve even removed or messed with, everything I’ve done has been below it!

Maybe it was wire no.2 (latching circuit) in the top left diagram, and it was friction latching it on when the box was vertical? Weird. Put it down to the phantom fiddler.

I’m still running the ancient, original Dubilier capacitor in the Wolf grinder, so agree if it works, use it