rewiring old induction motor - how might termination have been done?

to followup my own post ... ie. I was hoping it is *not* like this, but fearing that it might be:

I was about to reply that you would be unlikely to find a terminal block, what you see is usually the ends of the winding wires.

But that might not apply to any particular device, of course!

What are the wires insulated with? If they're insulated with PVC, I'd guess they might not run that inside the motor (too bulky and they'll get hot and maybe melt the insulation). If they're insulated with something heatproof (enamel, ceramic, ...) then it's more likely they run in the stator.

Theo

Hi Theo the 'external wires' seem to be PVC or similar, according to the vintage of the motor. They are multistrand, perhaps 1.5mm^2 conductor. I presume the stator windings are likely to be single strand enamelled wire ... it is what form the join between those two takes that I am wondering about.

I suspect that there's only one way of finding out what you've actually got.

Very possible.

My current plan is to make a (ahem) temporary repair by joining good new flex wires and shrouding each wire with heatshrink sleeving, as far back as I can go but without dismantling the motor. That will at least tell me if the issue that led me down this rabbit hole was due to the wires shorting where the insulation failed (I could see when this when poking around).

If that doesn't work, or when I have more time on my hands, I will look at removing and dismantling the cable and stripping back 'all the way', whatever that means.

J^n

No.

The likelihood is that the supply cables will simply disappear inside the varnished wrapping of the winding coils. You'd have to strip back that wrapping (having softened it in an oven) before connecting new cables to the terminal points within the windings.

You have it.

I can be reasonably sure. I served an apprenticeship within the electrical engineering industry, majoring in small 3-pahse and single-phase electric motor rewinding.

That is a 3-phase basket-wound stator, but the connections are pretty representative of what you would find if you dismantled your single-phase motor.

The coils themselves are connected up in series, with the ends brought out via the attached properly insulated cables. Two on each of the start and running windings.

Have you tested the insulation between the run and start windings (and between each of them to earth - the chassis) with a suitable instrument, such as a Megger?

Is there a general smell of burning about the windings?

(thanks for your various replies, I have collected them together here)

Good point - I no longer have access to a Megger, but I could at least check with a DVM.

Are you able to say anything about the respective resistance of the 'main' and 'auxiliary' windings? I need to recheck this, but IIRC one was about half the resistance of the other.

I don't think so. There was a slight whiff when things initially failed ... that was another reason to suspect the run capacitor. However that seems to measure OK, and there is no smell to it.

There did turn out to be a slight smell inside the contactor housing ... I am taking that to be due to the short I subsequently discovered between the wires to winding 1 and winding 2. This is perhaps 100mm from the body of the motor. I can't detect any smell nearer to the motor itself.

This motor actually brings out three cables (+ earth): main winding, aux winding, and common.

Yes, I think so. Without dismantling, I can't see any room within the motor housing for a terminal board or similar.

Thanks J^n

That'll help.

The windings are not the same as each other. I would expect the DC resistance of the start winding to be less than that of the run winding because it is smaller.

That's OK - it's effectively the same as two for each.

I'm sure you'll feel your way towards a solution. Don't even think about trying a rewind though! It requires skill and insight gained over a period in a workshop, plus a quite formidable amount of physical resources in the way of tools, machinery, insulating materials, a varnish bath and baking oven plus winding wire (probably two separate gauges in a single-phase motor).

Thanks for the interesting background info. I am very much hoping not to have to go in that direction; if my partial rewire of the external wires does not work then I will have to have a big rethink.

(thinks: this takes me back to my early hobby experiments at age eight or nine, which included making electric motors, galvanometers etc, using cotton-covered wire, a needle pushed through a cork, and so on...)

J^n

Brief update/success report:

I remade connections from the old/perished flex to short lengths of new, with a combination of crimping and soldering, and sleeved the cable to as close to the motor as I was able, with heat resistant sleeving.

I checked that the was no path to ground of all the windings (with a DVM, was not able to borrow a Megger) - all seemed good. The secondary winding (which has the capacitor on it) has a resistance of about 7.5 Ohms, the primary winding has a resistance of about 16 Ohms.

I fitted it all back together within the contactor housing (a bit of a faff with the extra bulk of the sleeving), reconnected the original capacitor ... and the motor starts and runs fine!

So I think I am done. Quite pleased that I didn't have to replace the 45 year old capacitor.

Thanks for the advice/pointers here.

J^n

Well done!

Glad to hear all is OK.

I am unsure if there is a centrifugal switch - i didn't dismantle the motor far enough to tell.

IIRC there is (sometimes?/always?) a bit of a funny slight 'ping'-type noise a second or two after the motor starts. I was thinking it was something to do the the bandsaw blade, perhaps momentarily 'adjusting' itself as it gets up to speed. But perhaps that is a switch ... I will listen more carefully next time.

OTOH there is a little schematic for the bandsaw which simply shows two windings and a capacitor. What determines whether a centrifugal switch is needed, please?

Thanks, J^n

PS: The capacitor is rated 12uF, so according to my reading is a 'run' capacitor, intended to be in series all the time. So no centrifugal switch?

The switch throws itself to "off" under the centrifugal force of the motor being up to speed.

Possible. Tends to be the case only with smaller motors. Single-phase motors used for heavier work (there used to be a lot of it in east London) normally use the start winding only to get the motor started and up to speed, when the switch opens under centrifugal force and leaves only the run winding in circuit.