ML7 motor – Tyco Crompton – wiring/burnt out?

[matt_ml7Participant @matt_ml7]

Hi,

I have a lathe with an electrical issue that I'm trying to diagnose. It's a Crompton 370W single phase motor. The owner's son in law told me there was water ingress at the sockets on the wall and last time the lathe was used there had been a pop/flash – I don't know where it was.

Initial checks:

• spindle turns freely – sounds a bit dry.
• plug fuse (5A) – ok.
• connectivity from plug to switch housing – ok
• switch housing – some damp internally, dried out
• overload cutout was tripped, now reset – ok
• locking mushroom Isolator, one terminal badly corroded, return spring fallen out – now repaired – ok

I tried powering it up, the motor hummed for a second or two then blew the 5A fuse. I replaced the fuse, tried again spinning the motor shaft by hand just before powering – same, hummed for a second or two then blew the 5A fuse. In hindsight, I might have performed the checks below before powering it up.

I unplugged and removed the cover plate on the back and noted the wiring:

• motor BLUE — Z2 — supply NEUTRAL
• motor YELLOW — S
• motor BLACK — U2
• motor RED — U1 — supply LIVE

Additionally:

• link wire U1 — S
• link wire U2 — Z2

The label on the inside of the cover plate:

• BLUE — Z2 — START
• YELLOW — S — START
• BLACK — U2 — RUN
• RED — U1 — RUN

So I assume U1/U2 is the primary winding, S/Z2 the start winding. There are no capacitors.

I removed the supply LIVE and NEUTRAL wires and both link wires.

• U1/U2 measures approximately 5 Ohms.
• S/Z2 measures as an open circuit.

My limited knowledge suggests the wiring is correct and either the start winding has burned out, or the centrifugal switch is in a failure state.

Can anyone help? Do I disassemble to inspect the switch and measure start winding resistance beyond it?

(I overhauled a similar motor a number of years ago and have a couple more sitting, so subject to shaft diameter for the pulley wheel, I may have a spare.)

[matt_ml7Participant @matt_ml7]

[EmgeeParticipant @emgee]

Mat, you have confirmed either the start winding is burnt out or the centrifugal switch is not closing.

Remove the end plate to check the connections on the centrifugal switch, you may need to remove the connection plate or other cables first.

Emgee

[Neil WyattModerator @neilwyatt]

> centrifugal switch is in a failure state.

Quite likely. Worth a careful look.

Actually, well worth a look inside as we have had people report older motors where rubber insulation has perished.

Neil

[matt_ml7Participant @matt_ml7]

Posted by Emgee on 06/02/2018 20:16:47:

Mat, you have confirmed either the start winding is burnt out or the centrifugal switch is not closing.

Remove the end plate to check the connections on the centrifugal switch, you may need to remove the connection plate or other cables first.

Emgee

Thanks, Emgee/Neil.

I am told the motor may have been a replacement at some point. Without further disassembly, there is a spring switch along the inner edge of the connection board, in very good condition – the fixed end tests ok to S (yellow). Testing conductivity between there and the spring arm using pressure on the probe to open and close, it looks like it tests ok. It's closed at rest, and very clean looking.

So unless there is more than one electrically significant moving part, it must be the winding that's gone.

[Pete RimmerParticipant @peterimmer30576]

Could be that old favourite – swarf in the gubbins. Worth pulling the end bell off just to check for that. My first lathe gave a little lightning show because I didn't at the time realise the importance of keeping the swarf out of the vents.

[matt_ml7Participant @matt_ml7]

Posted by Pete Rimmer on 06/02/2018 20:34:36:

Could be that old favourite – swarf in the gubbins. Worth pulling the end bell off just to check for that. My first lathe gave a little lightning show because I didn't at the time realise the importance of keeping the swarf out of the vents.

As I have to remove the motor to install another, I will certainly open it up and take a look.

Thanks, as I expected to be fair but wouldn't it be nice to find something yet…

[Speedy Builder5Participant @speedybuilder5]

I think a lot of ML7 users have the 1/2 Hp motor in preference to the smaller 1/3Hp. This may be the time to upgrade ?

[matt_ml7Participant @matt_ml7]

Posted by Speedy Builder5 on 06/02/2018 21:21:33:

I think a lot of ML7 users have the 1/2 Hp motor in preference to the smaller 1/3Hp. This may be the time to upgrade ?

It's a half that was on it. Is there a disadvantage to 3/4 – too much motor? I'm not sure exactly what I've got to replace it – have to have a look…

[John OlsenParticipant @johnolsen79199]

If I had to replace one of those I would look at putting on a 3 phase motor and a VFD. I'm not sure how the prices would compare…a 3phase motor should be cheaper for the same power, but then you have to pay for the VFD. But overall you end up with a better job. I have the Newton Tesla setup on my ML7, but you can do it cheaper than that if you try.

John

[matt_ml7Participant @matt_ml7]

Posted by John Olsen on 06/02/2018 21:51:18:

If I had to replace one of those I would look at putting on a 3 phase motor and a VFD. I'm not sure how the prices would compare…a 3phase motor should be cheaper for the same power, but then you have to pay for the VFD. But overall you end up with a better job. I have the Newton Tesla setup on my ML7, but you can do it cheaper than that if you try.

John

Thanks, John.

Yeah, that would be nice. I did look into it for my wood lathe a few years ago but never got round to it. Money's tight and I was gifted the lathe, so short term I'm hoping to use one of the single phase motors I've got.

Edited By matt_ml7 on 06/02/2018 22:17:30

[matt_ml7Participant @matt_ml7]

Start winding measures 9.2 ohm's, switch checks out ok too. Bearings feel very smooth.

I can only really clean the contacts, tidy it up and reassemble.

If it was a break and I've shorted it by moving something, I doubt the resistance would read so well…? Also, it sounded dry/squeaky – I don't believe it's the bearings given how they feel, perhaps the centrifugal plate against the pad of the switch? I could put a drop of silicone spray on it or something.

[EmgeeParticipant @emgee]

Matt, squeaky sound is common with the switch arrangement.

Emgee

[matt_ml7Participant @matt_ml7]

Posted by Emgee on 07/02/2018 11:40:05:

Matt, squeaky sound is common with the switch arrangement.

Emgee

Thanks, Emgee.

Reassembled, motor starts and runs fine. Confirm there is a dry-sounding rub when the centrifugal switch closes again before stopping, but sounds like a clean running motor.

Thanks for the comments, all.

Matt.

[EmgeeParticipant @emgee]

Matt, it would be a good idea to make sure the new sounding rub is not any part rubbing on a wire, just in case.

Emgee

Edited By Emgee on 07/02/2018 14:02:02

[matt_ml7Participant @matt_ml7]

Posted by Emgee on 07/02/2018 13:51:31:

Matt, it would be a good idea to make sure the new sounding rub is not any part rubbing on a wire, just in case.

Emgee

Edited By Emgee on 07/02/2018 14:02:02

Thanks for the nudge, good point.

No, wires are all held clear of moving parts, I can poke a finger in to depress and release the plate and then twiddle the spindle a few degrees back and forward. I think I'm comfortable with it.

[Robert TretheweyParticipant @roberttrethewey]

Perhaps this image may well assist you – please look up ritasears.blogspot.co.uk and go to the workshop tab then go to the bottom of the page last image should be what you're looking for.

If you by chance have a coolant pump which has a 4 position switch on the electric motor top and could send me an image of the internal wiring I'd be so grateful.

Bob Trethewey

[Author]

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