Skomo Table Saw - Brook Crompton motor wiring weirdness

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

Misterm

New member
Joined
9 Apr 2021
Messages
3
Reaction score
0
Location
West Sussex
Hi

I wonder if anyone can help.

A while ago, for very little money, I bought a cool little bench saw (Photo 1) with a Brook Compton motor (Photo 2) from the son in law of a deceased amateur carpenter. I saw the saw working at the home of the late amateur carpenter and was looking forward to using the saw at home.

The saw sat in my garage for a few months before I got around to using it. The first time I plugged it in, the saw tripped the RCD it was plugged into. I opened the panel on the back of the motor to see if I could see what was going on (see photo 2). I saw 6 terminal points : From top to bottom : K, Z, AZ, S, A, T. Terminals K & S are blank. The plug was wired: A to Earth, T to Live and I found Neutral loose in the compartment (the wire going off to the right of Photo 3); not connected to anything, assuming it came off on the trip home. Z only has 1 nut, so I assumed the neutral wire had fallen off of AZ as the end looked flat, as if squeezed between 2 nuts (no ring crimp attached and no empty ring crimp on any of the terminals) so tried wiring neutral to that as a test (Photo 3) using new cable and mains plug, wiring earth to A and Live to T as it was wired originally, plugged into an RCD socket all the while. Certainly looked odd to me, what with no earth to the case (bottom left of the compartment) The RCD tripped again.

I removed my wiring to try and make sense of it (Photo 5) I added the letter "T" to phpt, as the printed letter is obscured by the bottom wire.

I have seen similar Crompton motors in other posts on line, but with only 4 wires ( T and Z to the switch and AZ and A to run) (Wiring Up a Brooke Crompton Single-phase Lathe Motor (Myford Lathe) ) but my motor has 6 wires.

The seller knows nothing about this saw, as it belonged to his father in law. He thought the only way to use it was to plug it in... it runs.. unplug it... it stops. So I was just intending on putting a switch box in the cable, until this became very confusing. It looks like A definitely shouldn't be connected to earth, so how on earth was it running at point of sale? I thought of connecting neutral to A... but guessed that would pop the RCD again, as A was already connected to earth and I don't want to damage anything.

There's a momentary switch on the bottom of the motor (Photo 6). I am guessing this switches between 2 speeds? I didn't even notice that when I bought it.

Any pointers would be very helpful.

Many thnaks.

Confused Bob.

P.S. I tried posting this in another forum, but it didn't seem to work... apologies if this is a duplicate.
 

Attachments

  • 1.jpg
    1.jpg
    113.8 KB · Views: 56
  • 2.jpg
    2.jpg
    82.7 KB · Views: 57
  • 3.jpg
    3.jpg
    59.4 KB · Views: 52
  • 4.jpg
    4.jpg
    62.1 KB · Views: 49
  • 5.jpg
    5.jpg
    86.3 KB · Views: 52
  • 6.jpg
    6.jpg
    110.4 KB · Views: 53

Tris

What am I doing here?
Joined
28 Nov 2018
Messages
565
Reaction score
373
Location
Moreton in marsh
Bottom left corner of photo 5 has an earth symbol above what appears to be a blue screw head in a recess. Can't help with anything else on that one though, sorry.

Lookin at the way it's wired at the moment I'd get a friendly electrician to take a look unless you are familiar with electrics
 
Last edited:

guineafowl21

Established Member
Joined
28 Oct 2015
Messages
687
Reaction score
221
Location
Inverness
I suspect the earth wire was being used as a feed from a remote capacitor. When you saw it working, was it plugged into a normal socket, or a ‘mystery box’? Either way, running it unearthed is risky.

The momentary switch may well be a replacement for a broken centrifugal switch - switch on with button depressed, run motor up to decent speed, let go of switch.

Given the above, I would be inclined to strip the motor down and see what’s going on in there. If you can fix the switch, and return the terminals to the standard A and AZ (run), Z and T (start), you have a fighting chance.
 

Spectric

Established Member
Joined
19 Feb 2015
Messages
3,880
Reaction score
2,047
Location
North Cumbria
Hi

You need to either take the motor to someone who repairs motors or strip it down and take a look inside and post more pictures but really Tris has the right idea about seeking help. The fact the supply cable protective conductor is not used for it's intended purpose is a BIG safety issue, if you have any motor insulation issues then because the machine is not earthed it can become live with reference to ground.
 

Sideways

Established Member
Joined
26 Dec 2017
Messages
1,917
Reaction score
702
Location
United Kingdom
Read this thread.
I'm not familiar with this labelling from the old BC motors - haven't played with one yet - but there are a couple of threads around the web discussing them if you google for "A, AZ, K, T, Z motor terminals" or similar.
This seems one of the most useful as someone has gone to the trouble of trying to draw it out.

If you're not already experienced with modern single phase motors, all the usual caveats apply. AC mains can damage you worse than a 16" table saw if you don't know what you're doing...
Cheers
 

Misterm

New member
Joined
9 Apr 2021
Messages
3
Reaction score
0
Location
West Sussex
Thank you for your helpful replies.

I catually posted this in another forum, as I got an error message when I posted here and I thought it hadn't worked.

I connected Live to AZ and neutral to A and earthed the blue earth screw bottom left and the machine runs and runs very quietly. Having read your replies here and on the other forum. I will connect Live to the T terminal, as this is meant to be a thermal switch which then connects to AZ anyway. I also need to get a motor starter connected to this machine to help it run a little more safely.

Tris: Yes, that is now earthed correctly.

guineafowl21: The sockets in the garage looked like regular sockets to me, there wasn't much else in the garage when I was there. If there was a capacitor as yo usuggest, maybe this was in the house near the consumer unit or something. The switch didn't seem to do much, although I only pressed it lightly, I will try it again tomorrow, but I'm happy to control the saw from a motor starter box if I'm honest.

Spectric: I'm having a qualified electrician install a consumer unit in my new workshop soon, so I'll get him to take a look at the machine and my wiring too, to be safe. He also certifies other people's work, so should be great.

Sideways: Great link, thanks, I'll read through that in a wee while. I know the basics of single phase motors, but thanks to you and others that have replied, my learning curve has just become very steep.

Many thnaks again fo ryour replies and have a great Christmas.

Bob
 

guineafowl21

Established Member
Joined
28 Oct 2015
Messages
687
Reaction score
221
Location
Inverness
Could the ‘momentary switch’ be a resettable thermal cutout? If so, there probably never was a capacitor. It must start by having a much higher resistance start winding, with the resultant difference in Q giving the phase shift.
 
Top