240v 1ph 1600W Table saw motor wiring question

[Steve Blackdog]

Hello everyone,

This is my first post, so please bear with me. I am trying to work out how the motor to my table saw is wired. I bought it second hand and I think the previous owner has messed with it. One of the brush holders had been forced in and was touching the commutator. I am wondering whether to bin the motor and replace it with a new one. I have replaced the brushes and sorted the brush holder, and the motor turns by hand, but not when switched on.

The winding had burnt out the winding at the connection, which I have desoldered. I am pretty confident that the winding is now ok. The previous owner had replaced the 5amp plug fuse with a 13amp one, presumably when the fuse blew. So it is not surprising that the winding would fail before the 13amp fuse.

I am confused about the way the field windings are connected.

The field has two thicknesses of winding, a very fine one (underneath) and a thicker one on top of the finer one.

The cable from the switch (an NVR, overload switch and small flat capacitor) to the motor has three cores plus earth. I can't work out from the switches how the mains cable two core plus earth, changes to 3 cores plus earth.

One core goes to the one of the carbon brushes and one goes to the thin winding wire and one to the thicker winding wire. There is another wire coming from the thicker winding wire which goes to the second carbon brush.

What I can't work out is:-

1) why is there a single wire going to the thin winding wire? What is the other end attached to? Is this a starting winding or something? Is this 'extra' wire a switch wire?

2) is the wire coming from the thicker winding wire to the carbon brush attached? Is the thicker winding attached only to the incoming and out-going core?

3) are both the cores coming into the field both live or neutral? (or one of each?)

4) if I wanted to bypass the switches and the mains cable and connect the motor up to a 12 v car battery to try it out, can I connect two of the cores to the + and one to the - ? Or is that a bonkers idea?

Thanks in advance,

Steve

 

[Darkwood]

Welcome to the forum

Your post although imformative with regards to your confusion it doesn't tell us much about the motor, there are so many variations out there we cannot really advise you on the info given, is there a motor plate with details, single phase motors can work in many ways so unless we can figure out what you have then we will be guessing with our replies.

 

[Steve Blackdog]

Thank you for the kind welcome!

If only there were any available info about this motor. It is a SIP 01930 table saw. The motor has no plate on it at all. It is made in China to a low budget

I will try to take some photos to show what I am talking about.

Best regards

Steve

 

[Lucien Nunes]

The extra field could be either for self-excited regenerative braking, or a compensating winding; In either case I would expect the motor to run as a series universal without it. If there are no electronics involved e.g.soft-start, the switching probably applies mains between one end of the series (heavy) field and the armature (you indicate that the series field is in one piece, rather than half in each brush lead as is more usual.) I would take a guess that the leads from the motor are line, capacitor and common neutral.

Disconnected from the switching, the main circuit (arm and series field) might well run slowly from a car battery if there's not too much friction. But testing continuity through the circuit should be sufficient to find the fault (resistance will be variable due to carbon brushes).

FWIW Lucien's Second Rule of DC Machines states that most machines of any voltage between about 1/2cwt and 2 tons weight will do something when connected to a car battery. See: https://www.youtube.com/watch?v=oTeAp6sEHSI

 

[Steve Blackdog]

Here's an update, I am pretty sure everything is wired as it should be. I have checked all the connections and they seem to be ok. The capacitor is around 30mm x 20mm x 6mm. It says on it 0.33mF K X2 and under that MEX GPF 40/085/21/C 250v~ 275v~.

Now I reassembled everything, the motor hums loudly, for a couple of seconds max, doesn't turn then it blows the fuse in the plug.

I note the following advice on a motor manufacturer's trouble shooting page:

Stator is shorted or went to ground. Motor will make a humming noise and the circuit breaker or fuse will trip.

Disassemble motor and inspect windings and internal connections. A blown stator will show a burn mark. Motor must be replaced or the stator rewound.

I was wondering if the capacitor needed replacing, but this points the finger at the motor. Unless it is something obvious, I'm not sure I will be able to spot a problem with the stator windings.

I can get a replacement new motor from the saw manufacturer for around £75, so I am thinking that may be the best way forward.

I don't want to do that if it could be something cheap (e.g.capacitor) causing the problem. Also I don't want to spend £75 to find it makes no difference

Any advice very gratefully received.


Cheers

Steve

 

[Steve Blackdog]

Hi Folks,

Me again.

To save anyone spending their valuable time answering my questions, I've decided to go ahead and buy a new motor.

If nothing else, this exercise has revealed to me:

a) there's no such thing as a real bargain;
b) if I'm having to ask questions like this, I probably shouldn't be messing around with electricity!
c) I have forgotten much of what I knew when I did O' level physics; and
d) this seems a really nice forum with sensible folk, who have time for newcomers

Many thanks for help.

Steve

 

[PEG]

Hi dude,
a) 99% correct
b) possibly
c) I can remember most of it,loved the subject! (did 5 years of French-couldn't give you 10 words...)
d) a hasty,rash and sweeping statement...but pleasant,and gives us something to aspire to...

Good luck with your equipment,and take care,a young pal of mine,removed his thumb,to the wrist,with such a device! :nonod:

 

[sean_ork]

Disconnect the drive from the motor, so there nothing attached to its spindle - with mains off check its mechanically free to rotate - apply mains and hand spin the spindle (before the fuse goes) - if it runs then it might be needing a start capacitor - this might have been removed for replacement.

Google start capacitor for an image

 

[Lucien Nunes]

It's a universal motor though Sean, so doesn't have a start capacitor!

I can see the sense in cutting losses and replacing - sometimes life is just too short (-circuited). The troubleshooting guide that made that rather broad generalisation is probably referring to induction motors, not universals, but the result is coincidentally similar - windings probably FUBAR. It was worth a quick look though.

 

[sean_ork]

Seems daft spending close to £100 for a new motor when the entire saw can't be much more than £180 ?

And its a crap saw

 

[Steve Blackdog]

Seems daft spending close to £100 for a new motor when the entire saw can't be much more than £180 ?

And its a crap saw

Mmm. Maybe you're right - it is a crap saw and perhaps I should cut my losses.

 

[Barry White]

quote Lucien "It's a universal motor though Sean, so doesn't have a start capacitor!"

Lucien may have hit the nail on the head its not a start capacitor its for suppression and it may have broken down and shorted (seen this a few times)
May be worth just removing it.

The fuse 1600/230=6.95A so should have a 10A or 13A fuse (but should have run without load on a 5A)

It does sound like the windings or more likely the armature windings are shorted

 

[Steve Blackdog]

I have just been in the garage and messed about with the motor a bit more.

I discovered two things:-

1) if I take the armature out and turn on the power, there is no short, or it is not apparent - the fuse does not trip. Is that to be expected? I also put a multimeter on the armature and can find nothing obviously wrong - no sign of melting etc

2) I took the brush holders out and noticed that they are both slightly melted at the business end. I also noticed that they can be moved in and out. They were in as far as they would go.

Tomorrow, I am going to see what happens if I reassemble the motor, but with the brush holders 2-3 mm further out. I am wondering whether the brush holders are actually touching the commutator, or the brushes are pushed so hard against the armature that it is not turning as easily as it should.

Does that sound like it will make any difference?

Cheers

Steve

 

[Barry White]

1) amature or brushes missing = open circuit as the armature is in series with the coils
2) The brush holders should be clear of the armature which should spin freely
If you have a simple meter that measures ohms you could check the armature like this https://www.youtube.com/watch?v=qNV7TylIzSY

 

[Steve Blackdog]

Ah! Of course! I think I will try it with a light bulb between the wires going to the brushes.. that should prove whether the fields are ok, won't it?

 

[Steve Blackdog]

Update:

I stripped everything down again and put it back together, but this time with the brush holders clear of the commutator.

I can see now that the brush holder fouling the armature was the root of the problem, as with the brush holders back (with new brushes), the armature spins freely and much more smoothly. However, I'm not sure whether this has done irreparable damage to the motor.

I switched it on and hey presto... the motor turned.

2 issues though,

a) it was very slow, seemed to slow; and
b) smoke started to come out of the motor.

The fuse no longer blows. I ran it for maybe ten seconds or so.

When I opened it up, I couldn't see the source of the smoking. The armature was pretty cold and the field was warm to the touch.

The fine wire in the field was burnt out when I got the motor, I soldered it up again and I don't think it is shorting, but I guess it could be somewhere inside the coils.

What do I do next?

Cheers

Steve

 

[Lucien Nunes]

Probably bin it. When a coil has blown open at an exposed point, it is likely that the inner turns of the coil that cannot dissipate heat as easily will have damaged insulation between turns. In an AC motor, even if only a small percentage of the coils short together, transformer action induces heavy current in them, resulting in overheating and general failure. This is probably why the field was hot when the armature was cold. It's difficult to prove that without test equipment, and the cost and time involved out of proportion to replacement.

 

[Steve Blackdog]

Thanks for the advice. In the bin it goes!

 

[Steve Blackdog]

Final update:

Lucien may like to know that I followed his advice and bought a replacement motor. All's well that ends.

Thanks again for the help guys.