Table saw - single phase motor burnt out

[r8hil]

Hi,
I was hoping some motor experts might be able to help me here. I have a single phase motor off an old Kity Table saw that has recently started to struggle speeding up and I opened it up to find some of the windings burnt. I'm not sure how this happened but it's no longer running effectively and I guess it needs a rewind? Has anyone done this themselves, and if so, how do you varnish the windings?
Thanks

 

[dalmatiangirl61]

Have you ohm'ed the windings to verify they are burnt, or do they just look burnt? Theoretically you can rewind it yourself, but most of us would leave it to a motor rewinder. The windings are varnished in a tank with a vacuum applied so that there are no air bubbles trapped in the windings. There are vids on diy'ing a motor rewind if you want to give it a try. If its a plain jane motor its probably cheaper to just replace with a new/used motor.

 

[r8hil]

Some of the run winding's coating appear black, what do I need to check for with the multi meter?
Thanks

 

[dalmatiangirl61]

Main thing is checking for continuity thru each winding, next check that each run winding has the same ohm reading. A burnt winding can show as an open circuit (no continuity), or it can short to another wire so it shows continuity, but ohm reading is off. Also check for continuity between windings and the case, there should be none. If single phase, check contacts in centrifugal switch. If capacitor start/run, replace capacitors.

 

[r8hil]

That's another thing I'm confused about! It's a single phase, there is a starter capacitor, but not a centrifugal switch so I don't understand how the starter coil gets disconnected when up to speed?

 

[dalmatiangirl61]

I've seen that, but not qualified to explain it, you can probably find more info by researching it. If ohm readings look OK, slap a new capacitor on it and give it a try, if there is any doubt about the bearings, replace while you are there.

 

[r8hil]

I've seen that, but not qualified to explain it, you can probably find more info by researching it. If ohm readings look OK, slap a new capacitor on it and give it a try, if there is any doubt about the bearings, replace while you are there.

The run winding cables are 2ohms and the start windings are 14ohms? I have already changed the capacitor as I originally thought that was the problem but it made no difference.

 

[dalmatiangirl61]

Sounds like you will need a new motor, or get that one rewound. Quick research says that saw is made in France, so are you in Europe or USA, or????? Add your location to your profile, this is a perfect example of why.

 

[JST]

That's another thing I'm confused about! It's a single phase, there is a starter capacitor, but not a centrifugal switch so I don't understand how the starter coil gets disconnected when up to speed?

It presumably is NOT a "start" capacitor, but a "start/run" capacitor. That also suggests that the motor may be a "PSC" type, which needs no start switch.

Those are quite popular in europe, possibly because they use a more reliable capacitor type than typical capacitor start motors, have a better power factor, and they avoid the mechanical switch. They tend to have low start torque, but that may not be important for a saw, which is allowed to start up before any particular load is put on it.

If it is in fact burnt, rewinding is probably not an option if in the US. It tends to be very expensive, and most motor shops in the US will only do it for larger motors, or in some cases, motors that are very hard to replace. What the situation for rewinding is in other places, I do not know.

 

[r8hil]

I am in the UK, I will look into rewinding. I can't really replace the whole motor as the spindle and pulley size is very specific to this machine!

 

[r8hil]

Sounds like you will need a new motor, or get that one rewound. Quick research says that saw is made in France, so are you in Europe or USA, or????? Add your location to your profile, this is a perfect example of why.

Sorry, I just realised I previously measured the resistance with the switch open! Both windings now have infinite resistance. Does that seem right?

 

[JST]

Sorry, I just realised I previously measured the resistance with the switch open! Both windings now have infinite resistance. Does that seem right?

The descriptions are confusing..... No it does not seem right.

It would help to see some pictures of the inside of the wiring connections box on the motor, and of the original capacitor. Also of the "burnt" windings.

If it really is a capacitor start motor, the original measurements seem backwards. Usually the capacitor winding is lower in resistance, and there usually is less difference between them. Motors that do NOT use a capacitor usually have a higher resistance in the start winding, the resistance creates the necessary phase shift vs the main winding.

It is possible that the motor might be of the type that has a higher resistance start winding and has an electronic switch unit. Those electronic "switches" can be hard to locate, and may not look like anything special, often just a blob of plastic with wires or connections coming out of it. Might even be confused with a capacitor.

"Special" motors, if not available as replacements, are worth rewinding as long as the cost is less than replacing the whole machine. That may be the case with yours.

But some pictures are really needed to identify what you have, help determine if there really is a problem, and possibly identify it.

 

[r8hil]

The descriptions are confusing..... No it does not seem right.

It would help to see some pictures of the inside of the wiring connections box on the motor, and of the original capacitor. Also of the "burnt" windings.

If it really is a capacitor start motor, the original measurements seem backwards. Usually the capacitor winding is lower in resistance, and there usually is less difference between them. Motors that do NOT use a capacitor usually have a higher resistance in the start winding, the resistance creates the necessary phase shift vs the main winding.

It is possible that the motor might be of the type that has a higher resistance start winding and has an electronic switch unit. Those electronic "switches" can be hard to locate, and may not look like anything special, often just a blob of plastic with wires or connections coming out of it. Might even be confused with a capacitor.

"Special" motors, if not available as replacements, are worth rewinding as long as the cost is less than replacing the whole machine. That may be the case with yours.

But some pictures are really needed to identify what you have, help determine if there really is a problem, and possibly identify it.

I did previously try to attach some pictures but it said the files were too large? I'll try again.

 

[r8hil]

 

[r8hil]

 

[r8hil]

 

[jbacc]

In my humble and non-expert opinion, the windings look like toast. I would be looking to have a motor shop rewind it or find a replacement.

 

[JST]

One winding is clearly very toasted. The other was apparently not fried, so there is likely a problem with whatever system is used to start the motor.

it looks as if replacement or rewinding is needed.

The switch... is it only a switch, or might it have added functions?

Do you have any parts list etc for the saw that may call out the switch?

Any pic of the capacitor you replaced?

 

[r8hil]

I believe the blackened wires are the run windings. The switch is very basic inside, don't think it has any other functions. Also, the yellow wire from the switch box has perished insulation and is corroded, but still has continuity? Not sure if that's an issue.

 

[JST]

I believe the blackened wires are the run windings. The switch is very basic inside, don't think it has any other functions. Also, the yellow wire from the switch box has perished insulation and is corroded, but still has continuity? Not sure if that's an issue.

That was as I figured.... bad start system, so run winding is the only one getting current, and it fries due to the motor not starting.

No sign of any other possible switching for the other (start/run) winding suggests that either there was an open capacitor, or some other interruption of the circuit to it (bad connection, etc). Also possible that a stall when using the saw led to excess current, since the capacitor may not allow enough current to burn the winding.

Cause is not the first issue, which is to get a good motor, or fix that one.