Help with a single phase 2KW motor that's blowing fuses

[TopLoser]

Ok...

I have a saw bench with a 2.2KW 220Vac motor. A few weeks ago it began to have trouble starting up, very slow spin up to speed and kept blowing 20A fuses in the process. A few days ago it made a nasty smell and didn't start at all. A quick look revealed that the motor start capacitor (250uF) had popped. Replaced it with a new one and now the motor spins up to speed nice and quick but still blows the 20A fuse.

Any clues please?

[Andy Watson]

Is there a moror-run capacitor too?
Assuming there's switch to remove the capacitor when the motor is up to speed - does it operate?

[TopLoser]

Yes there is a motor run one as well, no obvious swelling of that one.

 Not checked the centrifugal switch yet, I need to drop the motor out of the saw bench to check that.

[johansen]

my guess is the start switch has failed shorted, and that is probably the reason the old motor start cap blew up.

[SeanB]

If the start cap has died, and it still is tripping the mains then either the motor run cap has died ( very likely in these, as they tend to use a cheap one there) and you need a new one. If the capacitor is fine, and the only way to check is to do a capacitance check, and if it is not within 5% of marked value, and it will do an insulation resistance check pass ( 20M with 500VDC applied and no nasty crackling noises from the unit), and the start switch is fine ( a 40W incandescent lamp across the start cap) and opening as the motor runs up to speed then the only other likely fault is a shorted winding.

If the start switch is cactus then you can remove the start capacitor and buy a hard start kit for a compressor. This is a 200uf capacitor and a PTC thermistor, intended for a 6 cycles per hour duty cycle in an airconditioner. Does help with older compressors that get a little grumpy as they age, and for a motor with limited starts it also works well.

[TopLoser]

Thank you all, that's a good set of tips to start off with. I'll give it a try in the morning when it's light.

When the motor trips it takes out all the light in the room and the fuse is about 100m away in a different building, good old fashioned fuse wire not a circuit breaker ffs

[james_s]

I'd definitely suspect the start switch, usually you can hear a distinct click as it re-engages while the motor is spinning down to a stop. It's possible for the contacts to weld themselves and remain closed even if the centrifugal mechanism is still functioning though.

[TopLoser]

Took it apart today and it doesn't look good.

New motor required I think!

[james_s]

Yikes, yeah that's bad. Looks like the start switch failed and burned out the winding. They can be rewound but I'm not sure if anyone does that anymore.

[nctnico]

I can smell it from here so yeah that needs replacing!

[Vtile]

Yep. Magic smoke is gone. 

Rewinding might come to question if the motor is some exotic type (which I don't believe it is).

[james_s]

When I was a kid there was an old fashioned hardware store run by a family friend. They had a guy in back who rewound electric motors of all types, stuff was a lot more expensive back then and it was worth doing. It still might be a fun project, I've never had a motor burn out like that but if I did I'd be tempted to try.

[TopLoser]

A new one is only £100, about $125.

There was no centrifugal switch in this one, just an encapsulated electronic control module with a thriac plus some stuff in it that connected to the two windings. I assume that was what failed and burnt the start winding and capacitor our.

[james_s]

I think I'd be inclined to find an old used motor in good shape over a $125 new one. A friend of mine got a cheap 2HP motor a few years ago and it was crap, the thing ran hot and hummed loudly, he ended up finding a motor from the 1970s that didn't look as nice but it sure runs a lot nicer. Maybe not all the cheap new ones are that bad though.

[Skimask]

...that should buff out...

[Red Squirrel]

The problem is only one winding is burnt, you need to burn the others too so they're all even and you should be good to go. 

[calexanian]

I know that smell. My condolences on your loss. Fortunately those motors are common as can be.

[james_s]

Yeah that smell is one of the worst smells ever, it permeates everything, handle it and your hands will still smell the next day. The only worse smell I recall was a burning out selenium rectifier.

[Gregg]

Sawdust is a major killer of motor windings.  It holds moisture and the moisture leeches chemicals out of the sawdust that attacks the insulation.  If you replace it, you might consider a totally enclosed fan cooled motor.  A used 220 volt motor should easily pass a 1000 volt megohm meter test if it is any good.