Defective Electric Motor

[James-W]

I had a 220 volt 3hp single phase motor on my table saw and it took a dive a few days ago. I bought the motor back in 1999 so it is 16 years old and had worked flawlessly for all that time. I bought a new motor (same type) and replaced the defective one, but I was thinking that maybe, depending on the problem, I might be able to repair the bad one and keep it as a spare.

The motor runs, but it runs really slow and it hums really loud. I thought maybe one of the capacitors (there are two of them) might be bad. I can remove the capacitors, take them to the place where I used to work, and test them with the capacitor checker they have. But I have my doubts that a bad capacitor would cause the problem of running slow. I would think a bad capacitor would cause it not to start to turn, but once it started turning it would run normally. But not being an "electric motor repairman" I am not sure of that. Any ideas as to what the problem might be?

 

[Zrexxer]

Bearing(s)?

 

[larry_g]

I think maybe a bad cap will cause your problem. A blown cap can be obvious if it has vented, so looking at it may reveal the problem. Also do not ignore the start switch as it may be open and not putting the start cap in the circuit. The start switch not opening can also cause the start cap to stay in circuit and cause the cap to blow.

lg
no neat sig line

 

[C96]

It might also have a bad centrifugal switch, check that as well

 

[C96]

Oops, beat me to it...

 

[nehog]

...
The motor runs, but it runs really slow and it hums really loud. I thought maybe one of the capacitors (there are two of them) might be bad. I can remove the capacitors, take them to the place where I used to work, and test them with the capacitor checker they have. But I have my doubts that a bad capacitor would cause the problem of running slow. ...

A bad cap will do just that.

 

[James-W]

I will remove the two capacitors and take them to where I worked and check them out, hopefully tomorrow if all goes well. The motor shaft turns easily with no noise at all (turned the motor shaft by hand with no power applied) so the bearings are apparently OK. If the capacitors are OK I will take the motor apart and check the start switch and anything else that may look suspicious.

 

[Tdubyac]

If you can spin the shaft by hand you can bypass the centrifugal switch and starting cap and the motor should start fine, if the starting cap is the problem.

 

[Alan Douglas]

You can "pull start" the motor with a few turns of rope around the shaft, but DO NOT loop the rope around your hand for a better grip.

 

[Mr. T]

If it's a 16 year old motor I'd spend the extra $30 and replace the bearings while you're at it. Easy job and cheap insurance if you plan on using it again.

 

[James-W]

I removed both capacitors today and took it to where I used to work. They have a capacitor checker and I checked both capacitors and they are both OK. I took the rear cover off the motor and checked the start switch, but nothing was wrong there either. The switch contacts are good and the centrifugal mechanism moves freely without binding. I am not sure what to check next.

 

[LS6 Tommy]

Ohm it out. Probabaly a bad winding. Check resistance each from Start, Run and Common to the motor chassis. If any one of them gives you low impedance continuity, that winding is shorted to ground. If it reads infinity, the winding is open. It could be a winding-to-winding short, but without knowing the factory impedance of the windings it's hard to check.

Tommy

 

[James-W]

Ohm it out. Probably a bad winding. Check resistance each from Start, Run and Common to the motor chassis. If any one of them gives you low impedance continuity, that winding is shorted to ground. If it reads infinity, the winding is open. It could be a winding-to-winding short, but without knowing the factory impedance of the windings it's hard to check.

Tommy

I agree, without knowing what the readings should be it would be difficult to know if I found the problem or not.

The thing is, I already bought a new motor so I don't really need to repair this one. It would be nice to have a spare 3hp motor, but for all I know I may never need it. If I could repair it cheaply, I would do it in a heartbeat. But if it is going to be expensive then I am not so sure it would be worth it.

 

[Mr. T]

Have you run the motor by itself, unattached to a load, while measuring how many amps it's drawing?

 

[Dagny]

Get a new start capacitor any way. bet it starts.

 

[James-W]

Have you run the motor by itself, unattached to a load, while measuring how many amps it's drawing?

I have tried running the motor with no load, but I have not tried to measure the amps it was drawing. The motor runs slowly and has a very loud hum.

 

[James-W]

Get a new start capacitor any way. bet it starts.

I can buy a new start capacitor and install it, but with all due respect, I seriously doubt it will make any difference. I have never known the Sencore capacitor checker to be wrong in all the years I was working. That checker is a very expensive piece of test equipment and it performs several tests for capacitors, coils and transformers. For capacitors it tells you the exact amount of capacitance, leakage, dielectric absorption and ECR of the capacitor.

 

[Alan Douglas]

You're sure the centrifugal switch contacts are closed, using an ohmmeter?

 

[James-W]

You're sure the centrifugal switch contacts are closed, using an ohmmeter?

I didn't use an ohmmeter to check them, but the contacts are not burned or corroded and they do close. I can check them with my meter later today but everything looked just fine when I had it apart. I used a small high intensity LED light to get a really good look at pretty much everything inside the motor, to include the windings, but nothing appears to be burned or shows signs of a problem.

 

[G_P]

Disconnect the centrifugal switch, and then power up the motor and spin it by hand to start it. If it spins up to full speed and does not hum loudly, then you have a problem somewhere in the start circuit. Either a bad cap, bad switch, or bad winding.

 

[James-W]

Disconnect the centrifugal switch, and then power up the motor and spin it by hand to start it. If it spins up to full speed and does not hum loudly, then you have a problem somewhere in the start circuit. Either a bad cap, bad switch, or bad winding.

OK, what I did is, I wrapped some black electrical tape around the contacts of the start switch so it cannot work. Then I put the motor back together again and tried to start it by hand. I tried to start it by hand and it turns slowly and hums loudly. I am about ready to put it away in a corner and forget about it for awhile. I am thinking it is a lost cause, but maybe it could be a project for when I have nothing else to do.

 

[Alan Douglas]

It may need to turn a little faster to start.