Help Fixing AO Smith Motor

[kristof]

I have an AO Smith S48B74A03 motor that I'd like to fix.

Here's the story on the motor. It was running fine on the pool but the impeller seal was bad causing a severe leak. I tore everything down and did my best to remove the old impeller from the shaft. Nothing worked. Since the motor and pump assembly was so old we decided to buy a new unit. Thanks to the excellent advice I received on this forum the new install went great and we're very happy with the result.

Since this old AO Smith motor was still in good running condition I decided to cut off the old impeller and see if I could repurpose it for something other than pool use. While I was fiddling around I noticed that there were a few dried up mini wasps nests jangling around inside the motor. Not putting much thought into it I tore the back off and removed the nests. I reassembled and put a cord on to test the motor. It ran fine but after 5-10 seconds it began to smoke. I removed power immediately.

I took it apart to see what I'd done. I had mis-assembled the centrifugal switch mechanism. The ring wasn't properly seated on the little arms so the switch couldn't trip when the motor was up to speed.

I fixed the centrifugal switch and inspected the motor. The coils look fine. The strings holding the coils look fine. The strings show no signs of burning. I reassembled everything and gave it another try.

Same thing. Starts wonderfully, gets up to speed, and smoke after 5 seconds. I immediately removed power.

My only thought is that I fried the capacitor when I ran it with the centrifugal switch mis-assembled. I think this is the cap:

Thoughts? How much is a replacement capacitor?

 

[JohnT]

The cap is used to generate a phase delay to start the motor. A bad one won't let the motor start spinning.

Do you hear the click of the switch closing as the motor winds down after it is shut off?

 

[Ohm_Boy]

That device in your second photo is a thermal protector.

 

[Ohm_Boy]

I think I would still suspect the centrifugal switch. It's function is to engage a starting winding to provide a phase kick to start the motor, and then remove it from the circuit once the motor spins up. It sounds like it is not disengaging the start windings.

Your motor may or may not have a capacitor. Caps are often inside a hump or bubble enclosure mounted on the side of the motor like a shark fin, or may be hidden inside the back motor cover. A photo from a little further back showing the whole motor might help.

 

[kristof]

Do you hear the click of the switch closing as the motor winds down after it is shut off?

Yes, the click of the centrifugal switch is very loud. I can hear it pop back as the motor spins down. Also, the brush that rides up against it makes a sound when the motor rotates. It's a cross between a whoosh and a scraping sound. By listening for it on startup I can tell that the centrifugal switch is initially off. When the motor gets up to speed the sound disappears and I can tell that the centrifugal switch has engaged. Then there is the loud click on spindown.

I think I would still suspect the centrifugal switch. It's function is to engage a starting winding to provide a phase kick to start the motor, and then remove it from the circuit once the motor spins up. It sounds like it is not disengaging the start windings.

Maybe I don't have the brush riding on the correct side of the wheel on the centrifugal switch assembly. The wiring to the brush isn't visible without removing a plate. I didn't note which side the brush was on when I first disassembled the motor so I took a guess. I believe that I have the brush riding on the back of the wheel, back being the side facing the rear of the motor.

If I take that plate off I should be able to trace the wiring and figure out which side of the wheel the brush should be on.

Your motor may or may not have a capacitor. Caps are often inside a hump or bubble enclosure mounted on the side of the motor like a shark fin, or may be hidden inside the back motor cover. A photo from a little further back showing the whole motor might help.

There definitely isn't a hump on my motor. If there is a cap it would have to be in the rear cover. I'll take the cover off and get some more shots of the inside.

 

[kristof]

Sorry about the delay. Other things took precedence, one of which was the pool opening. :-D

Ok, here are some pics showing the motor body. No visible hump for a capacitor. Looks like my motor has the booster windings instead.

Here are shots of the centrifugal switch and wiring:

The two black wires coming up from the bottom of this next picture are the temporary "for testing only" 110V power cord.

I have a multimeter and electronics experience so I'm able to do testing that way if anyone has some suggestions.

 

[JamesW]

http://www.aosmithmotors.com/AboutUs/de ... spx?id=841

http://www.aosmithmotors.com/WorkArea/l ... &libID=609

 

[kristof]

http://www.aosmithmotors.com/AboutUs/default.aspx?id=841

http://www.aosmithmotors.com/WorkArea/l ... &libID=609

Awesome. I'll take a look at these. Thanks!

 

[kristof]

After reading the repair manuals I seems that the problem is probably in the circuit board. The manuals stress that the finger thing and contacts on the board should never be bent, and that they need to be within certain tolerances.

When I reassembled the motor the first time with the centrifugal switch mechanism improperly put together I probably bent the finger in a way that it wasn't meant to go...too far back I think.

Thanks for the help everyone. It's not worth any more of my time so I'm going to leave it out for the local junkman.