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Old 11-07-2010, 01:53 PM   #1
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Single Phase AC motor overheating with no load after 1 min


I picked up a little 1/12 HP single phase AC motor a couple days ago for a project.

In prototyping, it ran fine for 5-10 min under small load with no overheating.

Then I bolted it down to a mount stand. During the process the bottom plate came off. No big deal (I think), put it back on and good to go. I tightened it down to the stand. Upon inspecting it, I noticed more up/down play in the shaft, but didnt think anything of it at the time.

I hooked it up with no load on, bolted to the stand, and it sounded like something was rubbing a little bit. The shaft didn't stay exactly center-line, it would slightly move up and down while turning. Then after about a minute it powered down and the motor casing was really hot to the touch. I let it sit for about 5 min, and when I came back the motor started back up. So I know the thermal protection circuit is turning it off.

But what boggles me is this has no load but now is overheating after about a min of running.

I think I somehow moved a bearing or something else to mis-align the internal shaft so there is some physical resistance inside that is causing this. If this is the cause, how do I re-align it?

Does anyone have any suggestions?

Thanks!

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Old 11-07-2010, 05:35 PM   #2
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Single Phase AC motor overheating with no load after 1 min


Sounds like a bad bearing. The shaft should not move up and down.

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Old 11-07-2010, 06:58 PM   #3
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Single Phase AC motor overheating with no load after 1 min


I found out the issue with the shaft play. There was a washer that has a spring characteristic that was missing on the bottom bearing housing. I greased it up and put it back in and now there is no more play.

However, the motor is still overheating at about 1 min of run time with or without a load.

The motor requires a start cap. I am using a recommended start cap of 72-88 uF. I know its working because the motor spins up easily.

Could it be possible that the internal centrifugal switch that cuts off the cap is shorted, causing more heat?

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Old 11-07-2010, 07:05 PM   #4
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Single Phase AC motor overheating with no load after 1 min


I don't know a lot about motors but I'm pretty sure some motors are designed to have a specific load. If not enough load it will run too fast, if too much load it will force too much.
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Old 11-07-2010, 07:09 PM   #5
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Single Phase AC motor overheating with no load after 1 min


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Originally Posted by Red Squirrel View Post
I don't know a lot about motors but I'm pretty sure some motors are designed to have a specific load. If not enough load it will run too fast, if too much load it will force too much.
That is what I was wondering, so I ran it with a load and had the same result.

It also ran fine previously with no load.
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Old 11-07-2010, 07:30 PM   #6
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Single Phase AC motor overheating with no load after 1 min


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Could it be possible that the internal centrifugal switch that cuts off the cap is shorted, causing more heat?
Tested and confirmed as the issue!

I started the motor with the cap then disconnected the cap, and the motor ran for over 5 min just fine.

So for some reason the start winding is not being turned off, causing excessive heat in the casing. Now I know what the culprit is!

When the back came off and the shaft came out a little, it must have done something to the switch circuit. What should I look for when looking for the centrifugal switch?
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Old 11-07-2010, 08:58 PM   #7
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Single Phase AC motor overheating with no load after 1 min


Do you here the switch click as the motor spins up? If not then look for mechanical issue with the centrifugal switch.
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Old 11-07-2010, 09:40 PM   #8
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Single Phase AC motor overheating with no load after 1 min


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Do you here the switch click as the motor spins up? If not then look for mechanical issue with the centrifugal switch.
No, I dont actually.

I dont see anything that resembles a switching mechanism inside the casing or on the spinning shaft. However it might be inside the aluminum that is connected to the shaft itself. In that case I will not be able to get in there at all.
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Old 11-07-2010, 09:51 PM   #9
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Single Phase AC motor overheating with no load after 1 min


It should look something like this. The part on the right is on the shaft. The part on the left is attached to the end bell of the motor and is the actual switch. These links might also be useful.
http://www.tpub.com/ceb/107.htm
http://www.tpub.com/ceb/108.htm

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Last edited by joed; 11-07-2010 at 09:56 PM.
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Old 11-07-2010, 10:01 PM   #10
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Single Phase AC motor overheating with no load after 1 min


Thanks for the help! I will look and see what I can come up with.

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