# Troubleshooting and reviving an electric motor



## brainjuicy (May 4, 2018)

I am hoping there are one or two here on the forum that are wise in the ways of tool restoration when the electric motor is needing some love.

Summary: This electric motor was known to be in working condition a few months ago. Now, when it is powered on it sits and hums. Not good. I suspected the start capacitor. I tried replacing the start capacitor and that did not work. I need some advice on what to try next.






Background: I am setting up shop for the first time to spend some time with my son and learn something together. I was given a piece of machinery that I have tried to restore to working order. And for a while it did work just fine. Without notice, the motor would not spin up and simply hums. Not surprisingly, the imported machine has a motor that is clearly marked Made in China. I say that up front to get it out of the way and ask that people hold their opinions on such. The way I figure it, I was given some tools and beggars can't be choosers. That said, I don't expect any manufacturer support or even OEM parts to be available. Shoot, there aren't even manufacturer markings or model numbers noted.

Troubleshooting: When the machine would not start, I quickly powered it off and checked the obvious things like an obstruction, seized bearings, burned up windings, etc. Nothing to see there--all look good. While I had the motor housing open, I checked the centrifugal switch and it tested fine for continuity, moved smoothly and the contacts looked clean. This 110-125v, single phase, 12 amp motor only has a start capacitor. I check the start capacitor for tell tale signs that is was bad. It looked fine on the outside but this does not rule out the start capacitor being bad. I then tried giving the motor a swift spin while simultaneously powering the motor up to see if I could get it to spin on its own. The result: no go. So, being at the bottom of my bag of tricks, I resorted to looking for a replacement start capacitor as an obvious next step that was relatively cheap.

What I bought: Looking at the start capacitor, the markings are not super helpful or clear. 
	

		
			
		

		
	



	

		
			
		

		
	
 I first read it as 35 or 35.0 mfd but, to the best of my knowledge, start capacitors for 110-125v 60Hz motors should be rated greater than 70 mfd. So, I interpreted the markings to mean 350 mfd. I looked high and low for one that matched exactly and had no luck. So I ended up with the closest thing I could find...a 340-408 mfd 110-125v start capacitor. 
	

		
			
		

		
	



	

		
			
		

		
	
 I was pretty excited to hook it up and give a try. Sadly, the result was the same--humming and no rotation. My thought process brings me to believe that I might have something wrong with the start capacitor sizing but what I have read stated that what I chose should have worked.

Can one of you confirm if the 340-408 mfd range-rated start capacitor should (or should not) work as a functional replacement for the original capacitor?

If you have other suggestions for troubleshooting I would appreciate hearing it.


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## markba633csi (May 4, 2018)

There's a very slight possibility that the new capacitor is bad also, but more likely you have a winding problem- i.e. short or open start winding or similar
If you have a ohmmeter or continuity checker we could do some checks on it, but the end result may be buying a replacement motor
What type of machine is this for? Lathe? Mill? Other?
Mark
ps If you're lucky there may be a poor solder joint you might discover- would require opening up the motor case


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## brainjuicy (May 4, 2018)

I do have a good Volt/Ohm meter. I will look into instructions for checking continuity on the windings and report back.

The motor is from a wood jointer (sometimes also called a jointer planer).


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## machinejack (May 4, 2018)

Jumper 110Vac to it and short with a screwdriver should have a load pop and spark. If not bad cap.


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## brainjuicy (May 4, 2018)

The capacitor charges and discharges with loud pop and spark.


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## machinejack (May 4, 2018)

OK ruled that out. With spinning it and it not taking off. Sounds more like shorted or open winding. Check things out with a meter. If you had a megger that would help. My machine shop was next to a rewinding shop. I bored and sleeved end bell's and made new shafts for them. Know just enough to be dangerous with motors.


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## markba633csi (May 4, 2018)

Most likely open start winding (or something open in start circuit) 
Get that bad boy opened up and let's take a stab at troubleshooting it
Mark


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## brino (May 4, 2018)

brainjuicy said:


> Not surprisingly, the imported machine has a motor that is clearly marked Made in China. I say that up front to get it out of the way and ask that people hold their opinions on such. The way I figure it, I was given some tools and beggars can't be choosers.



First, Welcome to the Hobby-Machinist!
You have found a place where people try to help each other.
Therefore, you do NOT have to worry about machine or "country of origin" snobbery here.
We are are doing the best we can with what we have.

It sounds to me like you have a bad starter winding.
With the motor unplugged, is there a little access panel to open? 
If so, any guidance in there about how to reverse the direction of it?
The connections that you change to reverse the direction will be the starter winding.
Unhook those from everything else and get a resistance reading.

I wonder if you had it on a bench, wrapped a heavy string around the shaft, plugged it in and gave the string a good pull to get it started.
Is it possible to get it going fast enough to start that way?
@markba633csi, you seem to have lots of motor experience.......ever tried that?

What ever you do please do NOT lose a finger!
It ain't worth it.

If we were closer I'd invite you over to pick a replacement motor out of my pile of "used spares"....thought 1/2 HP may be largest in that stash.

-brino


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## markba633csi (May 4, 2018)

Hi Brino- I think our OP tried yank-starting it already- no go apparently


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## brino (May 4, 2018)

markba633csi said:


> I think our OP tried yank-starting it already- no go apparently



Thanks, I guess I missed that!


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## markba633csi (May 4, 2018)

Yeah, unless we can locate a bad connection or other simple fix it may be new motor time...
If it's a short deep in the windings I think we're pretty much hosed, but you never know
Mark


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## RJSakowski (May 4, 2018)

You could have a bad run winding.  check that winding for continuity.


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## hermetic (May 5, 2018)

could be bad contacts on the centrifugal switch if it has one. with the spin test, can you wrap some string round the shaft and pull it to see if it does pick up and run. If it does run ok, and will run in either direction by giving it a spin in that direction, the fault lies in the start circuit,  You are going to have to strip it down to fix the centrifugal switch.


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## brainjuicy (May 8, 2018)

Ugh, I had the time to checked the windings last night with my ohm meter and sure enough one of the two failed the continuity test. 

Thanks to the community for all the prompt help in diagnosing the motor. Although I am not happy to know that the winding is messed up, I am happy to know what the issue is and to be able to move forward.

Unless you all have some good advise on how to further diagnose or fix it, I guess I am off to different task...finding a replacement CCW 1hp motor that works in my budget.


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## markba633csi (May 8, 2018)

Hi Brain, your only hope is that maybe there's a splice in there that might be flakey- usually where the factory attaches lead-in wires to the windings.
You might start investigating there. Connect your ohmmeter and start wiggling and pushing on things while watching the meter.  You might get lucky.
This is easier with an old style needle type meter; easier to see when it jumps. Or a battery and light bulb continuity tester. Good luck.
Mark


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