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Lathe motor problem=frustrating weekend!

John Conroy

John Conroy

member
Premium Member
I have been helping a friend remove the cylinder head from his 1950 Bentley lately (more on this later) and needed to make some parts for a puller. My lathe has been working fine but when I went to use it Friday afternoon the motor just made a loud buzzing/growling noise and would not turn. Since the machine is only a year old I called Modern Tool, the dealer where I bought it for some help. The Edmonton branch has only 2 techs and they were both out on calls so they had no one around to give me any help. I really wanted to get the tools parts made so I decided to try and diagnose the fault myself. Here is a link to a video showing the symptom.


I knew the problem was in the motor, not the controls so I removed it from the machine, there was an odor of burned electrical stuff and I thought the motor was probably toast but I remembered PeterT had a similar symptom on his new mill. I think the problem turned out to be the centrifugal switch but the tech also replaced the start capacitor. The switch on this motor is inside but the start and run caps are external under metal covers so I removed them and found that the 150MFD start cap case had a big bulge in it and the 30MFD run cap case was broken. Broken or bulged cases are a sure sign of a capacitor failure but it was too late by then to locate parts so I had to continue Saturday. There are no electrical supply places open Saturday but I did manage to find the correct 30MFD run cap at Princess Auto but they did not have the 150MFD start cap. I thought about using a different size but everything I read online said you must use the original size so I was out of luck until Monday. I got the new start cap at Amree Supply this morning and got them both installed. The original were soldered to the motor leads but I installed spade terminals to match the ones on the new caps in case I ever have to do this again. The motor starts better now than it ever has so I think the start cap had been slowly dying since I got the machine. The motor runs more quiet now too so I think the run cap had been dead from the beginning, I measured it's capacitance with my Fluke meter at only 3MFD. I'm glad to be up and running again, hope I never have to lift that 90 pound motor out of there again. Here are some pics, you can see the damage on the old caps in the first 2.

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John - nice job on the troubleshooting and thanks for the vid and pics. That buzzing sound is pretty indicative of capacitor problems. You could provide the sound track for dozens of vids on YouTube with air conditioner issues. No need to even put your Fluke on those caps...they looked like they had a rough bar fight alright.

Again, nice job diagnosing and good planning to put the spade terminals on; I wish I had done that on my air conditioner...I'm on my third start cap. I don't think they are made as well and wear out even sooner.

Congrats. Feels great to save on the service call doesn't it?
 
but I remembered PeterT had a similar symptom on his new mill. I think the problem turned out to be the centrifugal switch but the tech also replaced the start capacitor

That's mostly right John. I had to replace the start cap because it had blown. Modern replaced the cap, checked the motor, the centrifugal switch, the CSA box & starter switch for any issues. It ran for a while, I was happy face again. Maybe a couple weeks I forget. Then one day it kicked the CSA breaker box switch off & failed to start. This time I wasn't getting the grindy cogging noise but maybe a very brief Bvvvt noise, then kicked the switch. While I was waiting on the next available service visit I removed the & examined the CS. Mine is here under the endbell cap & beneath the fan blade.

I checked & CS action was OK, cleaned the slightly pitted points & had this hunch maybe plastic fan assembly was slowly migrating down the shaft until it finally impeded the CS from fully opening. I moved the fan position up to allow max clearance, replaced some of the cheesy screw hardware & used Loctite (which I think was the problem) & presto it worked again. Cap was OK. Still running knock on wood.

It could be a chicken & egg thing. If the CS is bugging out slowly & intermittently its hard on the cap until it finally goes. Replacing the cap buys new life but may be shortlived if its the CS all along, But if you had a bum cap it would probably give same symptoms. Some say if it lasts its first year it should last 10. Modern said they couldn't recall any cap replacements on my mill motor which is why they had to <cough> find one for me. The mill hasn't changed in 10 years or however long they have been selling according to them. But I also was told by PM that they upgraded the cap on (same) 935 so whether just coincidental or underlying upgrade I cant say yet. Hopes this helps you. I can relate to frustration. Hang in there.

ps Matt wont sell you caps (or any parts) unless you have a PM machine.
 

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Thanks guys, if I ever have to replace caps again I can do it without removing the motor. Dave at Modern (Edmonton branch) is ordering me a pair of spares to keep on hand just in case. I've been thinking of installing a 3 phase motor and a vfd so that project may happen soon. It would be more complicated than it was on my mill where I just removed all the original switches and contactors. I was drawing the schematic in my mind all weekend.
 
Is there a sparky in the group here? I read somewhere that capacitors don't like short cycles. If the motor is started and stopped often, it strains the capacitors which shortens their life. Can someone confirm or deny?

OTOH, capacitors are a pretty cheap replacement item. The couple of times I've needed one, the local motor shop charged $10 to $20 IIRC.

Craig
 
Not a sparky, but grew up on a farm: there seem to be big differences between electric motor “duties” and their respective robustness and reliability. I have seen four types (maybe there are more): light, medium, heavy, and severe/farm duty. We had some equipment with medium or heavy duty motors. They did not last at all. We replaced all of them with farm duty ones. They are physically bigger and heavier for the same power. And cost way more!

The SM1340 came with a farm duty 3hp single phase motor (the original 575V 3phase was also included). Burning the caps was my main concern. I went to James Electric to see if they could rewire the 575V for 220V 3phase so I could run it off the RPC. It would have cost more than a new 3hp 3phase motor. So I did not do it. They said the single phase, farm duty, motor should give me many years of service, even with the constant ON/OFF/REV switching in a lathe without a clutch. It has -> have been using it for years without any issues whatsoever.

Maybe I just lucked out? If I was running this lathe in a commercial setting, I would switch it to 3phase.
 
Properly made motor should not burn out easily - one good test for capacitors is to simply disconnect them and start the motor with a drill or with well connected string wrapped around the shaft.

You can check capacity of a capacitor with multi-meter but that is test not under load - capacitor can pass it and still be faulty.

Finally, you can check the coils for short.

It is strange a capacitor went dead after just a year - usually they last for a very long time.
 
trlvn said:
I read somewhere that capacitors don't like short cycles. If the motor is started and stopped often, it strains the capacitors which shortens their life. Can someone confirm or deny?
Click to expand...
I've heard that too. Like don't pulse your switch on/off really quickly. As a rule I don't do that. Even if I realized I should have changed speeds I let it run a few seconds & turn it off, but maybe I'm chasing unicorns. I have seen others do it like power tapping on the mill. OTOH, guessing my lathe motor has similar centrifugal switch & start capacitor configuration & it has button push 'jog' mode which effectively must be doing the same thing. But I am under-qualified to say what is happening behind the scenes, maybe its a bypass of some sort.
 
Start capacitors are not rated to be run continuously - thus if you start and stop many times in a sequence you are simulating "running" for your start capacitors - which will cause them to fail.

There is no need to let the motor run for 3sec or something - you can stop it immediately - the problem is with starting it up again so soon after another start. Certainly starting the motor say 10 times in a period of less then 1 min will not be healthy for the start capacitor.
 
I've also read about where people have replaced what they considered inferior offshore/OEM caps with something better, obeying/exceeding the voltage & capacitance spec ratings. The usual issue is they dont fit the form factor of the protection blisters. Having had one go Bang 2 feet from my face was a change-your-shorts experience so I wouldn't skimp on the protection part. The cap had a split & some magic smoke & ooze came out. But I've also heard of remote caps that were in a protective box some (short) distance away. I really can say if this was rigged or electrically approved because in another application I was told that short leads to capacitor was important. But that may have been more related EMF background noise issues. In the case of our machines there is a wire from the plug points into the motor which must be a couple inches long.
 
Tom Kitta said:
There is no need to let the motor run for 3sec or something - you can stop it immediately - the problem is with starting it up again so soon after another start. Certainly starting the motor say 10 times in a period of less then 1 min will not be healthy for the start capacitor.
Click to expand...

Agree. That's a better way of saying it. And confirmation would be audible click of the centrifugal switch after power off indicating the start cap circuit has been reset.
 
Start capacitors are constructed for high intermittent currents. Run capacitors are designed to not heat up during long runs. There are many configurations and constructions to capacitors. You can never use a start capacitor as a run capacitor and visa versa, even if the values are correct. You will just burn them out.

A proper start capacitor is just fine with short cycles. The windings on your motor are more vulnerable than a premium start capacitor. Short start/stop cycles warm up the windings. However it would have to be very extreme to damage them.

Another weak part (that @PeterT found out about) are the contacts and spring in the centrifugal switch. When they are damaged, it can destroy your motor or capacitors with seemingly little use. since there is a little arc when the disconnect, a lot of short start/stop cycles can lead to premature failure (again only in extreme cases)
 
Good info, I guess I have been abusing mine when single point threading(metric) or with a tap. Although I did all the same work on my old 1.5hp lathe for 5 years with no problem.
 
Maybe you were abusing it maybe not - after all a compressor that works for 90s and rests for 90s does 20 cycles in 1h at 50% duty... If you do that 8h a day in August in California you may expect a bit shorter life span.

Or just get a more heat resistant capacitor...

https://www.fraunhofer.de/en/press/...pacitors--stability-at-up-to-300-degrees.html

Wow 300C.

Meanwhile best I found for us regular pp is something like this https://www.amazon.ca/CBB60-Motor-C...BFWJWHYBDG1&psc=1&refRID=S0T90EEVEBFWJWHYBDG1

80C... still not bad.
 
This is the start cap I used.

https://www.amresupply.com/part/35-145A125-CAPACITOR-145-175-MFD-110-125V

This is the run cap.

https://www.princessauto.com/en/detail/30-f-electric-motor-run-capacitor/A-p8513426e

Pretty wimpy for temp rating.

I probably should havebused this run cap, much higher temp rating. Although it is a dual cap and I would have only used 1 side of it.

https://www.amresupply.com/part/36-305R3744-CAPACITOR-30-5-370V-RUN

Here is a very good vidoe explaining cap function and testing.

 
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