What can cause a DC brushed motor to stall?

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Tereci

Member
Location
California
Occupation
Software Engineer
2021 seems to be a year of motor issues for me 😳I have this horizontal lapidary grinder and sometimes it slows down to zero with just a bit of pressure. That is not a normal behavior for the machine - I talked to a bunch of other owners and theirs work fine even with quite a bit of pressure, no slowing down.
The only info the company provides about the motor is "1/2" arbor and powered by a 1/4 hp, heavy-duty, ball bearing, DC motor. This motor is variable speed so you can work at whatever pace you desire, delivering speeds from 800 rpm to 3,400 rpm.". They don't sell a replacement motor. Any tips on what the issue might be? The sales rep mentioned that the motor is brushed so I wonder if an issue like the one shown on the video could be caused by bad brush contact?


The company doesn't want ppl doing repairs themselves so the only option they gave me is to send it back to them and pay them for shipping and a new motor. I am still thinking about giving it a try myself first. Are there motors where brushes cannot be replaced? The motor is mounted in the stand of the machine and has no markings on it and I have to yet figure out how to get it out of the plastic collar on top.

motor.jpg


If you have any thoughts/insights, let me please know! :)
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210317-2146 EDT

petersonra:

This web site may be primarily for code related issues, but there are a lot of discussions on non code items.

I don't known whether it is appropriate for Tereci to ask his questions here or not. But some information that can be gained from his questions may be generally useful to ordinary electricians.

I somewhat believe from the questions asked that Tereci did not study physics in high school, and quite possibly many electricians did not either.

If Tereci's DC motor is a permanent magnetic type and the field magnetics have not failed, then the most likely cause of his problem is loss of brush contact, or shorted, or open turns on the armature, assuming full voltage to the motor. Could also be a bearing problem. Either adequate torque is not produced, or there is excessive internal load torque.

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D

__dan

Senior Member
Pull the brushes out and check them. If they look good, make sure they slide freely in the brush holder.

It is possible the unit has a really hokey power suppy to either make the DC or acts as a speed control. By hokey, I mean it was built super cheap with some cheap part not intended to last.

I would bet the motor is not worn out or burnt out, especially if it's a hobbyist item without hard or commercial use. Bad brush contact or cheap part on the hokey speed control board gone bad would be my first two guesses.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210325-2254 EDT

Terecei:

Suppose the motor is a nominal 110 V DC permanent magnet motor and that it produce about 1/4 HP output power at 90% of no load speed.

A 1/4 HP motor has 746/4 W = 186.5 W shaft power output. Assume 75% efficiency. Thus, at 110 V DC input we have 186.5/0.75 = 249 W input, or I = 249/110 = 2.26 A. Actually input power is possibly somewhat greater depending how you interrupt the efficiency.

Put two DC meters at the DC motor input. One a 5 A full scale, and the other a 150 V full scale.

Load the motor to just before it stalls and measure the two values. Then stall the motor and what are the readings. Possibly you should use a 20 or 50 A meter to start because you have no idea what current is going to do.

Report back the results.

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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210326-0337 EDT

The brain does strange things at times.

"produce" should be "produces".

"interrupt" should be "interpret" (this is the strange one), and

"readings." should be "readings?"
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210326-1605 EDT

ptonsparky:

I still describe it as a brain problem.

How does the brain work? I don't know. My cat does not write or spell or read words. But my cat can think in certain ways. And she has some sense of the time of day. In the morning if I don't get up early my cat jumps up on the bed and comes near my head to tell me it is time to get up. If she wants her food shaken she takes me to the bowl. Only at night when I am ready to go to bed does the cat go to the pantry door indicating to me that I should add new food to the bowl.

When the cat wants to go outdoors she gets my attention, and takes movements toward the door, but stops and waits along the way to make sure I understand her.

When I work at the bench and read a voltmeter I do not read it and use the letters V O L T to understand that I am reading 6 volts. More likely I am thinking in terms of the spoken word. Then when I go to write down 6 volts I go to some internal dictionary where I find the spelling for the concept of a volt.

The word "beside" in English means something adjacent for one meaning. Trying to translate beside to French does not produce good results. Translating adjacent to French produces somewhat better results, a cote. Translation to German is much cleaner, neben.

I have had recent occurrences of loss of balance or motor control. I am totally awake and conscious, and have totally lost control to my left side causing me to fall. This is of short duration. In my brain I know what needs to occur, but i can not control the left side muscles to do what the brain wants. Is this a failure of my balance sensor, a calculation error in the brain, failure to send signals to the left side, or failure of the left side servo mechanisms? Don't know. These are rather short time occurrences, fractions of a second to a number of seconds. The neurologists don't have an answer. Get up slower, and drink more fluids is their suggestion. But one of my falling incidents was from when I was standing for a while, and just fell over with no control, a fraction of a second.

I am intermittently working on an experimental setup to do the shaded motor tests. This includes a brushless motor drive to a torque transducer. In this method I have a way to have a driven speed controlled load for the shaded pole gear box under test, and be able to measure test torque.

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