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12VDC motor problems

I have a 12VDC wiper motor that is being used in one of those animal cranes as a drive for a platter. It stopped working and I have taken it apart and am stuck. What it does is that it works for about a minute then starts slowing down and then stops. If I turn it off and wait a minute it will do it again. There doesn't appear to be any shorts on the wires wrapped on the inside. I am a little confused on how this thing works. It has three brushes, two are opposite at 12:00 and 6:00 and one wired to the frame is at about 8:00. There are also two blue Varistors??? wired between the ground and each brush, at the input plug, not in the motor. They have CNR written on them and then

100820k under that. What are they used for? And why the three brushes in that kind of configuration? And... how are you supposed to wire these up? being that there are three brushes? And why does it stop running. The manufacture in AU said that it was losing magnetic power??, is that possible? They want over $300.00 for a direct replacement. I found surplus motors that are close for $20.00. Any help on this one? Thanks so much Russ
Reply to
Uriah
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Hi Russ, Off hand I don't know your problem. Sometimes the third brush is for phase control and sometimes its for a tachometer arrangement. I really don't know. It sounds as though your electronics control devices are overheating. When it slows down, is anything hot?

Tom

Reply to
Tom Biasi

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y

No it doesn't get hot at all. I have it hooked to a bench power supply and it I can watch the current draw and as it slows it pulls a little more current. I cut the blue variistors off and it made no difference. I checked the three coils wired on the inside and best I can tell they seem ok. wonder how they would use the third brush being that it is a DC motor? Thanks for the help, Russ

Reply to
Uriah

Does it slow down and stop forever, or does it recover after sitting?

If it stops forever, and it's not a rare-earth magnet type, and if you're using way more current than the manufacturer calls out, then chances are you're demagnetizing the field magnets.

If it stops forever, and it's getting _really_ hot, then chances are you're overheating it and demagnetizing the field magnets (although I think you'd notice the smoke and glowing things before then).

If it grinds to a stop then recovers, have you tried manually turning it? It may be mechanically binding due to heat.

If it's not one of those, I'm stumped, too.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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Reply to
Tim Wescott

I would suggest that the 3rd brush is wired to another external terminal

- It's for running the motor at a higher speed - It's probably a 2 speed automotive windscreen (windshield) wiper motor after all. I've not seen one with the 'varistors' as described, but they're most likely to be small capacitors to provide brush noise suppression. Does the motor slow down when run from an alternative source? The problem may lie in the supply of the unit... What's an 'animal crane' btw?

Jon

Reply to
JonSutton

Pulling more current as it slows could indicate seizing ! Are you slowing it by adding load or is it slowing all by itself ? If the latter it sounds like its seizing up somewhere. The third brush is part of the speed control and may or may not be in use in your application.

--
Best Regards:
                     Baron.
Reply to
Baron

lt will slow down and stop forever, and not get hot at all. I have it on a small bench PS and it was pulling maybe 1 amp. I don't think I could get it going by turning it but I am not sure I tried that. It runs fine with 12VDC for about a minute then slows and within another minute stops. No load on the motor at all. I will check it again to make sure I didn't miss anything. Thank you.

Reply to
Uriah

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That makes sense. The third brush has to do with speed. It is a wiper motor. Those blue things could be caps. They show cap values and test open. Maybe I don't have a strong enough PS, so I will try from another larger PS. The crane is actually called Chocolate Factory by ICE. Very similar to regular animal cranes but you pick up candy bars. It drops them on a platter and then the platter pushes them forward and if your lucky some will fall in to your arms. That platter is the problem. You know you might be right about the PS not being strong enough. When I found the crane broken it lost the 12 volts and the platter wasn't turning. A bridge rectifier went out so I replaced that and I thought the motor caused the problem because of the results I got with it on the bench. So I had ordered a surplus motor that almost fits and the machine is back up working. I was just puzzled about this motor and why it stops. I will test again on a bigger PS today. I bet that that is the problem not enough amps..... Thank you Russ

Reply to
Uriah

"Uriah"

** So you did not even bother to monitor the DC voltage on the motor while testing ??

How about you use a car battery for you next damn test - eh ?

..... Phil

Reply to
Phil Allison

hile

It is a 12VDC wiper motor. It looks like it was the low amp output of my cheap little bench PS. I hooked it up to something bigger and it it seems to be fine. I guess what happened was somehow the bridge rectifier failed on the board supplying the 12VDC to the motor on the crane. It was after I fixed the board when I tested the motor on my bench and I assumed that because the motor would stop after a minute that the motor was the cause of the problem. I am glad I didn't spend the $350.00 for a replacement motor. I only lost out on about $22.00 for the surplus one. I wonder why the bridge failed? Thank you everyone. It is a learning experience and that is why I wanted to figure it out. Russ

Reply to
Uriah

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