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Electric motor failure modes?

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I have a small brushed DC electric motor which I think is only rated for 5-9 volts, but the seller claims 5-12 volts. I gave it 12 volts for some seconds and after I stopped to switch to 5 volts, the motor seems to have an internal short circuit.

Is this the normal failure mode for an electric motor when gives too high voltage? I thought it should burn over the brushes and show an open circuit. Why is the circuit shorted internally when it fails?
 

ronsimpson

Well-Known Member
Most Helpful Member
If the windings get too hot the insulation on the wire may melt away and short out.
Go back and see if 12 volts will run the motor. (short time)
It might be the 5 v supply does not have the current to start the motor.
 

MaxHeadRoom78

Well-Known Member
Check the resistance, it should be an Ohm or two.
If you only ran for seconds, I would not expect a failure, without a load, the only result of over-voltage is over speeding, not excessive current necessarily.
Max.
 

spec

Well-Known Member
Most Helpful Member
This is a long shot, but you could try cleaning the armature- sometimes carbon builds up and creates a partial short. You could also check to make sure that the brushes and brush holders have not come loose.

spec
 
When I try to run it with my Arduino project the PSU (a cheap ATX one) turns itself off when the motor (controlled by a MOSFET) is turned on.

I thought this was a wiring error in my project, but the same thing happens if I remove the motor from the circuit and power only the motor directly from the ATX PSU. I tried at 5 volts and 12 volts.

Then I tried with a bench DC PSU, at 5 volts and 12 volts. The bench PSU drops to 0 volts, as it does if you short it.

When I measure it, there is 0 ohms. I checked with a similar (but not identical) DC motor, this motor shows some resistance, and it varies depending on the angle at which the motor is stopped (if I just rotate it with my fingers and then stop it again it shows different readings).

I thought perhaps the motor had stalled and required so much current that both the ATX PSU and the bench PSU cannot power it. I've seen this before.

I tried wiring in parallel all the 12 V rails on the PSU (P4 connector, ATX connector and one molex connector) so the PSU wouldn't shut off due to a safety feature checking for too high load on a single rail. But this again just resulted in the PSU turning itself off.

I don't have a more powerful PSU to test with. But I doubt this is the problem given that I read 0 ohms. I just don't understand how 12 volts for some seconds (perhaps half a minute at most, but I'm pretty sure it was less) can destroy the motor.
 

spec

Well-Known Member
Most Helpful Member
Can the motor be spun freely by hand or is it jammed. Try putting a voltage on the motor and spinning it.

spec
 
Can the motor be spun freely by hand or is it jammed. Try putting a voltage on the motor and spinning it.

spec
I'd have to disassemble the pump assembly mounted on it. I'm not sure it is meant for servicing but since it doesn't work anyway I don't have anything to lose.
 

fezder

Well-Known Member
Interesting fault, with such low voltage increase. Then again, I heard that current usage increases quite rapidly when rated voltage is exceeded.....haven't bothered to chech whether this is complete bollocks.
But yeah, as others have pointed out, s/c between armature filaments, whiskers from carbon dust, loose contact of windings, loose brush spring (had two windscreen motors fail due this).
Short from vcc-pole to motor chassis, if chassis is grounded to minus-pole. Also, sometimes armatures parts which take contact with brushes come just bit loose which loosen during higher speeds and can cause havock to brushes.
This is all I can think of!
 

Les Jones

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Hi halleffector,
Now you mention that the motor has a pump fitted to it I was wondering if it could be the same as one I ordered from China over amonth ago and am still waiting for delivery. This is the one I ordered.

Les.
 

spec

Well-Known Member
Most Helpful Member
Hi halleffector,
Now you mention that the motor has a pump fitted to it I was wondering if it could be the same as one I ordered from China over amonth ago and am still waiting for delivery. This is the one I ordered.

Les.

DC 12V Motor Durable Electric Tea Pot Tea Tray Mini Gear Water Pump 5℃-45℃
What does this do for your teapot or tea tray?

spec

PS: note the confusing use of the hyphen.:arghh:
 
Last edited:

Les Jones

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Most Helpful Member
Hi spec,
I don't know what use they would be with a tea pot. The water needs to be at 100 Deg. C. for tea. I thought they looked ideal for watering indoor plants that we put in the garage when we are on holiday. I don't trust a timer and solenoid valve (Which I use for outdoor plants) with an unlimited supply of water inside the house. I don't believe the first line of the description which says "No carbon brush, no pollution, electronic commutation, long service life" They look like normal permenant magnet brushed motors.

Les.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Another failure mode is the shaft seizes. Turning it a few times may get it going again. This probably isnt it for this motor but thought i would mention it anyway.

It sounds like your motor is shorted. Why, we dont know yet. You can check the brushes etc. If you can get the armature out you can test that with an ohm meter. If that is shorted it is probably not repairable unless it has several branches. If it has several branches you may be able to remove the wire from that one shorted pole (usually only one will short). Doing that means the motor runs more unbalanced so vibrates a lot more and the rotation is not as constant. If you can replace the wire with the exact gauge used though then you can get it going again at full speed/torque.
 

MaxHeadRoom78

Well-Known Member
Another way to test is to back-feed it, spin it up from an external source if possible, the resultant voltage will be proportional to the RPM based on the RPM/volt.
e.g. if 5v and 1000rpm, = 200rpm/volt.
Max.
 

fezder

Well-Known Member
Max, like using it as generator? Neat trick!
 

schmitt trigger

Well-Known Member
On a cheap Chinese motor I bought a while back, I had a similar experience. It worked for a brief time, then it completely shorted out.

Since I had nothing to lose, I disassembled the motor.

I found a metal burr, welded across a pair of commutator slots, which judging by the color had come out from the bell housing.

What i think happened is that during stamping the housing was not properly deburred. It was assembled to the rotor, then the motor tested for perhaps a pair of seconds, packed and shipped.
During shipment, the burr must have come lose. When I first applied power to the motor, the loose burr eventually landed on an energized commutator, and the the available energy caused it to weld and short the rotor.
 

RODALCO

Well-Known Member
If that motor was rated 5 - 9 Volts it should work on 12 Volts for a short period with no issues at all, excessive heating and smoke emitting are the early warning signs before failure.
I have been using my old Makita 9.6 Volt cordless drill for years off a 12V24Ah battery with no issues at all.
It is more likely a quality issue and would ask for a replacement motor.
 
I don't understand anything. After I removed the kickback protection diode, the motor works fine!

I measured the diode and it seems to only allow current one way (duh). How can the diode mess up this circuit? I have it connected like this:

http://i.stack.imgur.com/QULAI.png

I've tried to simply invert the power, but it doesn't work in either configuration.

With the diode removed, the motor works with and without the power inverted.
 

RODALCO

Well-Known Member
I would get an other diode, get 1N4007 (1A) or 1N5408 (3A) rated.
 

RODALCO

Well-Known Member
Your circuit is drawn correct, you may have fried the diode when it was connected the wrong way.
 
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