Why did my electric motor stop?

April 19, 2011 12:08 AM

What in an overdriven, broken electric motor is "burned out"?

I've overdriven a few electric motors (magnets and windings type) to the point where they suddenly stopped working (i.e. turning under electrical power). When I go and inspect the busted motor, I can't see anything obviously different about it in contrast to how it looked when it worked. So what changed?

posted by telstar to technology (13 answers total)

You have probably melted a small section of winding, thus preventing a complete circuit from being formed. This won't be visible because the wire is coated with insulation.
posted by alby at 12:39 AM on April 19, 2011 [2 favorites]

The wire in motor windings is usually insulated with clear varnish or lacquer, so you won't notice where it has melted and shorted out.
posted by weapons-grade pandemonium at 12:56 AM on April 19, 2011

Thanks, guys! While we're here, is there any hope of effecting a repair/replacement-of-component under those circumstances? Or is the motor then unrepairable toast?
posted by telstar at 1:07 AM on April 19, 2011

The search terms you want are motor+rewinding.
posted by flabdablet at 1:20 AM on April 19, 2011

Thank you!
posted by telstar at 1:24 AM on April 19, 2011

To give you a better idea of what's happening check out this wiki picture of a universal motor, a common type found in appliances like vacuums, hair dryers, blenders, things like that. Look at the finer wire wound around the core around the shaft in the middle (not the coarser wire on the two coils fixed to the surrounding frame). When the motor overheats (run too long or under too high a load) the winding melts, as described above, and this usually happens deep inside the winding, where it's thermally the hottest. Air cools the surface windings and sometimes a fan is even attached to the shaft to increase airflow over the winding. But the inner windings are only cooled by convection, where the heat transfers through to the surface. So when it fails it's usually hard to see exactly where it happened unless you start unwinding the motor, as the surface often looks perfectly fine.
posted by 6550 at 1:57 AM on April 19, 2011

is there any hope of effecting a repair/replacement-of-component under those circumstances? Or is the motor then unrepairable toast?

I think they'd have to be fairly large, expensive motors to be worth fixing.
posted by jon1270 at 2:41 AM on April 19, 2011 [1 favorite]

But the inner windings are only cooled by convection, where the heat transfers through to the surface

I would have thought that for this to be true, the inner windings would need to be liquid or gaseous and that this would be indicative of a severe overload rather than normal running, during which I would expect the inner windings to be cooled by conduction.
posted by flabdablet at 2:54 AM on April 19, 2011

Yeah, conduction not convection
posted by 6550 at 3:09 AM on April 19, 2011

The insulation in the commutator (usually mica) can also degrade, leading to shorts between the commutator bars.
posted by workerant at 7:07 AM on April 19, 2011

Brushes can also degrade/burn/melt if the motor is the type with brushes.
posted by SpecialK at 9:18 AM on April 19, 2011

It's also possible to demagnetize natural magnets in motors with excessive heat. I believe I've done this to a Dremel tool before.
posted by Dmenet at 11:56 AM on April 19, 2011

is there any hope of effecting a repair/replacement-of-component under those circumstances

What actually happens most of the time is not the wiring melting, but the insulation on the wiring overheating and burning off, which shorts the windings and gives burnt out stuff a characteristic smell.

Motors can be rewound, and there are places that do it professionally, but as mentioned previously, it may not be worth it for cheap motors.
posted by HiroProtagonist at 7:25 PM on April 19, 2011