Series wound DC motor.

[Dave Plowman (News]

Or rather a series wound motor running on 12v DC. Or so.

It's about car starter motors. I had one with a faulty drive (it's a reduction gearbox type) and was given an identical one with a supposed burnt out motor - having had been abused trying to start a dead engine.

Made one good from the two and tested it on the bench using a jump start pack. Seemed just fine.

Put all the bits left over back into one. But did strip the burnt out motor to have a look at it. It's a pretty bog standard series wound type. No smell of anything burnt, and the windings I could see looked just fine. Cleaned it all up and made sure the brushes were clean and free to move, and reassembled the lot.

And then tested it in the same way. It appeared to run at exactly the same speed as the good one. And got up to speed as quickly - ie near instantly. The voltmeter on the pack showed the same as with the good unit - if it had shorted turns I'd have expected to see a difference?

Or can anyone guess at a fault which would make the owner say it has burnt out - but now works? I've seen a fair few burnt out starter motors, and you can usually see obvious signs of overheating.

Oh - I found the fault on the drive from the original gearbox too. A thrust washer had broken up and disappeared. A new spare part sorted that.

[newshound]

I had one which must have had shorted turns once, presumably in the rotor. I could not spot any sign of overheating. It would spin OK when free but did not have all that much torque, although it would still turn the engine over for a bit. It turned out to be taking massive current (confirmed on a garage tester in the days when I did not have a clip on meter), which explained why it would flatten the battery quite quickly, even though the main leads did not get obviously hot. This was in the days before electronic ignition so I guess the voltage at the coil was well down, although I never checked that explicitly

A replacement motor sorted it out just like that (after *much* faffing with terminals, batteries, and solenoids).

[The Natural Philosopher]

One of te intriguing things that flying model planes with permanent magnet motors taught me is that as they demagnetise from heat, they want to spin faster and draw even more current - positive feedback, magic smoke and that's the end of the motor...

[harry]

It's back emf that limits the current in a DC motor

[Cursitor Doom]

Most obvious thing that springs to mind is that the motor was fine all along; just the solenoid for it had failed short-circuit. They're not built to last long for extended periods of peak current.

[Dave Plowman (News]

Did another check. Don't have a DC current meter, but the jump start pack is only 20 amp.hr or so and reasoned the voltage drop with the motor free running would give an indication. Both are near as dammit the same, measured with a decent DVM.

[Brian Gaff (Sofa]

You would think, would you not that some kind of thermal cut out might be included in such things to stop the unit overheating due to being stalled. Brian

[Dave Plowman (News]

Given the very high currents involved it would be a pretty massive (and pricey) device?

[tabbypurr]

and unreliable. DAMHIKT.

[Vir Campestris]

My experience with model railway locos back in the day was that as the magnet got weaker they'd produce less output for the same input, and would gradually slow down - not speed up.

Of course this means you turn the power up, and if it stalls - magic smoke...

"recharging" the magnet can transform an old loco.

Andy

[The Natural Philosopher]

Haryy is an example of a little knowledge.

The back EMF drops as the magnet gets weaker. So for a given voltage the motor has to spin faster to generate it.

That meand for a given load the current must increase.

That heats the motor even more and so takes the magnets into permanent loss of magnetism.

The point here is that you were operating with a current limiting rheostat in series. So what you noticed was the drop in efficiency for a given current.

On model planes we have a constant voltage* output speed controller. If you keep the voltage constant the motor speeds up

+1

*by which I mean the voltage is constant under load, - but of course varied by the throttle.

[harry]

Or the brushes sticking.

[Dave Plowman (News]

Experience of PM motors as used on cars says modern magnets - developed for computer HDs, etc, absolutely transform the perforamnce. I did some tests between a Lucas PM wiper motor and a modern Valeo - both pretty well the same size and designed for the same job. At the same current, the Valeo developed 4 times the torque. Of course the Lucas would have aged - but even when new were a bit marginal as the screen dried. Same thing with central locking motors.

[Vir Campestris]

Ah, all makes sense.

As the magnet gets weaker you're automatically winding up the power to keep the voltage the same.

Andy