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.

--
He who laughs last, thinks slowest*

Dave Plowman London SW
To e-mail, change noise into sound.

On 21/06/2020 14:38, Dave Plowman (News) wrote:
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.


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).

On 21/06/2020 15:20, newshound wrote:
On 21/06/2020 14:38, Dave Plowman (News) wrote:
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.


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,Â* batteries, and solenoids).
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...


--
"Corbyn talks about equality, justice, opportunity, health care, peace,
community, compassion, investment, security, housing...."
"What kind of person is not interested in those things?"

"Jeremy Corbyn?"

On Sunday, 21 June 2020 16:59:56 UTC+1, The Natural Philosopher wrote:
On 21/06/2020 15:20, newshound wrote:
On 21/06/2020 14:38, Dave Plowman (News) wrote:
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.


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,Â* batteries, and solenoids).
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...

It's back emf that limits the current in a DC motor
https://en.wikipedia.org/wiki/Counte...romotive_force

On Sun, 21 Jun 2020 14:38:38 +0100, "Dave Plowman (News)"
wrote:

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?

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.

In article ,
newshound wrote:
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

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.

--
*One of us is thinking about sex... OK, it's me.

Dave Plowman London SW
To e-mail, change noise into sound.

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

--
----- --
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"newshound" wrote in message
o.uk...
On 21/06/2020 14:38, Dave Plowman (News) wrote:
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.


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).

In article ,
Brian Gaff \(Sofa\) wrote:
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

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

--
*Wood burns faster when you have to cut and chop it yourself.

Dave Plowman London SW
To e-mail, change noise into sound.

On Monday, 22 June 2020 11:19:19 UTC+1, Dave Plowman (News) wrote:
In article ,
Brian Gaff \(Sofa\) wrote:

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

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

and unreliable. DAMHIKT.

On 21/06/2020 17:21, harry wrote:
On Sunday, 21 June 2020 16:59:56 UTC+1, The Natural Philosopher wrote:
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...

It's back emf that limits the current in a DC motor
https://en.wikipedia.org/wiki/Counte...romotive_force


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

On 22/06/2020 21:37, Vir Campestris wrote:
On 21/06/2020 17:21, harry wrote:
On Sunday, 21 June 2020 16:59:56 UTC+1, The Natural PhilosopherÂ* wrote:
One of te intriguing things that flying model planes with permanent
magnet motors taught me is that as they demagnetise from heat,Â* they
want to spin faster and draw even more current - positive feedback,
magic smoke and that's the end of the motor...

It's back emf that limits the current in a DC motor
https://en.wikipedia.org/wiki/Counte...romotive_force


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.




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...

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

"recharging" the magnet can transform an old loco.
+1

Andy

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

--
"It is an established fact to 97% confidence limits that left wing
conspirators see right wing conspiracies everywhere"

On Sunday, 21 June 2020 18:00:25 UTC+1, Cursitor Doom wrote:
On Sun, 21 Jun 2020 14:38:38 +0100, "Dave Plowman (News)"
wrote:

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?

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.

Or the brushes sticking.

In article ,
Vir Campestris wrote:
On 21/06/2020 17:21, harry wrote:
On Sunday, 21 June 2020 16:59:56 UTC+1, The Natural Philosopher wrote:
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...

It's back emf that limits the current in a DC motor
https://en.wikipedia.org/wiki/Counte...romotive_force


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.

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.

--
*No sentence fragments *

Dave Plowman London SW
To e-mail, change noise into sound.

On 23/06/2020 05:18, The Natural Philosopher wrote:
On model planes we have a constant voltage* output speed controller. If
you keep the voltage constant the motor speeds up

Ah, all makes sense.

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

Andy