Motor peculiarity

[kellys_eye]

I have a permanent magnet 12V motor that simply refuses to take current!

I can stall it at 8A but an identical motor draws 30A+ under stall conditions - yet I cannot see or find anything wrong with the '8A' motor.....

New brushes, no damaged rotor windings (that I can see or measure anyway), clean commutator, good wiring etc. But I can stall this motor (almost) using my fingers whereas the other can't be stalled with a wrench!

What gives??? Would the loss of magnetism give this sort of result? I realise this would cause a loss of torque but not a drop in stall current??????

 

[WHONOES]

Has the timing shifted?

 

[bushtech]

My 2c, and it really is 2c as I don't know nuffink about these things but aren't the windings picking up a problem under load.

 

[kellys_eye]

Timing can't change as the brushes are fixed to a 'keyed' plate.

Bearings are free-running, clean and no noise/vibration.

The motor runs at 1.5A no-load and I can stall it to 8 or 9A (either direction) - it gets hotter of course - but it 'should' give a lot more torque (it's from a golf trolley that no longer 'pulls' as it should). A similar - but not identical - motor pulls 30+ amps under load.

This motor is some 12.5cm long, 7cm diameter - not visible markings on the casing.

The motor tests are done straight from the battery (bypassing the controller which is working perfectly anyway) and the battery voltage drops from 12.6 to 11.8 at the stall current so the battery is ok (I've tested it on another battery too, same results).

I can see all the winding in the rotor and none appear burned nor is there any visible physical damage - they measure <1Ω segment-to-segment on the commutator so the stall current 'should' be higher than the 8-9A I can make!

Do brushes 'change' (they measure near-as-damn-it zero ohms)? The commutator is lightly scratched but otherwise clean and unworn. Brush pressure???? They are located using springs which seem ok too.

I'm still perplexed?

 

[WHONOES]

What is the resistance across the input wires?

 

[Minder]

The only reliable way of testing the armature resistance is to use a low voltage supply with a locked rotor and use a ammeter to obtain resistance.
You may have to have the armature tested with a growler for exact condition.
M.

 

[Bluejets]

Armature is more than likely wound with a smaller gauge wire.
Or as in some, like we used to do with slot car motors, wound 2 in hand.
If that is the case it is possible one of the 2 windings in parallel is open circuit.
Was the 30a motor in good condition or half burnt out?
Same number of armature segments i.e. 3 ??

 

[Externet]

With motor cold, apply power for 1 second while holding shaft. Sense by touch which parts got hot. Like a brush wire, supply terminal...

 

[kellys_eye]

Without the appropriate test equipment (growler and/or magnet tester) I'm having to declare the motor BER - too much time spent on it and no results.

Still, for such a simple device it has caused me more trouble than most things 'electronic' that I've had to tackle in my past! Must have had the motor apart at least a dozen times thinking I've 'done something wrong' or missed something........ sigh....

Thanks for the input(s) anyway.

 

[Bluejets]

Know how you feel.
Chased an intermittent open in a starter motor used for a hydraulic hoist on one of the work utes.
Finally found it, traced down to aluminium rivets that hold the brush-holder and transfer the armature current.

Thing was, every time it would go belly-up, fault would disappear the moment the test gear appeared.

 

[Minder]

It might be interesting to see if it still generates the correct voltage, IOW it should show the rated terminal voltage when rotated at the rated RPM, or a pro-rated value at anything less in rpm.
M.

 

[kellys_eye]

Sadly I have no spec or other details for the motor so nothing with which to make readings or comparisons.

 

[WHONOES]

Spose you could always try re-winding it with a thicker wire if you feel so inclined!
Have done so myself on a few motors and it doesn't take as long as you might imagine!!! Presumably it is just a 3pole type.

 

[kellys_eye]

I also have a link to a Youtube video that's 10 hours of 'watching paint dry' - don't know which would enthuse me the most.......

 

[WHONOES]

It's nowhere near as boring as watching paint dry. A simple re-wind only takes and hour or so.

 

[Tha fios agaibh]

I agree you need to test the armature.

One thing I've had success with is just hooking it up to a power supply and let it run on the bench for a long period of time.
It alows the new brushes to wear in to make better contact with the comutator, and the heat generated can remove moisture that may be present in the armature.

I'd even hike the voltage way up slowly with a variable power supply while monitoring that current doesn't exceed motor full load.

 

[WHONOES]

I agree you need to test the armature.

One thing I've had success with is just hooking it up to a power supply and let it run on the bench for a long period of time.
It alows the new brushes to wear in to make better contact with the comutator, and the heat generated can remove moisture that may be present in the armature.

I'd even hike the voltage way up slowly with a variable power supply while monitoring that current doesn't exceed motor full load.

Worth a try.