Originally Posted by: zoooctan 
Hello, thank you all for such an informing thread. I'm quite fascinated by the topic of motors getting magnetised or having been affected. I too have a few locomotives with strange behaviour. One for example, (Rio Grande 4-4-0) seems to stall often. But when pushing the motor amatures with a toothpick, they start running once at a moderate speed. if I go to a slow speed or often from a stop, the motor has trouble starting.
Can someone share more with me about what happens when a motor gets demagnetised? What are the symptoms and what are the possible rectifications?
What I can see so far, is that cleaning alone is not enough?
Thank you
It’s 50 years since I covered electric motor theory but if I remember correctly:
(Simple) motors such as these have a rotor and a stator.
The rotating part is the armature which in our case has 3 lobes for pre 2000 locos and 5 lobes for post 2000.
The stationary part consists of a pair of magnetic poles with magnetism provided by a shaped permanent magnet.
Since like poles repel each other (eg 2 norths) and unlike poles attract this is one of the principles of motor theory, electromagnetism theory has two parts: 1) pass current through a wire and it creates a magnetic field and 2) move a wire through a magnetic field and it generates a current in the wire.
Each lobe’s armature winding will have current passed through it creating a north pole say, then as the armature rotates the commutator (on which the carbon brushes press) will switch the direction of current flow to create a south pole in that lobe and so on as it turns.
These created poles react with the (permanent) field (a north and a south pole) of the permanent field magnet so each lobe of the armature will be alternately pulled and pushed to give a turning effect (torque).
Older types of permanent magnet are not really permanent; heat, mechanical shock, time and lack of a keeper * will cause the magnet to lose its strength.
The keeper in these motors is the steel armature between its poles. If the armature is removed from the field magnets they have lost their keeper and therefore some of their strength.
When the field strength is reduced, the push and pull effect with the armature and its changing poles is also reduced as is the back electromotive force in each lobe created by moving the wires in the lobes through the field from the permanent magnets is also reduced allowing more current to flow in each lobe.
So reduced power of the field magnets means: 1) reduced torque and 2) increased current in each lobe and therefore also 3) excessive heat generated.
* The remedy is to re-magnetize the field magnets using a remagnetizer which flashes a very powerful magnetic field around them, this is done with the motor fully assembled with the armature acting as the keeper in place.
With pre digital HO Märklin this was not a problem since the field magnets were also electromagnets not permanent ones - this is also why they would work with both ac and dc.
Long and wordy I know, a white board would have helped!
ChrisG