PM motor demagnetization 1

[thinker]

We plan to use a PWM inverter driven PM motor for high transient torque application (a dynamometer for internal combustion engine torque pulse simulation). One of concerns is a theoretically possible loss/reduction of magnetizing capability of rotor mounted magnets due to high transient overheating caused by transient torque. The PM motor supplier can only evaluate thermal processes based on steady state torque application. Is anybody familiar with this kind of problem or might be able to refer to the source of information?

 

[sreid]

I'm not a world class expert in this area but the following basic information might be useful.

The ability of rare earth magnets to resist demagnetization decreases with temperature. Samarium Cobalt magnets are much better in this reguard than the Neodyminum magnets. They are also much more expensive. It also depends on the grade of the material. The MCE website shows curves for all their materials. What one is looking for is a straight line demag curve at temperature.

http://www.mceproducts.com/

The thicker the magnets are relative to the air gap length, the better the magnets resist demagnetization.

I'm told at the limit, the magnets start losing strength at their edges.

Having said that, thick Samarium Cobalt magnets are quite difficult to demagnetize.

I believe, for a price, MCE will do magnetic FEA if motor manufactures can provide detailed design information. A static analysis works if the magnet temperature is set and demag is checked at all rotor-stator angles.

Another possibility is to have a custom motor designed and built to your specifications. I can recommend Carlsbad Magnetics.

http://www.carlsbadmagnetics.com/

You might also get additional information by calling both MCE and Carlsbad.

 

[thinker]

Thank you very much, sreid. Now we have a good starting point.

 

[Clyde38]

All of the magnet materials used to manufacture PM DC motors will suffer "reversable" demagnetization to some extent with an increase in temperature. If you use motors with ferrite magnets, the irreversable demag due to temperature alone is in the opposite direction. You mention "high transient overheating caused by transient torque" and this is another issue. High transient torque implies high transient current. You may demagnetize the motor due to current rather than high temperature.