demagnetizing and remagentizing a servo motor

[reebelec]

I have been asked to do some research on servo repairs, especially on remagnetizing a servo if needed. Can you guys give me any insight as to how it is done. What kind of equipment is required. What type of power are we looking at do this, etc...

thanks

 

[UKpete]

Generally speaking remagnetizing is not easy. The biggest problems occur when you have to magnetize 'in-situ', when feasibility depends on the physical space around the magnets, and pole patterns in a multi-pole design. It is much easier if you can remove the magnets and place them between the pole-pieces of a magnetizer, but this is not usually possible.

Particularly for rare-earth magnet types that you might find in a modern brushless-dc motor (samarium-cobalt or neodymium-iron-boron) and to a lesser extent ferrite types, the magnetizing force required is very high and requires a current pulse magnetizer with a specially designed fixture (magnetizing coil). There are numerous suppliers of magnetizers e.g.

http://www.magnet-physics.com/

- but the fixture design may require a specialist design company. They may use empirical design methods or in extreme cases, time-stepping finite element analysis.

Magnetizers are usually capacitive discharge type and deliver kA pulses (the pulse duration must be long enough to ensure that the field pattern fully penetrates the magnet if this is a conducting material - this takes a finite time due to induced eddy currents). Pulses are used because a steady current would melt the fixture. They may have to be designed to withstand high impulse forces.

It may be possible to fully remagnetize by trial-and-error (hit it with a large enough pulse) provided you have a measuring method that can confirm full magnetization has been achieved (e.g. using a Hall probe, increase pulse magnitude until no further increase in flux density is achieved).

Hope this helps.

 

[mc5w]

Most manufacturers of servo motors are not going to tell you what kind of current pulse you will need to remagnetize the permanent magnets. They are looking to make a buck too.

Mike Cole, mc5w@earthlink.net

 

[cn100]

reebelec
i work in a servo repair shop (10 years plus) and we have a large magnetizer mostly used for pancake motors, tachos, and d.c.motor magnets.everything is trial and error but if you can wrap a coil around a magnet you can pulse it and de-magnetize as well as magnetize. we start out with low voltage from 400 volt input transformer and see how much kick comes out and increase in stages. output arrives from large cables from capacitor bank plus and minus. never try to magnetize rare earth brushless servo magnets on rotor as good quality ones never de-mag unless current driven to almost destruction.cheapo dc ferrite motor magnets will be way overspeed when de-magnetized.very easy with gear to re-mag or better new motor. have been to siemens and seen how they do it and believe me leave it to them.