I have some dc permanent magnet motors that lost some percantage of their magnetic capabilities. On these motor there is a thick cupper wire around the magnets. And this cupper can be reached without dismounting the motor. I heard that this type of magnets can be magnetised by "capacitor discharge" method.

Can anyone tell me what kind of circuit I need and how can I calculate required components sizes.

DDlker YILMAZ

 

The magnets in PMDC motors are normally charged in a special charging fixture. This fixture may be several times the size of the actual motor. I
have never heard of having a special winding in a motor for charging magnets after the motor is assembled.
"Capacitive discharge" and "half cycle" magnet charging systems provide a brief pulse of power to the magnet charging fixture, typically in the range of thousands of amperes. The motor manufacturer may charge the magnets before or after they are assembled into the motor (depending upon various criteria about the motor or their manufacturing process).
I suggest that you contact the motor manufacturer to see what they can do to help you. Even if it is at all possible to re-charge or replace the magnets, it may still be cheaper to simply replace the motor. An important question to address though, is why the magnets seem to be losing their charge.

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Michael Griffin
London, Ont. Canada
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Dear Michael,
some users dismantle the PM motors to make maintenance(!). Altough, I tell my customers not to open such a motor by themselves, still some "too much knowing" technicians take the rotor out and grease the bearings. The result is 30-40% moment loss. They regard such motors as AC squirrel-cage motors.

 

Whether or not you can dismantle a PMDC motor without special precautions depends upon the magnet material. Some types of magnet material will lose its charge, and some types won't. The more exotic materials seem to be particularly prone to this problem. Generally it is safest to assume that you can't remove the armature.

Some other causes of de-magging include excessive heat and excessive armature current. Again, the type of magnet material used will determine how easily this will occur.

I am rather curious how much of a struggle your customers had to get the motor apart in the first place. I should think that once they got one of the end caps off, the armature would pull to one side and cling to one of the magnets with great force. The magnets are fairly brittle, and quite easily chipped or broken which would only add to the excitement. You can solve all this with the proper tooling, but that would take a bit more forethought than evidently went into their efforts in the first place.

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Michael Griffin
London, Ont. Canada
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I agree that is it technically very difficult to open a PM AC or DC motor, but they manage it!
We'd rather ask, why do they do it?
Electric motor is an interface between the electrician and the mechanic.
The mechanic sometimes behave a servo motor like an ordinary motor without knowing the facts an electrician would know.

 

its not so that never open the motor. We faced same problem in a techo generator used for feedback in hoist motor. we opened the tacho to change its bearing and then found the voltage generated was less compared to earlier. Still we are having such tacho kept useless. What can be done to normalise them?

 

I am a scholor in sharif university of technology.
I will be very glad if you send me any information you have about this matter, for me.

Hady Tavakoli Nia

 

Actually there are PM motors that have windings built inside for magnetizing and de-magnetizing. I have worked with such motors for many years. On these, Westinghouse, motors it would be impossible to separate the two halves without that feature. The motors are separated for maintenance reasons, to inspect the PM for chips & cracks, due to heavy currents and to service or replace the armature. I do not know the actual make-up of the magnetizing/De-magnetizing device but the process only takes a couple seconds and is automatically timed.

Roger