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My father and I have a company that services electric motors. We can repair every ac, dc, induction motor.. But these days factories are replacing there dc brush motors with servo motors. So if I want to stay and work for the factories I need to start repairing servo motors. The winding itself isn't different then normal induction motor, but the rotor has permanent magnets, encoder at the end,,, etc.My biggest problem is that once I dissmental the motor and put it back together it doesn't work anymore. I am thinking that is has something to do with the position of magnets in rotor and position of the encoder on the shaft. But I need to learn more.
So any help or info would be very appreciated.

My email is boris.matijasevic @ siol.net

Regards
Boris

The feedback device must be aligned properly on a BLDC servo motor. You may find hall effect sensors, UVW channels on an encoder, an encoder index mark, a sinusoidal encoder, a resolver, or a "smart" encoder. Or some combination of the above. It can be complicated. Some manufactures provide phasing info, some may not. Good luck.

There are a number of possible problems. One is that there are usually sensors that have to be aligned correctly. Sometimes when you take a servo motor apart and put it back together again, these sensors end up getting moved. Servo motors still have to be commutated. This is just done electronically in the drive instead of with a commutator. The sensors in the motor provide this commutation postion to the servo drive. How to do this properly varies according to the servo motor design. You would have to get this information from the manufacturer (or some other source).

Another problem (and there is a good chance that this is the problem you have) is that the magnets used in servo motors are often different from those used in normal permanent magnet DC motors. Often, if you break the flux path by disassembling the motor, the magnets lose their flux. This happens instantly on removing the armature. Whether or not this happens depends upon the magnet material in that model of servo. In order to disassemble the motor without losing flux, you would need to insert an alternative flux path which is inserted as part of the process of removing the armature.

Once the flux is lost, all that can be done is to recharge the magnets with a magnet charger. Even if you have a magnet charger with the right number of poles, you would have to know how the original manufacturer did things (outside in? inside out? with the sensors present or removed? etc.).

That's not even the whole story. Many motors have exacting requirements for commutation angles and there is a specific procedure to align them after repair. This may or may not be published and is very likely specific to each brand or configuration. I have seen some for Siemens motors that required special equipment and some that looked doable on the floor. I would expect that once you get the idea of what they are setting up, you might be able to tune them in a pinch without the specs. We do have local shops that manage to fix them with good results most of the time.

Regards
cww

Boris,

You are quite correct, that servo motors are differend from standard motors. BLDC motors are mainly synchronous 3 phase motors. They have no slip rings for curent commutation. Thus the 3 phase currents are commutated in the servo amplifier. It is a requirement that the current commutation be synchronized with the position of the motor armature. The trend is to use a resolver on the motor shaft and it must be synchrinized to the motor armature position In the manufacturing of these motors, the sychronization is done on special fixture. Once this is done, the position transducer (resolver or digital device) should not be changed. This position signal is used to control the current commutation, and to create a synthetic velocity loop by differentiating the position signal.

In actual use, some service people do not know this and can not figure out why the motor does not work. In our machine tool company, a bldc motor that has had the position transducer moved, goes back to the factory and gets reset. The manufacturer has the test equipment to readily reset the position transducer. It can be done in the field IF one knows what they are doing, but it is cheaper and more expedient to send the motor back to have the transducer realigned.

In your case, I recommend that you contact a motor manufacturer and explain that you need to know how to reset the resolver, etc., so you can do it in your business. It is not a major problem to reset the resolver. I am sure you can do it with some instruction and instrumentation. In our case we just did not want to bother with it, with the many drives being installed daily.

Regards

George Younkin, P.E., MSEE
Life FELLOW IEEE
Bulls Eye Research, Inc.
Fond du Lac Wisconsin, 54935
gwyounkin@charter.net