Encoder replacement of brushless servomotor 2

[elinBG]

Hello,

In our repair shop a customer brought SIEMENS brushless servomotor with permanent magnet rotor.
They disassembled the whole motor, but only the brake seemed to had a problem, we rewind it and since then the system works again, but now the motor is heating, 90 degrees Celsius frame temperature.

The stator winding is with perfect insulation 4000Mohms and same resistance of the coils

They doubt in the encoder (attached you will find photos). They bringed a second one smaller WORKING, with the same encoder.

Now they want to replace the encoders of the motors. I have two questions:

1) Is there any special way to place the encoder in exact position or it doesnt matter?
2) Do you think that the encoder could be the heating reason?

Thank you in advance for your answers!

 

[elinBG]

http://www.staehleprecision.com/dat...sition Encoders for Servo Drives brochure.pdf

Page 33, scared me?
Is this "PWM 8 phase angle measuring device" the only way?

 

[mikekilroy]

wow. of course not aligning the feedback device to the motor poles is the problem and DOES cause heating - you appear just luck it runs at all.

Get educated on how to phase feedback to motor before going on. your feedbk is probably fine; buying a new one and not knowing how to align it in the motor is ludicrous. I suggest you make arrangements with a servo motor repair shop to do these repairs for you.

 

[elinBG]

Thank you!

I resend them to servo-motor and automation specialist, who should phase the motor with the ENCODER.

The thing which I learned is that the encoder should be aligned with the motor, but there is another device, called resolver, which is "automaticaly recognized by the machine".

The difference is that the encoder have a mirror-disk and the resolver does not.

I will be grateful if somebody have some document to read about encoder-motor alignement.

Thanks

 

[cswilson]

The controller needs to know the angle of the rotor of a brushless servomotor in an absolute sense in order to know how much current to put in each phase, and in what direction, to get the torque that it wants. It reads the position sensor to get this information.

The torque per unit current from the motor is proportional to the cosine of the error in this measurement, so of course is best when the error is 0, as cos(0)=1.0

If the error is relatively small, the motor can still appear to function, but the servo loop responds by asking for more current. If the error is 30 degrees, cos(30)=0.866, and the motor will require 1/0.866 = 115% of the current to do a given task. At a 60 degree error, cos(60)=0.5 and the motor will require 200% current. It should be easy to see why this can result in the motor running very hot.

What is really dangerous is an error greater than +/-90 degrees, because the torque is generated in the wrong direction, causing unstable positive feedback and a runaway condition. I have known a couple of incidents of this that could easily have caused a fatality. So this is tremendously important to get right.

Curt Wilson
Delta Tau Data Systems

 

[ozmosis]

Depending on the type of Siemens servo motor, there is usually a zero mark on the motor that matches with the correct position sensor. However, and as mikekilroy suggest, I recommend you get the motor checked out by an authorised repair centre.

 

[JadeYork]

1. Encoder alignment is critical and can cause many problems including over temp if it is not set correctly. Check out this article to find out more about issues with encoders; it might help you understand issues with encoders more.

2. Was the windings mapped so it could be rewound with the same coil placement?

 

[elinBG]

Hello JadeYork,

1) Thank you for your answer.
2) The motor was not rewinded, only its brake was.

But doest it mean that if needed to rewind such a motor "the same coil placement" should be made?