servomotor problem

[Alex S.A]

hi everyone.
I had a problem on servomotor. When i pressed the switch to go to the left, it does not go and the deviation on the CNC screen rise up fastest and the CNC shows deviation error but when I pressed the switch to go the right, the motor moves correctlly. I exchange one driver for other one and the problem followed the motor.
I just changed the motor, but I would like to know what happened with the motor.
Was the problem on brush? Tacometer?
May someone help me?

Alex S.A
Brazil - Rio de Janeiro

[Al_The_Man]

Is there an encoder on the motor. A way to test it would be just to hook up a source of DC to the motor , a car battery even, and see if it responds in both directions at the same speed, monitor the output of the Tach for the same dc output in both directions. Usually if there is a problem with the tach it is in both directions and either the motor will jump or the motor will take off at high speed.
Al

[Alex S.A]

Yes, there is a encoder on motor.

[Al_The_Man]

If you have changed the motor and it is now ok, I would perform the test I mentioned, this tests the motor and tach to some degree, if everything looks ok, then the other thing to test is the encoder, It helps to have a 5vdc supply and a 'scope if you can get hold of one. Also it would not hurt to check the motor and tach brushes while you have the motor out.
Al

[ViperTX]

Sounds like your driver thinks the direction is changing, but it's not....I suspect the motor does not move at all in that direction? If that is so and all your connections are okay....then I suspect that half of your driver bit the dust...

[ViperTX]

Ooops...didn't read all your post....since the winding are all the same for CW or CCW the driver just changes the polarity of the applied armature voltage.....then I would try a new encoder.

[Al_The_Man]

Alex, with encoders at a few hundred dollars a pop I would do the simple test first, especially if you have the motor replaced already, you have the luxury of time, I had a similar problem a couple of years back with a DC motor than would run ok in one direction but very intermittent in the other, It turned out the brush carbon had completely worn away down to the spring, the spring made contact in one direction but not in the other.
Al

[Alex S.A]

Thanks for the replay.
Before took this decision to change the motor I checked the brush and they were okay.
The motor did not move to one direction ( It was completly stoped when I pressed the switch) and the error counter (deviation error) on the CNC screen shooted and the deviation error happened. On the other direction the encoder worked pretty well.
When I changed to the direction that the motor did not move I could hear some strange noise inside the motor like a relay been closed. I think it is a little bit strange the description of failure but it was what happened.
Do you guys think that the motor was not damaged ?

[ViperTX]

Originally Posted by Alex S.A Thanks for the replay. Before took this decision to change the motor I checked the brush and they were okay. The motor did not move to one direction ( It was completly stoped when I pressed the switch) and the error counter (deviation error) on the CNC screen shooted and the deviation error happened. On the other direction the encoder worked pretty well. When I changed to the direction that the motor did not move I could hear some strange noise inside the motor like a relay been closed. I think it is a little bit strange the description of failure but it was what happened. Do you guys think that the motor was not damaged ?

Is this an AC servo motor?

[Alex S.A]

Not. It is a DC.

[HuFlungDung]

If the motor turns a little bit, then clunks in one direction, I'd look at the physical components such as perhaps the pole pieces, brush housings, etc to make sure that they are all tight against the housing and not dragging on the rotor. Also, check the bearings.

Is the commutater smooth and concentric? No loose bars?

Are you getting any kind of overload condition?

[ViperTX]

Okay, so we know that the problems follows the motor, you've changed drivers and motors.

So, if you just want to experiment....reverse the armature windings and the problem should still be there (just in the opposite direction), if this is so...then I'm with Hu....something in the motor is keeping it from reversing direction....brushes do not seat properly on the commutator....if you can see the brushes with the end cap removed...manually spin the motor in each direction and see if you notice any deflection in the brushes, holders, etc.