Can I disassemble DC servos without ruining them? How best to clean commutator?


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Thread: Can I disassemble DC servos without ruining them? How best to clean commutator?

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    Default Can I disassemble DC servos without ruining them? How best to clean commutator?

    I have always been told to never disassemble a step motor as it ruins them - reduces their output torque significantly by demagnetizing them.

    So, is there a problem if you disassemble a permanent magnet DC servo motor? I am thinking that this is not a problem with DC motors in general because I have disassembled many DC starting motors without problems. However, servos may be different - so I ask:

    Is it OK to disassemble a DC servomotor to clean it up inside, to replace a bearing, clean the commutators, or etc.?

    My servos show very erratic ohms readings across the motor terminals. Turning the motor shaft to different rotational positions gives ohms readings from 3 to over 120 depending on where the shaft stops. I am thinking that this will cause erratic operation as a servo. Am I correct to be concerned? The commutator does not appear to be burned up or out of round or grooved, just very badly blackened by use. There was carbon dust in the brush holder as well. The brushes are nearly full length and in good condition and move freely.

    If I can disassemble these motors without reducing their performance, how should I go about cleaning up a dirty commutator? Is it OK to polish them with fine emery paper? Will polishing the bars with abrasives embed grit in the copper and cause excessive brush wear? Do they have to be turned in a lathe to clean them up? How do you guys handle servo maintenance and what should I do to bring these motors up to their peak performance before I install them?

    I have a used set of Siemens 1HU3056-OAC01-0ZZ9 and a used set of Baldor 4203623 motors.

    I wold appreciate any tips for maintaining these motors.

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    Default Re: Can I disassemble DC servos without ruining them? How best to clean commutator?

    Yes, you can disassemble and clean a DC servo, but it is an expert job. Much care needed. Consider discussing with a local motor rewind shop.

    Cheers
    Roger



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    Default Re: Can I disassemble DC servos without ruining them? How best to clean commutator?

    If you attempt it, take care when removing the armature as once it leaves the end plate bearing it is attracted to the P.M. field and can damage them if care not taken.
    BTW, using an ohm meter to take a resistance reading is a little futile, the positive way is to lock the rotor and use a low voltage supply and simultaneously measure the current and voltage.
    Many get concerned over a black com but it is normal to some extent.
    If the com is worn it can be turned down on a lathe if enough material is there, then it may need undercutting between the bars if it has the old style Mica material, later com's have material that wears with the copper,
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Default Re: Can I disassemble DC servos without ruining them? How best to clean commutator?

    The test I did involved measuring the resistance between motor leads, rotating the spindle slightly, and repeating with the rotor stationary. I understand that the resistance while turning will vary dramatically and is of no real consequence, but I am concerned about the variation in resistance when the rotor is stopped at different positions.

    Back around 1980 I worked for a company that made the automatic cabin climate control (combined heat and air-conditioning and dehumidification and ventilation and windshield defrost and blower speed and damper position and etc.functions) for high end Mercedes cars. This control had a complicated PCB/wiper switch assembly driven by a small DC servomotor that was made in Japan. After years of good reliability they began failing at a very high rate. I managed the Engineering laboratory and got the problem of determining why. We determined that there were dead spots on the commutators and if the servo landed on one - the unit would stop functioning as the motor could not move off that spot. Electron microscope examination of brushes and commutator bar surfaces showed that there was a difference in commutator bar surfaces. The older reliable ones had a grooved surface with rounded troughs and crests something like a sine wave. The ones that were failing had rounded troughs but sharp peaked crests. Turns out that the Japanese manufacturer had changed suppliers of their abrasive papers used in finishing the commutators and that resulted in a difference in surface finish. Going back to the original abrasives solved the problem. This automotive temperature controller servo was a low voltage and very low power servo so that experience may not apply to the high voltage high power servos used in machine tools, but it makes me cautious. Once burned twice cautious! Thus the question.

    So far as using a DMM to measure the resistance - you may have a good point there. I do have an HP computerized laboratory grade power supply that can measure both voltage and current simultaneously and very accurately. I may get it out and do the test just to satisfy my curiosity.

    WRT disassembly; I had a second motivation for doing so - besides plain curiosity about how they were designed and built and what condition they were in. I am building a dual use CNC/manual machine and I need a brake to hold the spindle down feed in position when I am using the mill in manual mode. Adding one to the spindle down feed motor would be the simplest method to do this and I have an applicable electrically released disk brake I could use. I needed to open up the motor to see if I can adapt it for this use.

    The motor I disassembled is pretty packed with carbon dust. The space between the rotor and the field poles is full. Clearly someone has been blowing the carbon out of the brush holders and into the motor itself. Where else would it go when you do this? that makes me question the whole concept of blowing out the brushes. Of course, this motor is 35 years old so................



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    Default Re: Can I disassemble DC servos without ruining them? How best to clean commutator?

    If the motor is 35yrs old then it most likely requires some attention to the com, if there is sufficient copper there to turn it down, also most likely require undercutting.
    Hence the popularity of BLDC servo's now, quite a few advantages.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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Can I disassemble DC servos without ruining them? How best to clean commutator?

Can I disassemble DC servos without ruining them? How best to clean commutator?