DC Brush Motor failure

Discussion in 'General Electronics Chat' started by JohnBedell, Oct 15, 2012.

  1. JohnBedell

    Thread Starter New Member

    Oct 15, 2012
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    Hello everyone -

    Would anyone share with me some reasons why a small 12v DC Brush Motor would work for awhile and then not?

    I am experiencing some motors that when power is first applied or if they are stopped and then re-started, will appear to "seize" up. I can still physically rotate the motor shaft will no effort so I am assuming that the failure is not mechanical in nature (i.e.bearings). These motors are new and I am trying to diagnose the problem - but after bending back the housing tabs and removing the end cap, I am not sure what I am looking for.

    I have ruled out the motor controller circuit since replacing the motor in the project solves the problem.

    Any direction or insight is appreciated.

    Thank you.
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    More information would help.
    What are the motors rated for? volts? amps?
    What are you using to power them with? Battery/ Half wave dc?
    What is the load on the motor?
    How long do they run before they give up the ghost?
    Motor info part #
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Small brushed motors are generally three phase, ie the armature has three separate windings connected to 6 contacts on the commutator.

    It is a common failure that one winding will go open circuit, and/or one commutator segment. If the motor stops at that position it will not re-start. Roughly a 1 in 3 chance of stopping there. ;)

    Sometimes cleaning a commutator might fix it. If you want to get fiddly you can get some small pointy probes and measure the continuity of each of the 3 windings on the armature.
     
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  4. Audioguru

    New Member

    Dec 20, 2007
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    I have RC electric model airplanes. The cheap small ones use motors with brushes and the brushes or commutators wear out after about 3 hours of running time. I have replaced many of these cheap little motors.
    The motors are tiny like the vibrator ones in cell phones or like the tiny motor used to open and close a CD tray.

    My larger RC model airplanes use brushless motors that keep on running and running and running and running and running and running and running and running and running and running and running and running and running and running and running .....
     
  5. JohnBedell

    Thread Starter New Member

    Oct 15, 2012
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    Thank all of you for replying so fast.

    Yes, the motors I am dealing with have three windings and are powered by an unregulated 12vdc .500mA AC to DC wall adapter. These motors are used in a product that will pass inital testing at the time of manufacture and then will fail within' 4 weeks after being in the field - sometimes right out of the box! I have also had some fail out of the suppliers box - prior to assembly/use.

    Many times I can get the motor to resume proper operation by turning the motor on/off a few times - it will still failure later, but for some reason I can get it going again. My first suspicion was that one of the bearings or the housing crimp was binding the armature up - but as stated previously, I can rotate the shaft with no effort.

    Could a motor with three windings run on only two for a period of time? What The RB posted seems very plausible in my situation.
     
    Last edited: Oct 15, 2012
  6. Audioguru

    New Member

    Dec 20, 2007
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    Many of my tiny worn out brushed motors would not start without a "push" because one winding or a commutator contact has disconnected. Then when running, the power is 1/3rd low with a lot of vibration.

    Sure the motors are not reliable. They are made "over there" and are VERY cheap.
     
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  7. debe

    Well-Known Member

    Sep 21, 2010
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    If the motors are failing when new & within 4 weeks, then i would say they are of poor quality. Brush motors should last a long time, car fuel pumps are brush motors & generaly last for 200,000Ks & they are imersed in petrol. Another example are old audio cassete player motors, seen them last for a lot of hours running. Have cut a few fuel pump motors apart to see why they failed, it was due to the comutator worn out. When this happens they generaly become noisy, if not replaced they eventualy wont run the next time you try to start the vehicle.
     
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  8. JohnBedell

    Thread Starter New Member

    Oct 15, 2012
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    Just an update.

    I marked a line on the gear and the housing and have taken resistance readings at the six "stops" (where the armature and permanent magnet attract) and have recorded readings of 7, 7, 1.2M, 7, 7 and 1.1M ohms. When the gear mark is lined up to any of the "7" positions and power then applied, the motor operates normally. If I line the gear mark up to one of the 1 Meg ohm positions and then apply power - no motor movement at all.

    I am assuming that the 1.2M ohm readings are an indication of an open winding and are the cause of my motor issue (in this instance).

    I also have performed the above on another motor and have recorded "normal" readings and two measurements that were less than 2 ohms. This motor would start if lined up at a normal reading position, but then quickly stop moving as it passed through the low resistance position. If I am not mistaken, this would be considered a shorted winding.

    If you've made any sense out of my rambling and can concur, I'd be most appreciative.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    On a lot of those cheap motors, the wires or the windings are not soldered to the commutator segments. A tab on the segment is simply bent over the wire which has had a section of the enamel scraped off. If section of wire with the enamel scraped off is not positioned correctly before the crimp, or if the crimp isn't tight enough, no connection.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    My tiny brushed motors are "coreless" and have very strong magnets.
    The smallest motor for one RC model airplane is 5mm diameter and 15mm long.
    The larger more powerful brushed motor is 7mm diameter and 20mm long.
    These motors use gears to drive the propeller so the motors spin very fast.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I think your test results tie exactly with the symptom of having one phase failed in the armature as previously discussed. :)

    Do you know what the cause of failure is? If the commutator looks darkened then it could be from excessive current, usually occuring at startup with a high mechanical load. Something as simple as a small series resistor could save your motors and add greatly to their life, assuming of course that you are not requiring high torques.

    Also good designs usually have a small ceramic cap 68nF or so across the motor terminals to reduce arcing and commutator wear.
     
  12. cork_ie

    Member

    Oct 8, 2011
    348
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    This is correct, and a very likely cause of your problem.
    It is a common problem in windscreen Wiper motors and fan motors.

    You could try soldering the risers (wires from the armature) to the commutator segments. Stick a bit of stiff paper in the slot between adjacent segments while soldering to avoid the solder bridging the segments.
     
  13. Gdrumm

    Distinguished Member

    Aug 29, 2008
    684
    36
    Have you torn one of the failing motors apart?
    I've found broken wires going to the brushes, broken springs, broken brush bodies, dirty commutator rings, etc., on many small motors from China.
     
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