Anyone understand dc motors

Discussion in 'Electricians' Talk' started by Radders, Jun 29, 2020.

  1. Radders

    Radders Member

    Hi, anyone help, I have a running machine not working, pretty sure its the drive motor, I've tested and left connected the 180v dc feed ( from the control board ) but didnt appear to have a neutral from the same board. I connected directly to the main neutral which then lit up my tester correctly but did'nt run the motor.

    One thing I did notice was a "1. ( or OL)" reading when doing a resistance check until I applied pressure on the motor brush to make better contact. Should the brushes be in contact permanantly?
     
  2. MGW

    MGW Screwfix Select

    Yes the brushes should be in permanent contact, some brushed have a plastic plunger and spring inside the brushes so once it reaches the wear limit the plunger forces the brush off the commutator so stopping arcing and resultant damage, if you need to push on the brush you need new ones, although you may get it running again it will likely damage the commutator if they are not renewed.
     
  3. Pete Jones

    Pete Jones Member

    Would it require a N if it was DC? a Neg may be but a neutral?
     
  4. Radders

    Radders Member

    Ah, you've rumbled my limited skills, understand ac but dc always confused me. How would I confirm the motor is gone or not, the 'live' has clearly traveled through the circuits
     
  5. Radders

    Radders Member

    yes I see the spring and I assume daylight between brush and commutator, but could I 'force' a connection just to prove the motor?
     
  6. Heat

    Heat Screwfix Select

    You could try to push the brushes in against the commutator and then put power to motor to prove brushes are worn.
    But if brushes are clearly worn and have reached their limit of the spring tension, then they need replaced.
     
  7. MGW

    MGW Screwfix Select

    If a supply has one leg bonded to earth that leg is neutral, it really does not matter if AC or DC, and also of course if neither leg is bonded to earth as with a UK 100 volt supply then we don't have a neutral just line 1 and line 2, this is why 110 volt plugs are not marked L and N and why the cores are brown and black with green/yellow not brown and blue.

    Let him with no sin cast the first stone, we all know even if that should be the case they are marked L and N and the colours are brown and blue, and I have never yet found the blue over sleeved with black. So are you Positive it is Negative and not also neutral? Or should I not say anything and sit on the fence and be Neutral.
     
  8. spinlondon

    spinlondon Screwfix Select

  9. Lectrician

    Lectrician Screwfix Select

    The motor must have two cables to it. Measure voltage between them.

    The brushes probably pose no problem at operating voltage, but with your testers tiny 9v, you probably would need to encourage some continuity.
     
  10. Bob Rathbone

    Bob Rathbone Screwfix Select

    A photograph of the motor and the brushgear would help.
     
  11. Radders

    Radders Member

    Thought I'd sorted it - bush spring had 'sprung' cleaned and refit, tested with a dewalt 18v battery I now have a motor that turns.

    put motor back onto treadmill, powered up set to 'go' - nothing

    removed black & red cables from circuit board, dewalt test, still turns

    dc meter test between red terminal and earth 170v ( motor rated at 180v ) - on circuit board

    dc meter test between red and black 10.6v - on circuit board

    stuffed now, beyond my skill sets... so close
     
  12. spinlondon

    spinlondon Screwfix Select

    10.6V seems an odd number.
    Is there anyway of increasing the output from your power supply, perhaps to 12V?
     
  13. Radders

    Radders Member

    problem is I dont really understand dc, I have 230v ac coming in, transformed down to 180v dc, this is arriving at the motor, it's the negative I dont get, if I have 170v between 'L and earth' but only 10.6v between L and N, my logic suggests a problem with N, but previous answers suggest dc dont work like that.

    How would I increase power, and more importantly what reading would you expect to get between the two terminals?

    Just a thought, if I hook up the live back to the circuit board ( 170v) then fix the black negative to earth rather than the board would anything go bang?
     
  14. Bazza-spark

    Bazza-spark Screwfix Select

    Yes it does.

    Not a good idea. Please don't try it as you will be putting a DC current on your earth which may adversely affect the operation of any RCD's or RCBO's in your house.

    @Radders think of a 9v battery. You have 9v between positive (+) and negative (-) with nothing to earth.

    In your case you have an ac supply transformed and rectified to dc with 170/180v between + and -. I am surprised there is anything to earth, but it is possible they have earthed the negative (-) side of the supply.

    Somewhere you are high resistance on the black -ve side of the power supply. You should be able to trace it fairly easily I would think.
     
  15. spinlondon

    spinlondon Screwfix Select

    Just dawned on me that this is a treadmill.
    It sounds like the Negative or return is being varied for speed control.
     

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