Large DC Machine Armature Dilemma!

Discussion in 'General Electronics Chat' started by TheBod, Jan 28, 2014.

  1. TheBod

    Thread Starter New Member

    Jan 28, 2014
    Hello, Thank you for your time!

    Here is the issue:
    • Average segment to segment resistance is 780 micro-ohms.
    • After a commutator skimm, one segment to segment resistance measurement was measured at 970 micro-ohms (rest all remained at 780 micro-ohms).
    • Full load segment to segment voltage is 12V. Full load segment to segment current is 330A continuous.
    • Each segment to segment armature circuit has 6 parallel bars (3 larger bars and 3 smaller bars).
    • The bars are TIG welded to the riser.
    • The segment to segment with a higher resistance has a parallel resistance consistent with one of the larger bars going open circuit, or close to open circuit.
    • Unable to NDT to check for cracks.
    • Load testing is not an option.

    Appreciate any advice on the risks of armature failure, and likely failure mechanisms, when we eventually put it all back together and run it.

    Is arcing through a cracked bar the main failure scenario to be worried about?

    If 1 parallel bar is open circuit (or close to it), and the other 5 bars are sharing the increased load (20% increased current), are the I squared losses a worry for life of the armature?

    If the failure scenario is one of the bars having high connection resistance, say 1 ohm in series with the 4700 micro-ohms, could I squared R losses at the 1ohm result in a failure?

  2. vrainom


    Sep 8, 2011
    Arcing is a real serious problem in armature rotors with an open segment, under test you can usually se a green spark from the copper slowly vaporizing, and the heat ends up damaging the commutator. But at least that's true about smaller motors with a winding per segment, I have no experience with your type of motor.
  3. ifixit

    Well-Known Member

    Nov 20, 2008
    Can you post a picture of this armature?
    What is this motors application? Do lives depend on it?

    Your assessment is likely correct in that you have a cracked bar. Electrically, it will likely work well enough, but depends on the application it is in.

    However, mechanically there are huge magnetic forces acting on the bars as they rotate. If there is a danger that these forces will cause more cracking and perhaps allow the bar to break lose completely and jam with the stator, then the armature needs replacing now.

    You could call the manufacturer of the motor for advice.

  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    if you have found a bad segment or winding, it needs to be fixed. centrifigual force will throw it apart when it gets up to speed, and destroy your motor.
  5. MaxHeadRoom


    Jul 18, 2013
    Replacement commutators used to be available, pain to replace, but not impossible.
  6. TheBod

    Thread Starter New Member

    Jan 28, 2014
    Visually there are no abnormalities to see. However the TIG welds on the riser are obviously under quite a bit of resin.

    We have surge tested the suspect segment to segment at 50V, and then using sensitive listening equipment, listened for any crackling, indicative of sparking and heard nothing.

    I'd like to load test it in a workshop, to de-risk the schedule, but the expense to set that up is prohibitive.

    The application is very important.

    The armature bands were recently replaced (to prevent possible band failure due to a design issue which questioned the strength of the OEM bands), not long before the commutator skimm. Everything went fine with that work. Prior to the band replacement, the motor ran fine for many years.
  7. bwilliams60

    Active Member

    Nov 18, 2012
    What is this armature used on? Do you have any pictures you can share with us on this unit. Generally when a winding is going open, there will be a dark mark along the edge of one of the commutators where the brush has been arcing on it. It may be hard to spot if it is just starting to open up.