DC motor strange behavior

Discussion in 'General Electronics Chat' started by cce1911, Jun 18, 2013.

  1. cce1911

    Thread Starter Member

    Jun 11, 2013
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    I have a 24 vdc brushed motor (~750 watt) driven by a LiFePO4 battery pack with a BCM. My problem is that when I put a load on the motor, it will run for less than a minute and then stop. Below is a plot of the voltage and amperage while under load. As you can see, the pack voltage stays around 25 vdc while the motor draws between 4 and 8 amps. I have a PWM speed controller on this motor, but the problem existed before I put the speed controller on it. You can see me changing speed in the first 3 steps and then stopping and starting the motor around .8 min. After less than a minute, I get this huge current spike (49 amps) and this causes the BCM to trigger the 40 amp cut-off.

    I'm inclined to think the problem is with the motor, not the pack or BCM. What would cause this kind of behavior? I've cleaned the commutator and the brushes are new. Seems like some sort of thermal event is causing a short or something. Anyway, I'm looking for ideas!
     
  2. skip.ele

    Member

    Nov 27, 2011
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    It might be interesting to see a plot of this data without the speed control involved.
     
  3. cce1911

    Thread Starter Member

    Jun 11, 2013
    41
    0
    It's fairly involved to get this plot since the motor is submerged and I have to use a data logger inside the pressure vessel. Before I had the data logger, I had a solid state relay and the same thing would happen. I changed out the SSR with a mechanical relay with the same result. The PWM controller was a final attempt to rule our the relay being the problem.

    This is a long winded way of saying the plot would look like this one from the .5 min mark forward.
     
  4. skip.ele

    Member

    Nov 27, 2011
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    Well I'm shooting in the dark here but it looks like the voltage is in a steady decline during the majority of the motor run time and when it reaches approximately 24 volts the problem occurs. Is it possible that the BCM is reacting to a bad cell/battery? If it is monitoring each cell/battery and one of them drops below 2v then it is supposed to switch into some protection mode...I have no idea what it is but maybe that is happening?

    I have a hard time believing that a motor rated at 750watts is going to suffer from any heating problem while running at only 25v@4amps (100watts) for less than one minute...but I could be wrong.

    Here are some ideas and then I'll leave you to the more senior members:

    1.Can you test with a different power supply?
    2.Can you test the voltage at each cell/battery while running the motor to see if one is not holding up?
    3.Does the BCM provide any feedback indicating what the failure/protection mode is when one trips? It appears to offer a number of protections...
    4. Assuming the batteries prove to be ok and assuming you are measuring the voltage at the motor and not at the power supply, how long are the wires connecting the motor and what is the wire size? I'm wondering if the measured voltage drop is due to line drop?
    5. I'm assuming your battery pack is capable of handling this load.

    Good luck to you. I have a few motor projects in my future as well and I'm curious what the outcome here will be.

    Skip
     
  5. cce1911

    Thread Starter Member

    Jun 11, 2013
    41
    0
    Skip,
    I can test with anther battery pack, it is NiMh and does not have a BCM. I've got no way to test each cell while the motor is running. The BCM does not provide any feedback as to what trips it. The voltage is measured with an Eagle Tree data logger mounted between the BCM and the speed controller. The total length of the wires between the BCM and the ESC, including the data logger is about 12 inches and I'm using 10ga wire. The battery pack is a 30 amp-hr pack made from 16 40152S 15Ah 10C LiFePO4 Cylindrical Battery Cells in a 2P8S configuration. It can handle discharge rates much higher than what this motor requires.
    -Capel

    P.S. the LiFe pack voltage drops off rapidly in the first few minutes and then should hold steady for a long time.
    P.S.S. Would a short in the armature exhibit this behavior?
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    What is the load? Is it continous? Do you vary the speed while under load?
     
    Last edited: Jun 18, 2013
  7. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi cce1911,

    Sounds like a motor armature winding is shorting out under load. When under load, there is a strong magnetic force pushing on each of the windings as there commutator pads receive current. If they are not secured well into the armature core with shims and epoxy they can work loose and short to the core, which is likely grounded, or short to each other.

    Inspect the armature closely. Use a lighted magnifier. Or try a new motor.

    Regards,
    Ifixit
     
  8. skip.ele

    Member

    Nov 27, 2011
    40
    0
    It is possible that an intermittent short in the motor could be the problem. It just seems unlikely given the low power and short time frame but again I could be wrong. Is the timing of your failure reproducible?

    My only thought at this point is to attempt to rule out the motor by testing the power pack and BCM with a static load of 100watts or more and see what happens. I'm not sure where you would get them but I'm thinking of a wire wound high power resistor maybe a 200watter just to give you some room @ 5ohms. If my math is right that would pull 5 amps at 25volts. Better double check me on that, it's late here and I'm sleepy...

    Skip
     
  9. cce1911

    Thread Starter Member

    Jun 11, 2013
    41
    0
    The PWM speed controller is programmed for 3 speeds (50%/75%/100%), so yes the speed can vary under load. But, this problem existed before I put the ESC in place. I do have a couple of big power resistors that I was going to make a burn tester with. This is a good idea to rule out issues with the battery and BCM!
    -Capel
     
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