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Electric motor question

  1. Mar 13, 2012 #1
    if you are spinning a large permanent magnet motor at high speeds and add a reverse pulse to the motor for use as a break does this wear or damage the coils in the long run?
    I was under the impression from what I know that it added heat and slowly wore on the wiring.
    Similar to holding the voice coil still on a speaker that was being driven.
     
  2. jcsd
  3. Mar 13, 2012 #2
    This method has potential to damage the coils, but whether it actually does damage or not is dependent on some factors. If the inertia of the rotor is very Large, it would take considerable time to stop the motor, during which high braking current might damage the winding due to overheating. Even if this overheating don't damage the winding immediately, the repetitive high temperatures can degrade the insulation, resulting in eventual failure.

    Another issue is, if the Motor isn't designed for such braking, then the mechanical jerk during plugging might throw out windings from the slots. (I am not very much sure on this, may be more knowledgeable person, (jim hardy?:) ) will come and explain. )
     
  4. Mar 13, 2012 #3

    FOIWATER

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    how are you adding a "reverse pulse" to the field?
     
  5. Mar 13, 2012 #4

    jim hardy

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    The forces are felt by the conductors.
    That's why it's important to have them physically secured lest they abrade away the insulation.

    Small ones can do with good varnish and twine to hold the wires in place
    large ones are blocked with wedges of insulating material such as "Micarta", a trademarked product.
    http://en.wikipedia.org/wiki/Micarta
     
  6. Mar 13, 2012 #5
    I am assuming you mean a DC motor - better to switch a "Brake" resistor across the +/- . MOSFET <100A<300V or IGBT higher, of course you need to "turn off" the supply as well.

    In an AC case with a drive this can be done a similar way but needs to be in the DC Link. As for a Direct Drive PM Motor - DC Injection or the Resistor Brake ( but that is getting complicated)

    A reverse pulse could be an option - particularly if you need extreme braking, but this will stress the motor - not impossible, just not my first choice. Realize you are needing to dissipate all of the energy stored in the momentum of the motor and load - as well as in the field- this is often a LOT of energy.

    Really all depends on the applicaiton
     
  7. Mar 13, 2012 #6
    Well i just needed someone to back me on this, even though i already know it to be correct, i have a background in electrical but have a co-worker who thinks he knows physics and electronics but he doesnt. The situation i am speaking of is the spindle motor in a CNC mill(Milltronics VM22), i am not sure if it is AC or DC, I know it is controlled by a PWM system, it does full torque any speed, forward or reverse 10,000 rpms and yes it weighs a lot.
    The system itself already when stopping from a high rpm spins down significantly before breaking, and our Mr. Awesome co-worker thinks that putting a short reverse motor pulse in the machining program will instantly slow the motor as apposed to leaving it with the factory spin down before it breaks itself, he doesnt understand that if they could do it they would have engineered it to do it already.
    It is damaging to the wiring/coils, anyone disagree, if so explain physics.
     
  8. Mar 13, 2012 #7

    jim hardy

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    That'll be a brush type motor. They can get violent.
    I echo Winadct's cautions :
    It could be capable of fracturing its mounts if armature is shorted or reversed into substantial current source while field is present (All the time for PM).

    i'd want to understand how the factory brakes it and proceed gingerly .

    There's this consideration - if that is a feature available through ordinary control panel , good design should disallow self destruction.
    But never intentionally challenge your backup protection.
    Read the user precautions carefully.
     
  9. Mar 14, 2012 #8
    The Drive should have a max torque setting ( as a limit) - could just be a max current limit already- so you could give it a neg speed instruction ( not quite the same as a direct negative pulse) - but this would be the same as just putting in a 0 speed instruction - because it will (should) only provide as much stopping current as it's limit allows. This is the same as giving it a 10,000RPM instruction from 0 speed - accelerating or decelerating - it will probably be applying max current/torque as allowed my the OEM
     
  10. Mar 14, 2012 #9
    Thanks for the post guys, but it is correct that a reverse to the motor is damaging as stated , correct, I know there are ways around it to apply a break as you mention but the argument is that a reverse pulse to the already spinning 10k motor isnt damaging to the coils or wiring, but it is, correct?
     
  11. Mar 14, 2012 #10
    Hello Justin - my point being that if you try "putting a short reverse motor pulse in the machining program" - the machine will probably only drive in reverse so hard. The OEM should have protection there to prevent the machine from damaging itself - due to a programming error. This is not really the same as "add a reverse pulse to the motor" as I think everyone here interpreted it.
    A reverse pulse from the program should be controlled - a reverse pulse applied directly to the motor is not a controlled process.
     
  12. Mar 14, 2012 #11
    yes I understand what your getting at there are, or should be software to hardware limitations, but a co-worker started to argue the motor point, ie directly, speaking in terms of if the machine would let you do it what would happen, if I have a PWM supply running a motor at hi speed in 1 direction then reversed PWM direction of rotation to drive motor in reverse and hit go,until it stopped. it would be bad, if not immediately in the long term, the argument is a physics argument and I said it would cause damage, he believes it would just stop.
    He doesnt believe that the spinning magnetic force applied to the opposing electromagnetic force would yield any heat or change in energy other than a dead stop in electron flow. From what i know of physics and engineering he is incorrect? these motors also index im not sure how it works but I assume different coils engaged at opposing forces index it to that point, but never looked into it just a hunch.
     
  13. Mar 14, 2012 #12

    jim hardy

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    So now it's academic?

    What i've surmised from the description* your equipment:

    Reversing applied polarity to a "separately excited" DC machine, which your sounds to be(permanent magnet), would cause a reversal and huge increase in current.
    Motor would deposit its Iω2 inertial energy into the external voltage source and interconnecting wires..
    Large magnetic forces would hammer the internal conductors and field magnet.
    How big a hammer? Determined by size of wires and strength of field.

    Hopefully the machine is built so you can't make it do that.

    But if it did that, an almost immediate and ungraceful stop would result.

    This was something we demonstrated when i took machinery lab ~ 1965.
    We applied a solid short circuit to a 7.5 hp machine but only after it had had rolled nearly to a stop. Still the "THUD" was impressive. So was the instantaneous halt.

    This application note, Microchip AN905,
    ww1.microchip.com/downloads/en/appnotes/00905a.pdf

    suggests that motors designed for PWM may be "braked" by using the H-bridge as a low resistance load to decelerate the motor. That'd be the designer's job to match braking resistance to motor characteristic.

    That's my best guess from far away !


    * description:
    OMG that VM22 is a 16,000 pound 24 hp machine !

    http://www.milltronics.net/Product_VM22_MillingMachines.aspx
     
    Last edited: Mar 14, 2012
  14. Mar 15, 2012 #13
    That is a ridiculous assertion by your co-worker. Whenever there is current there is electron flow, and without any current, there can't be any force.

    As windadct pointed out, Accelerating from 0 to 10,000 rpm or Decelerating from 10,000 rpm to 0 rpm demands equal energy (though in former we are opposed by friction and in the later case we are assisted by friction)
    So, you can safely apply reverse voltage to decelerate the motor provided that your reverse_applied_voltage don't produce current more than the maximum rating.

    When the motor is spinning at around the rated speed, Even applying 0 Voltage (let alone Reverse voltage) would create very high current than is allowed.
    So, during that time, applying reduced +ve voltage (not reverse) is required. It results in regenerative braking, but I'm not sure, if that works for PWM system.

    As the motor speed drops, the magnitude of applied voltage should be reduced (if we want to maintain a constant braking torque)
    At any given time, the armature current I = (Eb-V)/R
    where Eb is the back-emf which goes on decreasing with Speed.
    So, V should be decreased in proportion with Eb, and when the speed has decreased considerably, even applying reverse voltage might be Ok.

    Applying a Fixed Full Reverse Voltage directly Across the Motor, isn't Ok.
     
  15. Mar 15, 2012 #14
    Justin - so you were discussing hacking into your $30K vertical mill and driving the motors back wards - or just a purely hypothetical question?

    If you are issuing the stop command and it seems to take a while to get the spindle to stop - it may actually be part of the OS to cool the motor - before stopping. If you go from a hard cut ( maximum current) and instantly stop the motor, this will probably cause some thermal damage over time.
     
  16. Mar 15, 2012 #15
    Thanks guys for the back up, as i thought hahah, Some peoples kids.
     
  17. Mar 16, 2012 #16
    30k Mill try $120k the one in question 100K , Haha now you see why I said not to even try, software limits or not. Laterz CHEERS
     
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