exceeding voltage rating on a motor

Discussion in 'General Electronics Chat' started by strantor, Mar 22, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I have a friend building a pinewood derby car. he wants to put a motor on it and make it super fast. He was asking me for ideas so I found a dual shaft 12VDC motor online that he could just put wheels on the ends of the shafts and a 250V capacitor. I was thinking we could just charge the capacitor all the way up to 250V (~20X the rating of the motor) with the megger and discharge it across the motor. My thinking is that the motor should have a quick burst of crazy speed (20X the rated speed) and then promptly run out of juice which is exactly what he wants. I'm thinking that since it's all so quick, there won't be time for the motor to burn up. Am I thinking right, or am I going to roast some good parts?
    Thanks
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    how about pour some gasoline into a milkjug, point it backwards and light it for a little jet engine effect. Kind of the same thing, expect your cap will be more like a firecracker.

    Just kidding of course, DON'T play with gasoline
     
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ya !! You'll probably burn the coils after a few attempts.
     
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  4. someonesdad

    Senior Member

    Jul 7, 2009
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    <said in Christmas Story whiny voice> You'll burn the winding out...
     
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  5. dha

    New Member

    Dec 25, 2008
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    Well, for one thing, you could easily break down the insulation in critical places in the motor with those high voltages. The result would be arcing and burning within the windings, brushes, etc. Probably not a good thing. Most likely you would not get the burst of power you expect as a result. This is not a practical way to achieve your goal.
     
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  6. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    Haha, ok I'm seeing a trend here. So, i guess the textbook answer to my next question would be "12 volts", but more off the record could someone let me know about what is the max voltage I could put on a cheap 12V motor and reasonably expect it to not burn up? I know in alot of products things there is safety factor or an underrating factor

    Thanks
     
  7. strantor

    Thread Starter AAC Fanatic!

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    What would make the cap blow? I wouldn't be charging it higher than it's rated. but hey the milk jug thing gives me an idea.... I once built this potato gun that used hairspray to.....
     
  8. awright

    Well-Known Member

    Jul 5, 2006
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    There is no answer to a question like this other than trial and error. Even if you had identified the motor beyond, "...a cheap 12 volt motor...," no one would have a useful answer for you. Things can happen from an overvoltage like you propose other than simple thermal heating of windings. For one thing, you could simply blast the segments off the commutator, burn the commutator, vaporize wires, demagnetize permanent magnets, etc.

    That degree of overload is a very bad idea.

    Keep several spare motors on hand and try your idea out by gradually increasing voltage on the capacitor. Calculate your stored energy and figure out what you can expect to accomplish. Wear eye protection.

    awright
     
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  9. R!f@@

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    Apr 2, 2009
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    Motors cannot even handle twice it's rated voltage much less than 30% more.
    Caps blow if you exceed it's rated voltage, even when operated at rated voltage it's life reduce drastically.

    A motor should not operate above it's rated voltage even though it could handle 50% more for short duration provided you let it cool down.

    Caps should be rated around 10-20% more for the desired operating voltage
     
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  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    cap probably wouldn't blow, but the motor's windings would make a 'popping' sound as they errupted.

    I second the vote for a propellant, although hairspray probably isn't that environmentally friendly.
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You probably would not even get that much of a power boost. Magnetic saturation puts a pretty hard limit on how much a motor can handle, beyond that all you get is more losses (heat) for no significant gain in output.

    A permanent magnet or series wound DC motor could theoretically avoid saturation if its speed increased sufficiently, but with a 20-fold increase this would almost certainly result in the armature flying apart. That could be lethal, best avoided in my opinion.
     
  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are you sure the Pinewood Derby allows motors, even gravity motors? John
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Their is always the combat robots approach (lightweight division). Buy 5 or so motors, and if they keep burning out back off the voltage a little (just a little).

    Overvolting motors is a time honored tradition with that hobby.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    What about traction?
    If you give a blast of power to the motor then won't the drive wheels simply slip?
    Then inertia holds the car from moving.
     
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