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Thread: Questions about electric motors.Volts vs Amperes

  1. #1
    Member E-tron's Avatar
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    Questions about electric motors.Volts vs Amperes

    Can somebody explain me....

    Since the increase in voltage decrease the amperage, why a 48v motor can't run at lets say 96v?

    What destroys the coils?The high voltage or amperage?

    What determines the max power of a motor?Thicker winding wire at the coils can gives more Amps at the motor?

    Waiting for your quantum physics knowledge

  2. #2
    I ain't got no physics, but I do got pictures.

    This is what happens if you run a 36V hub motor at 84V.

    My simple-minded understanding is that the motor draws as much amps as it needs for the load, so that may be where you're having trouble getting the concept- it was certainly where I lost it.

    The amps doesn't decrease, it's limited only by the system- the batteries etc. If the system can feed the motor more amps than it can handle, at low load it's fine, but when you load it it's going to draw more than it can cope with. The coil windings will melt, is my guess. In the case of volts, the increased volts makes the motor spin faster- possibly beyond it's RPM limits, and it will melt bearings or break apart physically. In my case, with a brushed PMDC motor, the brushes burned up because they couldn't handle the arcing- I always try to remember voltage is potential- or pressure- like in water. If you imagine feeding a waterwheel more pressure than it can handle, it's going to overspin and the vanes will blow out water and leak...

    Now all those physics guys can slap me upside the head for talking like a photog...

  3. #3
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    agree with what Ted says,

    Because current = volts / resistance,
    If you were to double the voltage, then the current would double!

    Because Power = volts x amps
    Doubling the voltage (and hence amps) would produce 4 x the power.

    In reality however, the motor controller will limit the current and hence the power.

    The one to watch for though is that the rpm of the motor is proportional to the voltage, so
    doubling the voltage means you could potentially be running the motor at twice its rated rpm
    and it may well 'fly apart'.

    As for what kills coils, because power also equals current squared x resistance, if you double
    the current then the heat generated in the coils goes up by x4

    What determines max power .
    Because power also equals rpm x torque ..
    then it depends on how much current (torque) and how much voltage (rpm) the motor can handle.

  4. #4
    yeah. what he said.

  5. #5
    I love this forum. I learn more by reading what you all post than reading a hundred books.

  6. #6
    Empulse R #24 frodus's Avatar
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    Just do add, All of this is true if we're talking about Brushed motors (PMDC, Sepex and Series wound). AC acts a little different when it comes to RPM and the volts/hertz ratio. The power handling still remains the same, it can only handle its rated power.

    But yeah, spot on. The motor draws whatever current it needs to handle the load. If its a heavy load, it'l draw lots of amps. If the controller doesn't limit it, the motor will be overloaded. If the voltage and amperage exceeds the power rating for the motor, it will melt down.

    As far as RPM goes, if you overvolt a 36V motor to 72V, you get twice the RPM. If its rated at 4krpm at 36V, then it'l do 8krpm at 72V. Most Brushed motors don't do well above ~5krpm without reinforcement on the armature windings. They will fly apart. If its rated for 2krpm at 36V, then it'l probably be fine.


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