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Re: Why motor gets hot with PWM ?

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  • Alan Jones
    The reason PWM heats motors is because of the Wave form of the pulse. I cant remember the precise scientific explanation, but PWM is square wave, so it is
    Message 1 of 3 , Aug 29, 2001
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      The reason PWM heats motors is because of the Wave form of the pulse.
      I cant remember the precise scientific explanation, but PWM is square
      wave, so it is basically like swithching the motor on and Off very
      quickly. When pure DC is used, this only happens once, so there is no
      heat. Other DC controllers are pulsed, but dont use square wave, this
      also does not produce heat because the motor is accelerated from 0
      then decelerated gradually to 0, and so on, rather than being
      swithched on and off.

      --- In z_scale@y..., kim@b... wrote:
      > I have been thinking about why the motor gets hot with PWM and not
      DC
      > Here is my theory (feel free to challenge it)
      >
      > The heat in the motors are generated due to resistive loss in the
      > inductor (I*I*R)
      >
      > Case 1: PWM
      >
      > Assumptions:
      > 1. PWM is single polarity 0 to 10V 50% duty cycle (average is 5VDC)
      > 2. The motor resistance is 1 ohm
      >
      > Calculations:
      > Instantaneous current when the PWM is ON:
      > I=V/R
      > I=10/1
      > I = 10 AMP
      >
      > Power lost in heat is:
      > P = I*I*R
      > Power = 10*10*1
      > Power lost in heat = 100 Watts
      >
      > Since the PWM is ON only half (50% ) the time the
      > Actual power lost in heat is 100/2 = 50 Watts
      >
      > Case 2: DC
      >
      > Assumptions:
      > 1. DC voltage is 5V (because average DC with PWM is 5V)
      > 2. The motor resistance is 1 ohm
      >
      > Calculations:
      > Current when DC is ON:
      > I=V/R
      > I=5/1
      > I = 5 AMP
      >
      > Power lost in heat is:
      > P = I*I*R
      > Power = 5*5*1
      > Power lost in heat = 25 Watts
      >
      > Since the DC is ON all the time the
      > Actual power lost is 25 Watts
      >
      >
      > Hence compared to PWM, in this example generated 50W of heat but DC
      > only generated 25W of heat therfore DC runs cooler than PWM.
      > Kim
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