6V/6A DC motor on a 12V line

Discussion in 'General Electronics Chat' started by Jony130, Aug 19, 2015.

  1. Jony130

    Thread Starter AAC Fanatic!

    Feb 17, 2009
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    Can I run a 6V 6A DC motor with PWM and power it with 12V power supply?
    Is this safe for the motor ?
     
  2. crutschow

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    Mar 14, 2008
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    It is if you limit the PWM to no more than about 50% duty-cycle steady-state, or limit the motor current to 6A (if 6A is the maximum steady-state motor current rating).
     
  3. RichardO

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    May 4, 2013
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    I would also use a fuse to protect the motor in case the PWM circuit fails continuously on giving a 100% duty cycle.
     
  4. MaxHeadRoom

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  5. ian field

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    Oct 27, 2012
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    The impression I get is; the TS is wondering about applying 12V to a 6V motor at no more than 50% duty cycle - if the pulse repetition rate is high enough, the motor's inductance will protect it.

    With a "proper" PWM PSU, you'd have an inductor feeding a filter/storage capacitor and you could design the regulation so the voltage out of the filter never exceeded 6V.
     
  6. Jony130

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    In fact I was planning to use this circuit

    pwm.png

    Where the PWM frequency is around 36kHz. But I'm bit worried that the engine will not withstand this 12V at 50% duty cycle and will overheat and burn.
     
  7. ian field

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    At 36kHz, there may even be too much inductive reactance in the motor to get its full rated torque at 12V on high duty cycle.
     
  8. MaxHeadRoom

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    The 'Overheat' will not be as a result of voltage, but torque/load/current, 50% duty cycle on its own does not mean much.
    In drives where the voltage is quite a bit higher than the rated, some kind of current monitor/ limit is implemented.
    Max.
     
  9. alfacliff

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    the 36 khz will not get to the motor, usea rectifyer and filter.
     
  10. crutschow

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    The motor inductance acts like a filter to the PWM frequency, giving a DC current proportional to the average DC voltage from the PWM. A high inductance does not affect that fact.
    36kHz is fine although you could use a frequency in the low kHz region.

    Because of the motor inductance the 12V peak of the PWM will not cause high peak currents, so the motor won't overheat.
     
  11. MaxHeadRoom

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    Picmicro recommend a starting point of around 4.5Khz 5khz in their app notes for PWM motor control.
    Max.
     
  12. Jony130

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    Hi, today I test if 12V DC motor can survive the 24V DC. And yes, the motor survive 15 minute run without overheating too much.
    I also build a op amp (LM358) based PWM for test purpose. And by accident the output frequency was 4.3kHz. And If I rememberer correct when I change the cap to lower the frequency (980Hz) the motor speed up for the same duty cycle.
    As for 36kHz and ATtiny13 - this frequency is way to big because I have a lot of trouble with it. Even if I connect a PC fan. The fan and my PSU start behavior very strange. Maybe because I build it on the breadboard.
     
  13. alfacliff

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    Dec 13, 2013
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    the pc fan isnt a real dc motor, it has a small inverter circuit inside, its called a brushless dc motor. the frequency of the pwm and the frequency of the internal inverter might fighbt each other.
     
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