Motor Controller with adjustable load

Discussion in 'The Projects Forum' started by PwrOverload, Aug 19, 2008.

  1. PwrOverload

    Thread Starter New Member

    Aug 19, 2008
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    The title pretty much says it all. I am looking to design a motor controller with adjustable load with a Z8 microprocessor. Thinking of using a PWM control for the motor voltage and dynamo load and having a USB data upload function to PC.
    I have tried and failed with a simple design using a DC motor controller design.
    Possibly, a open drain power FET with diod prot might work. Also a band pass filter and zero crossing detector might be needed. I feel that LPF and amp will also be needed.

    Any suggestions?
    Advise much appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sure you don't mean an adjustable current supply instead?
    Usually, the motor draws the current necessary to maintain a certain RPM vs a certain load, with the back-EMF of the motor providing a stabilizing influence. Without the back-EMF, a DC motor under no-load conditions would continue to increase RPM's until it flew apart due to centrifugal force.

    I'm not familiar with Zilog's Z8 line. The last time I programmed something in Z80 Assembly Language was probably about 18 years ago.
    So, you're going to use the motor to drive a generator? :confused:

    Why don't you post that design?

    The drain is where the load should be connected. An open drain infers that no load is connected. While it may function, it won't drive the motor.

    You need a low-value non-inductive current sense resistor connected from the power MOSFET's source to ground. Current through the circuit can then be measured as voltage across the sense resistor.

    A low-pass filter for the gate drive is not what you would want, as a square wave is the fundamental frequency plus all of the higher odd harmonics of the square wave. The drive to the MOSFET should be as "square" as possible. This requires considerable bandwidth; the broader the bandwidth, the better the square wave.

    There is really no point in using a zero-crossing detector in a DC circuit. Unless you're just not telling us something.
     
  3. PwrOverload

    Thread Starter New Member

    Aug 19, 2008
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    I intended it to be to be a adjustable voltage supply. I know that the motor should provide the adequate current necessary. It will be driving a generator of a sort but no actual definite source. The zero-crossing detector was just a thought. I was thinking it was required but with more thought, it not be required.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Make a PWM to control the motor speed and take a feedback from the generator output to control the speed of the motor. For example, if the output voltage of the generator is 240 VAC it must be constant and not change when the load on the generator changes (current drawn by the generator changes). This is done by adjasting the speed the generator rotates (thus the speed of the motor) according to the current drawn.Thus you will read the value of the output voltage of the generator and if it is less than 240 then you have to increase the pulse width as to increase the speed and recover the output voltage. However, if the voltage is over 240 then you have to reduce the pulse width as to reduce the motor speed to recover the voltage.
     
  5. PwrOverload

    Thread Starter New Member

    Aug 19, 2008
    5
    0
    mik3,
    Is it possible for you to provide me with a circuit design?
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I will try to post it
     
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