How a chopper circuit works (s.o.s)

Discussion in 'Homework Help' started by tracker, Jun 8, 2011.

  1. tracker

    Thread Starter Member

    Jun 1, 2011
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  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Reffering to the Conceptual model of the Chopper circuit diagram from the linked article.

    First thing to understand.

    When the transistor cuts off the ground path, the Motor Coil will still conduct because of the collapsing magnetic fields stored CEMF. It will conduct through the diode loop; because for the collapsing magnetic field the diode will no longer be reversed biased.

    Second thing to understand.

    As current changes the through the sense resistor it will trigger state changes in what might be a comparator which will have hysteresis, or the further through the control loop, it might be the AND gate that has Schmitt trigger inputs to provide the hysteresis.

    That hysteresis is important to prevent the transistor from being controlled as a follower instead of a chopper circuit.

    In a follower circuit the transistor would directly limit the current by dropping part of the voltage which would require an expensive high power transistor, be very inefficient, and reduce the motors power response.

    As a chopper, the transistor turns on until current reaches a control level applied at Vref of the comparator, then turns off until the current has fallen below a lower level.

    See the first thing I asked you to understand. Because of the continued conduction of the motor through the diode loop, the chopping ranges the current instead of actually turning it on and off.

    Note that this circuit as drawn was not realistic.
    The sense resistor should be in place to sense the motor current. There is a type of chopper that is close to this configuration. It uses a capacitor in parallel with the resistor and uses the RC timing, to average the motor current.
     
    Last edited: Jun 8, 2011
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  3. tracker

    Thread Starter Member

    Jun 1, 2011
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    thank you very much for the explanation
    1. is the "activate signal" is the STEP SIGNAL from computer or ...
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Google "PWM" Chopper stepper drives use PWM (pulse width modulation) to control the amperage to the motor. Instead of a steady flow of current to the motor, the signal is 'chopped' up into short pulses. This allows the voltage to stay high but the amperage to be controlled.

    Stepper motor operate at many times the name plate voltage, but amperage must be kept at or slightly below the rated amperage. If not the motor will over heat or in worse case demagnetize the rotor.
     
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  5. tracker

    Thread Starter Member

    Jun 1, 2011
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    YES thank you very much i appreciate;)
     
  6. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Steppers often use Pulse Frequency Modulation (with PWM or with Fixed Pulse Width.)
     
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  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    There was not one shown but there could be an enable/disable line in a circuit.

    The Vref at the comparator would be one place to inject an analog control signal.

    An analog control signal could be something like a potentiometer with the wiper connection as a tapped voltage divider.

    For a computer control to a stepper the frequency of pulses could be sent from a MCU that is monitoring a rotation sensor and adjusts the motor frequency to match a target rotational velocity.
     
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