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Back EMF protection

Discussion in 'General Electronics Discussion' started by Siegmeyer, May 3, 2017.

  1. Siegmeyer

    Siegmeyer

    3
    0
    May 3, 2017
    I have a 48v, 12.5a dc supply powering a stepper motor driver. The hardware manual for the stepper driver says to add a capacitor between the power supply and stepper driver to protect against back EMF. I have heard of people blowing out their stepper motor drivers when engaging an emergency stop switch while their motors are running so I figured it would be good to follow the instructions.

    Technical support from the manufacturer said to use a 63v, 2200uF capacitor.
    Will this do the trick?
    Would it make more sense to use some sort of diode circuit?

    Thanks!

    upload_2017-5-3_10-37-4.png
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The recommended method is a shunt regulator.
    A capacitor is the second best solution. It will take up some of the energy of the back emf. This works together with the recommendation to use only 54 V max. as operating voltage. On the other hand, a good power supply will perform the same functin.

    The use of a suppressor diode, which may spring up as an alternative solution to limit back EMF, does not lead to the expected result:
    Let's assume 54 V operating voltage and a diode from this series (diodes from other manufacturers or other series will behave similarly).
    One the one hand you'l need a 'reverse standoff voltage' higher than the nominal operating voltage plus some tolerance (you don't want the diode to become active with a little bit of surplus operating voltage). Let's assume 5% tolerance, that gives you a reverse standoff voltage of >=56.7 V. The nearest available value is 58 V (PTVS58VP1UP).
    This diode will break down between 64.4 V and 71.2 V which is above the limit of the controller.
    A suppressor diode is not precise enough - unless you drastically reduce the nominal operating voltage, in which case you may not need aditional back emf suppression at all.

    The recommended shunt regulator is probably the best solution.
     
  3. Siegmeyer

    Siegmeyer

    3
    0
    May 3, 2017
    Thanks for the info.
    Any suggestions on what I should look for in a shunt regulator or how to select one for this application?
    I've never used or selected one before.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,360
    1,900
    Nov 17, 2011
    As you don't need a full regulator, but an overvoltage protection, the following circuit could be a starting point:
    upload_2017-5-4_7-14-47.png
    This is a crowbar overvoltage protection. As the voltage on vmotot exceeds ~60.6V, the transistor is turned on and dissipates the excess energy. R3 is required in this simulation to limit the current from the voltage source. With a real voltage source with built-in current limiting this resistor is not required.
    Transistor Q1 will dissipate a lot of energy when turned on. A heat sink may be required, depending on the on-time. For short back-emf pulses no heat sink is possibly required.
     
  5. Siegmeyer

    Siegmeyer

    3
    0
    May 3, 2017
    I'll have to do a bit of reading to understand all of that but it put me on the right track.
    Thanks for the help!
     
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