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How to eliminate the DC motor back emf?

Discussion in 'Electronic Design' started by Electronic Swear, Mar 4, 2005.

  1. A DC motor on starting and stopping, the back emf is very large even
    it is in a very short period.

    Motor supply is from a rectified [email protected] without filtering
    capacitor.
    I would like to know, is there any filter (R,C) to eliminate this back
    EMF?
    If i place a mono-capacitor in between two motor terminal, will the
    result is become better and reduce some back emf from motor?

    If so, what capacitance is needed?

    Thanks~
     
  2. Tim Wescott

    Tim Wescott Guest

    Perhaps you're confusing the inductive voltage spike with back-EMF?
    Back EMF occurs because a DC motor acts like a generator, so when it's
    turning it generates a voltage that reduces the drop across the windings
    and hence the motor current. Or perhaps you're talking about the radio
    frequency energy generated by the commutator and brushes?

    Inductive kickback occurs with any coil, whether it's within a mile of a
    DC motor or inside.
    Do you mean a single capacitor? A large cap in parallel with the motor
    terminals will reduce the inductive spike (_not_ the back EMF) when you
    turn off the motor, but would consume a great deal of current on
    startup. Smaller caps, however, suppress RF quite nicely.
     

  3. When I see the motor signal in CRO, they show out this tpe of waveform:



    <-----high voltage
    .....*.......................
    .....*.......................
    .....*.......................
    .....*.......................
    .....**.......................
    .....**.....************....
    ..*...*.. *................
    ****...*..*.................
    ........ **..................
    .........*...................

    I don't know the peak of the instant voltage come from. It is from the back emf?
    Or from other reason. I want to reduce or elminiated of that voltage.
    Thanks~
     
  4. Alan Turner

    Alan Turner Guest

    Hi,

    The motor has a fairly large inductance. Inductors have the following
    v~i characteristic:

    V = L * (dI/dt)

    In words:

    "The voltage across the terminals of an inductor is
    proportional to the rate at which the current through
    the inductor changes".

    When you turn the IGBT off you rapidly change the current through
    the inductance of the motor. This makes dI/dt high, and you see a large V
    across the motor terminals.

    A common solution is to connect a diode across the motor terminals which
    is reverse biased when the IGBT is on. The diode creates a path for the
    inductor current when the IGBT is off. Diodes used in this way are often
    called "flyback" or "flywheel" diodes.

    Regards,
    Alan
     
  5. Ken Smith

    Ken Smith Guest


    That spike is due to the inductance of the motor windings. If this is a
    moderately large motor, there can be a fairly large amount of energy
    stored in the inductance. All this stored energy must be dealt with.

    I assume your circuit looks like this:


    V-------------
    !
    [Motor]
    !
    +------------- Scope
    !
    !-
    !
    !-
    !
    GND--------------

    In the DC case, the easiest answer is to put a diode across the motor so
    that the voltage on it can't reverse. In the AC case, you are going to
    need a capacitor with prehaps a resistor in series.


    If you just use a capacitor, the energy that ends up in the capacitor is
    what was in the inductor so:


    E = 1/2 * I^2 * L <- energy in inductor


    E = 1/2 * V^2 * C <- energy in capacitor


    1/2 * I^2 * L = 1/2 * V^2 * C

    I^2 * L / V^2 = C <- so given a voltage limit we can find C
     
  6. Jamie

    Jamie Guest

    that looks like the effects of both the brushes and maybe flyback
    effects of non filtered rectifier between the zero crossing periods.
    you may want to try a small cap coupled across the + & - terminals.
     


  7. I have already added a reverse biasing diode at the terminals of the
    motor.
    The motor is a very high power DC motor, so it has high inductance. I
    am using a relay to switch on motor. And for the input signal of the
    motor, I use a triac to chop in different angle to control the speed.
    Because the chopped signal is in AC, then i rectify it by a bridge and
    use a filter capacitor to smooth it.

    I just use a small capacitor (0.015uF 1000Vdc) on ripple filtering, I
    don't know the capacitor is small or not. But I want to know if I
    increase the filtering capacitance of the cap. will have a better
    performance or not.

    Or using a R-C filter before the motor input terminial to suppress the
    large surge at start-up. I am afraid the R must in a very small value
    with high power range in order to not reduce the performance of the
    motor.

    Any suggestion?
     
  8. Jamie

    Jamie Guest

    i can only assume you must be using a DC PM or shunt more ? or maybe
    even a compound/universal type ?
    any ways. unless your reversing the direction, you should be able to
    use a large cap.
    the best thing to do is use both large electro caps for filtering
    and low ESR & ESL non polorized cap coupled together.
    also. using a reactor is a plus.
     
  9. Ken Smith

    Ken Smith Guest

    I think you can do an LC filter



    From rectified SCRs ---+---)))))---------+---- Motor
    ! !
    --- ---
    ^ ---
    ! !
    ----+-----------------+----


    At low SCR conduction angles the current from the SCRs is a bunch of think
    spikes. The motor isn't really intended to have those high frequencies on
    it. The inductance needs to be just enough to keep the SCRs safe from
    their di/dt rating and their max current with whatever capacitor you use.

    The capacitor doesn't have to be all that huge to make a fairly major
    impact. You don't need to spread a pulse out that much to really lower
    its high frequency content.
     
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