Choosing Capacitors

Discussion in 'General Electronics Chat' started by crazyengineer, Oct 16, 2013.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    So I'm a little confused about the concept of ripple current for a capacitor. Let's say you're trying to control a simple brushed motor that will draw 22A. I understand you need a bulk capacitor to handle the energy going to the motor based on max voltage and ripple current, but I'm confused how do you select a capacitor based on ripple current in this application.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For decades DC motor controllers have operated without any smoothing at all, with either a 100Hz/120Hz ripple or 360Hz ripple on 3phase using SCR technology, phase angle control.
    If you look at the KB Drive site you can get an idea of the type of drive.
    There are PWM power supply types that do have a capacitor bank for the supply.
    With capacitor selection, it all depends on what % of ripple you think you will need at maximum current.
    If your supply originates from a supply transformer, aiming for a small % ripple will influence the Kva sizing of the transformer, which can be considerable for that amount of current.
    Max.
     
  3. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Right now the drive is a simple h-bridge. I'm not planning on PWMing the inputs. Also, this will be connected to a battery, not a transformer so I do not know what ripple current rating I need for the capacitor.
     
    Last edited: Oct 17, 2013
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I may be wrong, but ripple is a by product of rectifying AC, bridge rectifier or other means. Your battery is DC so how are you getting ripple?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I agree, Where is the ripple from a battery?
    How are you controlling the RPM, if at all?
    Max.
     
    Last edited: Oct 17, 2013
  6. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Sorry for not being specific. Lemme explain a little better.

    I am planning to buy a 36V 800 watt brushed DC motor I wish to run it at its max rpm (I currently do not know the rated RPM). I plan to control it using 2 p-channel mosfets on the top and 2 n channel mosfets on the bottom. I plan to put .1amps to the gate of each transistor (since I do not care how fast the drive can switch on or off, I figure I can put a small amount of current to each gate). The motor will be powered by three 12V batteries wired in series.

    When I was talking to a friend about this project, I was told that when controlling a brushed motor, voltage spikes occur when the motor is suddenly turned off due to the fact the motor is inductive. So you need to add a bulk capacitor to reduce this effect.

    However, the same person told me that not only do you need to choose the right voltage rating on the capacitor to reduce this effect, but you need to also look at its ripple current rating.

    Since I'm assuming that the motor will draw 22-24 amps with appropriate loading, I'm trying to decide whether or not I should select several 50V-63V bulk capacitors with a ripple current rating of 4-8amps (I know there's a ripple compenstation factor for capacitors, but right now this is a guess) and wire them in parallel to make sure it handles 22-24amps, or is this overkill?
     
    Last edited: Oct 17, 2013
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Mosfets are voltage driven not current.
    I assume your friend was talking about the BEMF of the motor?
    When the motor is turned off it carry's on generating a voltage, the level of which is proportional to its RPM, if the motor is allowed to slow of its own accord this voltage decays at the RPM rate, you can also use this generated voltage to brake the motor by placing a some kind of shunt in parallel with the armature, either electronically or mechanically be means of a relay.
    If running this motor at MAX RPM then when turning off the H bridge the motor voltage will decay as the rpm slows.
    The only protection usually taken in a Motor H bridge is to place reverse EMF devices across the switching devices.
    And possibly a EMI filter across the armature.
    Al.
     
  8. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    1) Ideally MOSFETs are driven by voltage not current, but they have a capacitor inside it that will effect how fast the transistor will turn on or off. Higher gate current means it can be turned on or off a lot faster. (http://www.ti.com/lit/ml/slup169/slup169.pdf)
    2) You're suggesting that instead of turning off the motor instantaneously, there should be a way to slowly turn it off so that way the voltage spike will not occur?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You are using a bridge so are you planning on reversing the motor direction? If so, how fast will that occur? Will you let the motor stop before it is reversed?

    Any significant capacitor size across the motor will add large turn-on transient spike currents through the MOSFETs which add to their dissipation. Normally you have reverse-biased diodes across each MOSFET drain to source (in some cases you just use the normal MOSFET drain-source parasitic diode) which will carry the transient current from the motor and direct it back to the power supply and ground. You should have a large capacitor across the bridge power to ground to absorb the transient current at that point.

    You can add a small non-polarized capacitor (<1uF) directly across the motor to absorb the high frequency edges of the spikes and any brush noise.
     
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  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The capacitor at the top of the h-bridge is to negate the effect of inductance in the power supply leads. If the power supply leads are fairly long this inductance will let the voltage rise at the input to the H when the motor is turned off. You can reduce this spike by twisting the power and ground together and keeping them short. A large capacitor is often used to absorb and supply this current. It is difficult to calculate since you probably don't know the inductance of the lines, so bigger is better.
     
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