Help with capacitors

Discussion in 'The Projects Forum' started by cts_casemod, May 15, 2013.

  1. cts_casemod

    Thread Starter Member

    May 14, 2013

    I am making a final year project of a traction inverter for a car.
    What I would like to talk about are the capacitors.

    Typical industrial drives often use big electrolytic capacitors, however in Automotive applications, since space is critical not only high quality metal film capacitors are used instead, their capacity is also much smaller.

    I would like to hear you experience on this, whenever I should only take RMS current into account or should also take capacity. Any help or advice would be helpful.

  2. #12


    Nov 30, 2010
    Start by figuring out how much capacitance you need, then find the maximum voltage. Add a safety margin of 10% to 100% on the voltage.

    Find capacitors you can buy with those specifications and check for overheat limit due to ripple current and ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance) especially for high frequency applications. Sometimes you have to put capacitors in parallel to dissipate the heat they suffer.

    (Generic question gets generic answer.)
  3. cts_casemod

    Thread Starter Member

    May 14, 2013

    I understand that on an AC application it is desirable to have a given capacity to reduce the DC-Link Voltage ripple.

    Since I use a battery it is unclear what this capacity would be. I still have a ripple, but caused by the internal resistance of the battery and not by lack of the supply voltage as on a rectified AC supply.

    Having used a bank of 6 electrolytic capacitors rated at 10.000uF @ 800V for a total of 150A RMS capability I am now replacing those with Metal film counterparts capable of 900V @ 300uF. They are rated for 100A RMS each or 600A in total since they are paralleled.

    The Inverter is rated at 300Amp nominal, so my RMS current is just fine.

    The question is based on the capacity: On the Electrolytics I need to have such a capacity in order to get the desired ripple current.
    On a metal Film I have the required ripple current but at a reduced capacity and hence I am afraid I may not have the required reserve, but again I am operating from a DC Supply...

    This is all quite confusing and I cant find much information on the subject.
    Any help greatly appreciated.
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011

    Yeah for me too. Post a schematic or a picture or something showing what you are doing.

    Your sense make words really not.
    #12 likes this.
  5. cts_casemod

    Thread Starter Member

    May 14, 2013
    I may, if you let me know which part doesn't make sense to you.

    When calculating an electrolytic capacitor capacity on a inverter there are two things to have in account: One is required Ripple capability, the other one is the reserve to keep supplying the load after the AC supply reaches the peak and starts going back to zero.

    Most of this is carried out when the incoming supply is a rectified AC supply. No issues here, however I am working on a DC Supply and my concerns are DC-Link inductance caused by the turning on and off from the IGBTs at high frequency Typically 20Khz) and reactive power going back to the battery.

    I cant find literature on this, which is not to wonder since such applications have started to mass hit the market in the last decade or so.