Brushless dc card and questions

Discussion in 'The Projects Forum' started by jm-a, Feb 20, 2011.

  1. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi all,

    Found this brushless dc card and, as usual, a few questions.

    What's the use for :

    - L1
    - L2
    - L3

    Inductors to reduce EMi or ?
    Is there a " rule of thumb " to know right value for a 1500 W bldc?

    About blue rectangle, labelled RC : same questions.
    Use and how-to-math value?

    If you have good links to understand these two circuits, would be great...

    Thanks a lot.

    jm
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Those are the three outputs to the BLDC motors' windings.

    What "right value" are you asking about?

    The jumper block JP1 allows selecting different levels of power output sensing. The capacitor values aren't documented, nor is the resistor value for 1.1kW.

    We don't even have the link to where you found the schematic. Maybe there's a theory of operation at that site?
     
    jm-a likes this.
  3. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi,

    First thanks a lot for your clear and fast reply - as usual - .

    I thought of inductors values for L1 to L3; I had in mind added inductors for reducing EMi,- between igbt output and bldc windings - so bad question.

    About RC:i only wanted to know, if it is possible,values to give for a 1500 W brushless dc, so choice 1,6 kW ' second setting ' .
    Written on the scheme 220R and ? for C77


    This schematic was given to me; i think i'm going to study MC33035 and 33039 datasheet....


    Thanks a lot.

    jm
     
  4. jm-a

    Thread Starter Active Member

    Oct 20, 2010
    105
    2
    Hi all,

    Maybe something about use of L1 L2 and L3

    http://sci.tech-archive.net/Archive/sci.electronics.design/2007-10/msg00932.html

    Mook Johnson wrote:
    Gents,

    I'm looking at ways to bullet proof a motor driver. I'm using the standard triple half bridge configuration and want to protect against the motor phases shorting to each other or to the case (HV return). Driver is 600V bus at 1.25amps (~ 1HP).

    I'm using 1200V 15 amp IGBTs for this and have a current sense resistor in each of the three legs and the HV return on the driver. These signals are monitored by the controller IC and turns off the gates if a current threhold is exceeded.

    There is some propogation delay (~10uS) between when the current spike is generated across the sense resistor and when the gates actually turn off. The current in the IGBTs goes very high for this period of time.

    I'm thinking of adding some local current sense resistors to the emitter of the 3 high side IGBTs and connecting a npn transistor to is such that it shorts out the gate drive immediately on a overcurrent condition ( npn NPN base and emitter across the sense resistor and the collector to the gate)

    This that this would cause the IBGT to limit the current for the 10uS until the logic can shut it off.

    This current limit (~5A) would be set in the SOA for the IBGT with 600V applied and the current steering diodes (already present) would prevent reverse current flow through the high side sense resistors.

    See ascii sketch attempt below.

    Any gottchas in doing this?



    600V
    ____
    |
    |C
    G /
    /\/\----| <--High Side IGBT
    c| \
    \ b |E
    npn|---|
    / \
    e| /
    | \
    | /
    -----|
    |-------------- Motor phase
    V
    to low side IGBT






    Won't that just force the IGBTs into the linear region and result in huge dissipation?

    Firstly speed up your control circuitry. 10uS is a bit sluggish. You should be able to get down to 100nS without any real effort and much quicker if you really try. Then put an inductor in each output line which will limit the rate of current rise to below your circuitry protection time delay. That's the way I've always done it and it works well.

    --
    Gibbo


    Jm

     
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