IGBT Short Circuit Fault in 3 Phase invertor

Discussion in 'General Electronics Chat' started by wahidbashir, May 19, 2010.

  1. wahidbashir

    Thread Starter New Member

    May 19, 2010
    Dear Forum,
    I have a 3 phase servo motor with seimens.
    In its invertor stage, there are 3 IGBT modules each have two one for upper and the other is for low side.
    The invertor out put phases are U,V and W
    IGBT on low side for V phase has been shorted thrice a time.
    On inspection I have found gate drives for these 6 IGBTs which are optically isolated from controller. 5 optocouplers give reading on diode test of 1.5 V
    but the one for low side of V phase has a reading of 1.002V.
    Motor insultation is OK with the help of Meggar.
    Problem is inside drive. Is that could be the reason (optocoupler)
    Please guide in this regard.

    Wahid Bashir
  2. mik3

    Senior Member

    Feb 4, 2008
    If possible, post a circuit schematic.
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Why not contact the drive manufacturer or their representatives in your locality? Siemens would hopefully have a strong (helpful???) technical support department. There's a reasonable chance they might respond, particularly to questions with a specific query such as yours.
  4. BillB3857

    Senior Member

    Feb 28, 2009
    What you have is a |-|-| configuration (3 phases with H bridges) and from your description, if the Low V opto is too sensitive, you could be seeing punch-through if the High side is still on when the low side erroneously triggers on. Since the high side would already be in saturation when the low side falsely triggers, the low side would take the heat (pun intended)

    That's my theory but others may have other ideas.
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    I would imagine if both the high and low side IGBT's were both being driven fully on then there is risk of an effective short circuit on the DC bus via the two IGBT's in series - so both devices would be equally at risk from over-current failure.

    It is possible that if the lower IGBT is being driven marginally close to or just into conduction and is operating in a quasi-linear state this would subject that device to risk from excursions outside of its SOA.
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Hope you didn't do the meggar insulation test with the drive output connected to the motor.