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C curve type MCB trip due to high transformer inrush current

Discussion in 'Power Electronics' started by Lemon_Foam, Jan 13, 2019.

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  1. Lemon_Foam


    Sep 18, 2017
    Hi, i have a circuit that consist of transformer to step down input ac voltage from 220 to 100v for servo motor driver.

    I'm taken inrush current for transformer only (without any load connected ) by using inrush current meter, the inrush current get is not consistent, within 10~200A. The higher inrush current always cause the C curve type MCB to trip.

    Is there any method to tackle this issue without changing the MCB? Such as add some component to lower down the inrush current draw by transformer to energizing?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Circuits for inrush current limiting are available online, e.g. this one or this one.
  3. Bluejets


    Oct 5, 2014
    We had exactly the same problem on Clipsal LEM transformers for operating rooms ( a bank of 15) and were instructed by Clipsal to change the breaker type.
    Never worked, and they had no other suggestion.
    I might add that sometimes (depending on where in the cycle the power reconnected I imagine) the surge would take out the main switchboard sub-main 250A 3 phase breaker.

    What I finally came up with was fairly simple and has been in place now for over 15 years.
    Idea was to allow enough time for the transformer to saturate the core before switching in to full supply current.
    Each transformer primary was fed via a ceramic 10 ohm 5 watt resistor in series.

    This was by-passed by a timer arrangement set for around 2 seconds on power failure reconnect.

    Naturally because there were 15 transformers, I used a contactor type set of relays (four poles per relay) * 4 relays.
    The resistor were simply connected across each relay contact for each transformer.
    On power reconnect, resistor in circuit, after 2 seconds resistor by-passed via relay contacts via timer which holds itself out (time out)while power present.
    Last edited: Jan 13, 2019
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