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  1. #1
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    3 phase voltage stabilizer MCCB tripping problem

    I have made several 3 phase servo voltage stabilizers in last few months. Each one with same design working fine. Only one unit is having unexpected problem. Need suggestion if I'm missing anything to check.

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    This is the common circuit for each one. In our 3 phase servo voltage stabilizer we are using 3 individual servo voltage stabilizer and combined it is working as 3 phase SVS. When we test each one we check supplying single phase to single set of voltage stabilizer. If 3 sets works ok then we supply 3 phase supply.

    The problem with this one is, when each phase is checked individually, it works fine. No inrush current, no MCCB tripping. Even if we short 3 inputs and supply single phase line it works fine too. But if we supply 3 phase line the 63A 3 phase MCCB trips with spark. Even 100A one trips. But once it is powered up manually with MCCB bypass, it is taking only 0.6A in each phase without any load. Which is same as other ones.

    My question is, what may happen here? Am I missing something? I checked line to body conductivity for leakage. No, no leakage. checked for short, no short found.

    Notes:
    Input voltage range: 220V to 230V.
    Have 3 phase line with Neutral. Taking Neutral to any one phase for single phase line.
    With all 3 lines, it is working as 3 phase.
    Phase to Phase voltage: 380V to 400V
    Voltage stabilizer capacity is all same, 40KVA.
    Same auto transformer, Same connection.
    Same control circuit.
    Same Casing & fittings.

    Thanks in advance.
    I love my profession, I love my M's Lab. __MKDas

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    Re: 3 phase voltage stabilizer MCCB tripping problem

    Maybe you have reversed phase and neutral for one of the phases when going via the MCCB?



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    Re: 3 phase voltage stabilizer MCCB tripping problem

    Effective mains impedance is lower for 3-phase configuration, input current is higher. It's surely an inrush current problem. You can use a soft start circuit with series resistors and bypass contactor, as used in large VFD inverters.


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    Re: 3 phase voltage stabilizer MCCB tripping problem

    Do you have access to a current probe and a digital scope?

    If not, use three pairs(*) of incandescent light bulbs, each in series with each phase. If you have a current surge, or a wiring error exists, the faulty phase bulbs will light up. Use at least 60 watt bulbs, 100 watt would be better, such that they can pass the transformer's magnetizing current.

    (*) You require a pair of bulbs in series, because if a short exists, the full phase to phase voltage could be applied to the bulbs.
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  5. #5
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    Re: 3 phase voltage stabilizer MCCB tripping problem

    Checked and found some mistakes, Phase Neutral was not aligned for 3 auto transformer. Tested after correction, same result. Then I used 3 lamps of 100W in series to check the auto transformer inrush current. The lamps flicks in the first moment then the auto transformer is consuming only 40VA+-/5VA (as measured). Only taking a short inrush current. A 63A MCCB is tripping only for a auto transformer which consuming 40VA only at noload. But same type of auto transformers are working well in other machies.

    I've made a soft starter circuit for auto transformer just to reduce inrush current.

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    The maximum load through the auto transformer is 5-8A. I used here a 30A relay. Resistor: 1K/5W 4 in parallel. I tested one transformer with and without this circuit.

    Without soft starter, inrush current is over 30A (can not detect so far with an analog meter).
    But with soft starter, inrush current is almost 0A. No such response found in analog ampere meter.

    I hope this will solve all the problems.
    Last edited by Mithun_K_Das; 28th April 2019 at 12:09.
    I love my profession, I love my M's Lab. __MKDas



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  6. #6
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    Re: 3 phase voltage stabilizer MCCB tripping problem

    Problem is solved with the use of soft starter.
    Simple circuit, can be helpful to others too.
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    I love my profession, I love my M's Lab. __MKDas



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