1) Transformer inrush currents is property of magnetic circuits which is an inherent characteristic     of Power Frequency Transformers having magnetic core with B-H magnetizing curve which has     a limited initial linear portion followed by a flattened saturable portion.

2) Any transformer during switching on time initially offers virtually zero impedance only winding     resistance - no self inductance) during the first half cycle as at that instant there is no back         EMF as no flux is yet developed .

3) Self inductance of transformer primary is a property which offers a resistance to the applied       voltage by generating an equal and opposite back EMF.

4) In order to develop adequate back EMF a sufficient flux dФ/dt has to be developed.  (E=n x         dФ/dt)

 5) If during normal running the flux level required is having a value Фm , then during initial              energisation particularly at voltage zero instant the flux required is of the order of 2 Фm.

 6) In case the core has a residual flux from last turn off then the net flux requirement is                    2Фm + Фr. This requirement of flux results in operation of the core far
     beyond the linear portion of BH curve resulting in very heavy saturation. As the magnetic            core is not able to support such level of flux, the dФ/dt  is limited. and the back EMF is also        not developed  adequately.

 7) This results in the winding resistance (which is of the order of m-Ohms) as the only                    opposition to the applied voltage and therefore the resultant inrush current level is                    phenomenally high upto 6-8 times of the rated full current or even more.

 8) If the transformer Is energised at Voltage zero of the cycle the inrush current is maximum and      at voltage peak the same is minimum. The instant of energisation on each switching is                different and repetitive readings may vary largely.

 9) The source impedance plays a major role in this phenomenon. The expected levels of
     inrush current are mainly determined by source impedance and instant of switching.

 10) The stronger the source the lower the source impedance and hence higher the inrush current        level.
 11) Therefore it is not possible to specify any typical value of inrush current as the site
       source impedance unavailable.

 12) The inrush current can be marginally controlled by the design flux density.

 13) Therefore there is no way to limit /control the inrush current at the  starting first half                 cycle other than charging the transformer through external series impedance such as ballast          resistors which have to be subsequently shunted by a switch or contactor

 14)  Therefore  ultimate solution is to incorporate a soft start module in series with  the primary side of               the transformer


15) The above method is called as a Soft Start Feature which gives complete solution to the

inrush problem providing a soft start with limited inrush levels of 2-3 times the rated


 16) It is also advisable to provide  the following arrangement  to mitigate the  effect of  inrush

currents.Provision of a TVSS  as well as  AC type   of  capacitor network of 100Mfd 500V AC

in Delta mode    through  MCB 63A3POLE, helps to mitigate the voltage surges during

energizing resulting out of inrush currents.

causes and remedies


Datsons provides an optional feature to control this problem, called as a Soft-Start Module. It has two different versions 

The scheme used is to energies the transformer through current limiting ballast resistors in each phase connected in Soft-Start path thru auxiliary contactor. The main power contactor bridges the soft-start path to connect the transformer directly to the mains by an ON delay timer after around 1 second. The soft start path aux contactor turns off by another on delay timer and disconnects the same.This results in two step energization with in-rush current limited to not more than 50-60Amps .The SLD schematic below illustrates the scheme.   

The scheme used is to energies the transformer through current limiting ballast resistors in each phase connected in Soft-Start path parallel to main MCCB through auxiliary contactor which turns off within 4-5 seconds by an ON delay timer. The soft-start operation is indicated by an audio visual alarm prompting to switch on the main MCCB immediately. The MCCB in the main line has to be switched on manually, which bridges the soft-start path to connect the transformer directly to the mains. This results in two step energisation with in-rush current limited to not more than 50-60 Amps.

Warning: Kindly note the following limitations   in Manual Soft start. · Manual soft start module requires manual attendance and operation for every failure and 
resumption of source.
· In case of an un-intimated source failure and resumption , soft – start is not available on next
restart and it will not work as required. · If transformer with soft-start module is used at UPS input side and sourced from mains / generator
there is probability of frequent and unintimated interruptions and resumption  of the mains source
and  hence manual soft-start is not advised
. Only auto soft-start may be used in such case. · Manual soft start will be suitablefor UPS output application only , as the UPS output is
uninterrupted and rare to fail and resume without intimation.  

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