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what is inrush current in transformer?
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Answer Posted / s das
When a transformer first energised, the transient input
current can be 10-50 times larger than the normal operating
current for first several cycles. This is called inrush
current. If the trans. is energised at voltage zero
crossing the maximum flux is requirred to support the
voltage is double the normal flux. This is because the rate
of change of the flux(decides the voltage drop against the
inductance) should be same as steady state condition to
support same input voltage, but the flux is started from
zero in case of transient and started from negative maximum
value of flux in case of steady state for zero crossing
voltage of positive cycle. Tran. normally operats at the
knee point of hysteresis loop, the tran. will enter into
saturation zone and a huge current will be requirred to
generate double flux. In saturation zone, the inductance of
the core will be decreased and a considerable amount of
voltage(compared to drop against the coil) will be dropped
against the coil resistance9due to high current). The
voltage drop against the inductance of the coil is less
compare to normal condition and flux is not requirred to be
double. This could be severe if there is positive residual
flux at the time of energise at zero crossing of voltage's
positive half cycle. So the inrush current depends on the
time of energise, resistance of the coil and the residual
flux.
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