Mehran University Of Engineering & Technology, SZAB Khairpur Mirs Campus ENGR. AHSANULLAH MEMON LECTURER DEPARTMENT OF ELECTRICAL ENGINEERING MUCET KHAIRPUR MIRS PROBLEMS AND INRUSH CURRENT

LOAD DIVISION BETWEEN TRANSFORMERS IN PARALLEL

INRUSH CURRENT  Inrush current is produced when a R-L series circuit is connected to AC source and witch is closed.  When AC source is applied to transformer, the current will have source free response called inrush current and forced response called steady state component.  Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on.  Inrush current decays rapidly within 5-10 cycles to normal no load current in transformers but it may exceed the 25 times the full load rating during first half cycle.  For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases.

 Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.  Power converters also often have inrush currents.  Magnitude of insrush current depends upon the magnitude and phase angle of the voltage wave at the instant when switch is closed and magnitude and direction of residual flux in the iron core.  If there is no residual flux and the switch is closed at the movement when voltage has maximum value, the current will be limited to transformer no load current and there will be no inrush current.  Inrush current will be maximum when switch is closed at the movement when voltage wave is at minimum value.  Inrush current may be maximum,minimum or value in between, It also depend on type and magnitude of load connected to secondary of transformer.