The document discusses inrush current in transformers. It defines inrush current as the maximum, instantaneous input current drawn by an electrical device when first turned on. When a transformer is energized, inrush current occurs as both a source-free response and a forced response to the applied AC voltage. The magnitude of inrush current depends on factors like the residual flux in the transformer core and the phase of the AC voltage wave when the switch is closed. Inrush current can be several times the normal full-load current and decays within 5-10 cycles but may exceed 25 times the full-load rating initially.

1. PROBLEMS AND
INRUSH CURRENT
Lecture Notes by Dr.R.M.Larik 1

2. Lecture Notes by Dr.R.M.Larik 2

3. Lecture Notes by Dr.R.M.Larik 3

4. LOAD DIVISION BETWEEN TRANSFORMERS IN PARALLEL
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5. Lecture Notes by Dr.R.M.Larik 5

6. 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.
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7. ➢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.Lecture Notes by Dr.R.M.Larik 7

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