# We know that all machines or devices working on AC have high value of time constant so that there is n damage to inductor while curent reversal & high value of time constant means increase in time to reach steady state as we can see in fans , coolers , etc. but the tube-lights also have a choke coil but it give immediate response which means time constant has small value , so in that case it should be damaged..!! what is the reason exactly , I am not getting this thing..

Grade:11

## 1 Answers

ROSHAN MUJEEB
askIITians Faculty 833 Points
4 years ago
When the circuit in Fig 4.5.1 is switched on current changes rapidly from zero, this sudden change creates a rapidly expanding magnetic field around the inductor coils, and in doing so induces a voltage back into the coil. This induced voltage (called a back EMF) creates a current (the green arrowhead in the circuit diagrame) flowing in the OPPOSITE direction to the original current (the blue arrowhead in the circuit diagrame) applied by the battery.

See the Back EMF current and supply current change during the time illustrated by the video in Fig 4.5.1 The result of the sudden change voltage as the circuit switches on is that the rate of change in circuit curent, instead of causing a sudden increase in current from 0V to Maximum current increases ay a slower rate than it would in a totally resitive circuit. If the initial rate of change of current in the LR circuit were to continue in a linear fashion, the current would reach its maximum or steady "state value" in a time (T) given by:

T = L/R seconds.

T is the TIME CONSTANT and is measured in seconds
L is the INDUCTANCE and is measured in Henrys
R is the TOTAL CIRCUIT RESISTANCE and is measured in Ohms.

Seconds and Henrys are usually far too large for most electronics measurements, and milli and micro units are commonly used, but remember when calculating to convert any of these sub units to seconds or Henrys for use in formulae.

The rise in current is not linear however, but follows a curved "exponential" path, and in one time constant (indicated in Fig 4.5.1 by the vertical dotted line) the current (indicated by the horizontal dotted line) will have only risen to 63.2% of its maximum (steady state) value. After two time constants it will reach 86.5%, after 3 time constants 95% and so on until it reaches 99.5% which is regarded as its maximum value after 5 time constants.

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