Question

If no. Of turn is increased then what happen on time constant

Answer 1

When a current is applied to an inductor it takes some time for the current to reach its maximum value, after which it will remain in a "steady state" until some other event causes the input to change. The time taken for the current to rise to its steady state value in an LR circuit depends on:

•The resistance (R)

This is the total circuit resistance, which includes the DC resistance of the inductor (RL) itself, plus any external circuit resistance.

• The inductance of L

Which is proportional to the square of the number of turns, the cross sectional area of coil and the permeability of the core.

An Inductor opposes CHANGES in current flow

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 coil, and in doing so induces a voltage back into the coil. This induced voltage (called a back EMF) creates a current flowing in the OPPOSITE direction to the original current. The result of this is that the initial rate of change of the circuit current is reduced. If this initial rate of change were to continue in a linear fashion, the current would reach its maximum or steady "state value" in a time 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 the current will have only risen to 63.2% of its maximum (steady state) value. After five time constants it will reach 99.5%, which is regarded as its maximum value

Discharge

If the circuit is switched off, current does not immediately fall to zero, it again falls exponentially, and after one time constant period will have reached 36.8% of the previous steady state value (i.e.the steady state value -63.2%). It is considered to reach zero in five time constant periods.

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