INDUCTIVE AC CIRCUITS

Inductors, like capacitors, oppose current flow in AC circuits. They may also introduce a phase shift between the voltage and the current in AC circuits. A large number of electronic circuits are composed of inductors and resistors.

INDUCTORS IN AC CIRCUITS

Inductors in AC circuits offer opposition to current flow. When an AC voltage is placed across an inductor, it creates a magnetic field. As the AC voltage changes polarity, it causes the magnetic field to expand and collapse. It also induces a voltage in the inductor coil. This induced voltage is called a counter electromotive force (cemf); the greater the inductance, the greater the cemf. The cemf is out of phase with the applied voltage by 180° and opposes the applied voltage. This opposition is as effective in reducing current flow as a resistor.

The applied voltage and the induced voltage are 180° out of phase with each other in an inductive circuit.

 

The amount of voltage induced in the inductor depends on the rate of change of the magnetic field. The faster the magnetic field expands and collapses, the greater the induced voltage. The total effective voltage across the inductor is the difference between the applied voltage and the induced voltage. The induced voltage is always less than the applied voltage.

Figure below shows the relationship of the current to the applied voltage. In a purely inductive circuit, the current lags behind the applied voltage by 90°. 

The current lags the applied voltage in an AC inductive circuit.

 

 Another way of stating this is that the applied voltage leads the current by 90° in a pure inductive current. This can be represented by the acronym ELI. Voltage (E) leads current (I) in an inductive (L) circuit.

The opposition offered to current flow by an inductor in an AC circuit is called inductive reactance and is measured in ohms. The amount of inductive reactance offered by an inductor depends on its inductance and the frequency of the applied voltage. The larger the inductance, the larger the magnetic field generated and the greater the opposition to current flow. Also, the higher the frequency, the greater the opposition the inductor has to current flow.

The impedance of a circuit containing both inductance and resistance is the total opposition to current flow by both the inductor and the resistor. Because of the phase shift caused by the inductor, the inductive reactance and the resistance cannot be added directly.

The impedance (Z) is the vector sum of the inductive reactance and the resistance in the circuit. The impedance is expressed in ohms and is designated by the letter Z. Impedance can be defined in terms of Ohm’s law as: