Muhammad Usman Shahid EE-1057

Linear Circuit Analysis

The Corona Effect

When an alternating voltage is applied to two conductors lying near each other, the medium between

the conductors gets polarized. This is due to the attraction between the atoms of the medium and the

charge on the conductor.

Under normal circumstances, when the voltage is low, nothing is observed. However, when the voltage

across the conductors is increased, the polarization of the atoms keeps on increasing as well, until such a

time when the voltage is high enough to completely ionize the atoms of the

medium (which then become charged ions). This ionization produces

light and sound, as energy is released when the charged ions neutralize

the charged conductors.

When this phenomenon occurs between two conductors, in air, whose

diameters are small as compared to the space between them, its

known as the Corona Effect.

The voltage at which the Corona Effect occurs is called the Critical Disruptive Voltage

and is about 30kV which is the breakdown voltage of air.

The Corona Effect in Transmission Lines

In a Power System, the Corona Effect is a major disadvantage because it causes enormous power losses

in the system. Such a system usually deals with generating huge voltages and transferring them from the

generators to the consumption areas via closely laid wires (conductors). This high voltage causes the

Corona Effect and results in huge power losses as energy is converted into light and sound due to the

ionization of the air. The light can be seen as a faint glow along the wires and a loud hissing noise is

heard as well.

Furthermore, the ionization of air produces copious amounts of ozone (because of the oxygen present in

air) which causes corrosion of the wires as it reacts with copper to form copper oxide.

Muhammad Usman Shahid EE-1057

Use of Capacitor in the Ceiling Fan

A household ceiling fan uses a single phase induction motor. Unfortunately, a single phase induction

motor cannot produce enough torque to overcome the inertia of the blades and hence the ceiling fan

can either start at all or can start very slowly and is unable to get to its full speed.

The capacitor is therefore used to split the single phase from the mains supply into two so that the

induction motor produces enough torque to start the fan.

Since a capacitor is a transient device, theres a delay small delay before the voltage reaches a constant

state. This time delay, determined by the time constant (t = RC), causes a time difference of t seconds

between the original source voltage (Vs )and the voltage across the capacitor (Vc). This time difference is

seen as a phase shift between Vc and Vs as the AC waveform from Vs is either leading or lagging behind

the Vc by ts.

The voltage across the capacitor and the original voltage from the mains supply together produce a two

phase voltage which when applied to the induction motor causes it to start and thereby starting the

ceiling fan!