Page 403 - Basic Electrical Engineering

Z is known as the equivalent impedance or ‘dynamic impedance’ of the
0
parallel resonant circuit. It can be noticed that I = I cos ϕ is in phase with
L
L
the supply voltage. This shows that the circuit behaves like a resistive circuit
only since the reactive component currents cancel each other. The impedance

Z is therefore resistive only. Since current is minimum, impedance of the
0
circuit, Z = L/CR is maximum under the resonant condition. Since current at
0
resonance is minimum, a parallel resonant circuit is often referred to as a

‘rejector circuit’ meaning that a parallel resonant circuit tends to reject
current at resonant frequency.

It may be noted that the current drawn from the supply at resonance, i.e., I
= IL cos ϕ is minimum. The current circulating through the capacitor and the
L
inductor, i.e., I which is equal to IL sin ϕ is very high (I >> I or I cos ϕ L
C
L
C
L
Since I is many times more than I, we can say that parallel resonance is a
C
case of current resonance. Here we recall that series resonance is a case of
voltage resonance as voltage across the capacitor or the inductor is many
times higher than the supply voltage

Q-factor of parallel circuit

The ratio of circulating current between the two parallel branches, i.e., the
capacitor and the inductor to the circuit line current is called the Q-factor or
current magnification factor of the parallel circuit. Thus,

Now,