Electrical Engineering ⇒ Topic : Resonance in Parallel A.C. Circuits (Parallel Resonance)
Resonance in Parallel A.C. Circuits (Parallel Resonance)
A parallel a. c. circuit containing reactive elements (L and C) is said to be in resonance when the circuit p f is unity i.e. reactive component of line current is zero. The frequency at which it occurs is called the resonant frequency fr. It is called parallel resonance because it concerns a parallel circuit
Expression for fr. Consider the most practical parallel circuit consisting of a coil shunted by a capacitor as shown in Fig. a (i). The phasor diagram of this circuit is shown in Fig. b (ii). The circuit will be in resonance when the reactive component of line current is zero i.e. I c - IL sin ΦL = O. This can be achieved by changing the supply frequency because both and IL sin ΦL are frequency* dependent. At some frequency fr, called resonant frequency, the reactive component of line current will be zero and resonance takes place.
At parallel resonance, the circuit condition is
Because Zr is very high, Ir will be very small. The small current /r is only the amount needed to meet the resistance losses in the circuit. The parallel resonance is also current resonance because the current circulating between the two branches of the circuit is many times greater than the line current taken from the supply. Parallel resonant circuit is also called rejector circuit because it rejects (or takes minimum current) that frequency to which it resonates.
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