Series Resonance Circuit
Related Terms
In the RLC series circuit, when the circuit current is in phase with the applied voltage, the circuit is said to be in Series Resonance. The resonance condition arises in the series RLC Circuit when the inductive reactance is equal to the capacitive reactance XL = XC or (XL – XC = 0). A series resonant circuit has the capability to draw heavy current and power from the mains; it is also called as Acceptor Circuit. The series resonance RLC circuit is shown in the figure below.
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Effects of Series Resonance
The following effects of the series resonance condition are given below
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At Resonance condition, XL = XC the impedance of the circuit is minimum and is reduced to the resistance of the circuit. i.e Zr = R
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At the resonance condition, as the impedance of the circuit is minimum, the current in the circuit is maximum. i.e Ir = V/Zr = V/R
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As the value of resonant current Ir is maximum hence, the power drawn by the circuit is also maximized. i.e Pr = I2Rr
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At the resonant condition, the current drawn by the circuit is very large or we can say that the maximum current is drawn. Therefore the voltage drop across the inductance L i.e (VL = IXL = I x 2πfrL) and the capacitance C i.e (VC = IXC = I x I/2πfrC) will also be very large.
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Parallel Resonance Circuit
The resonance of a parallel RLC circuit is a bit more involved than the series resonance. The resonant frequency can be defined in three different ways, which converge on the same expression as the series resonant frequency if the resistance of the circuit is small.
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Firstly, if the supply frequency is low, below the resonant frequency ƒr then the condition shown in Fig 10.3.1 exists, and the current IL through L will be large (due to its comparatively low reactance). At the same time the current IC through C will be comparatively small. Because IC is smaller than IL the phase angle θ will be small. Including IS in the diagram shows that it will be lagging on VS and therefore the circuit will appear to be INDUCTIVE. (Note that this is the opposite state of affairs to the series circuit, which is capacitive below resonance).
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The other important point shown in Fig 10.3.3 is the size of the phasor for IS compared with IC and IL. The supply current is much smaller than either of the currents in the L or C branches of the circuit. This must mean that more current is flowing within the circuit than is actually being supplied to it!
This condition is real and is known as CURRENT MAGNIFICATION. Just as voltage magnification took place in series circuits, so the parallel LCR circuit will magnify current. The MAGNIFICATION FACTOR (Q) of a parallel circuit can be found using the same formula as for series circuits, namely;