In an RLC series circuit, when the circuit current is in phase with the applied voltage, the circuit is called series resonance. When the inductance reactance is equal to the capacitance reactance X L=X C or (XL - X C=0), resonance conditions will occur in a series RLC circuit. A series resonant circuit has the ability to draw high current and high power from the main line. It is also known as the receptor circuit. The series resonant RLC circuit is shown in the following figure

At  resonance :  X L  -  X C = 0  or  X L = X C

Impedance is

Among them, Zr is the resonant impedance of the circuit.

By substituting the value of X L – X C=0 into formula (1), we will obtain

Zr = R

Current I=V/Zr=V/R

Due to resonance, the opposite direction to the current is only the resistance (R) of the circuit. In this case, the circuit consumes the maximum current

The effects of series resonance (also known as series variable frequency resonance) are as follows

1. Under resonance conditions, XL = XC， The impedance of the circuit is minimized and reduced to the resistance of the circuit. That is, Zr=R

2. Under resonance conditions, due to the minimum impedance of the circuit, the current in the circuit is maximized. Ir=V/Zr=V/R

3. Therefore, due to the maximum value of resonant current Ir, the power consumption of the circuit is also the highest. Namely Pr=I 2 Rr

4. Under resonance conditions, the current drawn by the circuit is very large, or it can be said that the maximum current is drawn. Therefore, the voltage drop across the inductor Lie (V L=IX L=I x2 π frL) and capacitor Cie (V C=IX C=I x I/2 π frC) will also be very large.

In the power system, under resonance conditions, excessive voltage may accumulate on the inductive and capacitive components of the circuit (such as circuit breakers, reactors, etc.), which may cause damage. Therefore, the series resonance condition in the power system is avoided. However, in some electronic devices, such as antenna circuits and tuning circuits in radios and television receivers. The series resonance condition is used to increase the signal voltage and current at the desired frequency (fr).