Series resonance (also known as variable frequency resonance) circuit is an AC circuit with resistor R, inductor L, and capacitor C elements. The voltage at both ends of the circuit is generally different from the current phase; If we adjust the parameters or power frequency of circuit components (L or C) to make their phases the same, the entire circuit will exhibit pure resistance, and reaching this state is called resonance. In the resonant state, the total impedance of the circuit reaches or approximates its maximum value.

When in series resonance, the inductance voltage and capacitance voltage are of equal and different signs, that is, the inductance and capacitance absorb reactive power of equal and different signs, making the reactive power absorbed by the circuit zero. Both electric field energy and magnetic field energy are constantly changing, but they increase and decrease, compensating each other. This part of energy oscillates between the electric field and the magnetic field, and the total energy of the electromagnetic field in the entire circuit remains unchanged. The energy supplied to the excitation circuit is fully converted into resistance heating. In order to maintain oscillation, the excitation must continuously supply energy to compensate for the heating consumption of the resistance. Compared with the total electromagnetic field energy in the circuit, the less energy is consumed per oscillation, and the better the quality of the circuit.