Series resonance (or variable frequency series resonance withstand voltage device) mainly refers to the voltage and current of a series circuit being in the same phase, that is, the inductance of the circuit is equal to the capacitance reactance of the capacitor in numerical value, making the studied circuit exhibit pure resistance characteristics. Under a given terminal voltage, the maximum current will occur in the studied circuit, and the active power in the circuit will be the highest.  

When in series resonance, the virtual impedance of the circuit is equal to 0, z=r+jx，x=0，z=r， So i=u/z=u/r. 

Resonance definition: The energy of the L and C components in a circuit is equal. When one reactance module in the circuit releases energy, the other reactance module must absorb the same energy, resulting in energy pulsation between the two reactance components; In order to generate resonance, the circuit must have inductance L and capacitance C components; The corresponding resonance frequency is resonance, or resonance frequency, represented by fr.

The conditions for a series resonant circuit are as follows:

When Q=QI2XL=I2XC. The resonance condition of R-L-C series circuit when XL=XC.  

1. When resonance occurs in series or parallel, there is complete energy exchange between LNCs, which means that the released magnetic energy is completely converted into electric field energy and stored in capacitors, while the discharge of capacitors is converted into magnetic energy stored in inductors.

2. In a series resonant circuit, energy exchange occurs through changes in voltage polarity due to the same current passing through series - L and C. 

3. Inductance and capacitance remain two components during resonance. Otherwise, energy exchange cannot occur, but from the perspective of equivalent impedance, it becomes a component: zero resistance or infinite resistance.