1.Characteristics of series resonance
The circuit is purely resistive, and the terminal voltage is in phase with the total current. At this time, the impedance is the smallest and the current is the largest. High voltage may be generated on the inductance and capacitance many times larger than the power supply voltage. Therefore, series resonance is also called voltage resonance. In power engineering, series resonance will cause overvoltage and large current, which will damage electrical equipment. Therefore, series resonance should be avoided. The phenomenon that the terminal voltage of the parallel circuit is in phase with the total current of the circuit occurs in the circuit in which the inductance coil is connected in parallel with the capacitor is called parallel resonance. The total impedance of the parallel resonant circuit is the largest, so the total current of the circuit becomes the smallest. However, for each branch, its current may be much larger than the total current. Therefore, current resonance is also called current resonance. Parallel resonance will not produce resonance overvoltage endangering equipment safety, but each branch will produce overcurrent.
2. Application range of series resonance
(1) Understanding method of impedance characteristics at resonance working principle analysis of LC series resonant circuit needs to be divided into three frequency points and frequency bands, that is, at resonance, the input signal frequency is higher than the resonance frequency and the input signal frequency is lower than the resonance frequency.
When the input signal frequency is equal to the resonant frequency, the circuit resonates. The impedance of LC series resonant circuit is at the minimum state and can be equivalent to a pure resistance. At this time, the signal current flowing through the whole resonant circuit is the largest. This is the most important point in circuit analysis.
(2) When the input signal frequency is higher or lower than the resonant frequency, the LC series circuit is in the detuning state, and the circuit impedance is greater than that of the resonant state.
3. Tuning of series resonance test system
By adjusting the air gap of the reactor core, the inductance of the reactor continuously changes from the maximum value when the iron core is fully closed to the minimum value when the iron core is fully opened (generally, the reactor can be tuned within the range of 4% to 90% of the air gap of the iron core). When the reactor reactance is equal to the loop reactance, the system reaches resonance.
At this point ω L=1/ ω C
Where: l is reactor inductance; C is load capacitance; ω Is angular frequency（ ω= 2 π F, f is the input power frequency).
