By hvhipot 
The elements of a series resonance (also known as a series resonance withstand voltage test device) reactance instrument are current flow value, voltage drop value, and inductance value. The following introduces the three elements of a series resonance reactance instrument.
The first factor is to choose a fixed AC current value, with a focus on heat generation. Mainly set the long-term operating current value of the reactance instrument from this aspect. At the same time, sufficient high harmonic component values should be given priority consideration, and the actual current flowing through the output reactance instrument is the output current value of the variable frequency instrument motor loading.
The second factor is the voltage drop value, which mainly refers to the time when the voltage reaches 50 Hz. The actual voltage drop value on both sides of the reactance instrument coil corresponding to the actual rated current. The typical range of voltage drop values chosen is around 4 volts to 8 volts.
The third factor is the corresponding fixed current inductance value, and the rated inductance of the series resonant reactance instrument is a very important parameter. If the inductance value is not appropriate, it will directly affect the voltage drop value under the fixed current, leading to serious faults. The inductance value mainly depends on the cross-sectional area of the iron core of the reactance instrument and the number of turns of the coil. The inductance value can be adjusted by simply adjusting the air hole gap to determine the actual inductance value.