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The Q of a FRT will raise by increasing driving frequency to the point where system nearly drops out of the ferroresonance. Such drop happens suddenly and sharp....-pick-.
This part reflects non-linear system behaviour too.
Hence if you lost the FR by increasing the driving frequency over the mentioned point, you will have to lower driving frequency again down till near standard resonance frequency...to the point where resonance sets in again, and then increase frequency again to the point where system nearly drops out of the FR condition.
This is usually the point where Q is highest.
You will not be able to tune into this point that way if your driving system does not provide means for a "stepless" frequency tuning and a continuous (uninterrupted) driving source.
The only other way is setting up the target frequency first, then increase input power above rape-limits, till secondary LC is enforced to follow the input frequency in FR mode, and afterwards lower input power to the point where system drops nearly out of the FR condition. But that's rather brutal approach.
The more power you drive into the system, the more far you will be able to increase the frequency above the standard resonant (non-FR) condition.
Long ago I did tests with a modified HF transformer, taken out of an inverter-MWO, having primary coil H-bridge-driven, magnetic shunt with small air gap and resonant secondary (LC).
Tuning into the mentioned point (was for me round about 25 kHz), I was able to drive the system with 21 watts input and got more than 3.4 kilowatts (9.3 Amps at near 370 Volts RMS) FR oscillation in the secondary (Q = 160, if you want to put it like that).
Cheers!
Ronald