Resonance Frequency Drift

Question

I'm working on a wireless power transfer project. The coils that I'm using are made out of Litz wire with a value of 50 uH. For the capacitors, I'm using 68 nF capacitors. This puts the resonance frequency at about 86.314 kHz. The capacitors are metal film type capacitors made specially for high Q resonance circuits. The ESR of the coil is about 33 mΩ. I'm not really sure what the ESR of the capacitor is nor the amplifier used. I do know though that the Q for the circuit is really high. I was getting around 200 V peak to peak from 1 V peak to peak at some point. The issue is that just moving the coil around causes the resonance frequency to shift around drastically. I found that just moving the coil a little bit causes the resonance to shift up or down by upwards of 2 kHz. I did find that adding a series resistance of 6 Ω does seem to stabilize that.

Is that normal behavior when the Q is that high?

Answer 1

Is that normal behaviour when the Q is that high?

Yes this is normal behaviour when the Q is high. Any slight deformation of the coil can cause the inductance to change and this will cause the resonant point to change. Also, moving the coil towards other conducting objects will create eddy currents in those objects and reduce the effective inductance of the coil.

Adding a series resistor lowers the Q and inevitably reduces this effect.