The choice between drive an ultrasonic transducer in the series or parallel resonance is determined to a large extent by the behavior of the acoustic load. In cases where there are abrupt and frequent variations (eg, ultrasonic welding) it is more convenient to drive it at the parallel resonance. In cases where there are only small variations and the transducer is always loaded, it is easier and cost effective to drive it at series resonance. In the parallel resonance, when the load is removed the electrical impedance increases what protect the generator from overload automatically (provided the generator is able to track the parallel resonance frequency). In series resonance, the electrical impedance drops at no load condition overloading the generator, that is why ultrasonic cleaning equipment is damaged if operated with no water. To drive a transducer at series resonance, the frequency tracking is not as critical as when driving it at the parallel resonance and electronics may be less complex and cheaper. You can measure how much you transducer impedance changes in dependence of the loading using a impedance analyzer or horn analyzer (see the file attached).

With regards the amplitude of vibration, you can have high amplitudes both at series and parallel resonance. The difference is that, for the same power output, at series resonance the electrical impedance is low and the transducer will demand high current, and at the parallel resonance, the electrical impedance is high and it will demand high voltage.

Thank you for your answer Henrique Alves, that certainly steers my reading in the right direction - I will have to read up more about series and parallel resonance, and how the tracking differs. Is the frequency tracking at parallel resonance more critical because the varying load changes the resonant behavior more severely?

Referring to the horn analyzer brochure (and I've seen the Butterworth-van-Dyke model several times in other literature), are the component values of C0, C, L and R exactly the same for the series and parallel resonance equivalent circuit?

I've also seen that constant amplitude operation is beneficial for certain applications, and that requires constant current. Can that be applied whether you operate at series or parallel resonance?

Thanks again for sharing.

Estine Clasen, tracking the parallel resonance is more critical because it is surrounded by “cliffs” while the series resonance is on a “mild valley” with regards the impedance curve. Please, see a curve example on linear scale at the file “Converter impedance curve.jpg”.

The BVD components values varies drastically from the series to the parallel configuration. Please, see an example on the file “BVD series vs parallel.jpg”. They are from the same transducer.

With regards the constant amplitude operation, I have no experience with this approach.