resonant frequency of transducers

Hi All,

I am building an ultrasonic cleaning machine, I got a 3Kw 40Khz generator and a bunch of piezo transducers of 40 Khz each.

To test my solution I decided to build a small machine (just 2 transducers 60w 40Khz) connected in parallel and when I turn on the generator the transducers start resonating. But I cannot pick any ultrasonic frequency on the water with my oscilloscope (Previously to test that i could use the oscilloscope to measure ultrasonics on the water i bought a cheap 100w 40 Khz ultrasonic machine from ebay and i manage to get a reading of 28 Khz in the water) I did as well the paper foil test which failed so I guess something is going wrong.

I did check the generator and the output frequency was 40 Khz and current drawn was 0.4 amps (which is good for 2 transducers 40 Khz 60w each) so I am running out of ideas and i have a couple of questions hopefully someone can help me with:

1. Could it be that the transducers i got were not 40 Khz? if that is true then would i still hear the resonant noise from the transducer?

2. Could it be that the transducers were connected with the wrong polarity? even when the transducer provider show that polarity is as i did connect them. But important to mention that the transducers provider is not very trustable.

If you have any other idea of what it could be going wrong please let me know

As well I heard that the resonant frequency does not have anything to do with the transducer frequency :-? if that is true then how can i tell what resonant frequency i should expect?

Thanks in advance.....

I wonder how you "pick up ultrasonic frequency on the water with an oscilloscope"?

Most likely there's some ultrasonic power transmitted. The real problem is about impedance matching, and it can't be checked this way.

When 60 W transducers are connected to a 3 kW generator without measuring and controlling delivered power and maximum AC voltage, there's a good chance to destroy the transducers faster than you get it...

Your transducer will look something like a 10 resistive part (actually delivering power to the water) and a say, 200 ohms of capacitive reactance that needs to be tuned out by a series inductor. So you have to measure the voltage across the transducer and alter the frequency and series inductor to get the peak voltage when you have done this, you are in business!!. Because your generator is tooooo big, I would put a series resistor of 100 ohms in series, this will not alter the resonant frequency, so the tuning stays the same but will limit the current to the transducer and save its life.
Frank

Hi, thanks for your answers....

Just to clarify, the generator that i am using is a 3Kw generator but the power is variable so I am testing using low power.

Chuckey, something similar told me the generator provider but he only asked me to tuned the generator by altering the frequency until I get a peak current and he advise me that using only 2 transducers then I should be looking at 0.4 to 0.5 Amp. I did that but again I can see that transducers are vibrating but they are not producing ultrasonic frequency.

I don't clearly understand yet the difference between the resonant frequency and the frequency of the transducer when I sarted this project I was expecting that the frequency sent to the water from the transducer was 40 Khz. Is this resonant frequency meant to be described in the transducer specifications?

The transducer will vibrate at any frequency you drive it at. Its resonant frequency, is the frequency at which it delivers the maximum power to the water. Off resonance the real part of its impedance could be 200 ohms, dropping to 10 ohms at resonance then going back up high again. Along with the real part there is a highly reactive component which normally has to be tuned out. The manufacturers data should explain all of this.
Frank

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The transducer will vibrate at any frequency you drive it at. Its resonant frequency, is the frequency at which it delivers the maximum power to the water. Off resonance the real part of its impedance could be 200 ohms, dropping to 10 ohms at resonance then going back up high again. Along with the real part there is a highly reactive component which normally has to be tuned out. The manufacturers data should explain all of this.
Frank