I have seen a 3 pin drum coil inductor with piezo buzzer. what is the the use of it?

 

Give us a better description, because that description does not tell me enough to make any sort of guess.

 

I have seen this type of drum inductor with piezo, what is the use of it?

 

Show us a circuit, or at least a picture. I have no idea what you are talking about.

 

 

It's a tapped inductor, for boosting the output voltage to drive the piezo, the primary side is about 22 ohms the secondary side is about 180 ohms, the centre tap goes to the positive supply.

 

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ok, IT APPEARS TO BE A tapped, BUT NOT CENTER-TAPPED, coil to serve as a step-up transformer in a oscillator circuit to provide a higher voltage to drive a piezo-electric sounder.
Providing a wiring diagram instead of a circuit schematic is certainly MUCH LESS USEFUL in discovering the circuit operation.

 

Circuit schematic is yet to be left to be made. I will update the schematic once I design.
But how to decide which center tapped inductor to use

 

Where did you get the circuit? It is useless without knowing the soecs of the inductor, and that probably depends on what piezo disk you are using.

You know there are piezo buzzers with the oscillator built in, right?

 

A) It is not necessarily center tapped. It can be tapped anywhere needed to get the desired voltage increase.
B) The wiring diagram in this simple case is almost as easy to read as a true schematic.

 

Based on the data in post #6 it is certainly NOT CENTER tapped. It IS TAPPED, but far from the center.

For what it is worth, which may not be much, the piezo alarm sounders in two of my digital alarm clocks have DCplus AC supplied while alarm sounding. The DC portion seems to be about 6 volts, and the peak, DC+AC, is about 8 volts.
My intention is to get a louder alarm sound without a lot more hardware. (Not intending to hijack this thread.)