Current Source

So I was asked to design a circuit with a constant AC voltage of 240V but the output varies from 0 to 30A. Does anyone have an idea of how to do so? Do I need to use an op-amp current source? Or is there any other possible methods of doing so?

What is the power source?

You can either have the voltage constant or the current constant, not both. What determines the output current?

A constant AC voltage will mean a Voltage Source, not a Current Source. Current sources maintain a constant current, with varying load voltage. A voltage source means a constant voltage, with varying load current. A series potentiometer will adjust the available current, but will also introduce a voltage drop of its own, and it will not be very precise. Also, depending on the current you plan to control, it may burn up. It would have to be rated for a high wattage.

Honestly, I'm not really sure what your teacher/boss/etc wants. A load will draw a specific amount of current, no more, no less. Having a variable current output really doesn't do anything. A 400mA device connected to a 400 amp supply will only draw 400mA. What would be the point of a variable current voltage source?

Matt