The correct answer is: A. voltage device.
A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. It does this by using electromagnetic induction to create a magnetic field that induces a voltage in a secondary coil. The voltage in the secondary coil is proportional to the voltage in the primary coil, and the current in the secondary coil is proportional to the current in the primary coil.
The power transformer is a type of transformer that is used to transmit or distribute electrical power. It is a constant voltage device because the voltage in the secondary coil is always equal to the voltage in the primary coil, regardless of the load on the secondary coil.
The other options are incorrect because:
- A current device is a device that controls the current in a circuit.
- A power device is a device that converts electrical energy into another form of energy, such as mechanical energy or heat.
- A main flux device is a device that controls the main flux in a transformer.