In electrical engineering, power engineering and the electric power industry, power conversion is converting electric energy from one form to another, converting between AC and DC, or just changing the voltage or frequency, or some combination of these. A power converter is an electrical or electro-mechanical device for converting electrical energy. This could be as simple as a transformer to change the voltage of AC power, but also includes far more complex systems. The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another frequency.