Control transformer and ordinary transformer principle is no difference. The transformer works on the principle of electromagnetic induction. A transformer has two sets of coils. Primary coil and secondary coil. The secondary coil is outside the primary coil. When the primary coil is connected with alternating current, the transformer core generates alternating magnetic field, and the secondary coil generates induced electromotive force. The coil turns ratio of a transformer is equal to the voltage ratio. For example: the primary coil is 500 turns, the secondary coil is 250 turns, the primary through 220V AC, secondary voltage is 110V. A transformer can reduce or increase voltage.