A Control Transformer is an Isolation Transformer designed to provide a high degree of secondary voltage regulation (stability), during the short period of overload condition. This period is typically referred to as inrush. These transformers supply power to control and/or auxiliary equipment not intended for direct connection to the main circuit.
This is an ideal transformer for stepping up or down the voltage or for installations which require a galvanic partition between the primary and secondary voltage. E.g. UPS, Electric motor, compressor, cooling plants, automatic washing machines. They're intended for change of voltages with galvanic isolation, attenuation of line disturbances.