A Direct Current Torque Motor for Application on a Variable Frequency Transformer

A wound rotor induction machine can be used in a variable frequency transformer (VFT) to allow interconnection of two power grids that run asynchronously to each other. The transfer of power across the air gap of the machine is controlled by variation of the torque that is applied to the shaft of the VFT. The magnitude of the power transfer is directly proportional to the shaft torque while the direction of power flow is dependent on the direction in which the torque is applied. The rotor of the VFT spins at a speed that is proportional to the difference between the two grid frequencies. When two grids with the same nominal frequency are interconnected, the torque on the shaft of the VFT must be controlled at stall or near stall conditions. DC motors are well suited to this type of duty and have been used in the past for applications with similar requirements; for instance, steel mill looper motors operate at near stall conditions. High torque low speed DC motors have also been applied to steel mill roughers and low speed steel mill finishers. This paper will describe some of the considerations that must be addressed when designing a DC torque motor for a VFT application. It will also outline the main features of a 20 pole, 2,796 kW, 48/93 rpm, 750 V dc motor that was built for the commercial installation of a VFT at Hydro- Quebec's Langlois substation.