The Application of Double-Fed Motors for Traction Drives

For very high power applications, supplying three phase variable frequency power to the rotor windings of an ac machine results in a double-fed configuration that allows for example for slip power recovery and speed control over a limited speed range. This paper explores the additional performance gains that are possible when both the stator, and rotor are connected to independently controlled three phase power sources. In particular, the use of two independently controllcd cycloconverters to feed such a double-fed motor (DFM) takes full advantage of the cycloconverters very high power and smooth torque capability at low speeds and doubles the upper speed limit available compared to a standard cycloconverter system. This configuration also allows the frequencies of both stator and rotor power to be chosen indepently of the rotor speed, with the result that both converters can continually operate in such a way as to minimize harmonic generation, avoid saturation problems at low speed and to control the power factor at the motor terminals. The paper presents the results with help of the circle diagram for the DFM, which is very useful for visualising the complex interaction between voltages, currents, frequencies, speed and torque production.