Field Weakening Alternative

Abstract

The most common machines for traction are induction and PM synchronous machines. The latter are efficient and small, but they are more expensive and less rugged than induction machines. Fault behavior is easier to manage with an induction machine. For induction machines, the back electromotive force vanishes rapidly when the stator voltages are removed. As shown later, the efficiency of a rerated induction machine can compete with that of a PM machine when both are operated over a wide speed range. The size of any electromechanical device is related to the internal force it generates. It is important to recognize that the size and mass of a motor in a traction system are based on the target torque level. Any induction machine can support a wide frequency and speed range. When operated by an inverter, the name- plate frequency rating of the motor becomes meaningless, as it can be operated at arbitrary speed by adjusting the inverter fundamental frequency. By keeping the voltage-to- frequency ratio constant, the flux level in the machine is constant, meaning that nominal torque can be obtained at any given frequency that supports this constant ratio. The speed and output power of the machine then change linearly with frequency. It works within reason, as long as physical limits on the currents (overheating) and the flux densities (core or tooth saturation) are maintained. According to the National Electrical Manufacturers Association (NEMA), the admissible maximum operating speed for a general-purpose induction machine is twice its synchronous speed.