Real Time Flux Control in PM Motors

Abstract

Significant research at the Oak Ridge National Laboratory (ORNL) Power Electronics and Electric Machinery Research Center (PEEMRC) is being conducted to develop ways to increase (1) torque, (2) speed range, and (3) efficiency of traction electric motors for hybrid electric vehicles (HEV) within existing current and voltage bounds. Current is limited by the inverter semiconductor devices' capability and voltage is limited by the stator wire insulation's ability to withstand the maximum back-electromotive force (emf), which occurs at the upper end of the speed range. One research track has been to explore ways to control the path and magnitude of magnetic flux while the motor is operating. The phrase, real time flux control (RTFC), refers to this mode of operation in which system parameters are changed while the motor is operating to improve its performance and speed range. RTFC has potential to meet an increased torque demand by introducing additional flux through the main air gap from an external source. It can augment the speed range by diverting flux away from the main air gap to reduce back-emf at high speeds. Conventional RTFC technology is known as vector control [1]. Vector control decomposes the stator current into two components; one thatmore » produces torque and a second that opposes (weakens) the magnetic field generated by the rotor, thereby requiring more overall stator current and reducing the efficiency. Efficiency can be improved by selecting a RTFC method that reduces the back-emf without increasing the average current. This favors methods that use pulse currents or very low currents to achieve field weakening. Foremost in ORNL's effort to develop flux control is the work of J. S. Hsu. Early research [2,3] introduced direct control of air-gap flux in permanent magnet (PM) machines and demonstrated it with a flux-controlled generator. The configuration eliminates the problem of demagnetization because it diverts all the flux from the magnets instead of trying to oppose it. It is robust and could be particularly useful for PM generators and electric vehicle drives. Recent efforts have introduced a brushless machine that transfers a magneto-motive force (MMF) generated by a stationary excitation coil to the rotor [4]. Although a conventional PM machine may be field weakened using vector control, the air-gap flux density cannot be effectively enhanced. In Hsu's new machine, the magnetic field generated by the rotor's PM may be augmented by the field from the stationery excitation coil and channeled with flux guides to its desired destination to enhance the air-gap flux that produces torque. The magnetic field can also be weakened by reversing the current in the stationary excitation winding. A patent for advanced technology in this area is pending. Several additional RTFC methods have been discussed in open literature. These include methods of changing the number of poles by magnetizing and demagnetizing the magnets poles with pulses of current corresponding to direct-axis (d-axis) current of vector control [5,6], changing the number of stator coils [7], and controlling the air gap [8]. Test experience has shown that the magnet strengths may vary and weaken naturally as rotor temperature increases suggesting that careful control of the rotor temperature, which is no easy task, could yield another method of RTFC. The purpose of this report is to (1) examine the interaction of rotor and stator flux with regard to RTFC, (2) review and summarize the status of RTFC technology, and (3) compare and evaluate methods for RTFC with respect to maturity, advantages and limitations, deployment difficulty and relative complexity.« less