1. Technical Field
The present invention relates generally to switch-reluctance motors (SRM's), and more specifically relates to a system and method of controlling the turn-on and turn-off angles of an SRM.
2. Related Art
Electric machines can be broadly classified into two categories on the basis of how they produce torque—electromagnetically or by variable reluctance. In the first category, motion is produced by the interaction of two magnetic fields, one generated by the stator and the other by the rotor. Two magnetic fields, mutually coupled, produce an electromagnetic torque tending to bring the fields into alignment. The same phenomenon causes opposite poles of bar magnets to attract and like poles to repel. The vast majority of motors in commercial use today operate on this principle.
In the second category, motion is produced as a result of the variable reluctance in the air gap between the rotor and the stator. When a stator winding is energized, producing a single magnetic field, reluctance torque is produced by the tendency of the rotor to move to its minimum reluctance position. As far as motors that operate on this principle, the switched-reluctance motor (SRM) falls into this class of machines.
The switched-reluctance motor (SRM) is under development for variable speed applications where the inherent characteristics of the SRM make commercial sense. To date, these applications include, e.g., sourcing aerospace power systems, starter/alternators for hybrid vehicles, and wind turbine applications. The aerospace and automotive applications are generally characterized by high-speed operation. The wind energy application is characterized by low speed, high torque operation.
The SRM produces torque through excitation that is synchronized to rotor position. The excitation is generally described by three excitation parameters: the turn-on angle θon, the turn-off angle θoff, and the reference current Iref. The turn-on angle θon refers to the rotor position where phase excitation (current) is initiated. The turn-off angle θoff refers to the rotor position where phase excitation is ended. The reference current Iref is the desired phase current during phase excitation. It should be noted that current continues to flow in the phase winding after θoff, but the phase current is monotonically decreasing by virtue of the state of the inverter switches. A computerized controller is typically utilized to control the parameters. Control of the excitation angles results in either positive net torque for motoring, or negative net torque for generating. A major challenge for implementing an SRM involves implementing a controller that effectively selects the turn-on and turn-off angles. Unfortunately, prior solutions have failed to provide a solution that allows for efficient operation of the SRM that is easily implemented. Such inefficiencies have lead to larger sizes, increased weight, and increased energy consumption.
Prior art examples include U.S. Pat. No. 4,933,620, issued to MacMinn et al., which is hereby incorporated by reference and discloses a control strategy for low speed operation of an SRM. The invention deals with closed loop control of the turn-on angle of the SRM at low speeds and its hardware implementation. The suggested method tries to adjust the turn-on angle with the closed loop control such that first peak of the phase current occurs at the desired position. This control breaks down at the elevated speeds where the turn-on angle has no effect on the place of the first peak of the phase currents. The invention also keeps the conduction angle constant and advances the turn-off angle as the turn-on angle advances, but the conduction angle also needs to be adjusted as the operating conditions change for efficient system operation.
U.S. Pat. No. 6,046,561 issued to Zup et al., which is hereby incorporated by reference, discloses a commutation control method for the SRM. The invention focuses on a hardware implementation of an approach commonly known as “phase advance” with a low cost position encoder. Phase advance means that the phase conduction interval is shifted relative to the angular interval over which the phase inductance is increasing. In U.S. Pat. No. 6,046,561, the phase advancement is made using only speed information and the conduction angle is kept constant in the invention. This assumption causes deterioration in the SRM efficiency. The most efficient turn-on and turn-off angles are nonlinear functions of the motor speed and motor phase currents and subject to change also with operating parameters.
Unfortunately, none of the prior art provides efficient control of the excitation angles of the SRM at all operating points without extensive machine modeling and simulation. Accordingly, a need exists for a controller that can achieve these goals with a simple and efficient implementation.