CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of Japanese Patent Application No. 2019-170038 filed on Sep. 19, 2019 with the Japanese Patent Office, the inventions of which are incorporated herein by reference in its entirety.

BACKGROUND

Field of the Invention

Embodiments of the present disclosure relate to the art of a drive control system for a motor configured to increase magnetic flux established by a magnet arranged in a rotor and torque derived from a rotating magnetic field acting on the magnet, by establishing field flux in addition to the magnetic flux established by the magnet.

Discussion of the Related Art

JP-A-2012-157182 describes a variable-field rotary electric machine comprising a rotor provided with a permanent magnet, an annular stator arranged around the stator, armature coils arranged in the stator in the circumferential manner at regular intervals, and a magnetic field generator connected to an axial end of the stator. Each of the armature coils establishes a magnetic field in accordance with a current supplied thereto. The magnetic field generator comprises a field yoke and a field coil. Specifically, the field yoke comprises a diametrically-larger section joined to an axial end of the stator, a wall section closing an opening end of the diametrically-larger section, and a diametrically-smaller section protruding from an inner circumferential portion of the wall section toward the rotor. The field coil is formed by winding a wire around the diametrically-smaller section. In the variable-field rotary electric machine taught by JP-A-2012-157182, the field flux established by energizing the field coil serves as a magnetic circuit passing through the rotor, the stator, and the field yoke. However, since the field flux makes a turn at a portion between the diametrically-larger section and the wall section, an eddy current loss is caused at the portion between the diametrically-larger section. In order to reduce such eddy current loss, according to the teachings of JP-A-2012-157182, the diametrically-larger section is joined to the wall section through a fixing member formed of material whose eddy current loss is smaller than the field yoke.

In the variable-field rotary electric machine taught by JP-A-2012-157182, a rotating magnetic field is produced in accordance with a current applied to the armature coil, and the variable-field rotary electric machine generates a base torque in accordance with strength of the rotating magnetic field. The base torque is increased by establishing a field flux acting in a same direction as the magnetic flux. That is, the base torque can be assisted by energizing the field coil. Thus, the variable-field rotary electric machine taught by JP-A-2012-157182 can generate torque not only by energizing only the armature coil, but also by energizing the field coil in addition to the armature coil. Specifically, the armature coil is energized with AC current, and hence electricity is supplied to the armature coil from a power source through a controller such as an inverter. On the other hand, the field coil is energized with DC current, and hence electricity is supplied to the field coil directly from the power source. Thus, the electricity is supplied to the armature coil and the field coil though different electric circuits. In order to achieve a required torque to be generated by the motor, therefore, it is necessary to control the torques generated by the armature coil and the field coil taking account of characteristics of the inverter.

SUMMARY

Aspects of embodiments of the present disclosure have been conceived noting the foregoing technical problems, and it is therefore an object of the present disclosure to provide a control system for a motor configured to control currents supplied to an armature coil and a field coil properly to achieve a required torque.

The control system according to the exemplary embodiment of the present disclosure is applied to a motor comprising: an annular rotor formed of magnetic body; a magnetic flux generator that creates a magnetic flux at a predetermined site along a circumferential direction of the rotor; a stator arranged around the rotor; an armature coil that is formed on the stator, and that is energized to create a rotating magnetic field with respect to the rotor; and a magnetic field generator having a field coil that is energized to generate a magnetic flux to enhance the magnetic flux created by the magnetic flux generator. In the motor, a torque of the rotor is controlled by controlling a base torque generated by energizing the armature coil, and a field torque generated by energizing the field coil. In order to achieve the above-explained objective, according to the exemplary embodiment of the present disclosure, the control system is provided with a controller that controls the base torque and the field torque. Specifically, the controller is configured to: calculate a target torque of the rotor; and control the base torque and the field torque such that a ratio of the field torque to the target torque is increased greater than that of a case in which the target torque is less than a predetermined torque, when the target torque is equal to or greater than the predetermined torque.

In a non-limiting embodiment, the predetermined torque may be set to a smaller value with an increase in a speed of the rotor.

In a non-limiting embodiment, a map determining the ratio of the field torque to the target torque in accordance with an operating point of the motor governed by the torque and a speed of the rotor may be installed in the controller. In addition, the controller may be further configured to: calculate the ratio of the field torque to the target torque based on the target torque and the speed of the rotor with reference to the map; and control the base torque and the field torque based on the ratio of the field torque to the target torque.

In a non-limiting embodiment, the controller may be further configured to determine the ratio of the field torque to the target torque in such a manner as to reduce a total value of: a loss resulting from energizing the armature coil to achieve the target torque; and a loss resulting from energizing the field coil to achieve the target torque.

In a non-limiting embodiment, the total value of the losses may include a copper loss as a resistive loss resulting from energizing the armature coil and the field coil.

In a non-limiting embodiment, the magnetic field generator may comprise: a field yoke as a magnetic body including an annular wall section opposed to an axial end surface of the rotor, and a cylindrical section projecting toward the rotor from a radially innermost portion of the wall section; and a field coil formed by winding a conducting wire around the cylindrical section.

Thus, according to the exemplary embodiment of the present disclosure, the torque of the motor is controlled by controlling the current applied to the armature coil to generate the base torque, and the current applied to the field coil to generate the field torque. When the target torque is greater than the predetermined torque, the controller controls the base torque and the field torque in such a manner that the ratio of the field torque to the target torque is increased greater than that of the case in which the target torque is less than a predetermined torque. That is, when a large torque is required, the field torque is increased. According to the exemplary embodiment of the present disclosure, therefore, the copper loss resulting from generating the base torque and the copper loss resulting from generating the field torque can be reduced, compared to the case of achieving the target torque only by the base torque. For this reason, energy loss resulting from generating the target torque can be reduced.

