1. Field of the Invention
The present invention relates to a controller for a motor that has two rotors each of which produces a field by a permanent magnet and can change the phase difference between the rotors.
2. Description of the Related Art
As a permanent magnet type motor, there has been known a double rotor type motor that has two rotors that are coaxially disposed and each have a permanent magnet for producing a field (for example, see Japanese Patent Application Publication No. 2002-204541 (referred to as Patent Document 1 hereinafter)). In the motor of this type, the two rotors can relatively rotate with respect to each other about the axis thereof, and the phase difference between the rotors can be changed by the relative rotation. By changing the phase difference between the rotors, the strength of a composite field (the magnitude of magnetic fluxes) of the fields produced by the permanent magnets of the rotors can be changed.
The motor disclosed in the Patent Document 1 is mechanically configured so that the phase difference between the rotors changes according to the rotational velocity of the motor. That is, the rotors are connected to each other via a member that is displaced in the radial direction of the motor by the action of a centrifugal force. One of the rotors is capable of integrally rotating with the output shaft of the motor for outputting the torque produced by the motor to the outside. When the member described above is displaced, the other rotor relatively rotates with respect to the rotor capable of rotating integrally with the output shaft, and thus the phase difference between the rotors changes. In this case, the permanent magnets of the rotors are disposed in such a manner that, when the motor is halted, the directions of the magnetic poles (the directions of the magnetic fluxes) of the permanent magnets of the rotors are the same, and the strength of the composite field of the permanent magnets is maximized. As the rotational velocity of the motor increases, the phase difference between the rotors changes by the action of the centrifugal force, and the strength of the composite field of the permanent magnets of the rotors decreases.
For example, for a vehicle on which a motor is mounted as a propulsion force generating source (an electric-powered vehicle), the required maximum value of the torque of the motor is high particularly in a low velocity range, and thus, a motor whose strength of the field of the permanent magnets of the rotors is high is typically used so that a large torque can be produced.
However, for the motor whose strength of the field of the permanent magnets of the rotors is high, a periodic change in produced torque (a so-called torque ripple) is likely to remarkably occur particularly in an operational state of the motor in which the rotational velocity of the output shaft of the motor is low, and the produced torque is low (sometimes referred to as low-velocity and low-torque operational state, hereinafter). For example, as shown by dashed line in FIG. 10, a torque change having a large amplitude relative to the magnitude of the produced torque is likely to occur. In FIG. 10, the axis of abscissas indicates the rotational position (rotational angle) of the output shaft of the motor, and the axis of ordinates indicates the torque.
Such a vehicle having the motor as the propulsion force generating source is likely to vibrate when the vehicle is creeping in situations, such as traffic congestion and slow speed driving, or immediately after start moving (when the vehicle runs on a low torque produced by the motor as an idling torque of the motor in a state where the accelerator of the vehicle is not manipulated). Complementally, in general, in an operational state in which the rotational velocity of the output shaft of the motor is relatively high, a change in torque produced by the motor is sufficiently reduced by the action of an inertial force produced by the rotation of the output shaft even if the produced torque is relatively low.
Therefore, it is desirable to reduce a change in torque (torque ripple) produced by the motor in the low-velocity and low-torque operational state.
However, ordinary permanent magnet type motor having a single rotor cannot change the strength of the field of the permanent magnet, and therefore, it is difficult to reduce a change in torque produced by the motor in the low-velocity and low-torque operational state.
Even if the double rotor type motor such as one disclosed in the Patent Document 1 is used, the strength of the composite field of the permanent magnets of the rotors in the low-velocity and low-torque operational state is substantially kept at the maximum strength. This is because the motor disclosed in the Patent Document 1 uses a centrifugal force to mechanically cause relative rotation of the rotors, and therefore, in the low-velocity and low-torque operational state where a centrifugal force hardly occurs, the phase difference between the rotors is substantially kept at the phase difference for which the strength of the composite field is maximized. Therefore, the motor disclosed in the Patent Document 1 cannot reduce a change in produced torque in the low-velocity and low-torque operational state.