1. Field of the Invention
The present invention relates to a rotary electric machine mounted on a vehicle, and in particular, to the rotary electric machine having a rotor and a stator equipped with multi-phase windings for making the machine function as a motor and a generator.
2. Description of the Related Art
In recent years, some vehicles have a function for stopping the idle rotation (i.e., idling) of the engine for saving consumption of fuel. In such vehicles, the engine is frequently turned off every time and started up every time the vehicle stops. Because the stop and start-up of the engine is repeated, it is desired that a generator mounted on the vehicle be motor-driven and serves as a starter which has a problem regarding the life of brushes incorporated in the starter. In these vehicles, important parameters necessary when quickly starting the engine are a start-up torque having no rotation and a predetermined torque at which the rotation increases to a value equivalent to the idle rotation.
It is desired that fluctuations in the engine-rotation can be stabilized by supplying torque from the generator to the engine. Thus, the generator is required to be motor-driven while the engine rotation can become unstable due to unexpected disturbance of torque of the engine. In these vehicles, motor torque is also required to be generated at least at the level of idle rotation.
In general, the vehicle generator is designed to supply required power at an idle rotation of about 1500 rpm. When the vehicle generator is designed in this way, the revolution speed at which extraction of a generated power current becomes possible is about 1000 rpm.
When the vehicle generator is motor-driven, a voltage suitable for charging the battery is generated at a revolution speed of 1000 rpm. Therefore, electric current cannot be sent from the battery during motor operation. Hence, a sufficient torque cannot be generated at the revolution speed.
As shown in FIG. 1, the torque decreases linearly from a non-rotation start-up torque to a power generation starting revolution speed. Although some torque can be extracted through weakening current-flowing in the field winding of the generator, the torque is not sufficient for the purpose described above.
Characteristics of a synchronous rotary machine such as that above are determined by an inductance of a stator, specifically, the number of windings in a stator winding. As shown in FIG. 2, in power generation characteristics, the minimum revolution speed required for initiating the power generation changes depending on the amount of the inductance (the number of windings), and torque and revolution characteristics during motor-driving also change in accompaniment.
In a vehicle rotary machine having both power generation and engine driving functions, it is difficult to actualize the characteristics required during power generation and the characteristics required for torque of the engine with the same inductance. A conflicting request is made in that, when the power generation characteristics are given priority, motor characteristics become a state in which the drive torque is insufficient. When the motor characteristics are given priority, the power generation output during idle rotation becomes insufficient.
As a conventional technology responding to such requests, a series-parallel conversion rotary electric machine in Japanese Patent Laid-open Publication No. Heisei 4-88900 is known. The series-parallel conversion rotary electric machine provides two sets of stator windings and changes impedance by switching the stator windings between serial and parallel. In another conventional technology, a rotary electric machine is known in which switching elements are disposed at a neutral point and between a high potential and a low potential of a Y-connected three-phase winding in the rotary electric machine. When an abnormality, such as disconnection, occurs in the three-phase winding, the rotary electric machine uses the neutral point in place of the winding of the phase at which the abnormality has occurred.
In the series-parallel conversion rotary electrical machine disclosed in Japanese Patent Laid-open Publication No. Heisei 4-88900, a plurality of sets of multi-phase windings are required to be mounted in the stator. Problems occur in that a configuration of the stator becomes complicated and cost rises. A large number of switching elements for switching the winding between serial and parallel is additionally required. For example, compared to six rectifying elements ordinarily required, nine switching elements are additionally required, thereby also contributing to the increase in cost.
In addition, the rotary electric device disclosed in Japanese Patent Laid-open Publication No. 2007-99066 is a technology used only during motor-driving and in response to an abnormal situation, and does not meet the above-described requirements. A conventional three-phase winding is driven by only a two-phase winding and three phases are imitated by use of the neutral point. Problems occur such as the rotation not being smooth.