1. Field of the Invention
The present invention relates to a three-phase alternating current generator (three-phase magnet generator) driven by an internal combustion engine.
2. Description of the Related Art
With respect to the principle of an alternating current generator, a flywheel to which permanent magnets are fixed is rotated by the power of an internal combustion engine, and rotating magnetic fields produced by the permanent magnets cut across a stator coil (generator coil), with the result that a voltage is induced in the stator coil by an electromagnetic induction effect. In this case, the magnets are fixed in a rotation direction of the flywheel and N-poles and S-poles are arranged at regular intervals. Thus, the number of magnets to be used becomes 2n (n is a positive integer (natural number)). In addition, the number of teeth of the stator coil becomes 3m (m is a positive integer (natural number)) because of a three-phase alternating current generator.
A conventional three-phase alternating current generator has a structure of n=m. In other words, when the number of magnets fixed to the flywheel is set to 2n, the number of teeth of the stator coil becomes 3n. A Y-connection is employed as a connection method for three-phase output (for example, Kokichi Okawa, “Design and Characteristic Computation Method for a Permanent Magnet Magnetic Circuit (II), Usage Volume,” Sougou Denshi Research, Sep. 30, 1987, First Edition, pp.479-481).
According to the conventional three-phase alternating current generator, when the number of magnets is 2n, the number of teeth is 3n, and a three-phase output is a Y-connection, it is possible to satisfy an output specification characteristic. However, a self-heating amount becomes very high. One of factors in which the self-heating amount becomes higher is the Y-connection.
In general, the Y-connection or the Δ-connection is employed as a connection method on the output side of the three-phase alternating current generator driven by an internal combustion engine. An output current in the Y-connection becomes (line current Iu=phase current Iu′) as shown in FIG. 5. An output current in the Δ-connection becomes (line current Iu/√3=phase current Iu′) as shown in FIG. 6.
Therefore, a current value required for one phase in the Y-connection becomes about √3 times higher than that in the Δ-connection. Here, the self-heating amount is computed from I2R. Symbol I denotes a phase current and R denotes a winding resistance. The self-heating amount becomes the square of the current value. Thus, the self-heating amount in the Y-connection becomes higher than that in the Δ-connection, so that it is ideal to employ the Δ-connection with respect to measures for suppressing the self-heating amount. However, in the case of the Δ-connection, the output current at a low speed rotation is reduced. Accordingly, because it is impossible to satisfy the specification, there is a problem that it is difficult to apply the Δ-connection to the conventional three-phase alternating current generator.