Conventionally, there is known an electric motor having first and second rotors which are coaxially provided around the rotation axis of the electric motor, in which the relative position, that is, the phase difference between the first and second rotors is controlled according to the rotational speed of the electric motor or to the speed of the rotational magnetic field generated in the stator (for example, see Patent Document 1).
In the electric motor, when the phase difference is controlled according to, for example, the rotational speed of the electric motor, the relative position in the circumferential direction between the first and second rotors is modified via a member which is displaced in the radial direction by the action of centrifugal force. Alternatively, when the phase difference is controlled according to, for example, the speed of the rotational magnetic field generated in the stator, a control current is passed through the winding of the stator in a state with the rotational speeds of the respective rotors maintained by centrifugal force, to thereby modify the speed of the rotational magnetic field. Thus, the relative position in the circumferential direction between the first and second rotors is modified.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-204541
In an electric motor according to one example of the above-mentioned conventional technique, when the phase difference between the first and second rotors is controlled according to the rotational speed of the electric motor, the control over the phase difference between the first and second rotors is possible only while centrifugal force according to the operational state, that is, the rotational speed, of the electric motor is applied. Therefore, there arises a problem that it is impossible to control the phase difference at an appropriate timing including a halt state of the electric motor. Furthermore, when the electric motor is likely to be subjected to vibrations from the outside, such as when the electric motor is mounted in a vehicle as a driving source, there arises a problem that it is difficult to appropriately control the phase difference between the first and second rotors only by the action of centrifugal force. In addition, in this case, the phase difference is controlled irrespective of a variation in the supply voltage in the power source to the electric motor. Therefore, there is a possibility that an unfavorable situation is brought about in which the relationship of the supply voltage and the counter electromotive voltage is reversed.
Moreover, when the phase difference is controlled according to, for example, the speed of the rotational magnetic field generated in the stator, the speed of the rotational magnetic field is modified. Therefore, there arises a problem that the control processing of the electric motor becomes complicated.