Controller for motor

A method of controlling a motor via phase difference changing driving means, the motor having a first rotor, a second rotor, each of which produces a field by a permanent magnet, and an output shaft capable of rotating integrally with the first rotor of the rotors, which are coaxially disposed, said second rotor being capable of relatively rotating with respect to said first rotor, the strength of a composite field of the fields of the permanent magnets of the rotors being capable of being changed by changing the phase difference between the rotors by the relative rotation of the second rotor, and the phase difference changing driving means having an actuator that produces a driving force to make said second rotor relatively rotate with respect to said first rotor, the method comprising:

• a demagnetization determining step of determining whether or not demagnetization of the permanent magnets of said rotors occurs during operation of said motor;
• and
• a rotor phase difference controlling step of controlling said phase difference changing driving means to change the phase difference between said rotors from a current phase difference to a phase difference that results in a higher strength of said composite field in a case where the result of determination in said demagnetization determining step is positive.