Motor drive system and its control method

A motor drive system, comprising

• an AC electric motor having a rotor with a permanent magnet attached thereon;
• a power converter for controlling power supply to said AC electric motor by on/off control of a plurality of power semiconductor switching elements under pulse width modulation control;
• a current detector for detecting a motor current flowing between said power converter and said AC electric motor;
• a ripple detecting unit for detecting a ripple current width of said motor current, corresponding to switching frequency of each of said power semiconductor switching elements, based on a detected value from said current detector;
• a reference setting unit for setting a reference value of said ripple current width such that magnet temperature does not increase to a temperature causing demagnetization of said permanent magnet, at least based on a temperature increase characteristic of said permanent magnet with respect to said ripple current width;
• and
• a frequency adjusting unit for controlling frequency of a carrier used for said pulse width modulation control, based on a comparison between said ripple current width detected by said ripple detecting unit and said reference value set by said reference setting unit,
• said frequency adjusting unit increasing frequency of said carrier to be higher than present frequency when said ripple current width is larger than said reference value and decreasing the frequency of said carrier to be lower than the present frequency when said ripple current width is equal to or smaller than said reference value.