PROTECTIVE DEVICE FOR ELECTRIC MOTOR SYSTEM

PROBLEM TO BE SOLVED: To provide a protective device for an electric motor system which can properly protect a power converting unit and an electric motor from overheating by monitoring the correlation between the temperatures of the power converting unit and electric motor.SOLUTION: In the protective device for the electric motor system for protecting the power converting unit 120 and the electric motor 130 against overheating by controlling the driving mode of a switching element provided in the power converting unit 120, the temperatures of the power converting unit 120 and electric motor 130 are obtained. In accordance with the correlation between the obtained temperatures, either mode is dynamically selected from among (a) a mode wherein the carrier frequency relating to PWM control of the switching element is set to a preliminarily determined frequency, (b) a mode wherein the carrier frequency is set to a frequency lower than the preliminarily determined frequency, and (c) a mode wherein a square-wave AC voltage that alternates between positive and negative values at every 180° progression of phase is applied to the electric motor 130. The driving of the switching element is controlled in the selected mode.