A power transducer which outputs AC power having a variable voltage and a variable frequency, the power transducer comprising: a converter for rectifying the AC voltage of an AC power supply to convert it to a DC voltage; a DC intermediate circuit having a smoothing capacitor for smoothing the DC voltage of the converter; an inverter for converting the DC voltage of the converter to an AC voltage; a detecting circuit for detecting the voltage of the DC intermediate circuit; a regenerative braking circuit provided in the DC intermediate circuit and connected to a regenerative braking resistor; a drive circuit for driving the regenerative braking circuit when the detecting circuit detects a specified voltage and for driving the inverter; and a control circuit for controlling the drive circuit, wherein the power transducer further comprises an operation panel for allowing the operator to individually set a permissible duty factor (%ED R) of the regenerative braking resistor and a resistance value R of the regenerative braking resistor, characterised in that the control circuit comprises a timer for accumulating the operating time of the regenerative braking circuit to obtain the cumulative operating time; wherein the control circuit is arranged to perform the steps of: calculating a value of a current which flows in a power semiconductor, which is a switching element, in the regenerative braking circuit from the resistance value set for the regenerative braking resistor; obtaining a generation loss of the power semiconductor from the calculated current value, calculating a permissible duty factor of the power semiconductor based on the obtained generation loss, comparing the permissible duty factor of the regenerative braking resistor with the permissible duty factor of the power semiconductor, and automatically disconnecting only the power semiconductor in the regenerative braking circuit without breaking the output of the power transducer when the cumulative operating time obtained by the timer reaches the permissible duty factor of the regenerative braking resistor or the permissible duty factor of the power semiconductor, whichever is smaller. a converter for rectifying the AC voltage of an AC power supply to convert it to a DC voltage; a DC intermediate circuit having a smoothing capacitor for smoothing the DC voltage of the converter; an inverter for converting the DC voltage of the converter to an AC voltage; a detecting circuit for detecting the voltage of the DC intermediate circuit; a regenerative braking circuit provided in the DC intermediate circuit and connected to a regenerative braking resistor; a drive circuit for driving the regenerative braking circuit when the detecting circuit detects a specified voltage and for driving the inverter; and a control circuit for controlling the drive circuit, wherein the power transducer further comprises an operation panel for allowing the operator to individually set a permissible duty factor (%ED R) of the regenerative braking resistor and a resistance value R of the regenerative braking resistor, characterised in that the control circuit comprises a timer for accumulating the operating time of the regenerative braking circuit to obtain the cumulative operating time; wherein the control circuit is arranged to perform the steps of: calculating a value of a current which flows in a power semiconductor, which is a switching element, in the regenerative braking circuit from the resistance value set for the regenerative braking resistor; obtaining a generation loss of the power semiconductor from the calculated current value, calculating a permissible duty factor of the power semiconductor based on the obtained generation loss, comparing the permissible duty factor of the regenerative braking resistor with the permissible duty factor of the power semiconductor, and automatically disconnecting only the power semiconductor in the regenerative braking circuit without breaking the output of the power transducer when the cumulative operating time obtained by the timer reaches the permissible duty factor of the regenerative braking resistor or the permissible duty factor of the power semiconductor, whichever is smaller. calculating a value of a current which flows in a power semiconductor, which is a switching element, in the regenerative braking circuit from the resistance value set for the regenerative braking resistor; obtaining a generation loss of the power semiconductor from the calculated current value, calculating a permissible duty factor of the power semiconductor based on the obtained generation loss, comparing the permissible duty factor of the regenerative braking resistor with the permissible duty factor of the power semiconductor, and automatically disconnecting only the power semiconductor in the regenerative braking circuit without breaking the output of the power transducer when the cumulative operating time obtained by the timer reaches the permissible duty factor of the regenerative braking resistor or the permissible duty factor of the power semiconductor, whichever is smaller.