An apparatus for restarting a medium voltage inverter configured to provide an inputted power to a motor, the apparatus comprising: a measurement unit configured to measure an input voltage of the motor; and a controller configured to change a voltage or a frequency from the medium voltage inverter in response to a predetermined voltage-frequency ratio of the medium voltage inverter based on the input voltage and a rotor frequency of the motor: characterized in that the apparatus further comprises: an estimation unit configured to detect the rotor frequency of the motor using the input voltage of the motor, wherein the estimation unit includes an extraction unit configured to extract a first voltage from a frequency of the input voltage of the motor and a second voltage lagging in phase by 90 degrees from the first voltage, a first detection unit configured to detect the rotor frequency of the motor based on the first and second voltages, and wherein the extraction unit includes a generation unit configured to extract the first and the second voltage, a second detection unit configured to detect the frequency of the first voltage, and a determination unit configured to determine a bandwidth of the rotor frequency of the motor, and wherein the first detection unit includes a first conversion unit configured to convert the first and second voltages inputted from the extraction unit to a rotation coordinate, a compensation unit configured to compensate an output of the first conversion unit in a proportional integral format, an adder configured to add an output of the compensation unit to an initial frequency, and an integrator configured to integrate an output of the adder. a measurement unit configured to measure an input voltage of the motor; and a controller configured to change a voltage or a frequency from the medium voltage inverter in response to a predetermined voltage-frequency ratio of the medium voltage inverter based on the input voltage and a rotor frequency of the motor: characterized in that the apparatus further comprises: an estimation unit configured to detect the rotor frequency of the motor using the input voltage of the motor, wherein the estimation unit includes an extraction unit configured to extract a first voltage from a frequency of the input voltage of the motor and a second voltage lagging in phase by 90 degrees from the first voltage, a first detection unit configured to detect the rotor frequency of the motor based on the first and second voltages, and wherein the extraction unit includes a generation unit configured to extract the first and the second voltage, a second detection unit configured to detect the frequency of the first voltage, and a determination unit configured to determine a bandwidth of the rotor frequency of the motor, and wherein the first detection unit includes a first conversion unit configured to convert the first and second voltages inputted from the extraction unit to a rotation coordinate, a compensation unit configured to compensate an output of the first conversion unit in a proportional integral format, an adder configured to add an output of the compensation unit to an initial frequency, and an integrator configured to integrate an output of the adder. an estimation unit configured to detect the rotor frequency of the motor using the input voltage of the motor, wherein the estimation unit includes an extraction unit configured to extract a first voltage from a frequency of the input voltage of the motor and a second voltage lagging in phase by 90 degrees from the first voltage, a first detection unit configured to detect the rotor frequency of the motor based on the first and second voltages, and wherein the extraction unit includes a generation unit configured to extract the first and the second voltage, a second detection unit configured to detect the frequency of the first voltage, and a determination unit configured to determine a bandwidth of the rotor frequency of the motor, and wherein the first detection unit includes a first conversion unit configured to convert the first and second voltages inputted from the extraction unit to a rotation coordinate, a compensation unit configured to compensate an output of the first conversion unit in a proportional integral format, an adder configured to add an output of the compensation unit to an initial frequency, and an integrator configured to integrate an output of the adder.