Turbocharger and marine vessel

1. A turbocharger, comprising:

a compressor to be driven by a turbine to boost ambient air to be used by an internal-combustion engine;

an electric motor which is coupled to a rotary shaft of the compressor;

a plurality of power conversion units;

wherein the plurality of said conversion units include at least a first power conversion unit and a second power conversion unit;

wherein the first power conversion unit converts a direct-current power to an alternating-current power to be used by the electric motor;

a smoothing capacitor coupled to direct-current bus buses, the direct-current buses positioned between and coupled to the plurality of said power conversion units; and

a control unit to control the first power conversion unit to reach a measured value of a generator motor speed to comply with a value of a generator motor speed command of an input signal to the electric motor, from an upstream controller, during an operation of the electric motor,

wherein the control unit includes a command change unit to change the value of the generator motor speed command to a value equal to or greater than the measured value of the generator motor speed if the value of the generator motor speed command of the electric motor is less than the value of the measured generator motor speed and a line-to-line voltage of the smoothing capacitor is equal to or greater than a predetermined first voltage threshold value that was set for that line-to-line voltage if the smoothing capacitor does not exceed a predetermined value of tolerance, during the operation of the electric motor.

2. The turbocharger according to claim 1 ,

wherein the control unit is to reset the value of the generator motor speed command to the value of the generator motor speed command of the input signal from the upstream controller if the line-to-line voltage of the smoothing capacitor becomes equal to or less than a predetermined second voltage threshold value,

wherein the predetermined second voltage threshold value is equal to or less than the predetermined first voltage threshold value from a time that the generator motor speed command is changed until after a predetermined period of time has elapsed.

3. The turbocharger according to claim 2 , wherein the command change unit is to decrease a value of the generator motor speed command if the generator motor speed command is to be changed at a frequency equal to or greater than a predetermined frequency.

4. The turbocharger according to claim 1 , wherein the command change unit is to set the value of the generator motor speed command to a greater value if the line-to-line voltage of the smoothing capacitor approaches a predetermined second voltage threshold value, even after a predetermined period of time has elapsed from a time that the value of the generator motor speed command is to be changed.

5. A marine vessel comprising:

a turbocharger including a compressor to be driven by a turbine of the marine vessel to boost ambient air to an internal combustion engine;

an electric motor which is coupled to a rotary shaft of the compressor;

a plurality of power conversion units;

wherein the plurality of said power conversion units includes at least a first power conversion unit and a second power conversion unit;

wherein the first power conversion unit converts a direct-current power to an alternating-current power to be used by the electric motor;

a smoothing capacitor coupled to direct-current buses, the direct current buses coupled to the plurality of said power conversion units; and

a control unit to control the first power conversion unit to reach a measured value of a generator motor speed to comply with a value of a generator motor speed command of an input signal to the electric motor, from an upstream controller, during an operation of the electric motor,

wherein the control unit includes a command change unit to change the value of the generator motor speed command to a value equal to or greater than the measured value of the generator motor speed if the value of the generator motor speed command of the electric motor is less than the value of the measured generator motor speed and a line-to-line voltage of the smoothing capacitor is equal to or greater than a predetermined first voltage threshold value during the operation of the electric motor.

6. A method to control a turbocharger, comprising:

controlling a first power conversion unit, of at least first and second power conversion units, to reach a measured value of a generator motor speed to comply with a value of a generator motor speed command of an input signal to the generator motor, from an upstream controller, during an operation of an electric motor, the electric motor being coupled to a rotary shaft of a compressor driven by a turbine to boost ambient air to an internal-combustion engine, wherein the first power conversion unit converts a direct-current power to an alternating-current power to be used by the electric motor; and

changing the value of the generator motor speed command wherein the generator motor speed command is changed to a value equal to or greater than the measured value of the generator motor speed if the value of the generator motor speed command of the electric motor is less than the value of the measured generator motor speed and a line-to-line voltage of a smoothing capacitor, the smoothing capacitor being coupled to direct-current buses between the first and second power conversion units of the at least first and second power conversion units, is equal to or greater than a predetermined first voltage threshold value during the operation of the electric motor.