Patent ID: 7218068

Claim:
A power source for an electric motor that drives a re-circulation pump provided in a boiling water reactor for supplying an alternating current, the power source comprising: a first bank having an AC-to-DC forward converter and a DC-to-AC inverter that are electrically connected mutually in series, the first bank outputting an alternating current that has a voltage and frequency different from the voltage and frequency of an input alternating current; a second bank connected in parallel to the first bank, the second bank having an AC-to-DC forward converter and a DC-to-AC inverter that are electrically connected mutually in series, and the second bank outputting an alternating current that has a voltage and frequency different from the voltage and frequency of an input alternating current; a speed controller for controlling the first bank and the second bank, thereby to control a speed of the electric motor; and a shut-off mechanism electrically connected in series to the first bank and the second bank, the shut-off mechanism being configured to connect and disconnect the first and second banks electrically, independently of each other; wherein the speed controller: temporarily stops the first bank and the second bank, thereby idling the re-circulation pump, while only the first bank remains in an inoperative state; causes the shut-off mechanism connected to the first bank to disconnect the first bank, while the re-circulation pump is idling; re-starts only the second bank when a speed of the electric motor reaches a predetermined speed after the first bank is disconnected while the re-circulation pump is idling, thereby causing only the second bank to control the speed of the electric motor; and re-starts the first bank after controlling the speed of the electric motor by means of only the second bank, thereby causing both the first bank and the second bank to control the speed of the electric motor.