PURPOSE: To reduce the adverse influence of the stop of a motor-driven water supply pump to a plant as much as possible by controlling the number of revolutions of a motor in response to a water supply control command when a variable voltage and variable frequency unit installed at the power source side of the motor is normal.

CONSTITUTION: A motor-driven water supply pump C15 is driven by a motor 18 controlled at its number of revolutions by a variable voltage and variable frequency unit 1. Normally, the unit 1 so varies its output voltage and frequency as to become the number of revolutions corresponding to water supply control command (number-of-revolutions command) during operating to obtain the water supply to a boiler. When the unit 1 is defected due to certain cause, a fault contact 3 is closed, and a switching relay coil 4 connected in series with the contact 3 is energized. Then, a switching relay contact 2a is operated, the unit 1 is disconnected, and the motor 18 is coupled directly to a power source. As a result, the pump C15 is continuously operated at a predetermined number of revolutions.