PURPOSE: To vary performance of a pump by providing a circuit for switching the connection of a stator winding in an electric motor through receiving an output from an alarm setter for detecting increase or decrease in a water supplying pressure.

CONSTITUTION: A pressure detector 6 detects a water supplying pressure to consumers 2 to output a water supplying pressure signal 6a. An alarm setter 7 receives input of the water supplying pressure signal 6a to compare it with a set value. When a supply pressure increases detecting signal 7a, operates, a voltage is applied to the coil of an auxiliary relay 8c for detecting a star connection time zone and a star connection command 8a during operation is attracted, while, when a supply pressure decrease detecting signal 7b operates, a delta connection command 8b during operation is attracted. Therefore, even in the case of decrease in water demand, it is possible to make the water supplying pressure approximate to a suitable value without providing any speed controller.