Certain polyphase loads work normally only when a determined phase sequence of polyphase electric energy is applied thereto. For example, three phase motor driven submersible pumps can function as pumps only when their impellers rotate in one direction with a determined phase sequence of electric power being supplied to the motors. With a reverse phase sequence, the submersible pumps will no longer perform their pumping action. However, it very often happens, particularly at a construction site that the impellers rotate in a reverse direction as the result of wiring and connection between the motor and an electric power supply. To change the rotational direction for pumping, an operator must change manually the phase sequence of electric power to be supplied to the three phase motor by, for example, changing the connections of wires to the supply terminals of an electric panel board.
The foregoing is due to the fact that conventional submersible motor-pumps do not have means which automatically corrects the phase sequence of the supplied power for pump operation.
Japanese Utility Model Application No. 56-133254 filed on Sept. 8, 1981 by the assignor of the present application discloses an automatic phase sequence correction device built in to a motor-driven submersible pump casing. More specifically, the device comprises a phase sequence sensor branched off from supply lines to the motor and including a relay coil which is sufficiently energized in response to a reverse phase sequence or rotation of the supplied three phase electric power carried by the supply lines. The device further includes a phase changing circuit connected in two phases of the supply lines and including two transfer contacts of the above relay connected in the respective phases thereby to assure that the desired phase sequence of electric power is applied to the motor. The above two phases of the supply lines further include normally open contacts which are closed when their associated auxiliary relay is activated after the position of the phase changing circuit has been established by the phase sequence sensor.
Whereas the above device assures the application of the desired phase sequence of electric power to the motor such as to cause the pump to operate, the device lacks open phase protection capability. More specifically when a supply line which is not associated with the auxiliary relay is open phase due to, for example, breaking of the line, the motor will run with a single phase resulting in damage to the motor.