To provide a method for preventing water shortage operation of a pump without a water level sensor and its device which effectively functions independent of an external variable factor.
A motor terminal voltage detecting circuit 5 and a capacitor terminal voltage detecting circuit 6 in a single phase induction motor 1 using a capacitor 4 are connected to a control box 1. A main circuit switch 8 opened and closed with an electric output signal from a control box 7 is annexed to a motor main circuit. A constant proper to the motor defined from a ratio to the motor terminal voltage in a no-load running condition is determined, and the motor terminal voltage and the capacitor terminal voltage in running are compared with the constant proper to the motor so as to break the motor power source by judging as the water shortage when it reaches a previously set condition value.
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