PURPOSE: To enable detection of accurate information without depending upon the delivery temperature of a compressor and the piping temperature of a heat exchanger as in a conventional method by a method wherein, provided a source current value detected by a current transformer is below a given current value when the working frequency of a compressor exceeds a given frequency, it is decided that the current transformer has failed.
CONSTITUTION: By detecting that a current value detected by a current detecting circuit 7 is below a given current value when the working frequency of a compressor 15 exceeds a given frequency, disconnection of a current transformer 8 is detected. By deciding that disconnection occurs after a lapse of a given time starting from the starting of a compressor, disconnection of the current transformer 8 is more reliably decided without being influenced by behavior of a current during the rise of running of the compressor. It may be theoretically detected that the comparing value of a detecting current value is higher than zero ampere. However, since a current can not be detected from a constitutional point of view of the current detecting circuit 7 until a current matching a voltage value exceeding the forward voltage of a diode bridge 9 flows, decision at a value near a zero ampere is impracticable, and decision to a detectable value is carried out in consideration of the noise margin of a circuit.
YAGI TOSHIAKI
NAKASUMI EIJI
