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A refrigerating machine having a compressor driven by a motor. The motor incorporates an armature with an iron core having a cage type conductor and a permanent magnet which is magnetized so as to enable the motor to operate as a synchronous motor. A bypass passage is provided between a discharge si

A refrigerating machine having a compressor driven by a motor. The motor incorporates an armature with an iron core having a cage type conductor and a permanent magnet which is magnetized so as to enable the motor to operate as a synchronous motor. A bypass passage is provided between a discharge side of the compressor and a suction side of the compressor, a shut-off valve is provided for opening and closing the bypass passage, a pressure detecting device is provided for detecting a discharge pressure from the compressor and a current detector is provided for detecting a value of drive current for the compressor. The shut-off valve is opened and closed in accordance with the discharge pressure and the value of the drive current.

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1. A refrigerating machine comprising a compressor driven by a motor, the motor incorporating an armature with an iron core having a cage type conductor and a permanent magnet which is magnetized so as to enable the motor to operate as a synchronous motor, a bypass passage between a discharge side o

1. A refrigerating machine comprising a compressor driven by a motor, the motor incorporating an armature with an iron core having a cage type conductor and a permanent magnet which is magnetized so as to enable the motor to operate as a synchronous motor, a bypass passage between a discharge side of the compressor and a suction side of the compressor, a shut-off valve for opening and closing the bypass passage, a pressure detecting device for detecting a discharge pressure from the compressor, and a current detector for detecting a value of drive current for the compressor, wherein the shut-off valve is opened and closed in accordance with the discharge pressure and the value of the drive current.2. A refrigerating machine according to claim 1, wherein the compressor is a scroll compressor.3. A refrigerating machine comprising a plurality of compressors each driven by a motor having an armature with an iron core incorporating a permanent magnet, and including a first compressor driven at a variable speed by an inverter, and a second compressor having the armature with the iron core incorporating a cage type conductor in addition to the permanent magnet, a bypass passage between an output side and an input side of the plurality of compressors, a shut-off valve for opening and closing the bypass passage, a pressure detecting device for detecting a discharge pressure from the plurality of compressors, and a current detector for detecting a drive current for the second compressor, wherein the shut-off valve is opened and closed in accordance with the discharge pressure and the value of the drive current.4. A refrigerating machine according to claim 3, further comprising a check valve provided in a passage between a discharge side of the second compressor to a discharge side of the first compressor, wherein the first compressor is preferentially started.5. A refrigerating machine according to claim 3, wherein the plurality of compressors are all driven by motors each having an armature with an iron core incorporating a cage type conductor and a permanent magnet which is magnetized so as to enable the motor to operate as a synchronous motor.6. A refrigerating machine comprising a plurality of compressors each driven by a motor having an armature with an iron core incorporating a cage type conductor and a permanent magnet, the motors for the compressors being capable of being driven by a source frequency of a commercial power source and a variable frequency of an inverter, a pressure detecting device for detecting a discharge pressure from the plurality of compressors, and current detectors for detecting drive currents of the respective compressors, wherein a differential pressure between an inlet side and an outlet side of the plurality of the compressors is reduced in accordance with the discharge side pressure and the drive currents.7. A refrigerating machine according to claim 6, further comprising power source change-over switches for selectively one of supplying directly a commercial power to the motors and through an inverter circuit, and a compressor drive circuit for driving the respective motors by one a source frequency of the commercial power source and a variable frequency of an inverter.