| 摘要 |
PURPOSE:To detect the abnormality of a motor quickly in a simple structure by winding coils around the common iron core in series and in parallel to the motor and by actuating a protective circuit by switching the contact with the composite flux of both coils. CONSTITUTION:A current coil 1a in series and a voltage coil 1b in parallel to a motor 2 are provided, both of which are wound around over a common iron core to form a differential relay 1. When an air capacity change-over switch 5 is turned ON, the motor 2 is driven via a contact 3C of an interrupting relay. A detecting contact 1C of the differential relay 1 is closed at a moment, but a coil 3a is not electrified because of a damping time constant RC. In normalcy, the flux of the coil 1a is denied by that of the coil 1b and the relay 1 is not actuated. If the motor 2 is burnt, a larger current pours into the coil 1a to close the contact 1C, by which the coil 3a is actuated to turn OFF a breaker 3 and connected to a contact 3d to self-maintain the coil 3a. Thus, the abnormality is quickly detected and the feeding of current interrupted, so that the overheating can be prevented.
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