| 摘要 |
PURPOSE:To remove an unnecessary variation in the output of a variable frequency power source when a signal line is defective by detecting the malfunction of the amplitude by a signal amplitude malfunction detector, and switching the voltage to a motor from the variale frequency power source to a commercial power source at the abnormal time. CONSTITUTION:A signal line 9 is disconnected, the amplitude of a discrete wave flowed to the line 9 decreases, even if no particular change occurs, (including zero) at the input side of a variable frequency power source 3. Accordingly, a signal malfunction detector 3B for detecting that the amplitude of the line 9 is displaced from a normal value is provided at the power source 3 side to judge whether the line 9 is disposed in the abnormal state such as a disconnected state or not, and if the result is abnormal, switches 2, 10 are opened, and a switch 11 is closed to switch the voltage applied to a motor from the power source 3 to a commercial power source 1. |
