| 摘要 |
PURPOSE:To detect an overload by sampling and integrating a digitized main circuit current signal at every predetermined period to obtain a mean current, comparing it with a momentary overcurrent criterion reference value, and operating an overcurrent relay when it is larger than the reference value. CONSTITUTION:A current value I detected by a current detector 24 is converted by an A/D converter 25 to a digital signal, and M pieces of digital signals are sampled at every predetermined time by a first averaging processor 26. Then, sampled M pieces of data are added, the added value is divided by the number M of additions to obtain an average current IA. Then, an overload detection is calculated, and an overload detection is executed by the obtained mean current IA. In this case, for the protection of a DC motor 21 against the overload, momentary overcurrent detection for specifying the rectifying capacity of a commutator a time limiting overload detection for specifying the temperature rise, deterioration of an insulator of an armature winding, and long time overload detection for specifying the temperature rise for causing an insulator breakdown are monitored.
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