Abstract: A load monitoring apparatus has a structure that the electric-power value of a motor is sampled in response to a sampling signal, the sampling cycle of which is changed at a predetermined recursive number. Moreover, the electric-power value is calculated in accordance with a result of the sampling is subjected to an absolute-value range and a relative-value range which is determined in accordance with the previously calculated electric-power value so as to determine an abnormal load of the motor. The sampling cycle is elongated in predetermined steps to correspond to the characteristic of, for example, a reduction gear until a stable driving state is obtained so that an abnormal state of the load such that the load is gradually increased or decreased is detected in accordance with the relative-value range before the abnormal state exceeds limits of the absolute-value range.

Abstract: A voltage detecting circuit is capable of detecting the voltage to be detected at high precision, is small in size, and is low in manufacturing cost. A first photo coupler and a second photo coupler are interposed between voltage input terminals to which the voltage to be detected is applied. The photo coupler has light emitting elements connected parallel reversely, first voltage generator for generating a voltage depending on the photo detector side currents of the first photo coupler and second photo coupler, a differential amplifier whose one input terminal receives the voltage generated by the first voltage generator. A third photo coupler and a fourth photo coupler having light emitting elements are connected parallel reversely for receiving the output current of the differential amplifier, and a second voltage generator for generating a voltage corresponding to the photo detector side currents of the third photo coupler and fourth photo coupler.

Abstract: An electric power detector for a three-phase AC voltage supply which measures the current of one phase and the voltage between the one phase and another phase and computes the phase difference between the voltage and the current, decides the phase rotation direction by comparing the phase difference with a specified angle, and obtains a power factor angle by adding or subtracting 30.degree. to or from the phase difference corresponding to the result of the decision to produce a power factor angle for computing three-phase power.