| 摘要 |
A solid state overload relay utilizes an apparatus that converts the current of a polyphase electrical load to a voltage signal approximately proportional to the RMS current of the load. In addition to providing the overload function, the voltage signal is used to provide an improved phase current unbalance protection. Conventional current transformers sense the current flow in each phase of the load. The current waveform for each phase is converted to a voltage by rectification and the peak value and the average value of the resultant voltage is determined. A summing circuit combines the two values at a predetermined ratio to create a DC voltage that is approximately equal to the rms value of the input phase current. A combination of the approximation circuit with a time integrating circuit and a level detecting circuit produces a low cost solid state overload relay in one embodiment of the invention. The phase current unbalance circuit determines the average of the DC voltages from each phase and compares this average with the DC voltage from each individual phase. It will output a trip signal if any of the individual phase currents exceeds the average by a predetermined percentage of the average for a set time interval. The improved device does not require compensation for non-sinusoidal currents, line harmonics, or transients which could cause erroneous results.
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