| 摘要 |
<p>PROBLEM TO BE SOLVED: To enable the measurement of a.c. currents with current transformers which saturates at the upper limit of the dynamic range of a current to be measured by generating output signals indicating the selected measurement values of a.c. currents from a secondary current which detects up to the saturation of cores in response to a detection sensor of the secondary current. SOLUTION: Current transformers 17A to 17C are provided with toroidal cores 19A to 19C respectively, and each-phase electric lines 7A to 7C pass through the toroidal cores 19A to 19C respectively to be a primary winding. Secondary windings 21A to 21C generate signals indicating each-phase current. These signals are brought into an A/D converter 23 and digitized at intervals controlled by a microcomputer (μp) 25, and theμp 25 protects a motor 3 from overloads through the use of these digitized signals. In this case, theμp 25 multiplies a coefficient which predicts a current value which does not cause saturation in the rest interval of a sampling period by currents which bring the toroidal cores 19A to 19C into saturation points. By measuring the secondary current up to the saturation points through this method, it is possible to measure the ac currents of the current transformers 17A to 17C.</p> |
