Comparison of methods of loading large A-C. and D-C. generators and rotary converters for factory temperature tests

A satisfactory test should duplicate as completely as possible the conditions under which the machine is guranteed to operate, and the conditions of test should be more severe and not less severe than the conditions of operation. Four methods of loading a-c. generators are described and illustrated by test results: (a) Separate open circuit and short circuit loading is unreliable because the test conditions are radically different from operating conditions but this method may be the only possible method on account of minimum testing equipment required. (b) Alternate open circuit and short circuit loading more nearly approaches operating conditions but results do not check with actual load unless test conditions are modified, from those theoretically determined. Experience is not, as yet, available to empirically determine proper test conditions. (c) Direct-current open delta loading still more closely approximates operating conditions in that current and voltage are simultaneously present. The method is somewhat limited in application due to the presence of direct current in the armature. The proper field current for use in the test is shown to be that corresponding to open circuit normal voltage. The test results check well with actual operation. (d) Zero power factor loading affords the nearest approach to actual operating conditions that has been suggested. It exactly corresponds with operating conditions except for the differences due to power factor, and these, in general, result in more severe conditions than actual loading. Test results are closer to results from actual loading than by any other method described. A direct check on field coil temperatures and exciting voltage is afforded. The application of the method is only limited by the expensive test equipment sometimes required. The zero power factor method of loading is recommended for adoption by the Institute as the preferable method. For d-c. generators and rotary converters, only one satisfactory method of loading has been suggested. This is the familiar loading-back method which is recommended.