Mr. Hanrahan's comments with regard to the assumption of constant stator winding and iron temperatures are a valuable criticism. H. Cooke of the TKM Electric Corporation has made the same comment and has suggested that the temperature rises of the stator winding and of the core of a typical 400-cps permanentmagnet alternator are on the order of 50 C, well below the allowable maxima.To illustrate a more realistic functional relationship between frequency and the various alternator characteristics, an analysis has been made assuming the full-load flux density B mi and the full-load demagnetizing armature mmf H d are constant. The results are shown in Table IV and in Fig. 9. Also shown in the figure for comparison are the other weight-frequency curves shown originally in Fig. 2 of the paper. These results show a very substantial decrease in weight with increasing frequency. Furthermore, the table shows that the winding and iron temperature rises increase significantly with increasing frequency. However, owing to the low values of temperature rise in the 400-cps model, the temperature rise in the winding is 115 C and the core is 216 C at 5,000 cps, values still below the allowable maxima for short-duty-cycle operation.The full-load flux density and direct-axis demagnetizing ampere-turns are assumed constant in view of the following reasoning. If the rotor is air-stabilized, the intersections of the magnet B-H curve and the out-stator permeance line 1 and of the B-H curve and the air-gap permeance line Fig. 9. Relative weight of alternators as a function of frequency Curve 1-Woundfield synchronous alternator Curve 2-Permanent-magnet alternator Curve, 3-Induction alternator The dashed curve shows curve 2 of ; Fi S . 2 2.0
INTRODUCTIONA unique test metbod for screening transformer insulation systems was presente cl in a paper (1) during the 1962 Electrical Insulation Conference. This test metbod referred to as "Progressive Temperature Testing" was based on increasing the ~est temperature in fmite steps until the insulation system fa1ls. The test temperatures and times were selected in accordance with the Arrhenius(2) relationship of the logarithm of life and the reciprocal of absolute temperature.The original purpose of the progressive temperature test was for screenin!( insulation systems for their temperature capabilities. This data could then be used in selecting temperatures for additional and more complete aging tests. The progressive temperature test proved to be very useful in comparing the aging characteristics of insulation systems. In most cases, this test method gave definite end points in a reasanably short time.Because of the Jack of supporting data, it was recommended that the pro!(ressive temperature test not be used for predieting thermal life. It was suggested that this test be used only as an evaluation tooi.Data has now been accumulated on the same insulation system evaluated by both the progressive temperature test and ~he conventional constant temperature test. The purpose of this paper is to describe these life tests, present the data obtained and compare the two test methods.
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