D URING the interruption of direct currents in circuits having induct ance, the presence of high-voltage peaks or "inductive kicks" is quite commonly recognized in a qualitative sense. There is however little quantitative data on this subject, particularly at the voltages, loads, and altitudes encountered on modern aircraft.Data on inductive voltages usually are obtained by taking cathode-ray oscillograms or by observing a cathode-ray os cilloscope. The former method is accu rate but relatively slow and tedious ; the latter, with the tubes available up to this time, has been somewhat questionable because of the difficulty of clearly deter mining by eye a single fast transient. In this investigation use has been made of a cathode-ray tube with a retentive screen-so retentive that the trace of a single half-cycle sweep of a 60-cycle wave takes several seconds to disappear. Using this tube it has been possible to take a large amount of data in a short time, so that, instead of relying on a few oscillograms, ten readings could be taken at each point and an average and range established for the voltage disturbances set up.
Scope of This PaperIt is the purpose of this paper to pre sent experimental data on the magnitude and variations of voltage peaks set up by d-c arc interruptions of inductive circuits. These voltages were measured on d-c systems of 30, 60, 125, and 250 volts with currents ranging from 0.1 to 220 amperes and simulated altitudes from sea level to 50,000 feet. The loads used included typical aircraft devices such as motor fields and contactor coils.The effect of the induced-voltage peaks on insulation puncture or flashover was not investigated. This phase of the sub ject requires a separate investigation.Because of limits in time, no attempt is made to explain theoretically the re sults obtained. Rather, emphasis is on the presentation of data taken to date.Paper 44-200, recommended by the AIEE com mittee on air transportation for presentation at the
torques usually present in salient pole machines? This second harmonic component of torque, especially under line-toline fault conditions, can constitute an appreciable percentage of the total torque. This is pointed out by Concordia (see reference 14 of the paper, p. 140).We feel that a more standard nomenclature might have been followed for distinction between the so-called d-c currents and the various fundamental frequency currents and their symmetrical components.The paper might have mentioned the maximum radial sole plate forces due to short-circuiting of one-half of the field poles. Our water-wheel generator foundations are always designed to meet the forces resulting from this eventuality.Mr. Ruskin's paper should stimulate a great deal of interest in the subject of generator sole plate reaction. It appears to be particularly valuable in providing the utilities with a practical method of evaluating these forces as given by the manufacturers, and in drawing attention to the magnitude of forces which may be developed by faulty single-and 3-phase synchronizing.Vernon W. Ruskin: The simplified equations include the effect of that part of the second harmonic torque, which is caused by the interaction of the rotating field due to the negative-sequence armature currents with the fundamental component of the rotor field. The simplified equations do not include that part of the second harmonic torque due to saliency, which is the reluctance torque due to the trapped armature flux in a salient pole machine.5 For a lineto-line short circuit this part of the torque can be taken care of by the K factor developed in Appendix I.As to forces due to short circuit of onehalf of the field poles, this event requires two simultaneous grounds in the rotor circuit, so that one-half of the rotor periphery is dead while the remainder is still energized. While such an o^currence is very uncommon, there have been isolated cases of trouble from this cause. The discussers ivisehy-design generator foundations to take care of this eventuality.
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