Lee A. Kilgore

Calculation of Synchronous Machine Constants- Reactances and Time Constants Affecting Transient Characteristics

Recent advances in the theory of synchronous machines have introduced a large number of new constants. The method of symmetrical components requires sequence reactances, and an accurate theory of transients requires transient and subtransient reactances and time constants. Most of the published discussion on the constants has been concerned with the application, rather than the calculation of values. In this paper, all of the most significant constants are calculated, except the subtransient time constant. A general method of calculation is discussed in which the reactances are accurately resolved into components. Comparisons of test and calculated values are given. The formulas for salient-pole machines and turbine generators are given in Appendixes A and B. The principle of superposition is applied to resolve accurately the reactances into components which can be readily calculated. The induced currents in the field and additional damping circuits are accounted for simply by applying the constant interlinkage theorem.

Transient Starting Torques in Induction Motors

A transient fundamental-frequency torque is shown to occur in the starting of all induction motors. For full-voltage starting on a general-purpose squirrel-cage motor, this torque may be as high as two or three times the pull-out torque. A consideration of such torques is particularly advisable for cases where induction motors are started and stopped frequently or continuously, since under such conditions mechanical failure of the motor or associated parts, gearing, couplings, etc., may occur if the stresses exceed the endurance limit of the material. A method of analysis is given for calculating the transient electrical torques acting in the case of a locked rotor; the results obtained by means of this analysis are compared with test data obtained on a special locked-rotor test setup. A method is also given for calculating shaft torque from the electrical torque in the usual application. Test results also indicate that this method is satisfactory for practical use.

Human factors in engineering

The opportunities open to engineers to move into positions of management are hampered by a lack of knowledge of the requisites of leadership and ignorance of the art of human relations. Basic rules are set forth that to be effective must be put into practice until they become habit.

Effects of Saturation on Machine Reactances

The effects of magnetic saturation on the various types of reactances used in calculations on synchronous machines are considered in this paper. Saturation factors for the important constants used in transient and unbalanced load calculations are presented in curve form. These data are taken from short-circuit tests on a large number of machines. The saturation factor for transient reactance under conditions encountered in stability calculations is difficult to test directly so the test data are supplemented by theoretical calculations.

Spring and Damping Coefficients of Synchronous Machines and Their Application

This paper develops simple formulas for spring and damping constants of synchronous machines. These formulas include the effects of induced currents in the damper winding and field. An approximation to the effect of primary resistance in producing a negative damping torque is given. An equivalent circuit is given which can represent accurately the mechanical system of several engine-driven generators or compressors operating in parallel. It is concluded that the conventional representation of synchronizing power as full-load power over full-load angle is sufficiently accurate for most cases, but that it is well to have a more accurate method for special cases.

Transient Torques in Synchronous Machines

The calculation of the alternating torque developed on single-phase short circuit has been discussed by previous authors.1,7 This paper extends the analysis of the electrical torque on short circuit to cover the transient torques due to losses, which although not so large as the alternating torques may be the most serious factor in the effects of short-circuit torques. The resulting mechanical torques in the various parts of the machine are analyzed, and some important conclusions reached: (1) That a rigid stator transmits all the electrical torque developed; while the shaft and coupling of a coupled set, or the springs in a spring-mounted stator in general need transmit only a fraction of the alternating components. The greater part of the alternating components of the electrical torque are absorbed by the inertias involved, when the natural frequency of torsional oscillation is below the rated frequency. This is the case always for spring-mounted stators, and almost always the case for coupled rotors. (2) That the sudden increase in torque on short-circuit conditions determined by losses in the negative sequence resistance (mainly the rotor losses) or in the resistance of the external circuit, may produce higher mechanical torques in shafts and couplings than the alternating components of the electrical torque. (3) That there are other transient conditions which may produce more serious mechanical torques than sudden short circuit.

Regulation of grid-controlled rectifier

This paper discusses several factors which have not been included in the previously published regulation formulas and which should be considered in the practical application of rectifiers. Approximate solutions are given to take system reactance, and the load inductance into account. The effect of grid pickup characteristics on regulation, and means of modifying the inherent regulation characteristics by regulators or compensators are discussed.

A Study of the Modified Kramer or Asynchronous-Synchronous Cascade Variable-Speed Drive

LARGE wind tunnels require a wide range of speed and accurate speed control and if their use factor is high, a high efficiency over the working range is desirable. Also minimum disturbance to the power system is important in many cases. To meet these requirements a speed-control system has been adopted which, while not new in principle, involved the solution of a number of interesting problems. The system as shown in Figure 1 consists of a wound-rotor driving motor (A) whose secondary winding feeds a synchronous motor (S1) driving a variable-speed d-c generator (DC1) which in turn drives a d-c — a-c set (DC2 and S2) to return most of the secondary power back to the line. The name “modified Kramer set” is suggested by the authors since the scheme involves conversion of the secondary power to d-c and field control for the speed changes as in the well-known Kramer set. The term “modified” was used because the Kramer set used a rotary converter and the d-c power was usually fed into a d-c motor on the same shaft as the main motor. Another descriptive name would be asynchronous-synchronous cascade.

Variable-Speed Drive for United States Army Air Corps Wind Tunnel at Wright Field, Dayton, Ohio

THE machinery manufacturers are being required to solve many design and production problems in connection with our national-defense program. All electrical manufacturing companies are exerting maximum effort to produce generators, motors, conversion apparatus, and other electrical equipment which are needed in meeting the requirements of a large and rapidly expanding industrial activity. There is a further need for machines of greater capacity, new combinations of apparatus and control devices, and more information on machine characteristics to accomplish new and difficult objectives.

A 3-million-kva short-circuit generator

The need for an additional 3 million kva of short-circuit capacity for a new circuit breaker laboratory was met by the construction of a 3,600-rpm generator with very low reactance. The general philosophy of designing for low reactance, which led to the two-pole design, is discussed and the various electrical and mechanical features of the generator are described