Commutation Phenomena Research Articles

CALCULATION AND MEASUREMENT OF COMMUTATION CURRENTS IN DC MACHINES

ABSTRACT The present paper presents a method of resolving differential equations that proceed from the modelling of the commutation phenomena in a direct current machine. The validity of the method is discussed by comparing the calculated curves to the experimental ones. This study is limited to the case of perfectly smooth currents with brushes covering three commutator bars.

A MULTIPLE-SCALE APPROACH IN THE STUDY OF COMMUTATION PHENOMENA: APPLICATION TO A CHOPPER SUPPLYING A STEP MOTOR

ABSTRACT In this paper, it is shown how a multiple scale approach can contribute to the study of commutation phenomena in machine supply. A brief recall of the method is given and a typical example is detailed. This approach proves to be fruitful to get easily a good description of the dynamic behaviour as well as to reveal the prevailing effect of some parameters.

NATURAL COMMUTATION IN SOLID ROTOR SYNCHRONOUS MACHINES ASSOCIATED WITH STATIC CONVERTORS

Abstract In this paper the natural commutation phenomenon in solid rotor synchronous machines is treated. An analytical approach is used, and the effect of the solid iron parts in the rotor is considered by an approximate method. The decrement time functions of the commutation currents due to the iron body are determined for the general case of salient pole machines. The results show that the rotor eddy currents modify considerably the values of the armature commutation currents and their duration.

COMMUTATION TRANSIENTS IN SELF-CONTROLLED SYNCHRONOUS MACHINES CONNECTED TO D.C. NETWORKS

ABSTRACT In this paper the commutation phenomenon in a synchronous machine (generator or motor) associated with a thyristor bridge is studied. The transient analysis of the different currents in the different windings is done for a salient pole synchronous machine with damper windings. The different expressions are given in terms of the classical machine reactances and time constants which can be directly measured or furnished by the manufacturer. Comparison between theoretical and practical results are given.

Discussion on “Copper drag on commutators”, “Flashover on d.c. machines” and “Commutation phenomena in electrical machines. Part 2: Further experiments in flashover”

Discussion on “Copper drag on commutators”, “Flashover on d.c. machines” and “Commutation phenomena in electrical machines. Part 2: Further experiments in flashover”

Commutation phenomena in electrical machines. Part 1: Basic experiments in flashover

Work on an experimental turntable rig built to simulate a commutator has demonstrated that, as a direct voltage between the two brush arms is increased, (a) the tendency to form short arcs from a brush to the segment leaving the brush increases and (b) the persistence of the associated bar-to-bar arcs increase suddenly. Invariably, the intersegmental arcs are initiated at the positive brush. Once a continuous series of intersegmental (bar-to-bar) arcs exists between the brush arms, a flashover takes place. High-speed ciné photography has shown also that the intersegmental arcs increase rapidly in intensity and that the discharges are propelled into the air above the surface of the commutator to a distance of about 1 in by a process known as ‘jetting’. Under these conditions, the arc current is increasing extremely rapidly, producing a mass of ionised gas above the commutator. This behaviour, and the damage produced, is consistent with features noted on large machines after flashover, and it is likely that this mechanism of flashover applies to such machines also.Having established that the major flashover mechanism in the experimental rig involves a continuous string of intersegmental arcs, the basic properties of such arcs and means of extinguishing them rapidly were investigated. Clearly flashover can be prevented either by restricting the formation of short arcs or by extinguishing them quickly. Two simple mechanisms of extinguishing the arcs have been studied: an electromagnetic method which propels the arc rapidly along the gap to the edge of the commutator where it is extended and extinguished and a method using an airblast.

Commutation phenomena in electrical machines. Part 2: Further experiments in flashover

In an earlier paper, it was established that a major mechanism of flashover involved the generation of a series of intersegmental arcs, and further relevant work has been carried out on the properties and characteristics of such arcs and on means by which they may be extinguished. The dependence of the flashover voltage (per gap) on gap width, the velocity of sliding and the brush grade have been investigated. This voltage increases markedly with increasing gap width; but, except at low speeds, it is independent of the velocity for a given gap width. The flashover voltage was found to be only marginally dependent on the brush grade when brushes selected from the main types were tried. Sequences of ciné photographs showed a phenomenon known as ‘jetting’, in which molten copper, formed at an upper edge of a segment, is thrown outwards by the pressure of vaporising copper. The method considered whereby short arcs could be extinguished was based on a transverse magnetic field. Under the earlier conditions, the arcs accelerated uniformly; but it was suggested that a terminal velocity should be reached, which has been shown to be correct using additional apparatus. A formula describing the dependence of this terminal velocity on basic parameters has been derived. Further work has been carried out on the practical aspects of extinguishing short arcs by means of an air blast.

Commutation phenomena in a static power convertor

A systematic experimental and theoretical study was made of the effect of various inherent and extraneous circuit parameters on the commutation oscillations and on the period of commutation overlap in a static power convertor. The initiation of oscillations by a voltage jump occurring across a convertor device at the start and at the end of commutation overlap was analysed for a variety of conditions, and the theoretical deductions were experimentally verified. The period of commutation overlap in a convertor without extraneous capacitances and in steady-state operation was shown to be determined by the sum of the instantaneous values of direct current prevailing at the start and the end of the overlap. Enforced reductions of the period of commutation overlap were found to be obtained by an appropriate adjustment of the magnitudes of the relevant parameters in a controlled-rectifier circuit either with capacitors connected between the anodes of the convertor devices or with capacitors connected between the input terminals of the convertor transformer in the presence of series inductances in the a.c. supply lines.

Dissymmetry in D-C Rotating Electric Machines

A new technique for detecting dissymmetries built or developed in rotating electric machines is introduced. It is presented with the intent of interesting machine manufacturers to investigate the ideas developed and the degree of practical advances that may be realized by measuring, analyzing, and minimizing detrimental nonuniformities found in such rotating devices. The method promises a simple and more accurate test and final machine adjustment tool. Heretofore unsuspected causes of harmful effects are shown to be mechanical, magnetic, and electrical machine unbalances. The internal electric transients revealed throw new light on machine sizes, efficiencies, radio interference, carbon brush evaluating techniques, and application, as well as on the commutation phenomenon itself. It indicates that exact duplicate machines are rare accidents. The easing of present operating conditions for carbon brushes and extended life for the versatile d-c system are envisioned.

A new method for observing the phenomena of commutation

The paper describes a new method for measuring the armature coil current in a d.c. machine as it reverses during commutation. A small search coil, fixed relative to the armature, is magnetically coupled with one of the armature coils. The e.m.f. induced in the search coil, which is proportional to the rate of change of armature coil current, is integrated to give a signal proportional to the current itself, and this is displayed on the screen of a cathode-ray tube. The method overcomes several of the difficulties associated with the use of a shunt for measuring armature coil current during commutation.A method is described for triggering the oscillograph time-base which permits examination of the current reversals as they occur in a number of consecutive revolutions of the machine.The paper is illustrated with oscillograms showing thearmature coil current during commutation in a particular aircraft generator, and the cause of poor commutation in this machine is deduced.

Discussion on “A new method for observing the phenomena of commutation”

Discussion on “A new method for observing the phenomena of commutation”

Superposition applied to mechanical rectifier contacts

CONTACTS of mechanical rectifiers physically make and break connections between the a-c phase terminals and d-c terminals of the rectifier, but do not contribute to the important phenomena of commutation. Commutating reactors in series with the contacts are used to produce the required conditions during ?make? and ?break? steps in which the contacts are able to perform their limited functions. The main power circuits may be seen in heavy lines in Fig. 1. Make and break phenomena are equally important, but this paper deals only with break problems, the make problems having been adequately solved in modern rectifiers.

Commutation of Large D-C Motors and Generators [includes discussion

This paper presents a method of analysis of commutation phenomena and application of the method to calculate the voltage appearing from bar-to-bar on the commutator during commutation. This is the voltage absorbed by the brush contact and which is zero for uniform contact voltage over the contact area of the brushes. The voltage is calculated as a function of time for a 2,250-horsepower 250 rpm 600-volt motor and curves are shown and compared with curves obtained by oscillograph tests. Oscillograph curves were obtained by connecting slip rings to each of a group of commutator bars and recording voltage from ring-to-ring under various conditions of load. Abrupt pulsations of voltage under the brushes appear in the calculated and test results. It is shown that perfect commutation in the sense of complete compensation of reactance voltage is associated with pulses of brush contact voltage so that nonuniform, pulsing contact voltage is a normal condition. Calculations of the type presented constitute new criteria for successful commutation.

Current and Torque of D-C Machines on Short Circuit

This paper presents formulas, in terms of easily evaluated coefficients, for the armature current, field current, and torque of d-c machines on short circuit. The method of the paper makes it unnecessary to calculate the mutual inductance between the armature and field windings. Heretofore, this mutual inductance (between, two windings which normally are in quadrature with each other) has made it exceedingly difficult to calculate current and torque on short circuit. Under short-circuit conditions, the armature and field windings are not, in effect, in quadrature with one another and are mutually inductive partly because of saturation of the flux path at one side of the poles and partly because of commutation phenomena. These circumstances create difficulties uniquely overcome in the paper. A basic feature of the paper is the expression for the relation between flux, armature amperes, and field amperes. Calculations cannot be made without establishing this relationship (plotted for a specific case in Figure 9) and a simple expression for it has not been presented before to the author's knowledge. Another important feature, also believed to be new, is the development of an expression for the resistance to current flow created by the reactance voltage of the alternating current in the armature winding. A third feature is a formula for estimating the time constant of the armature circuit using an average coefficient of inductance. The results are applied to several machines and the calculated performance compared with test data.

Current and torque of D-C machines on short circuit

This paper presents formulas, in terms of easily evaluated coefficients, for the armature current, field current, and torque of d-c machines on short circuit. The method of the paper makes it unnecessary to calculate the mutual inductance between the armature and field windings. Heretofore, this mutual inductance (between the two windings which normally are in quadrature with each other) has made it exceedingly difficult to calculate current and torque on short circuit. Under short-circuit conditions, the armature and field windings are not, in effect, in quadrature with one another and are mutually inductive partly because of saturation of the flux path at one side of the poles and partly because of commutation phenomena. These circumstances create difficulties uniquely overcome in the paper. A basic feature of the paper is the expression for the relation between flux, armature amperes, and field amperes. Calculations cannot be made without establishing this relationship (plotted for a specific case in Figure 9) and a simple expression for it has not been presented before to the author's knowledge. Another important feature, also believed to be new, is the development of an expression for the resistance to current flow created by the reactance voltage of the alternating current in the armature winding. A third feature is a formula for estimating the time constant of the armature circuit using an average coefficient of inductance. The results are applied to several machines and the calculated performance compared with test data. In most cases, particularly with large machines having bar windings, approximate calculations can be made without need of manufacturer's data.

Discussion on “Commutation phenomena and magnetic oscillations occurring in direct-current machines”

Discussion on “Commutation phenomena and magnetic oscillations occurring in direct-current machines”

Commutation phenomena and magnetic oscillations occurring in direct-current machines

Commutation phenomena and magnetic oscillations occurring in direct-current machines

Some phenomena of commutation

Some phenomena of commutation

Discussion on “Some phenomena of commutation”

Discussion on “Some phenomena of commutation”