In using the conception of electric quantity Q, we consider only the terminals of the lines of dielectric flux, that is, deal merely with the effect of the dielectric flux on the electric circuit which produced it. This conception is in many cases more convenient, but it necessarily fails, when the distribution of the dielectric flux in the dielectric field is of importance, such as is the case when dealing with high dielectric field intensities, approach- ing the possibility of disruptive effects in the field of force, or when dealing with the effect produced by the introduction of ma- terials of different permittivity into the dielectric field. There- fore, with the increasing importance of the dielectric field in engineering, the conception of electric quantity, or charge, is gradually being replaced by the conception of the dielectric flux and the dielectric field, analogous to the magnetic field, wh…
Dielectricity And Capacity Passage Atlas
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Why This Theme Matters
Section titled “Why This Theme Matters”This passage may show how capacity, charge, displacement, or dielectric storage enter circuit theory.
Candidate Passages
Section titled “Candidate Passages”A simple harmonic oscillation as a line discharge would require a sinoidal distribution of potential on the transmission line at the instant of discharge, which is not probable, so that probably all lightning discharges of transmission lines or oscillations produced by sudden changes of circuit conditions are complex waves of many harmonics, which in their relative magnitude depend upon the initial charge and its distribution - that is, in the case of the lightning discharge, upon the atmospheric electrostatic field of force.
A simple harmonic oscillation as a line discharge would require a sinoidal distribution of potential on the trans- mission line at the instant of discharge, which is not proba- ble, so that probably all lightning discharges of transmission lines or oscillations produced by sudden changes of circuit conditions are complex waves of many harmonics, which in their relative magnitude depend upon the initial charge and its distribution - that is, in the case of the lightning dis- charge, upon the atmospheric electrostatic field of force.
between iron or magnetite terminals, and requiring about 75 volts, is white and very brilliant, that is, has a spectrum with many lines about uniformly distributed over the visible range. We can greatly increase the temperature of the arc by using a high-frequency condenser discharge: in this case very large currents of very short duration exist as oscillations between the terminals, with periods of rest between the oscillations, very long compared with the duration of the current. In this case the duration of the current is too short to feed a large volume of electrode vapor into the arc stream, and as the current is very large during the short moment of the discharge, the vapor between the terminals is very greatly overheated. Oscil- lating condenser discharges thus offer a means of increasing the temperature of the arc stream very greatly beyond the boiling point of the material. When using a condense…
22. Usually in electric circuits; current, voltage, the magnetic field and the dielectric field are proportional to each other, and the transient thus is a simple exponential, if resulting from one form of stored energy, as discussed in the preceding lectures. This, how- ever, is no longer the case if the magnetic field contains iron or other magnetic materials, or if the dielectric field reaches densities beyond the dielectric strength of the carrier of the field, etc.; and the proportionality between current or voltage and their respective fields, the magnetic and the dielectric, thus ceases, or, as it may be expressed, the inductance L is not constant, but varies w^ith the current, or the capacity is not constant, but varies with the voltage.
22. Usually in electric circuits, current, voltage, the magnetic field and the dielectric field are proportional to each other, and the transient thus is a simple exponential, if resulting from one form of stored energy, as discussed in the preceding lectures. This, how- ever, is no longer the case if the magnetic field contains iron or other magnetic materials, or if the dielectric field reaches densities beyond the dielectric strength of the carrier of the field, etc. ; and the proportionality between current or voltage and their respective fields, the magnetic and the dielectric, thus ceases, or, as it may be expressed, the inductance L is not constant, but varies with the current, or the capacity is not constant, but varies with the voltage.
circuit current during the fraction of the half-wave, which the discharge over the multi-gap arrester lasts, is moderate, due to its very short duration, and can easily be absorbed and radiated by the arrester; so that even if lightning discharges rapidly follow each other for some time, they can be taken care of by the arrester with moderate temperature rise : assuming a vicious thunder storm, in which lightning flashes succeed each other practically continuously, several per second. Each discharge causes a short circuit over the lightning arrester, varying in duration from nearly a half-wave - if the discharge occurs at the beginning of a half-wave - to practically nothing - if the discharge takes place near the end of a half-wave - ^that is, in average, for one-half of one-half wave, or :- • sec, in a 60 cycle system. Therefore from two to three lightning dis- charges per second would still short circ…
former, the high-tension switches are opened at the generator end of the transmission line. The energy stored magnetically and dielectrically in line and transformer then dissipates by a transient, as shown in the oscillogram Fig. 41. This gives the oscillation of a circuit consisting of 28 miles of line and 2500-kw. 100-kv. step-up and step-down transformers, and is produced by discon- necting this circuit by low-tension switches. In the transformer, the duration of the transient would be very great, possibly several seconds, as the stored magnetic energy (L) is very large, the dis- sipation of power (r and g) relatively small; in the line, the tran- sient is of fairly short duration, as r (and g) are considerable. Left to themselves, the line oscillations thus would die out much more rapidly, by the dissipation of their stored energy, than the transformer oscillations. Since line and transformer are co…
former, the high-tension switches are opened at the generator end of the transmission line. The energy stored magnetically and dielectrically in line and transformer then dissipates by a transient, as shown in the oscillogram Fig. 41. This gives the oscillation of a circuit consisting of 28 miles of line and 2500-kw. 100-kv. step-up and step-down transformers, and is produced by discon- necting this circuit by low-tension switches. In the transformer, the duration of the transient would be very great, possibly several seconds, as the stored magnetic energy (L) is very large, the dis- sipation of power (r and g) relatively small; in the line, the tran- sient is of fairly short duration, as r (and g) are considerable. Left to themselves, the line oscillations thus would die out much more rapidly, by the dissipation of their stored energy, than the transformer oscillations. Since line and transformer are co…
The alternation of the field flux induces an e.m.f. of self induction in the field winding. In the shunt motor, this causes the field exciting current and with it the magnetic field flux to lag and thereby to be out of phase with the armature current which, to represent work, must essentially be an energy current, and thereby reduces output and efficiency and hence requires some method of compensation, as capacity in series with the field winding or excitation of the field from a quadrature phase of voltage. In the series motor the self-inductance of the field causes the main current to lag behind the impressed voltage and thereby lowers the power-factor of the motor. Thus, to get good power-factor, the field self-inductance must be made low, that is, the field as weak and the armature as strong as possible. With such a strong armature, and weak field, the commutating pole is not sufficient to control ma…
67. In the theoretical investigation of electric circuits the velocity of propagation of the electric field through space is usually not considered, but the electric field assumed as instan- taneous throughout space; that is, the electromagnetic com- ponent of the field is considered as in phase with the current, the electrostatic component as in phase with the voltage. In reality, however, the electric field starts at the conductor and propa- gates from there through space with a finite though very high velocity, the velocity of light; that is, at any point in space the electric field at any moment corresponds not to the condi- tion of the electric energy flow at that moment but to that at a moment earlier by the time of propagation from the conductor to the point under consideration, or, in other words, the electric field lags the more, the greater the distance from the conductor.
dielectric flux, dielectric field intensity, permittivity, as used in dealing with the electrostatic fields of high potential apparatus, as transmission insulators, transformer bushings, etc. The fore- most difference is that in the magnetic field, a line of force must always return into itself in a closed circuit, while in the electro- static or dielectric field, a line of force may terminate in a con- ductor. The terminals of the lines of electrostatic flux, ^ at the conductor, then are represented by the conception of a quantity of electricity or electric charge, Q, being located on the con- ductor. Thus, at the terminal of the line of unit dielectric flux, unit electric quantity is located on the conductor.
To estimate the current which discharges in the lightning flash, the conductivity of air in the path of the discharge, and the diameter of the discharge are required, and as both are unknown, any estimate must be very approximate only. The specific resistance of gases and vapors decreases with increasing temperature and with decreasing pressure. It is a few ohm centimeters at atmospheric pressure and the high temperature of the magnetite or carbon arc, and is also a few ohm centimeters at the low temperature and low pressure of a high current Geissler tube discharge. The mercury arc stream also gives a specific resistance of a few ohms. The temperature of the air in the lightning discharge probably is moderately high, but the pressure is also not far from atmos- pheric, so that lOO ohm centimeters may not be very far from the true magnitude of the resistance. Estimating one to two feet as the diameter of…
44. Such circuits with distributed series capacity are of great interest in that it is probable that lightning flashes in the clouds are discharges in such circuits. From the distance traversed by lightning flashes in the clouds, their character, and the disruptive strength of air, it appears certain that no potential difference can exist in the clouds of such magnitude as to cause a disruptive discharge across a mile or more of space. It is probable that as the result of condensation of moisture, and the lack of uni- formity of such condensation, due to the gusty nature of air currents, a non-uniform distribution of potential is produced between the rain drops in the cloud; and when the potential gradient somewhere in space exceeds the disruptive value, an oscillatory discharge starts between the rain drops, and grad- ually, in a number of successive discharges, traverses the cloud and equalizes the pot…
Assuming a thunder cloud to pass over the line. The ground below the cloud then assumes an electrostatic charge, corresponding to the opposite charge of the cloud. The trans- mission line, as part of the ground, thus also assumes a static charge, higher than that of the ground, since it projects above it. Any equalization of the potential distribution in the cloud by a lightning flash, as discussed in the preceding, requires a change in the electrostatic charge of the line, corresponding to the changed potential difference between ground and cloud above the ground, and the static charge thus set free on the line rushes as an impulse or wave along the line. The wave shape of such impulses induced by cloud discharges is in general not a smooth sine wave, but may be very irregular : during the equalization of the cloud potential by the lightning flash, the potential difference against ground, of the part of…
The wave length of oscillation thus depends on the length of the circuit in which the stored energy readjusts itself. For instance, in the short circuit oscillation of the system, the wave extends over the entire circuit, including generators and trans- formers ; and the entire circuit so represents one wave, or one- half wave, that is, the wave length is very considerable. If the readjustment of stored energy takes place only over a section of the circuit, the wave length is shorter. For instance, if by a thunder cloud a static charge is induced on the trans- mission line, and by a lightning flash in the cloud, the cloud discharges, the electrostatic charge induced by it on the line
In an ununiform electric conductor, the relation of the voltage to the length of the conductor does not determine whether the conductor is safe or whether locally, due to small cross-section or high resistivity, unsafe current densities may cause destructive heating, but the adaptability of the conductor to the current carried by it must be considered throughout its entire length. So in the dielectric field, the thickness of the dielectric may be such that the voltage impressed upon it may give a very safe average voltage gradient or average dielectric flux density, and the dielectric nevertheless may break down, due to local concen- tration of the dielectric flux density in the insulating material. Thus, for instance, in the dielectric field between parallel con- ductors, at a voltage far below that which would jump from conductor to conductor, locally at the conductor surface the concentration of elect…
Naturally, as soon as determinations of spark voltages became available, attempts were made to estimate the voltage of a lightning flash. Considering, in a lightning flash, the dis- charge as that in an ununiform field, similar to that between needle points, and so requiring about 10,000 volts per inch. In this case, a lightning flash of two miles, or about 10,000 feet length, would require a potential difference of about 1200 million volts. The existence of such voltages in the clouds does not appear possible: a potential difference of 1000 mil- lion volts would produce a brush discharge of about one-half mile in length, before the final lightning flash occurs. In the brush discharge the air is electrically broken down, and becomes conducting. But it is also mechanically and chemically broken down, that is, the molecules are dissociated and recombine after the discharge, in all possible combinations. Th…
The shunt motor on an alternating-current circuit has the objection that in the armature winding the current should be power current, thus in phas£ with the e.m.f., while in the field winding the current is lagging nearly 90 deg., as magnetizing current. Thus field and armature would be out of phase with each other. To overcome this objection either there is inserted in series with the field circuit a condenser of such capacity as to bring the current back into p>hase with the voltage, or the field may be excited from a separate e.m.f. differing 90 deg. in phase from that supplied to the armature. The former arrange- ment has the disadvantage of requiring almost perfect con- stancy of frequency, and therefore is not practicable. In the latter arrangement the armature winding of the motor is fed by one, the field winding by the other phase of a quarter-phase sys- tem, and thus the current in the armature…
causes an oscillation in which the lower frequencies predominate, that is, a low-frequency high-power surge. A spark discharge from the line, a sudden high voltage charge entering the line locally, as directly by a lightning stroke, or indirectly by induc- tion during a lightning discharge elsewhere, gives a distribution of potential which momentarily is very non-uniform, changes very abruptly along the line, and thus gives rise mainly to very high harmonics, but as a rule does not contain to any appre- ciable extent the lower frequencies; that is, it causes a high- frequency oscillation, more or less local in extent, and while of high voltage, of rather limited power, and therefore less destruc- tive than a low-frequency surge.
To limit the machine current which followed the light- ning discharge, and so enable the lightning arrester to open the discharge circuit, series resistance was introduced in the arrester. Series resistance, however, also limited the discharge current, and with very heavy discharges, such lightning arresters with series resistance failed to protect the circuits, that is, failed to discharge the abnormal voltage without destructive pressure rise. This difficulty was solved by the introduction of shunted resistances, that is, resistances shunt- ing a part of the spark gaps. All the minor discharges then pass over the resistances and the unshunted spark gaps, the
[[PDF_PAGE:49]] Report of Charles P. Steinmetz 43 Denotations e = nominal induced E. M. F. of alternator or group of alternators. Xii = true self inductive reactance of alternator or group of alter- nators. xn= external reactance of alternator or group of alternators, thus. Xi = xn+Xi2 = total self inductive reactance of alternators or group of alternators. xj = effective reactance of armature reaction of alternator or group of alternators, thus: Xo=xn+x 2 synchronous reactance of alternator or group of alternators. x = reactance (or impedance) between alternators. z = impedance of circuit between alternators. = Vi- 2+(2x!+x) 2 , where r = resistance of circuit between alternators. Or approximately a = phase angle of circuit between alternators, where: tan a = Or approximately : a = 90 degrees. w = phase displacement from mean, of oscillating alternators, thus: 2co = total phase displacement of oscillati…
Magnetic and dielectric fields are usually combined, since where there is a current producing a magnetic field there is a voltage producing a dielectric field. Thus the space surrounding a conductor carrying an electric current is an electromagnetic field - that is, a combination of a magnetic field, concentric with the conductor, and a dielectric field, radial to the conductor.
The frequency of oscillations occurring in electric cir- cuits varies over an enormous range: from low frequencies, very little above alternator frequency, up to hundreds of mil- lions of cycles per second ; and the effect of the oscillations in the system therefore varies accordingly: from the relatively harmless static displays; brush discharges, streamers, sparks, etc., of extremely high frequencies, down to the disastrous high power low frequency short circuit oscillations, in which even in 10,000 volt system*^, currents ^i many thousands of amperes may surge, which voltages approaching 100,000, and with which no protective device can cope, which does not have unlimited discharge capacity, that is, contains no resistance whatever in the discharge path.
extensively used, and monocyclic generators built. These were .^iriulr-pluisi’ alternating-eurrenl generators, having a small quadrature phase of high inductance, which combined with the main phase gives three-phase or quarter- phase voltages. The auxiliary phase was of such high reactance as to limit the quadra- < i < ti ■ poWCI and thus make the flow of energy essentially single- phase, that is, monocyclic. The purpose hereof was to permit the use of a small quadrature coil on the generator, and thereby to preserve the whole generator capacity for the single-phase main voltage, without danger of overloading the quadrature phase in case of a high motor load on the system. The genera] introduction of the three-phase system superseded the mono- cyclic generator, and monocyclic devices are today used only for local production of polyphase voltages from single-phase supply, for the starting of small siliEle…
8. Of the amount of energy consumed in creating the electric field of the circuit not all is returned at the disappearance of the electric field, but a part is consumed by conversion into heat in producing or in any other way changing the electric field. That is, the conversion of electric energy into and from the electromagnetic and electrostatic stress is not complete, but a loss of energy occurs, especially with the magnetic field in the so-called magnetic materials, and with the electrostatic field in unhomogeneous dielectrics.
that is, the effective value of the discharge current is propor- tional to the condenser potential, e0, proportional to the square root of the capacity, C, and the frequency of charge, fv and inversely proportional to the square root of the resistance, r0, of the discharge circuit; but it does not depend upon the induc- tance L0 of the discharge circuit, and therefore does not depend on the frequency of the discharge oscillation. The power of the discharge is
In the series repulsion motors, 6 and 7, a quadrature field also exfsts, just as in the repulsion motors, but this quadrature field depends upon that part of the total voltage which is impressed upon the commutating winding, C, and thus can be varied by varying the distribution of supply voltage between the two cir- cuits; hence, in this type of motor, the commutating flux can be maintained through all (higher) speeds by impressing the total voltage upon the compensating circuit and short-circuiting the armature circuit for all speeds up to that at which the required commutating flux has decreased to the quadrature, flux given by the motor, and from this speed upward only a part of the supply voltage, inversely proportional (approximately) to the square of the speed, is impressed upon the compensating circuit, the rest shifted over to the armature circuit. The difference between 6 and 7 is that in 6 the…
80. As the result of the phenomena discussed in the preceding chapters, conductors intended to convey currents of very high frequency, as lightning discharges, high frequency oscillations of transmission lines, the currents used in wireless telegraphy, etc., cannot be calculated by the use of the constants derived at low frequency, but effective resistance and inductance, and therewith the power consumed by the conductor, and the voltage drop, may be of an entirely different magnitude from the values which would be found by using the usual values of resistance and induc- tance. In conductors such as are used in the connections and the discharge path of lightning arresters and surge protectors, the unequal current distribution in the conductor (Chapter VII) and the power and voltage consumed by electric radiation, due to the finite velocity of the electric field (Chapter VIII), require con- sideration.
outer ones adjustable and set for about ^ in. gap. This lamp is connected across a high voltage 0.2-mf. mica condenser C, which is connected to the high voltage terminal of a small step-up trans- former T giving about 15,000 volts (200 watts, 110 •*- 13,200 volts). The low tension side of the transformer is connected to the 240-volt 60-cycle circuit through a rheostat R to limit the current. The transformer charges the condenser, and when the voltage of the condenser has risen sufficiently high it discharges through the spark gaps I by an oscillation of high frequency (about 500,000 cycles), then charges again from the transformer, discharges through the gap, etc. As several such condenser dis- charges occur during each half wave of alternating supply voltage the light given by the discharge appears continuous.
There obviously is no more sense in thinking of the capacity current as current which charges the conductor with a quantity of electricity, than there is of speaking of the inductance voltage as charging the conductor with a quantity of magnetism. But while the latter conception, together with the notion of a quantity of magnetism, etc., has vanished since Faraday’s representation of the magnetic field by the lines of magnetic force, the termi- nology of electrostatics of many textbooks still speaks of electric charges on the conductor, and the energy stored by them, without considering that the dielectric energy is not on the surface of the conductor, but in the space outside of the conductor, just as the magnetic energy.
There obviously is no more sense in thinking of the capacity current as current which charges the conductor with a quantity of electricitj^, than there is of speaking of the inductance voltage as charging the conductor with a quantity of magnetism. But while the latter conception, together with the notion of a quantity of magnetism, etc., has vanished since Faraday’s representation of the magnetic field b}^ the lines of magnetic force, the termi- nology of electrostatics of many textbooks still speaks of electric charges on the conductor, and the energy stored by them, without considering that the dielectric energy is not on the surface of the conductor, but in the space outside of the conductor, just as the magnetic energy.
Assume now that a ground, P, is brought near one of the IjneBi A, to within the striking distance of the voltage, e. A dischaigB then occurs over the conductor, P. Such may occur by the puiu>* ture of a line insulator as not infrequently the case. Let r «■ re- sistance of discharge path, P. While without this discharge path, the voltage between A and C would be ei = e (assuming sini^ phase circuit) with a grounded conductor, P, approaching line A within striking distance of voltage, e, a discharge occurs over P forming an arc, and the circuit of the impressed voltage, 2 s, now comprises the condenser, C2, in series to the multiple circuit of con- denser, Ci, and arc, P, and the condenser, Ci, rapidly discharges^ voltage, eij decreases, and the voltage, 62, increases. With a de- crease of voltage, ei, the discharge current, i, also decreaseSi and the voltage consumed by the discharge arc, e’, increases un…
in generator, line, and load does not represent the entire phenome- non. While electric power flows over the line A , there is a magnetic field surrounding the line conductors, and an electrostatic field issuing from the line conductors. The magnetic field and the electrostatic or “dielectric ” field represent stored energy. Thus, during the permanent conditions of the flow of power through the circuit Fig. 3, there is electric energy stored in the space surround- ing the line conductors. There is energy stored also in the genera- tor and in the load ; for instance, the mechanical momentum of the revolving fan in Fig. 1, and the heat energy of the incandescent lamp filaments. The permanent condition of the circuit Fig. 3 thus represents not only flow of power, but also storage of energy. When the switch S is open, and no power flows, no energy is stored in the system. If we now close the switch, before t…
in generator, line, and load does not represent the entire phenome- non. While electric power flows over the line A, there is a magnetic field surrounding the line conductors, and an electrostatic field issuing from the line conductors. The magnetic field and the electrostatic or ” dielectric ” field represent stored energy. Thus, during the permanent conditions of the flow of power through the circuit Fig. 3, there is electric energy stored in the space surround- ing the line conductors. There is energy stored also in the genera- tor and in the load ; for instance, the mechanical momentum of the revolving fan in Fig. 1, and the heat energy of the incandescent lamp filaments. The permanent condition of the circuit Fig. 3 thus represents not only flow of power, but also storage of energy. When the switch S is open, and no power flows, no energy is stored in the sj^stem. If we now close the switch, before…
For instance, if in an isolated high-potential transmission line, the ground is brought within striking distance of one of the line conductors - as by the puncture of an insulator. A spark dis- charge then occurs to the ground, and the arc following the spark discharges the line by a transient oscillation, that is, brings it down to ground potential (and the other two lines, in a three- phase sj^stem, then correspondingly rise in voltage to ground, from the Y to the delta voltage). As soon as the line is dis- charged the arc ceases, that is, the spark gap to ground opens, and the line then charges again, from the power supply of the system, and its voltage to ground rises, until sufficient to jump to ground again and start a second transient oscillation, and so on continual transient oscillations follow each other, as a ^‘con- tinual transient,” or ”arcing ground.” Oscillograms, Figs. 59 and 60, show su…