Theory and Calculation of Electrical Apparatus (1917) — part 11 of 21

In the reverse conversion, however, distributing a single-phase load between phases of a polyphase system, the voltage of the generator phase of the converter must be higher, that of the motor phase lower than that of the polyphase system, and as the gen- erator phase is lower in voltage than the motor phase, it follows, that the phase converter transfers energy only when the poly- phase system has become unbalanced by more than the voltage drop in the converter. That is, while a phase converter may reduce the unbalancing due to single-phase load, it can never restore complete balance of the polyphase system, in voltage and in the flow of power. Even to materially reduce the unbalancing, requires large converter capacity and very close voltage regula- tion of the converter, and thus makes it an uneconomical machine. . To balance a polyphase system under single-phase load, there- fore, requires the addition of a phase balancer to the phase converter. Usually a synchronous phase balancer, would be employed in this case, that is, a small synchronous machine of opposite phase rotation, on the shaft of the phase converter, and connected in series thereto. Usually it is connected into the neutral of the phase converter. By the phase balancer, the voltage of the motor phase of the phase converter is raised above the generator phase so as to give a power transfer sufficient

232

ELECTRICAL APPARATUS

to balance the polyphase system, that is, to shift half of the single phase power by a quarter period, and thus produce a uniform flow of power.

Such synchronous phase balancer constructively is a synchro- nous machine, having two sets of field poles, A and B , in quad- ' rature with each other. Then by varying or reversing the excitation of the two sets of field poles, any phase relation of the reversely rotating polyphase system of the balancer to that of the converter can be produced, from zero to 360°.

137 . Large single-phase powers, such as are required for single- phase railroading, thus can be produced.

(a) By using single-phase generators and separate single-phase supply circuits.

(b) By using single-phase generators running in multiple with the general three-phase system, and controlling voltage and me- chanical power supply so as to absorb the single-phase load by the single-phase generators. In this case, however, if the single- phase load uses the same transmission line as the three-phase load, phase balancing at the receiving circuit may be necessary.

(c) By taking the single-phase load from the three-phase system. If the load is considerable, this may require special construction of the generators, and phase balancers.

(cZ) By taking the power all as balanced three-phase power from the generating system, and converting the required amount to single-phase, by phase converter and phase balancer. This may be -done in the generating station, or at the receiving station where the single-phase power is required.

Assuming that in addition to a balanced three-phase load of power, P 0 , a single-phase load of power, P, is required. Estimating roughly, that the single-phase capacity of a machine structure is half the three-phase capacity of the structure — which probably is not far wrong — then the use of single-phase generators gives us Po-kw. three-phase, and P-kw. single-phase generators, and as the latter is equal in size to 2 P-kw. three-phase capacity, the total machine. capacity would be P 0 + 2P.

Three-phase generation and phase conversion would require Po + P kw. in three-phase generators, and phase converters transferring half the single-phase power from the phase which is loaded by single-phase, to the quadrature phase. That is, the phase converter must have a capacity of P/2 kw. in the motor phase, and P/2 kw. capacity in the generator phase, or a total

PEASE CONVERSION

233

capacity of P kw. Thus the total machine capacity required for "both kinds of load would again be P 0 + 2 P kw. three-phase rating.

Thus, as regards machine capacity, there is no material differ- ence between single-phase generation and three-phase genera- tion with phase conversion, and the decision which arrangement is preferable will largely depend on questions of construction and operation. A more complete discussion on single-phase genera- tion and phase conversion is given in A. I, E. E, Transactions, November, 1916.

CHAPTER XV

SYNCHRONOUS RECTIFIER

Self-compounding Alternators — Self-starting Synchro- nous Motors — Arc Rectifier — Brush and Thomson Houston Arc Machine — Leblanc Panchahuteur — Permutator — Synchronous Converter

138 . Rectifiers for converting alternating into direct current have been designed and built since many years. As mechanical rectifiers, mainly single-phase, they have found a limited use for small powers since a long time, and during the last years arc rectifiers have found extended use for small and moderate powers, for storage-battery charging and for series arc lighting by constant direct current. For large powers, however, the rectifier does not appear applicable, but the synchronous converter takes its place. The two most important types of direct-current arc-light ma- chines, however, have in reality been mechanical rectifiers, and for compounding alternators, and for starting synchronous motors, rectifying commutators have been used to a considerable extent.

Let, in Fig. 72, e be the alternating voltage wave of the supply source, and the connections of the receiver circuit with this sup- ply source be periodically and synchronously reversed, at the zero points of the voltage wave, by a reversing commutator driven by a small synchronous motor, shown in Fig. 73. In the receiver circuit the voltage wave then is unidirectional but pul- sating, as shown by e 0 in Fig. 74.

If receiver circuit and supply circuit both are non-inductive, the current in the receiver circuit is a pulsating unidirectional current, shown as i Q in dotted lines in Fig. 74, and derived from the alternating current, i, Fig. 72, in the supply circuit.

If, however, the receiver circuit is inductive, as a machine field, then the current, i 0 , in Fig. 75, pulsates less than the voltage, e Q , which produces it, and the current thus does not go down to zero, but is continuous, and its pulsation the less, the higher the in- ductance. The current, i, in the alternating supply circuit, how-

236

ELECTRICAL APPARATUS

ever, from which the direct current, i 0 , is derived by reversal, must go through zero twice during each period, thus must have the shape shown as i in Fig. 76, that is, must abruptly reverse. If, however, the supply circuit contains any self-inductance — and every circuit contains some inductance — the current can not change instantly, but only gradually, the slower, the higher the inductance, and the actual current in the supply circuit assumes

Fig. 77. — Differential current on rectifier on inductive load.

a shape like that shown in dotted lines in Fig. 76. Thus the cur- rent in the alternating part and that in the rectified part of the circuit can not be the same, but a difference must exist, as shown as i' in Fig. 77. This current, i', passes between the two parts

Fig. 78. — Rectifier with A.C. and D.C. shunt resistance for inductive load

of the circuit, as arc at the rectifier brushes, and causes the recti- fying commutator to spark, if there is any appreciable inductance in the circuit. The intensity of the sparking current depends on the inductance of the rectified circuit, its duration on that of the alternating supply circuit.

By providing a byepath for this differential current, i r , the sparking is mitigated, and thereby the amount of power, which can be rectified, increased. This is done by shunting a non-inductive resistance across the rectified circuit, r 0 , or across the alternating circuit, r, or both, as shown in Fig. 78. If this resistance is low, it consumes considerable power and finally increases sparking

SYNCHRONOUS RECTIFIER

2X7

by the increase of rectified current; if it is high, it has little effect. Furthermore, this resistance should vary with the current.

The belt-driven alternators of former days frequently had a compounding series field excited by such a rectifying commutator on the machine shaft, and by shunting 40 to 50 per cent, of the power through the two resistance shunts, with careful setting of brushes as much as 2000 watts have been rectified from single- phase 125-cycle suppty.

Single-phase synchronous motors were started by such recti- fying commutators through which the field current passed, in series with the armature, and the first long-distance power trans-

Fig. 79. — Open-circuit rectifier. Fig. 80. — Short-circuit rectifier.

mission in America (Telluride) was originally operated with single-phase machines started by rectifying commutator — the commutator, however, requiring frequent renewal.

139 . The reversal of connection between the rectified circuit and the supply circuit may occur either over open-circuit, or over short-circuit. That is, either the rectified circuit is first disconnected from the supply circuit — which open-circuits both — and then connected in reverse direction, or the rectified circuit is connected to the supply circuit in reverse direction, before being disconnected in the previous direction — which short-circuits both circuits. The former, open-circuit rectification, results if the width of the gap between the commutator segments is greater than the width of the brushes, Fig. 79, the latter, short-circuit rectification, results if the width of the gap is less than the width of the brushes, Fig. 80.

In open-circuit rectification, the alternating and the rectified voltage are shown as e and e 0 in Fig. 81. If the circuit is non- inductive, the rectified current, io , has the same shape as the vol-

I

238 ELECTRICAL APPARATUS

tage, eo, but the alternating current, i, is as shown in Fig. 81 as i. If the circuit is inductive, vicious sparking occurs in this case with open-circuit rectification, as the brush when leaving the

Fig. 81 . — Voltage and current waves in open-circuit' rectifier on non-induc- tive load.

commutator segment must suddenly interrupt the current. That is, the current does not stop suddenly, but continues to flow as an arc at the commutator surface, and also, when making con-

Fig. 82 . — Voltage and current wave in open-circuit rectifier on inductive load, showing sparking.

tact between brush and segment, the current does not instantly reach full value, but gradually, and the current wave thus is as shown as i and i 0 in Fig. 82, where the shaded area is the arcing current at the commutator.

Sparkless rectification may be produced in a circuit of moderate

SYNCHRONOUS RECITFIER

239

inductance, with open-circuit rectification, by shifting the brushes so that the brushes open the circuit only at the moment when the (inductive) current has reached zero value or nearly so, as

Fig. 83. — Voltage waves of open-circuit rectifier with shifted brushes.

shown in Figs. 83 and 84. In this case, the brush maintains con- tact until the voltage, e , has not only gone to zero, but reversed sufficiently to stop the current, and the rectified voltage then is shown by e 0 in Fig. 83, the current by i and i 0 in Fig. 84.

Fig. 84. — Current waves of open-circuit rectifier with shifted brushes.

140. With short-circuit commutation the voltage waves are as shown by e and eo in Fig. 85. With a non-inductive supply and non-inductive receiving circuit, the currents would b$ as shown by i and i 0 in Fig. 86. That is, during the period of short-circuit,

SYNCHRONOUS RECTIFIER

241

the current in the rectified circuit is zero, and is high, is the short- circuit current of the supply voltage, in the supply circuit.

Inductance in the rectified circuit retards the dying out of the current, but also retards its rise, and so changes the rectified current wave to the shapes shown — for increasing values of in- ductance — as io in Figs. 87, 88 and 89.

Fig. 88. — Current waves of short-circuit rectifier on inductive load at the

stability limit.

Inductance in the supply circuit reduces the excess current value during the short-circuit period, and finally entirely elimi- nates the current rise, but also retards the decrease and reversal of the supply current, and the latter thus assumes the shapes shown — for successively increasing values of inductance — as i in Figs. 87, 88 and 89.

Fig. 89. — Current waves of short-circuit rectifier on highly inductive load, showing sparking but no flashing.

As seen, in Figs. 80 and 87, the alternating supply current has during the short-circuit reversed and reached a value at the end of the short-circuit, higher than the rcetified current, and at the moment when the brush leaves the short-circuit, a considerable current has to be broken, that is, sparking occurs. In Figs. 86 and 87, this differential current which passes as arc at the com- mutator, is shown by the dotted area. It is increasing with in-

242

ELECTRICAL APPARATUS

creasing spark length, that is, the spark or arc at the commutator has no tendency to go out — except if the inductance is very small — but persists : flashing around the commutator occurs and short- circuits the supply permanently.

Fig. 90. — Voltage wave of short-circuit rectifier with shifted brushes.

In Fig. 89, the alternating current at the end of the short- circuit has not yet reversed, and a considerable differential current, shown by the dotted area, d, passes as arc. Vicious

Fig. 91. — Current waves of short-circuit rectifier with inductive load and the brushes shifted to give good rectification.

sparking thus occurs, but in this case no flashing around the commutator, as with increasing spark length the differential current decreases and finally dies out.

In Fig. 88, the alternating current at the end of the short- circuit has just reached the same value as the rectified current,

SYNCHRONOUS RECTIFIER

243

thus no current change and no sparking occurs. However, if the short-circuit should last a moment longer, a rising differential current would appear and cause flashing around the commutator. Thus, Fig. 88 just represents the stability limit between the stable (but badly sparking) condition, Fig. 89, and the unstable or flashing conditions, Figs. 87 and 86.

By shifting the brushes so as to establish and open the short- circuit later, as shown in Fig. 90, the short-circuited alternating e.m.f. — shown dotted in Figs. 90 and 85 — ceases to be symmet- rical, that is, averaging zero as in Fig. 85, and becomes unsym- metrical, with an average of the same sign as the next following voltage wave. It thus becomes a commutating e.m.f., causes a more rapid reversal of the alternating current during the short- circuit period, and the circuit conditions, Fig. 89, then change to that of Fig. 91. That is, the current produced by the short- circuited alternating voltage has at the end of the short-circuit period reached nearly, but not quite the same value as the recti- fied current, and a short faint spark occurs due to the differential current, d. This Fig. 91 then represents about the best condition of stable, and practically sparkless commutation : a greater brush shift would reach the stability limit similar as Fig. 88, a lesser brush shift leave unnecessarily severe sparking, as Fig. 89.

141. Within a wide range of current and of inductance — espe- cially for highly inductive circuits — practically sparkless and stable rectification can be secured by short-circuit commutation by varying the duration of the shoH-circuit, and by shifting the brushes, that is, changing the position of the short-circuit during the voltage cycle.

Within a wide range of current and of inductance, in low-in- ductance circuits, practically sparkless and stable rectification can be secured also by open-circuit rectification, by varying the duration of the open-circuit, and by shifting the brushes.

The duration of open-circuit or short-circuit can be varied by the use of two brushes in parallel, which can be shifted against each other so as to span a lesser or greater part of the circumfer- ence of the commutator, as shown in Fig. 92.

Short-circuit commutation is more applicable to circuits of high, open-circuit commutation to circuits of low inductance.

But, while either method gives good rectification if overlap and brush shift are right, they require a shift of the brushes with every change of load or of inductivity of the load, and this limits the

244

ELECTRICAL APPARA T US

practical usefulness of rectification, as such readjustment with every change of circuit condition is hardly practicable.

Short-circuit rectification has been used to a large extent on constant-current circuits; it is the method by which the Thomson-

Fig. 92. — Double-brush rectifier.

Houston (three-phase) and the Brush arc machine (quarter- phase) commutates. For more details on this see “ Theory and Calculations of Transient Phenomena,” Section II.

Fig. 93. — Voltage waves of open-circuit rectifier charging storage battery.

Open-circuit rectification has found a limited use on non-in- ductive circuits containing a counter e.mi., that is, in charging storage batteries.

If, in Fig. 93, e 0 is the rectified voltage, and e x the counter e.m.f .

• 6q — *6

of the storage battery, the current is it = 7 where r- = ef-

r

fective resistance of the battery, and if the counter e.m.f. of the

SYNCHRONOUS RECTIFIER

245

battery, equals the initial and the final value of e 0 , as in Fig. 93, Go — e and thus ?* 0 start and end with zero, that is, no abrupt change of current occurs, and moderate inductivity thus gives no trouble. The current waves then are: i and i 0 in Fig, 94.

Fig. 94. — Current waves of open-circuit rectifier charging storage battery.

142 . Rectifiers may be divided into reversing rectifiers, like those discussed heretofore, and shown, together with its supply transformer, in Figs. 95 and 96, and contact-making rectifiers, shown in Figs. 97 and 98, . or in its simplest form, as half-wave rectifier, in Fig. 99.

Fig. 95.-— Reversing rectifier with Fig. 96. — Reversing rectifier alternating-current rotor. with direct-current rotor.

As seen, in Fig. 99, contact is made between the rectified cir- cuit and the alternating supply source, T , during one-half wave only, but the circuit is open during the reverse half wave, and the rectified circuit, B , thus carries a series of separate impulses of cur- rent and voltage as shown in Fig. 100 as i±. Hownver, in this case the current In the alternating supply circuit is unidirectional also, is the same current, i. This current produces in the trans- former, Tj a unidirectional magnetization, and, if of appreciable

246

ELECTRICAL APPARATUS

magnitude, that is, larger than the exciting current of the trans- former, it saturates the transformer iron. Running at or beyond magnetic saturation, the primary exciting current of the trans- former then becomes excessive, the hysteresis heating due to the unsymmetrical magnetic cycle is greatly increased, and the transformer endangered or destroyed.

m

Pig. 97. — Contact-making rectifier Fig. 98. — Contact-making rectifier with direct-current rotor. with alternating-current rotor.

Half-wave rectifiers thus are impracticable except for extremely small power.

The full-wave contact-making rectifier, Fig. 97 or 98, does not have this objection. In this type of rectifier, the connection be- tween rectified receiver circuit and 0 0 00 I alternating supply circuit are not

synchronously reversed, as in Fig. 95

or 96, but in Fig. 97 one side of the T rectified circuit, B , is permanently

f J T~ B connected to the middle m of the

jfr T alternating supply circuit, T, while the

W other side of the rectified circuit is

™ tt u synchronously connected and discon-

Fig. 99.— Half-wave rectifier, J J J _

contact making. nected with the two sides, a and

b, of the alternating supply circuit. Or we may say: the rectified circuit takes one-half wave from the one transformer half coil, ma, the other half wave from the other transformer half coil, mb. Thus, while each of the two transformer half coils carries unidirectional current, the uni-

directional currents in the two half coils flow in opposite direc- tion, thus give magnetically the same effect as one alternating

%

SYNCHRONOUS RECTIFIER

247

current in one half coil, and no unidirectional magnetization re- sults in the transformer.

In the contact-making rectifier, Fig. 98, the two halves of the rectified circuit, or battery, B , alternately receive the two suc- cessive half waves of the transformer, T.

The voltage and current waves of the rectifier, Fig. 97, are shown in Fig. 100. e is the voltage wave of the alternating sup-

Fig. 100. — Voltage and current waves of contact-making rectifier with direct-current rotor.

ply source, from a to 6. and e 2 then are the voltage waves of the two half coils, am and bm , and i 2 the two currents in these two half coils, and i 0 the rectified current, and voltage in the circuit from m to c. The current, ii, in the one, and, i 2 , in the other half coil, naturally has magnetically the same effect on he pri- mary, as the current, ii + h = io, in one half coil, or the current, io/2 = i, in the whole coil, ab, would have. Thus it may be said: in the (full-wave) contact-making rectifier, Fig. 97, the rectified

248

ELECTRICAL APPARATUS

voltage, e Q , is one-half- the* alternating voltage, e, and the rectified current, i 0 , is twice the alternating current, i. However, the ib in the secondary coil, ab, is greater, by V2, than it would be with the alternating cur- rent, i — io/2.

Inversely, in the contact-making rectifier, Fig. 98,* the rectified voltage is twice the alternating voltage, the rectified current half the alternating current.

Contact-making rectifiers of the type Fig. 97 are extensively used as arc recti- fiers, more particularly the mercury-arc rectifier shown diagrammatically in Fig. 101. This may be compared with Fig. 97. That is, the making of contact during one half wave, and opening it during the reverse half wave, is accomplished not by mechanical syn- chronous rotation, but by the use of the arc as unidirec-

Fig. 101. — Mercury- arc rectifier, contact making.

Fig. 102. — Diagram of mercury-arc rectifier with its reactances.

tional conductor: 1 with the voltage gradient in one direc- tion, the arc conducts; with the reverse voltage gradient

1 See Chapter II of “Theory and Calculation of Electric Circuits.”

SYNCHRONOUS. RECTIFIER

249

— the other half wave — it does not conduct. A large induc- tance is used in the rectified circuit, to reduce the pulsation of current, and inductances in the two alternating supply circuits — either separate inductances, or the internal reactance of the transformer — to prolong and thereby overlap the two half waves, and maintain the rectifying mercury arc in the vacuum tube. A diagram of a mercury-arc rectifier with its reactances, Xi, x 2 , x 0 ,

Fig. 103. — Voltage and current waves of mercury-arc rectifier.

is shown in Fig. 102. The “A.C. reactances” Xi and x 2 often are a part of the supply transformer; the “D.C. reactance” x 0 is the one which limits the pulsation of the rectified current. The waves of currents, i, i 2 and io , as overlapped by the inductances, Xi, x 2 and x 0) are shown in Fig. 103.

Full description and discussion of the mercury-arc rectifier is contained in “ Theory and Calculation of Transient Phenomena,” Section II, and in “Radiation, Light and Illumination.”

250

ELECTRICAL APPARATUS

143. To reduce the sparking at the rectifying commutator, the gap between the segments may be divided into a number of gaps, by small auxiliary segments, as shown in Fig. 104, and these then connected to intermediate points of the shunting re-

Fig. 104. — Rectifier with intermediate segments.

sistance, r, which takes the differential current, i Q — i, or the auxiliary segments may be connected to intermediate points of the winding of the transformer, T, which feeds the rectifier, through resistances, /, and the supply voltage thus successively

Fig. 105. — Three-phase F-connected rectifier.

rectified. Or both arrangements may be combined, that is, the intermediate segments connected to intermediate points of the resistance, r, and intermediate points of the transformer wind- ing, T.

Polyphase rectification can yield somewhat larger power than

SYNCHRONOUS RECTIFIER

251

Fig. 109.— Quarter-phase rectifier with independent phases.

Fig. 110. — Quarter- phase ring-connected rectifier,

G. 111. — Quarter-phase rectifier with two commutators.

252

ELECl RICAL APPARATUS

single-phase rectification. In polyphase rectification, the seg- ments and circuits may be in star connection, or in ring connec- tion, or independent.

Thus, Fig. 105 shows the arrangement of a star-connected (or F-connected) three-phase rectifier. The arrangement of Fig. 105 is shown again in Fig. 106, in simpler representation, by showing the phases of the alternating supply circuit, and their relation to each other and to the rectifier segments, by heavy black lines inside of the commutator.

Fig. 107 shows a ring or delta-connected three-phase rectifier.

Fig. 108 a star-connected quarter-phase rectifier and Fig. 109 a quarter-phase rectifier with two independent quadra-

Fig. 112. — Voltage waves of quarter-phase star-connected rectifier.

ture phases, while Fig. 110 shows a ring-connected quarter-phase rectifier.

The voltage waves of the two coils in Fig. 109 are shown as ei and e 2 in Fig. 112, in thin lines, and the rectified voltage by the heavy black line, e 0 , in Fig. 112. As seen, in star connection, the successive phases alternate in feeding the rectified circuit, but only one phase is in circuit at a time, except during the time of ine overlap of the brushes when passing the gap between suc- cessive segments. At that time, two successive phases are in multiple, and the current changes from the phase of decreasing voltage to that of rising voltage. Only a part of the voltage wave is thus used. The unused part of the wave, e% 9 is shown shaded in Fig. 112.

Fig. 113 shows the voltages of the four phases, e 2 , ez, c 4 , in ring connection, Fig. 110, and as e 0 the rectified voltage. As seen, in this case, all the phases are always in circuit, two phases always in series, except during the overlap of the brushes at the gap between the segments, when a phase is short-circuited dur- ing commutation. The rectified voltage is higher than that of each phase, but twice as many coils are required as sources of supply voltage, each carrying half the rectified current.

SYNCHRONOUS RECTIFIER

253

By using two commutators in series, as shown in Fig. Ill, the ■ two phases can be retained continuously in circuit while using

Fig. 113. — Voltage waves of water-phase ring- connected rectifier.

only two coils — but two commutators are required. The voltage waves then are shown in Fig. 114.

Fig. 114. — Voltage waves of quarter-phase rectifier with two commutators.

A star-connected six-phase rectifier is shown in Fig. 115, with the voltage waves in Fig. 117. The unused part of wave e\ is

Fig. 116. — Six-phase ring- connected rectifier.

shown shaded. A six-phase ring-connected rectifier in Fig. 116, with the voltage waves in Fig. 118.

254

ELECTRICAL APPARATUS

144* As seen, with larger number of phases, star connection becomes less and less economical, as a lesser part of the alternat- ing voltage wave is used in the rectified voltage : in quarter-phase

N

hJ

Fig. 117. — Voltage waves of six-phase star-connected rectifier.

rectification 90° or one-half, in six-phase rectification 60° or one-third, etc. In ring connection, however, all the phases are

Fig. 118. — Voltage waves of six-phase ring-connected rectifier.

continuously in circuit, and thus no loss of economy occurs by the use of the higher number of phases.

Fig. 119. — Rectifying commutators of the Brush arc machine.

Therefore, ring connection is generally used in rectification of a larger number of phases, and star connection is never used beyond quarter-phase, that is, four phases, and where a higher number of .phases is desired, to increase the output, several

SYNCHRONOUS RECTIFIER

255

rectifying commutators are connected in series, as shown in Fig. 119. This represents two quarter-phase rectifiers in series displaced from each other by 45°, that is, an eight-phase system.

Three-phase star-connected rectification, Fig. 106, has been used in the Thomson-Houston arc machine, and quarter-phase rectification, Fig. 108, in the Brush arc machine, and for larger powers, several such commutators were connected in series, as in Fig. 119. These machines are polyphase (constant-current)

Fig. 120. — Counter e.m.f. shunting gaps of six-phase rectifier.

alternators connected to rectifying commutators on the armature shaft.

For a more complete discussion of the rectification of arc machine see ct Theory and Calculation of Transient Electric Phenomena / 7 Section II.

145 . Even with polyphase rectification, the power which can be rectified is greatly limited by the sparking caused by the dif- ferential current, that is, the difference between the rectified current, which never reverses, but is practically constant, and the alternating supply current. Resistances shunting the gaps between adjoining segments, as byepath for this differential cur- rent, consume power and mitigate the sparking to a limited extent only. A far more effective method of eliminating the sparking is by shunting this differential current not through a mere non- inductive resistance, but through a non-inductive resistance which contains an alternating counter e.m.f. equal to that of the supply phase, as shown diagrammatically in Fig. 120.

In Fig, 120, 61 to e 6 are the six phases of a ring-connected six- phase system; e\ to e'g are e.m.fs. of very low self -inductance

1

256 ELECTRICAL APPARATUS

and moderate resistance, r, shunted between the rectifier seg- ments. Fig. 121 then shows the wave shape of the current, i 0 — i, which passes ‘through these counter e.m.fs., e! (assuming that the circuit of e', r , contains no appreciable self-inductance) .

Such polyphase counter e.m.fs. for shunting the differential current between the segments, can be derived from the syn- chronous motor which drives the rectifying commutator. By winding the synchronous-motor armature ring connected and

/I

K _A

k n

k

Fig. 121. — Wave shape of differential current.

of the same number of phases as the rectifying commutator, and using a revolving-armature synchronous motor, the synchronous- motor armature coils can be connected to the rectifier segments, and byepass the differential current. To carry this current, the armature conductor of the synchronous motor has to be increased in size, but as the differential current is small, this is relatively

little. Hereby the output which can be derived from a poly- phase rectifier can be very largely increased, the more, the larger the number of phases. This is Leblanc’s Panchahuteur, shown diagrammatically in Fig. 122 for six phases.

Such polyphase rectifier with non-inductive counter e.m.f. byepath through the synchronous-motor armature requires as many collector rings as rectifier segments. It can rectify large currents, but is limited in the voltage per phase, that is, per segment, to 20 to 30 volts at best, and the larger the

SYNCHRONOUS RECTIFIER 257

required rectified voltage, the larger thus must be the number of phases.

146 . Any number of phases can be produced in the secondary system from a three-phase or quarter-phase primary polyphase system by transformation through two or three suitably designed stationary transformers, and a large number of phases thus is not objectionable regarding its production by transformation. The serious objection to the use of a large number of phases (24, 81, etc.) is, that each phase requires a collector ring to lead the current to the corresponding segment of the rectifying commutator.

This objection is overcome by various means:

1. The rectifying commutator is made stationary and the brushes revolving. The synchronous motor then has revolving

Fig. 123. — Phase splitting by synchronous-motor armature: synchronous

converter.

field and stationary armature, and the connection from the stationary polyphase transformer to the commutator segments and the armature coils is by stationary leads.

Such a machine is called a permutator . It has been built to a limited extent abroad. It offers no material advantage over the synchronous converter, but has the serious disadvantage of ’re- volving brushes. This means, that the brushes can not be in- spected or adjusted during operation, that if one brush sparks by faulty adjustment, etc., it is practically impossible to find out which brush is at fault, and that due to the action of centrifugal forces on the brushes, the liability to troubles is greatly increased, 17

258

ELECTRICAL APPARATUS

For this reason, the permutator has never been introduced in this country, and has practically vanished abroad.

2. The transformer is mounted on the revolving-motor struc- ture, thereby revolving, permitting direct connection of its secondary leads with the commutator segments. In this case only the three or four primary phases have to be lead into the rotor by collector rings.

The mechanical design of such structure is difficult, the trans- former, not open to inspection during operation, and exposed to centrifugal forces, which limit its design, exclude oil -and thus limit the primary voltage, so that with a high-voltage primary- supply system, double transformation becomes necessary.

As this construction offers no material advantage over (3), it has never reached beyond experimental design.

3. A lesser number of collector rings and supply phases is used, than the number of commutator segments and synchronous- motor armature coils, and the latter are used as autotransformers to divide each supply phase into two or more phases feeding suc- cessive commutator segments. Fig. 123 shows a 12-phase recti- fying commutator connected to a 12-phase synchronous motor with six collector rings for a six-phase supply, so that each sup- ply phase feeds two motor phases or coils, and thereby two recti- fier segments. Usually, more than two segments are used per supply phase. The larger the number of commutator segments per supply phase, the larger is the differential current in the synchronous motor armature coils, and the larger thus must be this motor.

Calculation, however, shows that there is practically no gain by the use of more than 12 supply phases, and very little gain beyond six supply phases, and that usually the most economical design is that using six supply phases and collector rings, no matter how large a number of phases is used on the commutator.

Fig. 123 is the well-known synchronous converter, which hereby appears as the final development, for large powers, of the syn- chronous rectifier.

This is the reason why the synchronous rectifier apparently has never been developed for large powers: the development of the polyphase synchronous rectifier for high power, by increasing the number of phases, byepassing the differential current which causes the sparking, by shunting the commutator segments with the armature coils of the motor, and finally reducing the number

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of collector rings and supply phases by phase splitting in the synchronous-motor armature, leads to the synchronous con- verter as the final development of the high-power polyphase rectifier.

For “ synchronous converter” see “ Theoretical Elements of Electrical Engineering,” Part II, C. For some special types of synchronous converter see under “ Regulating Pole Converter” in the following Chapter XXI.

CHAPTER XVI

REACTION MACHINES

147. In the usual treatment of synchronous machines and induction machines, the assumption is made that the reactance, x , of the machine is a constant. While this is more or less approximately the case in many alternators, in others, especially in machines of large armature reaction, the reactance, x , is variable, and is different in the different positions of the armature coils in the magnetic circuit. This variation of the reactance causes phenomena which do not find their explanation by the theoretical calculations made under the assumption of constant reactance.

It is known that synchronous motors or converters of large and variable reactance keep in synchronism, and are able to do a considerable amount of work, and even carry under circum- stances full load, if the field-exciting circuit is broken, and thereby the counter e.m.f., E i, reduced to zero, and sometimes even if the field circuit is reversed and the counter e.m.f., Ei, made negative.

Inversely, under certain conditions of load, the current and the e.m.f. of a generator do not disappear if the generator field circuit is broken, or even reversed to a small negative value, in which latter case the current is against the e.m.f., E o, of the generator.

Furthermore, a shuttle armature without any winding (Fig. 126) will in an alternating magnetic field revolve when once brought up to synchronism, and do considerable work as a motor.

These phenomena are not due to remanent magnetism nor to the magnetizing effect of eddy currents, because they exist also in machines with laminated fields, and exist if the alternator is brought up to synchronism by external means and the rema- nent magnetism of the field poles destroyed beforehand by application of an alternating current.

These phenomena can not be explained under the assump- tion of a constant synchronous reactance; because in this case, at no-field excitation, the e.m.f. or counter e.m.f. of the machine

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is zero, and the only e.m.f. existing in the alternator is the e.m.f. of self-induction; that is, the e.m.f. induced by the alternating current upon itself. If, however, the synchronous reactance is constant, the counter e.m.f. of self-induction is in quadrature with the current and wattless; that is, can neither produce nor consume energy.

In the synchronous motor running without field excitation, always a large lag of the current behind the impressed e.m.f. exists; and an alternating-current generator will yield an e.m.f. without field excitation only when closed by an external circuit of large negative reactance; that is, a circuit in which the current leads the e.m.f., as a condenser, or an overexcited synchronous motor, etc.