VOL. I No.6 JUNE 1936

Philips Technical ReviewDEALING WITH TECHNICAL PROBLEMS

RELATING TO THE .PRODUCTS, PROCESSES AND INVESTIGATIONS OFN.V. PHILIPS' GLOEILAMPENFABRIEKEN

EDITED BY THE RESEARCH LABORATORY OF N.V. PIDLIPS' GLOEILAMPENFABRIEKEN, EINDHOVEN, HOLLAND

A CONTROLLABLE RECTIFIER UNIT FOR 20000 VOLTS/I8 AMPERES

By J. G. W. MULDER and H. L. VAN DER HORST.

Summary. The use of relay valves enables the construction of highly-efficient controllablerectifier units. In this article a controllable rectifier uni t for 20 000 volts / 18 amperesis described for feeding the transmitting valve of a high-frequency furnace with valvegenerator.

IntroduetionGas-filled rectifying valves are particularly suit-

able for rectifying high powers. They differ fromthe high-vacuum valves, which have been givenpreference in the past, by their much lower internalresistance and correspondingly higher efficiency.At the same time the whole unit is of simple con-struction, since the cooling arrangements whichbecome very cumbersome with high-vacuum valves

for high tensions (dissipation) can be dispensedwith.

The relay valves represent an important advancein the design of gas rectifiers. These are gas-filledrectifying valves which, in addition to the cathodeand anode, also have a control grid with the aidof which the ignition voltage of the valve can beregulated within wide limits with a very smallpower consumption (low grid voltages and small

Fig. 1. General. view of the 20-kilovolt, I8-ampere rectifier unit. On the right is oneof the two high-tension transformers furnishing the anode voltage. On the left are therectifying valves, which are screened by metal walls against electric fields. The anodesare shown above. Below are the heating current transformers (left coils) and the gridvoltage transformers (right coils).

162 ,PHILIPS TEGHN~eAL REVIEW Vol. 1, No. 6

grid ~urrents)., By means of these, relay valves the.. .' '_ ,,,. ,,' .,..' '. . ~ "

continuous 'voltage furnished by the' unit can hec~ntr~llcd a~d regulated ht a verysimple ~anner,~thollt the need for opening or closing an:y con-tacts whatsoever in the high-Jerision circuit.l, '

A description 'is given below of a rectifying unitequipped with.relay valves (fig.l),whiç,~'feeds thetransmitting 'valve' 'of' a high-frequency' furnace~iih valve 'getieraiör -alrea~y described ill thisReview 1). On jts full load. of 20 amps this unitfurnishes a D.e. potentialof 20000 volts, whichcán, .however, be adjusted to' any desirable lowervalue merely by turriing a phase regulator:, '

• • #.

Lay-out of the Rectifying Unit.'

The lay-out '.of . the rectifying unit is showndiagrammatically in fig. 2. It is seen, that the plantis' made .up óf two similar components, eachof which furnishes half the voltage and canalso ,be'. ~sed separately, Each u~it is connectedto a transformer (Tl' T2) and contains six rectifyingvalves, which provide six-phase 'rectification on acircuit" de's'cribed by' ~r~~~z,. the ~~rr~n,t, in..each,.phase pà~sing:,through' ~ 'pair ~f. váfv.ê~ ,cbûrieç:ted

_., ... 't-. ~. '_ ..., ". ~ • •~ ." ., , •

" in- ser~es:, ,"', ..' . . ", ., " ". , ':The' t:fansförnier primaries are "éönnecteçl'iÎl delta

• " '"lIJ,' '.~' . "# ':. " • • : ", •• . .';'. •

(380 volts) and the secondaries ID a star ~ITCU1t,74QO- v;öiis' iag~~~st' 'tIie "neutral, ~:Thesé'.<ciróiritsare', b?tJ.i ' o{ exactly .idep.tical desigQ.,·although thesecondaries carry different voltages against earth.This offers, the advantage, that only one trans-former of half the capacity of the whole unit issufficient as a spare.The pulsating direct current charges the 2(J.F

condènser C, which at the same time is dischargedthrough the transmitting valve Z ~onnected inparallel. The anode current of the transmittingvalve must also pass through the '100-millihen"rychoke coil L2' which protects the rectifier againstthe high-frequency 'alternating voltage of approx-imately 7000 cycles generated at the transmittingv~lve. The large choke coil L1 (1 henry) serves forsmoothing the current which charges condenser Cand hence also the D.e. potential applied to the.transmit.ting valve. The re'sistance R = 50,ohms,which is. in series with Ll"limits the anode currentin the event, of any tran.sient irregularities at thetransmitting valve. .

The rectifying valves B] - B]2 can each furnisha mean D.e. of ,6 amps, so that .the whole, unitcan carry ,a'load of 18 anips. Th~se valves areshown only diagrammatically in the circuit diagramby arrows., The' detailed drawings. of. the valv~s

1) Philips techno Rev. 1, 53, 1936.

B3 and B6 show theindirect heating ofthe cathodeby means of a separate filament transformer foreach valve, as well as the supply of an alternatingvoltage to 'the" control grid through the t.ransfor-mers t3, t6 and the phase-shifters F], F2•

These phase-shifters used, consist of a stator and arotor; which' are adjusted and secured by means ofa worm transmission. The rotating field generated

, by the stator intersects the rotor windings earlier

Fig. 2. Electrical lay-out of the rectifying unit. The unitconsists of two Iû-kilovolt, IS-ampere components, which areconnected in series. The rectifying valves Bj to B12 (type DeG ,5/30) serve as relay valves. TIle grid voltage is taken fromthe phase-shifters F] and F2 through the transformers 11 to112', (In the circuit diagram only 13 and 16'are shown, since theremaining valves are indicated only diagrammatically). Thechoke coil L] and the condenser C serve for smoothing the •D.e. potential furnished. The choke coil [.2 protects therectifying unit against high-frequency oscillations from the. transmitting valve. .

. ,

or later, according to the position of the rotor with-reference to the stator. By altering the positionof the rotor, the phase displacement of the rotor -voltage. against the stator voltage can thus, bealtered. ".. The voltage systems of. the 'stato! and rotorar~ bot~ three-phase. The grids of the rectifyingvalves are connected to the rotor in. such a waythat the grid voltage of all the, valves has the samephase displacement against the corresponding anode

, voltage:' ,-

.-_---------------------------------- ---

JUNE 1936 CONTROLLABLE RECTIFIER UNIT 163

The action of the phase of the grid voltage inregulating the voltage of the rectifier is discussedin detail in the section dealing with this matter.

Constrnction of the Rectifying Valves

The rectifying valves used are Philips relay valvestype DCG 5/30 with hot cathode and with mercuryvapour as a gas filling, whose main electricalcharacteristics have already been described in aseparate article in this Review 2). The valve, whichis shown in figs. 3 and 4, is made up of two glassbulbs bI and b2, which are fused to the muchnarrower chrome-iron ring r. The top bulb b2contains the anode a, and the lower bulb bI the

a ----t--r........._jt-~~0~r -------.:~"T/"."l__t ---7.~w'""'

t----h

Fig. 3. Relay valve type DCG 5/30 with mercury vapourdischarge. a is the graphite anode, I, the indirectly-heatedcathode, and r the control grid subdivided by partitions,with the funnel-shaped extension t. The upper half of thebulb bI' is covered on the inner side with a conductive coatingwhich is connected to the control grid. The liquid mercuryis in the neck I, at the bottom.

2) Relay valves as timing devices in seam-welding practice,D. M. Duinker, Philips techno Rev. 1, 11, 1936. Sincethe publication of this article the construction of the DCG5/30 valves has been slightly altered. The inside wall ofthe bulb enclosing the cathode has been coated with aconductive layer, which is connected to the control grid.Owing to this change the characteristic (ignition voltageplotted as a function of the grid voltage) is no longeridentical with that shown in fig. I of the article citedabove. The grid voltage required for ignition in themodified valve is approximately 18 volts and is practicallyindependent of the anode voltage.

Fig. 4. Photograph of the DCG 5/30 rectifying valve. Thewarm air rising from the lower bulb is taken up by the micaplate above the anode, in this way heating the upper bulband preventing condensation of the mercury vapour.

indirectly-heated hot cathode k. The lower bulbterminates in a cylindrical neck h containing liquidmercury, the temperature of which is only slightlyabove the ambient temperature. The vapourpressure of the mercury in the cold state is a fewthousandths of a millimetre. The ring r serves asa control grid, its voltage differing from that ofthe cathode by not more than a few hundredvolts. In addition the presence of this ring increasesthe back-firing voltage ofthe valve. For this purposea funnel-shaped extension t is provided, which isshaped to conform with the conical anode a. Thedistance between -the funnel and the anode isroughly the same at all points, the optimum distanceto obtain a high back-firing voltage being fairlycritical, for in the negative phase practically thewhole reverse voltage is applied across the funneland the anode. If this distance is too large, back-firing may occur since the electrons in the gas-space are given every opportunity to ionise, whilstwith too small a distance the field strength and

PHILIPS _TECHNICALIREVIEW

Fig. 5. Ignition sequence of a relay valve. with the alternating anode voltage Va and thealternating grid voltage Vg' V. is the critical grid voltage. -a is the phase lag of thegrid voltage with respect to the anode voltage.L) al = 0 Current passes almost during the whole positive phase from OOI = rp

onwards.. Current passes from OOI = n/3 + rp onwards.Current passes from OOL = 2n/3 + rp onwards.. Grid voltage in opposing phase. No current passes.~ Current passes throughout the positive phase.

hence the danger of auto-electronic emission fromthe anode are considerably enh'anced.. The t.emperature of the ring is determinedmainly by the radiation of heat from the hotcathode and bythe anode current. To avoid exces-sive heating, when the valve is running on fullload, the width of the ring has had to be madelarger, than appeared desirable, to make sure ofsuppressing back-firing between the anode andcathode. It was therefore found advantageous tosubdivide the apêrture of the ring as shown infig. 3, 'so that. in place of an open ring there arethree parallel, narrow tubes.

Electrical Characteristics

The permissible mean working current of therelay valve DCG 5/30 is 6. amps. The voltage-dropat the discharge is about 16 volts and is practicallyindependent' of the current intensity. The ignitionvoltage is not appreciably greater with a sufficientlyhigh potential at the control ring. If, on the otherhand, the grid voltage is reduced below 18 volts,the ignition voltage will- rise abruptly to morethan 10000 volts, so that with the availabletransformer voltage ignition is no longer obtained.

\

2) a2 = n/33) a3 = 2n/3,~) a,s = :Tt;

5) as = 'Ln/36) a6 = 5n/3

\1

Vol. 1, No. 6

.When, however, ignition has once taken place thediscliarge cannot be extinguished by merelyreducing the control voltage. The discharge isonly extinguished, "Yhen the alternating voltageat the anode passes through zero.

Voltage Regulation

'The phase displacement of the grid voltage Vgof the relay valve with respect to the anode voltageVa determines the moment of ignition in' each.positive phase of Va and hence the average valueof the output voltage of the cycle. This is due to.the fact that, as already stated, ignition is onlyobtained when the grid voltage exceeds a certaincritical valuê Vc (of about 18 volts).

Fig. 5 shows the action of the phase-shifterin the case of one single valve Va is the anodevoltage, Vg the alternating grid voltage, Vc thecritical grid voltage, and a the phase-lag of thegrid voltage against the anode voltage. In case(1) Vg and Va are in- phase. The discharge is·obtained when Vc exceeds the critical value (wt = cp).To fix satisfactorily the moment of ignition, alsowith small fluctuations of Vc, the alternating gridvoltage should be several times greater than. the

~4723

The .protection of the high-tension tninsforu{ersagainst overloads is afforded by the adjustablemaximum-current relays (Tl~T4' fig. 2), which areinserted in two of the three' primary conductors.If one of the maximum-current relays operates themain switch is tripped.

The rectifying valves must be protected againstanode. voltage, being applied before the cathodehaving .reached operating temperature. For thispurpose a slow-acting relay. is provided, which isconneeted in parallel with the heating transfermers.The main switch can only he closed after this relayhas beèn under tension for 6 minutes. In' coldsituations it is advisable to extend this time from10 to 15 minutes. Although after 6 -rninutes' thecathode lias been sufficiently heated, the vapourpressure of the mercury is still very low. With toolow a vapour pressure the are voltage is too high,and as this might adversely affect the cathode,it is desirable to wait before switching 'on theanode voltage until the whole bulb, as well as themercury in the neck at the bottom, has becomesufficiently heated to raise .the vij.pour pressure' ofthe mercury to the required value. .

The transmitting valve is protected against over-heating by means of a so-called "water lock"which prevents the heating current or the anodecurrent from being applied, if. the flow of coolingwater is not adequate. Moreover, the heating currentcan only be switched on, when the regulatingdevice is adjusted to the minimum voltage. The'anode is protected in a similar way. Tlie an~devoltage can only be switched QP.,. when the phase-shifter for voltage control is' at its minimumsetting. Regulating the output voltage to its maxi-mum value by turning the phase-shifter takes onlya few seconds. The anode is also protected againstoverloads, which might occur if the anode voltageis applied before the valve can oscillate. This is,for instance, the case when the electrical connec-.tion with the furnace is broken, which takes placeautomatically when the furnace is tilted from itsvertical position for casting operations, .

JUNE 1936 CONTROLLABLE RECTIFIER. UNIT

critical voltage (e.g. 100 volts). The current passes.during the greater part of the positive phase ofthe anode voltage, as indicated by the shading.'As the phase retardation increases (a2 = n/3,a3 = 2n/3) the portion of the period during whichcurrent passes is reduced. If Vg and Va are i~ counterphase (a4 = n) no current at áll flows.Ilf the phaseretardation is further increased, then at a =.n + cpthe current suddenly commences to pass with itsmaximum value; it continues to flow during thewhole positive half of the cycle of the anode voltage,although the grid voltage has in the meantimedropped below the critical value and even hecomezero. When ionisation has once taken place, the gridis inactive. In .fig. 6 the average D.e. potential Vo

furnished by a single valve is plotted as a functionof the phase retardation a. The points 1 to 6 cor-respond to the cases represented in fig. 5.

va

5

/6722

Fig, 6. Average D.e. potential Vo plotted as u function of-the angle of lag a between the alternating grid voltage andthe .nltemating anode voltage. Points / to 6 corre~ond tothe special cases of fig. 5.

Safety Precautions in the Rectifying Unit

The operatien of the fairly extensive plant· ofhigh-tension transformers, rectifying valves, trans-mitting valve, high-frequency oscillating circuitsand melting furnaces has been considerably simpli-fied by the adoption of suitable safety precautions,and the provision of automatic auxiliary equipment.In addition these protect the transformers, therectifying valves and the transmitting valve against

• overloads, which might result, when the switchesare operated in the wrong order.