CHAPTER LIV

RECTIFIERS

The purpose of a rectifier is to change alternating current into a uni-directional or pulsating current. There are several classes of apparatus to which the term rectifier may be applied, as

1. Mechanical rectifiers; 2. Electrolytic rectifiers; 3. Mercury vapor rectifiers, or, mercury arc rectifiers; 4. Electro-magnetic rectifiers.

~Mechanical Rectifiers.~--By definition, a mechanical rectifier is a form of commutator operating in synchronism with the generator and commutating or rectifying the negative waves of the alternating current as shown graphically in figs. 2,076 and 2,078. The essential features of construction are shown in fig. 2,079.

~Ques. Mention some application of a mechanical rectifier.~

Ans. It is used on a compositely excited alternator as illustrated on page 1,192.

~Electrolytic Rectifiers.~--If two metals be placed in an electrolyte and then subjected to a definite difference of pressure, _they will_ (under certain conditions) _offer greater resistance to the passage of a current in one direction, than in the other direction._ On account of this so called valve effect, electrolytic rectifiers are sometimes called "valves."

~Ques. What metal is generally used for the cathode?~

Ans. Aluminum.

~Ques. What is generally used for the other electrode?~

Ans. Lead or polished steel.

Metals of low atomic weight exhibit the valve effect at high differences of pressure, and heavier metals at low differences of pressure.

~Ques. Describe the "Nodon valve."~

Ans. The cathode is of aluminum or aluminum alloy, and the other electrode, which has considerably more surface, is the containing vessel. The electrolyte is a neutral solution of ammonia phosphate.

~Ques. Describe its action.~

Ans. It is due to the formation of a ~film~ of normal hydroxide of aluminum, over the surface of the aluminum electrode. This film presents a very high resistance to the current when flowing in one direction but very little resistance, when flowing in the reverse direction.

~Ques. What is the effect when a Nodon cell is supplied with alternating current?~

Ans. Half of the wave will be suppressed and an intermittently pulsating current will result as shown in fig. 2,077.

~Ques. How may both halves of the alternating waves be utilized?~

Ans. By coupling a series of cells in opposed pairs as in fig. 2,080.

~Ques. Upon what does the efficiency of the film depend?~

Ans. Upon the temperature.

It should not for maximum efficiency exceed 86 degrees Fahr. There is also a certain critical voltage above which the film breaks down locally, giving rise to a luminous and somewhat disruptive discharge accompanied by a rapid rise of temperature and fall in efficiency.

~Ques. When an electrolytic rectifier is not in use for some time what happens?~

Ans. The electrodes will loose the film.

~Ques. What must be done in such case?~

Ans. The electrodes must be reformed.

~Ques. How is the loss of film prevented?~

Ans. By Removing the electrodes from the electrolyte and drying them.

~Ques. What attention must be given to the electrolyte?~

Ans. Water must be added from time to time to make up for evaporation.

This is necessary to keep the solution at the proper density.

~Ques. What is the indication that the rectifier needs recharging?~

Ans. Excessive heating of the solution with normal load.

~Ques. What is the indication that a rectifier is passing alternating current?~

Ans. It will heat, and if the solution be very weak, it will cause a buzzing sound.

~Ques. What harm is caused by operating a rectifier with a weak electrolyte?~

Ans. The electrodes will eat away.

A few of the so called electrolytic valves are here briefly described:

~The Audion Valve.~--This valve was invented by De Forest in 1900 and is practically identical with the Fleming oscillation valve, the latter being illustrated in fig. 2,086.

~Grisson Valve.~--In this valve the cathode is a sheet of aluminum, and the anode, a sheet of lead, supported, in the original form, horizontally in a vessel containing the electrolyte, consisting of a solution of sodium carbonate. Cooling is effected by circulating water through metal tubes in the electrolyte itself.

~Pawlowski Valve.~--This is an electrolytic valve employing a solid electrolyte. It consists of a copper plate which has been coated with a crystalline layer of carefully prepared copper hemisulphide, prepared by melting sulphur and copper together out of contact with air. The prepared plate is placed in contact with an aluminum sheet and the combination is then _formed_ by submitting it to an alternating pressure until sparking, which at first occurs, ceases.

~Giles Electric Valve.~--This consists of a combination of spark gaps and capacity used to protect electrical apparatus against damage due to atmospheric discharges and resonance surges. The spark gaps are formed between the edges of sharp rimmed discs of non-arcing metal. These discs are insulated from each other, and from the central tube, which provides a support for the apparatus and also an earth. The condenser effect is obtained by means of the annular discs and the tube; an adjustable spark gap, a high resistance, and a fuse all connected in series, complete the valve.

~Buttner Valve.~--It is of the Nodon type employing a cathode of magnesium-aluminum alloy, and probably iron or lead as anode, with an electrolyte of ammonium borate. Buttner claims that the borate is superior to the phosphate in that it does not attack iron, and will keep in good working condition for longer periods.

~Mercury Vapor Rectifiers.~--The Cooper Hewitt mercury vapor rectifier, as shown in fig. 2,093 consists essentially of a _hermetically sealed glass bulb filled with mercury vapor and provided with four electrodes_. The two upper electrodes are of solid material and the two lower of mercury.

The solid electrodes are the positive electrodes; the mercury electrodes are the negative electrodes.

The mercury pools of the two lower electrodes are not in contact when the bulb is vertical, but the bulb is so mounted that it can be tilted to bring these two pools temporarily in contact for starting.

The bulb contains highly attenuated vapor of mercury, which, like other metal vapors, is an electrical conductor under some conditions. The positive electrodes are surrounded by this vapor. Current can readily pass from either of the solid electrodes to the mercury vapor and from it to the mercury electrode, but when the direction of flow tends to reverse, so that current would pass from the vapor to the solid electrode, there is a resistance at the surface of the electrode, which entirely prevents the flow of current.

The alternating current supply circuit is connected to the two positive electrodes as shown in the diagram, and as the electrodes will allow current to flow in only one direction and oppose any current flow in the opposite direction, the pulsations of the current pass alternately from one or the other of the positive electrodes into the mercury.

As these currents cannot pass from the vapor into either positive electrode, they are constrained to pass out all in one direction through the mercury electrode, from which they emerge as a uni-directional current. The positive electrodes of the rectifier thus act as check valves, permitting current to pass into the mercury vapor but not allowing it to pass from the vapor to the solid electrodes.

~Ques. What condition prevails before the bulb starts to rectify?~

Ans. There appears to be a high resistance at the surface

~Ques. Describe the arrangement and operation of the condenser.~

Ans. The condenser is connected between one of the positive electrodes and a coating of tinfoil outside [Illustration: FIG. 2,102.--General Electric 2,200 volts, 60 cycle, primary, 6.6 ampere, secondary, 75 light, double tube mercury arc rectifier outfit with automatic shaking device, the case being removed to show parts. The constant current transformer is air cooled. The winding which supplies energy for the exciter transformer is located at the top of, and around the core of the constant current transformer. The exciting transformer is mounted on the base of the constant current transformer inside of the casing. It supplies low pressure currents to the starting anodes of the rectifier tube. This current establishes an auxiliary arc when the tube is shaken, which is necessary in order to start the rectifier. The exciting transformer is wound for 110 volts and it consumes about 200 volt-amperes. The direct current reactance is mounted on the base of the transformer and enclosed in the same casing. It is connected in series with the lamp load and its function is to reduce the pulsations of the circuit to a value most satisfactory for operation. The tube tank for holding the oil is mounted on the same base as the transformer. It is provided with a cooling coil; a tube carrier is provided for raising or lowering the tube in the tank. A thermometer is provided to gauge the temperature of the oil in the tank. The static dischargers consisting of horn gaps in series with resistance, are connected between the anodes and the cathode in order to protect the tubes and other apparatus from excessive electrical strains. The horn gaps open the circuit after discharge, and in case the resistance becomes damaged the discharge passes across the spark gap provided, thereby shunting the resistance.] the part of the bulb containing the mercury, and induces static sparks on the surface of the mercury which break down the negative electrode resistance.

The action of the rectifier will be better understood by reference to the diagram of current waves and impressed pressure as shown in figs. 2,103 to 2,106.

~Ques. Describe a mercury vapor rectifier outfit for series arc lighting.~

Ans. It consists of a constant current regulating transformer, a rectifier bulb, and a control panel containing the necessary switches, meters, etc. The transformer and rectifier bulb are mounted in the same tank.

~Ques. Describe the construction and operation of the mercury arc[11] rectifier shown in fig. 2,108.~

[11] NOTE.--The terms _vapor_ and _arc_ as applied to rectifiers, do not indicate a different principle; the Westinghouse Co. employ the former, and the General Electric Co., the latter.

Ans. Fig. 2,108 is an elementary diagram of connections. The rectifier tube is an exhausted glass vessel in which are two graphite anodes A, A', and one mercury cathode B. The small starting electrode C is connected to one side of the alternating circuit, through resistance; and by rocking the tube a slight arc is formed, which starts the operation of the rectifier tube. At the instant the terminal H of the supply transformer is positive, the anode A is then positive, and the arc is free to flow between A and B. Following the direction of the arrow still further, the current passes through the battery J, through one-half of the main reactance coil E, and back to the negative terminal G of the transformer. When the impressed voltage falls below a value sufficient to maintain the arc against the reverse pressure of the arc and load, the reactance E, which heretofore has been charging, now discharges, the discharge current being in the same direction as formerly. This serves to maintain the arc in the rectifier tube until the pressure of the supply has passed through zero, reversed, and built up such a value as to cause the anode A to have a sufficiently positive value to start the arc between it and the cathode B. The discharge circuit of the reactance coil E is now through the arc A'B instead of through its former circuit. Consequently the arc A'B is now supplied with current, partly from the transformer, and partly from the reactance coil E. The new circuit from the transformer is indicated by the arrows enclosed in circles.

~Ques. How is a mercury arc rectifier started?~

Ans. A rectifier outfit with its starting devices, etc., is shown in figs. 2,114 to 2,116. To start the rectifier, close in order named line switch and circuit breaker; hold the starting switch in opposite position from normal; rock the tube gently by rectifier shaker. When the tube starts, as shown by greenish blue light, release starting switch and see that it goes back to normal position. Adjust the charging current by means of fine regulation switch on the left; or, if not sufficient, by one button of coarse regulation switch on the right. The regulating switch may have to be adjusted occasionally during charge, if it be desired to maintain charging amperes approximately constant.

~Ques. In the manufacture of rectifiers, could other metals be used for the cathode in place of mercury?~

Ans. Yes.

~Ques. Why are they not used?~

Ans. Because, on account of the arc produced, they would gradually wear away and could not be replaced conveniently.

In the case of mercury, the excess vapor is condensed to liquid form in the large glass bulb or condensing chamber of the tube and gravitates back to the cathode, where it is used over and over again.

~Ques. In the operation of rectifiers, how is the heat generated in the bulb dissipated?~

Ans. In small rectifier sets the heat generated is dissipated through the tube to the air, and in large tubes such as used in supplying 40 to 60 kw. for constant current flaming arc lights operating at 4 or 6.6 amperes, the tubes are immersed in a tank of oil, and cooled similar to the arrangement used for oil insulated water cooled transformers.

~Ques. What results are obtained with oil cooled tubes?~

Ans. In practice it is found that the life of oil cooled tubes is greatly increased and temperature changes do not affect the ability to start up as in the air cooled tubes.

~Ques. In the operation of a rectifier, name an inherent feature of the mercury arc.~

Ans. A reverse pressure of approximately 14 volts is produced, which remains nearly constant through changes of load, frequency, and voltage. Its effect is to decrease the commercial efficiency slightly on light loads.

~Ques. What is the advantage of a rectifier set over a motor generator set?~

Ans. Higher efficiency and lower first cost.

~Ques. What is the capacity of a rectifier tube?~

Ans. 40 to 50 amperes.

~Ques. How is greater capacity obtained?~

Ans. When a greater ampere capacity is required, two or more rectifier sets can be joined to one circuit.

The rectifier may be joined in series for producing an increased voltage or two tubes can be connected in series in a single set.

~Ques. For what service is a rotary converter better adapted than a rectifier?~

Ans. For power distribution and other cases where a great amount of alternating current is to be converted into direct current, the rotary converter or large motor generator sets are more practical.

~Ques. For what service is a rectifier especially adapted?~

Ans. It is very desirable for charging storage batteries for automobiles from the local alternating current lighting circuit.

When the consumer installs and operates the apparatus for his own use and wear, there is considerable saving over motor generator sets because a small one to two horse power motor generator outfit has an efficiency of only 40 to 50 per cent. while mercury vapor rectifiers will have from 75 to 80 per cent.

~Ques. What precautions should be taken in installing a rectifier?~

Ans. It should be installed in a dry place and care should be taken to avoid dangling wires near the tube to prevent puncturing. If the apparatus be installed in a room of uniform moderate temperature very little trouble will be experienced in starting, while extreme cold will make starting more difficult.

[12]~Electro-magnetic Rectifiers.~--Devices of this class consist essentially of a double contact rocker which rocks on pivot (midway between the contacts), in synchronism with the frequency of the alternating current, so changing the connections at the instants of reversals of the alternating current that a direct current is obtained.

[12] NOTE.--The Edison electromagnetic rectifier is described in detail in GUIDE No. 4, pages 942 to 945.

Fig. 2,121 is a combined sketch and diagram of connections of a type of _electromagnetic rectifier_ that has been introduced for changing alternating into direct current. The actual apparatus consists of a box, with perforated metal sides, about ten inches square and six inches deep. This box contains the step down transformer P,S,S', and the condensers K and K', the magnets and contact making device about to be described being fixed on the polished slate top of the box, exactly as shown in the figure. The transformer primary winding P may be connected through a switch _s_ with a pair of ways on the nearest distribution box, or to a plug connection or lamp-holder, and the apparatus will give a rectified current of 6 or 12 amperes at 20 volts, according to the size.

S and S' is the secondary winding of the transformer, with a tapping _t_ midway, joining it to a series circuit containing two alternating current electromagnets E and E', whose cores are connected by the long soft iron yoke Y. Pivoted at P' is a steel bar SB, which is polarized by the two coils C and C' the current being supplied by a cell A. Fixed rigidly to SB, and moving with it, is a double contact piece CP with platinum contacts opposite similar ones on the fixed studs CS, CS'.

CP is flexibly connected through F to one of the direct current terminals T, to which also are joined up one coating of each condenser K and K'.

The other direct current terminal T' is connected to the center of the transformer secondary at _t_; and CS and CS' are respectively joined up to either end of the secondary winding and to the other coatings of the condensers.

When the alternating current circuit is broken, the springs SP, SP, carried by SB and bearing against the adjustable studs, keep SB, CS and CS'. The apparatus thus acts also as a _no voltage circuit breaker_, for should the supply fail, the storage battery A' under charge will be left on open circuit.

The action of the device is briefly as follows:

Owing to the direct current in the magnetizing coils C and C' one end of SB will be permanently of north and the other of south polarity; and since the polarities of the poles E and E' will alternate with the alternations of the transformer secondary current, SB will rock rapidly on its pivot, and contact will be made by turns with CS and CS'.

The purpose of the condensers K and K' is to reduce the sparking at these points. When contact is made at CS, the direct current terminals T and T' are connected to the S half of the secondary winding; and when contact is made at CS', they are connected to the S' half. Thus a rectified uni-directional current will flow from T and T', and it may be used to charge the battery A', work a small motor or for various other purposes requiring direct current.

When the rectifier is used for charging storage batteries, the separate cell A may sometimes be dispensed with, the winding C,C' being connected to one of the cells under charge.

The rectifier is adjusted to suit the frequency of the supply circuit by altering the distance of the poles of E and E' from the ends of the polarized armature SB; and also by changing the tension of SP, SP by means of the screw studs against which they bear.

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~ELECTRICAL GUIDE, NO. 1~

Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.

~ELECTRICAL GUIDE, NO. 2~

The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.

~ELECTRICAL GUIDE, NO. 3~

Electrical Instruments, Testing, Practical Management of Dynamos and Motors.

~ELECTRICAL GUIDE, NO. 4~

Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.

~ELECTRICAL GUIDE, NO. 5~

Principles of Alternating Currents and Alternators.

~ELECTRICAL GUIDE, NO. 6~

Alternating Current Motors, Transformers, Converters, Rectifiers.

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Alternating Current Systems, Circuit Breakers, Measuring Instruments.

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Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.

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~ELECTRICAL GUIDE, NO. 10~

Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.

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TRANSCRIBER'S NOTES

Silently corrected simple spelling, grammar, and typographical errors.

Retained anachronistic and non-standard spellings as printed.

Enclosed italics markup in _underscores_.

Enclosed bold markup in ~tildes~.

End of Project Gutenberg's Hawkins Electrical Guide v. 6 (of 10), by Hawkins