From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Reprint from 'The Electrical Times' discussing recent developments in mercury vapour rectifiers.
| Identifier | ExFiles\Box 32\5\ Scan002 | |
| Date | 9th June 1921 | |
| Reprinted from THE ELECTRICAL TIMES. [9th June, 1921. Mercury Vapour Rectifiers. SOME NOTES ON RECENT DEVELOPMENTS. EARLY in May Mr. Frank Ayton started up a mercury rectifier substation supplying the new works of E.{Mr Elliott - Chief Engineer} R.{Sir Henry Royce} and F.{Mr Friese} Turner, Ltd., Messrs. Turner's plant represents the first steel-cylinder mercury vapour rectifier operating in this country. The equipment has a capacity of 125 kilowatts at 470/475 volts. It has been supplied by Power-Rectifiers, Limited, and was installed by Turner's on Mr. Ayton's advice. The 3-phase supply at 3,000 volts and 50 periods is taken from the corporation a.c. mains, lighting supply for the works being given from a separate static transformer at 220 volts. The plant was started up without any trouble whatever, and we understand that Mr. Ayton is extremely satisfied with the way in which it has worked so far. We particularly inquired whether there were any "snags," but the answer is reassuring, and Mr. Ayton thinks that as soon as the price can be brought down there will be a very important demand for these large mercury vapour rectifiers. The starting up and shutting down of the plant is beautifully simple, and the skilled attention demanded by it is very small—in fact, no permanent attendant is required in the substation. The load which the plant has to carry is a somewhat severe test for it, because in the first place it has to supply electrical energy to a large foundry only, in connection with which there are travelling cranes, moulding machines, air compressors, cupola blowers, elevators, etc. Consequently, the variation in load must be fairly large. It is too early yet to say that the plant is an unqualified success, though, so far it has quite come up to Mr. Ayton's expectations. We hope to be able to give further particulars of this particular plant at a later date. As the power mercury vapour rectifier is little known in this country, the following notes will doubtless be found of value by many of our readers. The credit for the discovery of the valve action of the mercury vapour arc is due to Mr. Cooper Hewitt, and is embodied in the small glass-bulb rectifiers supplied by the Hewittic Electric Co., Ltd. These rectifiers are unsuitable for sub-station work purely on account of their limited capacity; 30 amps. d.c. is as much as can be obtained with the largest size at the present day, though it is hoped soon that an output of 100 amps. will be possible. The capacity can be improved by operating two or more units in parallel, but even then the field in application of the glass-bulb mercury vapour rectifier is limited to comparatively small power work, such as electric vehicle battery-charging, and so forth. They are also supplied for wireless work, a standard size giving 5 amps. at 5,000 volts. For power purposes the steel cylinder type of rectifier has been developed by Brown, Boveri and Co., and standard units are made by this firm of 300, 600, and 1,000 amps. capacity. By operating two or more units in parallel any desired capacity can be obtained. These rectifiers supply an alternative to rotary-converters, and motor-generators, corresponding in simplicity to the a.c. transformer. While a knowledge of the molecular and physical nature of the arc, the problems of which are very difficult to investigate, is of little practical operation of the rectifier, however, with which we are mostly concerned. A clear insight into the mechanism of the rectification may be be obtained from a study of the single-phase rectifier. Figure 1 shows the connections of a unit of this type in their simplest form. Fig. 1.—Showing the connections of a single-phase mercury vapour rectifier in their simplest form. The primary of a transformer is linked to the main supply, the secondary winding being taken out to the two anodes, 1 and 2, of the rectifier. The cathode of the rectifier forms the positive terminal of the d.c. circuit, the negative-point being the neutral point of the transformer secondary winding. When in operation the first half-wave is induced in the secondary travels to one anode, say 1, and through the arc to the cathode, while the second half-wave passes to the cathode via the second anode—2. The cathode always remains at the same sign, and a unidirectional pulsating current flows through the external circuit back to the neutral point of the transformer secondary. The mercury terminal must always be made live when connecting up, as otherwise no arc will be formed even with several thousand volts pressure. Correctly connected 20-30 volts will start the arc. Though both halves of the a.c. wave are utilised the d.c. current in the above-mentioned circuit would drop to zero at each half-period unless precautions to prevent this were taken. Quite apart from the undesirable characteristics of a wave form rising from zero to a maximum and falling again to zero in each half-period, steps must be taken to prevent even a momentary cessation of the current, as an interregnum of even 1/100,000th of a second would be sufficient to extinguish the arc. Separate excitation has the effect of establishing the arc, but the introduction of suitable choking coils (L) in the d.c. circuit will effectively flatten out the wave form and maintain it at all times above zero. When running, the anodes reach an even temperature of about red heat, while the cathode is a travelling spot on the surface of the mercury, the spot temperature being about 2,000 degrees C. The single-phase rectifier has a very limited field of application owing to the highly pulsating character of the d.c. wave produced. D.C. motors, for instance, can only be run off the rectifiers provided the momentary value of the rectified pressure is greater than the back e.m.f. of the circuit. The six-phase rectifier, which produces an approximately uniform d.c. wave form, is the most suitable for power, lighting, and traction loads. A diagrammatic sketch of a six-phase 300-amp, Brown, Boveri rectifier is shown in Fig. 2. A is the space in which the main arc operates, C being an anode of which there are six—one for each phase, and B the common mercury cathode. B is a space provided in which the ascending mercury vapour can condense. The anodes are surrounded by steel arc-guides which terminate in a steel disc fitted immediately above the cathode. The object of this is to encircle the arc in a well-defined space, which effectively overcomes the tendency of the arc to short circuit inter-falls. At one period in the development of power rectifiers internal short circuits were then most common failing, but this trouble has since been practically eliminated. Water-cooling is provided round both portions of the rectifier. In the 600 and 1,000 amp. sizes cooling of the anodes is assisted by radiation fins, as shown in Fig. 3. E, in Fig. 2, is the auxiliary ignition anode, which is used for starting the rectifier main arc. Closing a push-button switch energises the solenoid surrounding the spindle which terminates in the anode E.{Mr Elliott - Chief Engineer} The anode is forced down against the action of a spring until contact is made with the mercury cathode. Completion of this circuit short-circuits the solenoid coils, and the anode, E, is withdrawn under the action of the compressed spring. As it leaves the surface of the mercury a small arc is struck, which in a second or so vaporises sufficient mercury to form a conducting path to enable the main arc to start. In this condition a pressure of only 20-30 volts is sufficient to start the main arc. The operation of starting up a rectifier is thus a very simple matter, occupying only a few seconds. Fig. 2.—Section through a 300 amp. normal rating Brown, Boveri Rectifier. The construction of large steel cylinders, which will maintain the high degree of vacuum necessary for successful operation, has proved one of the difficulties in the development of the power rectifier. In modern practice all joints are made of asbestos-packing and mercury, which form such an effective seal; a vacuum of 0.01 mm. mercury is easily maintained nowadays. A two-stage mercury air-suction pump is an important part in the operation of the rectifier, but they are not necessitated by leakages through the joints, their function being to remove any gases given off by the anodes when heavily loaded. By this means the vacuum may be increased up to .005 mm. hg. in operation. This results in an increased efficiency. The importance of maintaining a high vacuum will be appreciated when considering the question of efficiency, but we may also mention that increasing the vacuum increases the overload capacity of the set. Losses in mercury rectifiers are due to the P.D. drop across the arc, anode, and cathode, and arc. The first two remain practically constant. This may be taken as 6.5 volts for iron anodes and 5.5 volts for the cathode, making a total of 12 volts. The drop in the arc depends on the vacuum, current density, and section of the arc. In the largest size rectifier 20 volts is about the average figure when assuming the arc length as 30 cm. long, or 0.4 volts per cm. This pressure of 20 volts is sufficient to maintain the arc, though a somewhat higher voltage is required to start it up, and the drop remains practically constant for all loads. It follows that the efficiency of the rectifier is almost constant at all times, the losses being made up of the product of the line current and the total drop from anode to cathode. Also the higher the d.c. pressure the higher will be the efficiency. For instance, at 220 volts it is about 91.6 per cent. ; at 550 volts 96.5 per cent.; and at 1,500 volts, 98.5 per cent. In the first case the drop across the arc is taken as 20 volts, while in the second and third cases 22 volts is allowed as the distance between anodes and cathode is slightly increased in the higher-pressure rectifier. The foregoing figures apply only to the rectifier proper and do not include the losses in the auxiliaries—such as vacuum-pump, and so forth. Fig. 3.—300 kw. 600 volt Rectifier at Asnières (Paris). Taking all losses into account the following table shows the running efficiency of rectifiers, comparative figures being given of rotary converters, and motor-generators of similar capacity. EFFICIENCY. Plant capacity. | 1/4 Load | 1/2 Load | 3/4 Load | Full-load. | Per cent. | Per cent. | Per cent. | Per cent. 500 kw. 550 v.{VIENNA} | Rectifier ...... | 92.5 | 93.5 | 93.8 | 93.6 50 periods | Rotary converter | 85.0 | 91.0 | 92.2 | 92.8 1,000 kw. 550v. | Rectifier ...... | 93.0 | 94.0 | 94.2 | 94.0 50 periods | Rotary converter | 87.0 | 91.4 | 92.8 | 93.0 | Mtr. gen. ...... | | 92.5 | 93.0 | 91.6 1,500 kw. 550v. | Rectifier ...... | 92.3 | 93.6 | 93.8 | 93.7 25 periods | Rotary converter | 87.0 | 91.5 | 93.2 | 93.9 1,500 kw. | Rectifier ...... | 95.2 | 96.2 | 96.3 | 96.1 1,500 v.{VIENNA} 25 pds. | The over-all efficiency of the rectifiers, it will be seen, is higher than that of rotary-converters or motor-generators and more uniform, remaining approximately constant from 1/4 to full load. Below 1/4 full load the losses in the auxiliary gear begins to be appreciably felt. The two-stage vacuum-pump, for instance, consumes about 1 kw. continuously. At light loads, say 1/5th normal, separate excitation by means of an auxiliary arc is necessary, as the main arc has a tendency to become unstable. The exciting arc is kept permanently in circuit, and consumes about 1-kw. per cylinder. With high d.c. pressures—1,500 volts it will be seen, excellent results are obtained. In general the power factor is from 90 to 97 per cent. The six-phase rectifier is used for the lighting, traction and power purposes, being built for two and three-phase a.c. supply. In the former case two two-phase transformers Scott-connected are used, and in the latter a single six-phase transformer. The wave form of the direct current produced by the six-phase rectifier is reasonably flat; and by the use of suitably-designed choking coils the undulation of the rectified wave form compared with the a.c. wave can be reduced. Fig. 4.—550 kw. 600 volt Rectifier at Orbe, Switzerland. | ||