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 developments in mercury vapour rectifiers.
| Identifier | ExFiles\Box 32\5\ Scan001 | |
| Date | 9th June 1921 | |
| X.1589. MERCURY VAPOUR RECTIFIERS. Some Notes on Recent Developments. Reprinted from THE ELECTRICAL TIMES, 9th June, 1921. POWER-RECTIFIERS Limited, TRAFALGAR HOUSE, WATERLOO PLACE, LONDON, S.W. 1. 4 Reprinted from THE ELECTRICAL TIMES. 9th June, 1921. [COLUMN 1] to about 8 per cent; for the single-phase rectifier it is about 60 per cent. Between the d.c. pressure and that of the secondary winding of the transformer a fairly definite relationship exists. Regulation of the d.c. voltage is obtained either by step-by-step regulation of the primary of the transformer or by means of an induction regulator. The latter may be either hand or automatically operated. Induction regulation is preferable for lighting circuits owing to the smoothness of the regulation, and with automatic control very little skilled attention is necessary. Assuming a standard regulation of the transformer Fig. 5.—300 kw. 600 volt Rectifier on Traction Load in parallel with four 250 kw. Rotary Converters at Asnières (Paris). an inherent regulation of the d.c. voltage of 18 per cent., from no load to full load is obtained. By a suitable arrangement of the connections, however, a much closer control can be secured. For traction loads the rectifier can be given a shunt characteristic with a 4 to 5 per cent. rise of pressure with falling load, by the use of a specially designed reactance coil connected to the neutral of the transformer. The relationship of the two currents of a polyphase rectifier is quite definite. To obtain the theoretical R.M.S. value of the phase current, the total d.c. is divided by the square root of the number of phases. In the case of a six-phase rectifier √6 is 2.45, so that for a 600-amp. rectifier the effective phase current would be 600 / 2.45 equals 245 amp. In actual practice a higher figure than 2.45 would obtain to allow for the overlapping effect produced by the series and regulating coils. From the foregoing it is clear the k.v.a. rating of a rectifier transformer must be higher than the corresponding d.c. output. The primary, on an average, is designed for a 20 per cent. greater input to allow for efficiency and power factor, and up to 30 per cent. greater output for the secondary is commonly allowed, making an increase for the whole transformer of about 1.5 times the d.c. output. This does not mean a corresponding loss of energy, but only that the secondary windings are unfavourably utilised. The rating of the rectifier itself depends on the current carrying capacity of the anodes, and, as already mentioned, three standard sizes, 1,000, 200, and 350 amp. continuous output, are made by Brown, Boveri and Co. These sizes are made for pressures up to 750 volts, and, with slight modifications of design, up to 1,500 volts. It is anticipated it will not be difficult to construct rectifiers for converting current at 5,000 and 8,000 volts, and very high efficiencies will be obtained at these pressures. As the voltage of the rectifier rises the current rating of the anodes must be reduced, and they must be also placed rather further apart. Any output can be obtained by operating two or more standard units, in parallel either with a common transformer or with one for each rectifier as conditions require. The frequency of the main supply has no effect on the d.c. current produced, so that there is no difficulty in running two rectifiers in parallel, though they be running off different sup- [COLUMN 2] plan, both as regards voltage and frequency. This is undoubtedly a valuable characteristic of the rectifier. Comparing the mercury vapour rectifier with other converting plant, there is no doubt of its higher efficiency, except possibly at low d.c. pressures; both as regards starting and running it is more simple to operate and requires very little skilled attention. Automatic operated rotary converters, of course, need little attention, but a probable development of the near future is automatic operation and remote control of the rectifier, and this should enable a considerable saving in running charges to be effected. As regards weight, it is lighter than a rotary converter of similar output, and, being less than for a 2,20- kw. set, in general occupies less floor space, no special foundations are needed, the only requirements being that the floor level is sufficiently strong to take the weight of the machine. There are, of course, no running parts to cause wear and tear, so that maintenance charges are low, while a minor but satisfactory feature is noiseless operation. The makers claim that the rectifier is better able to cope with fluctuating loads than a rotary converter, and Mr. Ayton’s experience at Ipswich appears to confirm this. It may also be mentioned that 1,100 and 1,200v. sets are now in operation on the Continent on traction loads, the operation of which have been so satisfactory that a similar 3,000v. plant is contemplated. Beyond doubt, the mercury vapour rectifier will find a very wide field of application, and the rapid growth which has characterised its development is likely to be accelerated in the near future. From 1912 up to the present time about 142 rectifier sets, totalling 76,000 kw., have been installed or are on order, the majority being on the Continent. The only set in actual operation in England is the one at Ipswich—120 kw., 475v. of 3-phase 5,000v. 50 periods supply—but the following are, we understand, on order:—Hertford, 240 kw. d.c. 490v.— 3-phase 2,800v. 50 periods supply; Birmingham in course of erection, 225 kw. d.c. 450v.—3-phase 5,000v. 25 periods; Wolverhampton, 220 kw. d.c. 440v.—3-phase 6,500v. 50 periods; Glasgow, 830 kw. d.c. 520v.—3-phase 6,500v. 25 periods; Battersea, 830 kw. d.c. 460v.—3-phase 6,000v. 50 periods; Knottingley (Yorks), 195 kw. d.c. 220v.—3-phase, 10,000v. 50 periods; and Liverpool 258 kw. d.c. 250v.—3-phase, 6,300v. 50 periods. All the foregoing are for supplying lighting and power loads. As regards d.c. output, the sizes of existing plant range from the 120 kw. set at Ipswich up to the 2,350 kw. set at Hayang, Lorraine. The plant with the highest d.c. voltage is the 600 kw. 1,200v. set at Eckeberg, which is used for traction work off a 3-phase 10,500v. 50 periods supply. Our information is due to the courtesy of Mr. R.{Sir Henry Royce} J.{Mr Johnson W.M.} Robson, of Power-Rectifiers, Limited, Trafalgar House, Waterloo Place, S.W.1, and to some correspondence with Mr. Ayton, of Ipswich. Fig. 6.—800 kw. 580 v.{VIENNA} Plant for Traction Load installed at Berne early in 1919. St.{Capt. P. R. Strong} Clements Press, Ltd., Kingsway, W.C.2. | ||