Document XOq9m3wLrpK8JbdXN3Q83BbwJ

EARLY MAIN AND PILOT EXCITERS on turblne-oen- adapted to fit the Job. TMa photograph ahowa a 8600-rpm erator aota were generally conventional do machines machine Installed about the time of the first world war Turbine-Generator Exciters Keep Highspeed exciters are no longer a standard dc machine connected to the turbine*generator shaft. They give increosed service, run as long as the main generator without shutdown By C IYNN, Manager, De-Engineering Depl, Wettinghonje fiec Co/p Shaft-drivcn kxcitebs provide ihe commonest method of furnishing excita tion for turbine-generators. The reason (or their wide use Is reliability of drive, and unity of construction and operation of turbine, generator and exciter. Any other source of excitation ia subject to failure of (he drive or other units intro duced Into the eystem. The one disad vantage of shaft-driven exciters is that the turbine-generator must be shut down, in case of trouble, to work on or remove the exciter. Alternate methods of excitation have until recently used rotating dc machines, usually motor-driven, although some times separate small turbines have driven the exciters. With a separate system o( excitation, an individual ex citer for each sc machine, or a common exciter, has sometimes been used. Before 1918, most 60-cycle turbinegenerators were built for 1800 rpm. Ex citer capacities were small, as gener ators themselves were not large. Be cause the exciters were small, it was not difficult to build shaft-driven excit ers for either 1800 or 3600 rpm turbinegenerators. By 1918. 3600-rpm generators had been built in capacities up to 6250 kva, requiring direct-connected exciters of 50-kw rating. By 1919,7500-kw sices had been sold, ond in 1927 a unit was built for 3600 rpm, rated at 12,500 kva. This machine hod a direct-connected 62V-kw exciter. To date, the largest 1800-rprn turbine-generator is 180,000 kva, with a 375-kw 250-v direct-connected exciter. The largest-3600-rpm unit ia 111,800 kvs and the largest direct-connected 3600-rpm exciter is 280 kw. Eorly Machines. At first ihsli-driven exciters wero nothing more than opentype conventional do mochines, modified to suit operation of higher-speed units. Such machines were usually one- or two-bearing type, driven from the'end of the generator shaft by a crude tort of flexible or solid coupling. At that time, turbine-generators, boilers and their auxiliaries were not very reliable. Units did not operate for sustained periods; shutdowns occurred In normal operation and gave sufficient time for regular maintenance work on the 'exciters. Photo above shows a 3600-rpm directconnected turbine-exciter built about the timo of the first world war. Most manufacturers of large-sited central-station 3600-rpm turblne-gener* ators have built direct-connected excit ers In sixes up to about 150 kw. Beyond this rating some manufacturers have built shaft-driven exciters coupled to the turbine shaft by a reduction gear with exciters operating at about onehalf turbine speeds. The redaction gear requires additional apace for (he. unit; also, a speed of 1800 rpm means an exciter physically twice as large as that for 3600 rpm. The commutator peripheral speed of the 1800-rpm ex citer Is only about 20% lower than far the corresponding 3600-rpm unit. With 70 ;ej POWER January 1948 fXCITER ACCESSIBILITY for maintenance of bruahta has plant equipment hat Increased. Closeup view of a modem become progressively more Important as service of power- exciter ehows the ease with whleh brush rlgQlng Is reached Pace With Increased Operation correctly designed current-collecting pgftj--comntulator and brush 'rigging ste the heart of any dc machine--there it no reason to build geared exciters for present-day cepscities. As equipment improved, units could run continuously for longer periods. Exciters were, therefore, modified to give more accessibility for brush re placement, os in the photo above. Direct-Connected Eicltert. It Is Im possible to get brushes that will last from shutdown to shutdown to a new line of direct-connected turbine-exciters .hss been designed for continuity of operation. These use pedestal-type bear ings, is shown obove. To shorten the distance between bearings, armature diameters were made larger and the eore length shorter. The ventilating fan was removed from the main exciter. Journal diameters wer - increased. These changes raise the critical speed, which has always been above operating speed, and add smoothness. To simplify ventilation, the exciter was turned around with the commutator toward the ac generator. The ventilat ing fan has two acta of blades with a common discharge. Large blades cool the exciter and the smaller ones draw ventilating air over collector rings and brushes of the ac generator. Kxciter and pilot exciter are e com plete unit on a new type of depressed bedplate. This permits removal of main and pilot exciters as a unit if the gen erator rotor has to be removed. Realign ment is effected by simply setting the exciter back on the sole plate and in serting dowels. By depressing the bed plate and building the frame and pedes tals on raised sections a large apace ia provided below the commutator. Thia new type of enclosed exciter gives more accessibility than open machines of comparable size. Impnred Sondes. Since service con tinuity is of 'prime Importance, other factors that normally require mainte nance ofl dc machines have been mini mized or eliminated. Armature coils are brazed to commutator necks Instead of soldered. This makes a joint as strong as the copper itself. Commutator necks are packed between and behind the bars with insulation so no creepage sur< face exista. This eliminates ell surfaces lor collecting flyash and coal dust. Rear end of armature windings, which are normally left bare, are completely in sulated. The only surfaces on the rotor that- can collect conducting material from the ventilating air are at the com mutator necks and next to shrink rings. These are exposed and, when the unit it on slow roll, can be wiped ofl occa sionally with o dry cloth. Other wind ings, usually of bare copper, are com pletely insulated with glass tape or other high-grade material. Large-sized JB00* and 3600-rpm ma chines require shrink-ring commutators because of stresses in tbclr high-speed operation. Shrink-ring commutators, built over 15 years ago, used massive rings for holding bars in place, binding them down on an Insulated barrier on the shaft. Mechanically, such com mutator* would not fly opart; neither would they stay smooth through tem perature cycles. Commutators, there fore, must be designed and constructed to go through temperature .cycles end not hove uneven bars. On the old con struction, as the commutator heated up, the copper expanded, and one or more bars bowed outward alighlly between rings. That type made no provision for longitudinal expansion. TTe shrink ring of a modern^ .com mutator, drawing, p 72, is relatively small In section, ond made of nickelmolybdenum alee). Rings are stressed so they always work within the elastic limit. These rings act like a rubber band around the commutator. Aa the commutator gets larger In diameter be cause of temperature expansion and EOWER January 1948 17) 71