Document 50EwJ29eVGKpy6KOM34Xj3B2D

centrifugal force these rings enlarge. They always stretch within their elastic limit, thus permitting the commutator to remain smooth as it increases uni* lormly in diameter. Brush HoUftrt Simplified. For about IS years, medium* and larger-sized dc machines hove used double brush hold ers lor two reasons. !. Two brushes, separated by a thin partition with Individual-pressure fin gers on each brush, reduce circulating current. This current, instead ol simply flowing from one commutator bar across the loco ol the brush into the next bor, Is forced to travel the full length of one brush through the shunt and back the length of the next brush. 2. Double brushes give better con tact between brush and commutator surface. This Is because inertia ol brush is less and irregularities of the com mutator surface do not necessarily lift both brushes. Individual fingers and springs permit separate brush wear. Pile* Kxcltars. For many years, stand is ample for normal fault contingencies. shunt field, fed from the station battery ard practice has used pilot exciters Electronic Eschars. Ignitron-type ex or any other source of approximately : mounted on the shaft extension of the citers have been supplied os a means of constant dc voltage. This provides sta main exciter, photo on p 70, to furnish excitation for Isrge-aized turbine-gener bility at low voltages during starting or excitation (or the main exciter. This ators. When the ignilron receives its any other time that the unit is under eliminates the main-generator field power from the generator It excites, spe- hand control. rheostat with its losses and apace re cial precaution* are used against Una Rheosibt arms In fields 1 and 3 ere quirement*. and provides absolute sta disturbances. This is necessary because mechanically connected for motor or bility of the mein exciter when operated the dc-voltago output of the exciter hand operation, as desired. As the arms at any voltage the ac generator requires. Various methods of eliminating the pilot bears a definite relation to the ac volt age. With some electronic exciters, a turn, resistance is cut into one field cir cuit and out of the other. Also, the / exciter have been tried, such as notch small separate ae generator coupled to rheostat for the separately excited field ing the poles of the mein exciter. The the shaft of the turbine-generator sup 3 is built so at dome point, about half ' weakness of these schemes la the poutbllity of instability when main exciter plies on Independent source of exciter power. voltage on the main exciter, the rheo stat Is open circuited and remains so as operates self-excited at low voltage, Both of these types of excitation sys it is turned further in a direction to in i \ The current per brush on pilot exciter U low--about 2% amperes. It eon be carried directly into the brush from the tems are more costly and take up con siderably more space than the directconnected exciter. The rectifier itself, crease the voltage. This (s done because field 3, the stability field, is not needed at higher exciting voltages- This re : tip of the brush finger without difficulty. Brushes for the pilot exciter arc stand- however, can be anywhere In the station. Electronic units are subject to norma) duces drain on the station battery. Shunt field 2 is excited from the roto- i' * ard but the shunts are cut off to within one inch of the brush as a means of re moving a worn brush. Flexible shunt* are used on the fingers, so spring end finger bearings do not carry current. hazards of rectifier operation end are oversized so they can operate with part of the unit not normally functioning. Rototrol Excitation. Rotating regulat ors, usually built to replace the conven trol pilot exciter. This scheme uses a "buck-boost" rototrcl pilot exciter. In diagram, facing page, rheostat on the main exciter is adjusted to provide a base value of excitation (or the ac \ Separate taclter*. In the past, when tional pilot exciter, constitute a recent generator. This can be set to maintain ,i *i i j: motor generators or separata steam turbine-driven dc generators were used as exciters, they were opeo-typo conven tional machines. Recent improvements on motor-generator exciters consist of an oversized induction motor and a direct-coupled fly-wheel. This allows the exciter to ride through system disturb- improvement applicable to rotating exeiters. Several schemes involving roto(rob have been -applied. In one type, the main exciter is o conventional unit and receives its full excitation from a rototrol pilot exciter, fn a modified scheme the main exciter is conventional but has three shunt-field windings, dia steady-state stability at any load desired on the ac generator- The pilot exciter operates so field 2 of the main exciter either adds to or subtract* from the ex citation o( field 1. This requires the pilot exciter to operate at positive or negeiivo polarity ond o( a magnitude to produce the necessary excitation. A i anecs where the motor receives Its power from the main generator being served. The ac voltage can drop to gram on facing page. In this system, exciter shunt-field winding number 1 provides most of the main exciter-field excitation end re generator static voltage regulator and the "buck-boost" rotolrol pilot exeiier produce excitation of the comet polar ity and magnitude to maintain the ac l 70% of normal value (or a relatively ceives its energy from the main exciter voltage in spite of changing loads. , long period, or to zero for 0.3 second armature circuit in the usual manner. This is belter than using excitation without loss ol stability. This provision Field 3 is a small, sepiraiely excited for the main exciter from the rototrol n |to) POWER January 1948 EXCITATION by buck-boost rototrol Is shewn In achcmatlc diagram, right pilot exciter because if anything hap pens to the pilot or any part ol tb volt age regulator, complete excitation U not lost, nor would continuity of load be dis turbed. In this cose, the main exciter, operating os a self-excited generator, provides excitation for (he ae generator at some predetermined voltage, say, 75%. If such a failure occurs and the load is other than 75%, the generator continues to carry whatever load Is plsced on It within its capacity. But the power factor changes, since the ae generator is under- or over-excited de pending on whether the load It more or less than the original MtUng. The equipment continues to (unction fa this manner, even though a short circuit or open circuit develops In the pilot ex citer output. This would not be Uue if fields 1 and 2 of tbo main exciter were combined, nor if (be "buck-boost'' rototrol pilot were connected In series with the circuit of self-excited field 1. If a failure docs occur, (ha excitation system can be operated manuallyla tbe diagram above, only two fields are used on the rototrol pilot exciter; a sel(energising series field, and a con trol field that combines a differential shunt field and a pattern field. Its ex citation Is secured from the turbinegenerator through a voltage-regulating Potential unit end a voltage-regulating Satomstio-control unit. These two dorices arc adjustable, but have no moving Ptrti in the sense of an ordinary voltage regulator of the contact-making or rheostatic type. Voftega Rtgulotion. The various com ponent parts of the voltaga-regufatlng potential un. and voltage-regulating sutoratiic-control unit function as fol lows: Current transformers are located on conductors A and C while a potential transformer Is connected to conductors A and B. Current-transformer A sup plies a filter reactor, whereas C i* con nected through a resistor to half ol the some primary winding of the filler re actor. Output volloge of the two cur rent transformers produced in the sec ondary. winding of the filter reactor is combined with the voltage output of the secondary of the potential transformer to make tbe voltage regulator re sponsive to three phase-voltage condi tions. Final ae output voltage is a meas ure of the voltage delivered by the ae turbine-generator, compensated for the load current being caniedrThis output voltage is fad through the two control switches and a voltage-adjusting resis tor into two impedance circuit*, one capacitive ond the other reactive, each terminating in o rectifier. The curve shows capacitor and r* actor impedances for a static voltage regulator. The relation between the applied so voltage and the current through a capacitor is a straight line OPC. Curve OPX, Is nonlinear and shows relation of the ac applied volt age and the current in a saturated re actor. With the proper capacitance and reactance, these two curves intersect at P, as indicated. This point (s chosen to correspond approximately to normal so voltage on the turbine-generator. The voltage-adjusting resistance unit per mits reasonable variations in this volt age, as desired by station operation. The two rectifier unit* are connected in series through a mld-tapped resistor. The equipment is adjusted so these two dc output voltages are equal when the ac generator output (a at normal voltage. Under this condition there U no volt age output at YY, In diagram above. If ac-generator voltage drops, volt age applied to the copacltor and reactor la reduced *o their delivered voltage changes in accordance with the curves above. This produces a different do output voltage from each of the two rectifier unit*. When this ocean, the difference fa the voltage that appears across point* YY and Is fed into the rototrol control field. The polarity is such as to cause the "buck-boost" rototrol to Increase its voltage, for a drop In the ao voltage. The pilot exciter thereupon increase* it* voltage and adds excitation into field 2 of the main exciter. This increases the excitation on the main exciter and, fa turn, excitation of the ac machine, until its voltage is again brought baek to normal. There is then no difference fa the output from the two rectifiers end thus no voltage b produced ot YY and the rototrol control field operates at zero excitation. If the voltage of the ac generator In creases, say, from a drop in load, the same action takes place, but of opposite polarity. Here, the polarity ol the yplu age from YY Is reversed and, when de livered (o the pilot rototrol control field, puls a voltage of opposite polarity on field 2 of the main exciter. This bucks it* voltage downward and decrease* the excitotton on the main exciter and on the field of the oo machine, bringing ae [Continued on page 134) ^OWER January 1948 (II) 71