Brophy, Jere E.

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 The Knowledge Bank at The Ohio State University Ohio State Engineer Title:  Control Generators Creators:  Brophy, Jere E.  Issue Date:  1942-02  Publisher:  Ohio State University, College of Engineering  Citation:  Ohio State Engineer, vol. 25, no. 3 (February, 1942), 9-11, 24-26.  URI:  http://hdl.handle.net/1811/35815 CONTROL GENERATORSJERE BROPHY, Engr. IDynamoelectric Amplifier—RototrolEver since power has been used intelligently it hasbeen harnessed. This has been the duty of the con-trol equipment designed by the control engineer.Control engineering grew up simultaneously with powerengineering, as is clearly demonstrated in electricalengineering. Among the various control devices de-veloped for use in this field are rheostats, startingboxes, contractors, relays, regulators, controllers, and,recently, control generators.Most of these methods of control have a low speedresponse to any sudden change, however, the controlgenerator may have both a high speed response andthe ability to control large amounts of power with asmall input.For this reason manymach ines using largeamounts of power wereoperated by mechanical,steam or hydraulic devicesuntil recently. This is par-ticularly true of the in-dustries engaged in themanufacture of heavy pro-ducts. Steel mills formerlyused steam power for re-versing, flipping and roll-ing their blooms and oper-ating the flying shears inthe finishing mill. Thiswas done to save time oneach operation in starting,stopping, reversing andaccelerating equipment andRototrol applied to a planer.work which had a high moment of inertia. Todaymuch of this work is being performed by large elec-tric drives since the large amounts of power requiredcan be quickly controlled by small, almost minute,amounts of power. The output of a photo-electriccell can be made to control the operation of a fourstand mill, if necessary.The machine which makes this possible withoutundue complexity is the control generator. It is es-sentially a motor-driven generator with an extra setof brushes for each set of poles. Any variation inthe input to this generator is greatly amplified in theoutput across the load brushes. This is the machinewhich amplifies a bit of control power of about onewatt to a controlling power of 2000 watts, 10,000 wattsor more, if necessary, with a high speed response.In external appearance, this dynamoelectric ampli-fier, or "amplidyne" or Rototrol as it is commerciallyknown, does not differ from the conventional genera-tor. Internally, it contains a set of short-circuitedbrushes in quadrature to the set of load brushes in theconventional generator.Before approaching the subject of motor controlCourtesy Westinghouse Electric &f Manufacturing CoRototrol Control on aPlanerFig. 6. Hot-strip mill installation using amplidyne exciter forof strip tension at reelwith the dynamoelectric amplifier, a bit of review ofsome of the characteristics of the direct current motormight not be amiss. The torque produced by a motoris a function of the product of the field flux andthe armature current. Thus, T=KfIa . Also, thespeed of a motor is directly proportional to its countere.m.f. and inversely proportional to its field flux. Thus,E V-IaRaS=K— or S = K where T is torque, K is af fconstant, f is field flux, E is counter e.m.f., la isarmature current, Ra is armature resistance, V is ex-ternally applied voltage across the armature and S isspeed. As may be readily seen from the aboveformulae, if the field flux is increased by increasingthe field current the torque is increased and the speedis decreased.The control generator takes advantage of this factin its regulation of the desired motor action. Thecontrol generator is used to supply the current neces-sary to produce the field flux required by the motor.Therefore, any variation in the control generator'soutput directly affects the operation of the motor.In the conventional generator a voltage is producedin the armature conductors by their cutting of thefield flux. The current in the armature is propor-tional to the armature voltage and the armature cir-V-Ecuit resistance. Cryptically, E=KfS and I a =R + Rawhere R is the load resistance. Another phenomenonwhich also occurs in the armature is the armature re-action. It is the flux produced by the armature cur-rent flowing in the armature and is, of course, pro-portional to the armature current. It is easily provedthat this flux is in quadrature, or at right angles, tothe field flux and is stationary even though the arma-ture is constantly revolving. This armature reactionis a nuisance in conventional generators since com-Court.-sy General Electric Co.constant-tension controlmutating field poles have to be Iprovided to compensate for it. IIn the case of the control gen-1erator this field is used ad-1vantageously.If the armature of a conven-tional generator is short-cit-1cuited the load resistance is al-lmost negligible. Since the ar-mature current is inversely pro-portional to the armature circuitresistance it can reach a vervhigh value with only a vervsmall field flux. The high ar-mature current produces a largearmature reaction or high ar-mature flux. If a set of brushesand poles are added to this short-circuited generatorthe armature reaction will produce an e.m.f. at thesenew brushes. This is a cross-axis excited generator.Schematic Development of a Control Generatorfrom a Conventional GeneratorIf a load is connected to the new set of brushesthe load current will, in turn, set up an armature fluxwhich will oppose the primary field flux since theload flux is in quadrature to the short-circuited arma- iture reaction. Another field is added to the circuitto neutralize the new load flux by placing a field coilin series with the load and winding it so that its fluxbucks the load current flux. Thus, the initial controlfield is unaffected and has only to form sufficientflux to produce enough e.m.f. for the short-circuitedarmature. Since this is almost pure resistance and ofvery low value the speed response is great. Whenthe cross-axis excited generator has the load-compensating field added to its other fields itis known as a dynamoelectric amplifier. This unit isLOAO AXISSHORT-CIRCUITAXISLOAD-COMPENSATINGWINDING•LOAOPOWER FLUX PRODUCEDBY SHORT-CIRCUIT CURRENT•-LOADCONTROL FLUXCourtesy General Electric CoFig. 1. Functional diagram of the control generatorPage 10 The Ohio State Engineer-POWER SUPPLYFig. 10. Photoelectric register equipmentusing the control generator for powersupply to Servo-correcting motorA: Scanning head.B: Electronic amplifier.C: Amplidyne generator.D: Control motor.Courtesy General Electric Ccthe machinewhich wasdeveloped un-der the super-vision of Dr.E. F. W.Al e xander-son.The addingof the load-compensatingfield and itsbalancing toneutralize theload flux isa d e l i c a t etask since anychange in thecontrol fluxmay cause changes 10,000 times as great in the outputcircuit. Moreover, the current flowing in the load cir-cuit is many times as great as the current in the controlfield. This makes the coil adjustment difficult.The amplification of a dynamoelectric amplifier maybe defined as the ratio of its output to its input to thecontrol field. This value usually ranges between 2,000and 10,000 but may go considerably higher.The dynamoelectric amplifier may be likened to atwo stage radio amplifier. The small current sent tothe control field sets up a small flux which causes asmall voltage in the armature conductors. As thearmature is short-circuited the current which flowsin it is very high. Thus, a small current with its indi-vidual idiosyncrasies has transmitted those peculiar-ities on a larger scale to a larger current. This mightbe considered as the first stage of amplification. Theamplification in the succeeding stage is smaller.In the second stage, the armature current producesa large flux called the armature reaction. This fluxcauses a high e.m.f. at the load brushes. Both stagesof amplification are now complete.In a typical machine theshort circuit voltage is low,about two volts. Very littlecontrol field flux is neededto produce this low voltage.The low resistance of theshorted armature permits ahigh armature current as aresult of the low short cir-cuit voltage. This currentwill produce an armature re-action sufficient to producean e.m.f. of 250 volts at theload brushes. The current inthe shorted armature is ap-proximately equal to theavailable load current. Since the short circuit voltageis so small the extra set of brushes introduces no addi-tional problems of communication.Amplification in a four kilowatt machine is approx-imately 175 in the first stage and 40 in the secondstage. The total amplification is the product of thetwo stage amplifications, approximately 7000.In another respect the dynamoelectric amplifier isanalogous to a radio amplifier; very high amplificationcan be secured at the expense of the time response.The sensitivity of the operation may be increased inthe ratio of 100 to 1 by permitting the cycle to take100 times as long to complete itself.Amplification can be further increased by introduc-ing a primary stage with "hard" vacuum tubes. Theoutput from such a stage is sufficient to control thegenerator since its power requirement is about onewatt. A high gain would result here since the controlenergy of the vacuum tube amplifier is measured inmicrowatts and its output is measured in watts. Thusminute forms of energy such as a beam of light, canbe utilized to control accurately and quickly largeamounts of power.More amplification can be achieved by cascadingthe power from the dynamoelectric amplifier into a con-ventional generator or another dynamoelectric amplifierwhose output is used to excite and control the finaldrive motor.The amplification of the control generator mayalso be increased by increasing the short circuit arma-ture flux. This may be done by adding field coilswhich aid the already existing flux. These coils maybe excited by shunting them across the load, by plac-ing them in series with the short-circuited armatureor both methods may be used simultaneously whennecessary. At times it may prove advantageous touse one of these methods to buck the primary controlflux and thus reduce the amplification.Since the control field needs a very small current{Continued on Page 24)Courtesy General Electric Co.Fig. 7. Steel-strip polishing apparatus using amplidyne exciter for reel motors to controlspeed and tension.February, 1942 Page 11CONTROL GENERATORS(Continued from Page 11)the physical size of the field poles and coils can bevery small. This permits several different controlfields to be placed on the same amplifier. It isreadily apparent that one field could be used as func-tion of speed; another of voltage, current or phaserelationship.At present, most dynamoelectric amplifier are beingused as exciters for final drive units in the heavy indus-tries. They are being used on synchronous motors tomaintain leading components and on steel mill, lowinertia motors and generators to secure a maximumreversal speed within proper limits of the current rise.Army mechanics may soon get to know the intrica-cies of the control generator from first hand expe-rience. Some of the new tanks wiil have dynamo-electric control. An airplane type motor will act asthe prime mover to drive a large, amplidyne-controlledgenerator which furnishes the power for the amplidyne-controlled driving motors.A large twin-head vertical boring mill in the Schen-ectady Works of the General Electric Company hasa simple but unique application of the dynamoelectricamplifier. There is a motor for each head whichcontrols its horizontal and vertical motion. Eithermotion may be selected by using the conventionalmechanical two-way clutch for operating either leadscrew.To start a cut the operator has only to engage theproper motion for the head with the clutch and pressa button. The head automatically moves to the cor-rect position for the cut and begins when the feedis engaged.This precise control is obtained by the accurateplacement of several limit switches which are actu-ated by the movement of the tool head. These switchesgovern the control field circuits of the amplifier.The power requirement for the control circuit is verylow which permits the use of small accurate switchesfor the limit positioning. Switches, half the size ofa match box, which require only a movement of twothousandths of an inch to operate them are largeenough to carry the control power of the amplifier.The control generator furnishes the armature powerfor the motor which drives the feed screw for the toolhead. The motor field is externally excited. Thus,the control generator may start, stop or operate themotor at any predetermined speed by varying the con-trol excitation to the motor. The excitation of thecontrol field of the amplifier comes from a tinygenerator that is driven by the motor for the toolhead. The output of this small generator passesthrough the limit switches and potentiometers to thedynamoelectric amplifier.(Continued on page 26)Page 24 The Ohio State EngineerTHE ENGINEER from the Chemical Workshad one of his usual headaches."We're pumping hot corrosive acidsthrough your glass pipe, and it lasts foryears," he moaned, "and the works bogsdown because the pumps can't take it!Can't you people build a glass pump?"It sounded impossible. Pump makers saidit couldn't be done. Such a pump requirednot only highly resistant glass but also in-tricate parts, accurate to thousandths of aninch! Even Corning had doubts but decidedto tackle the problem.Pooling its ideas with Nash EngineeringCompany's knowledge of pumps, Corningdevised new methods of glass manufacture,even a new type of glass for certain parts.And today chemical, food and beverageplants, and other industries handling cor-rosive solutions have a glass pump thatworks like a charm. Resistant to corrosion,it eliminates a cause of product contami-nation and undesirable chemical reactions.Resistant to heat shock, it may be cleanedwith hot acids. Transparent, it permits con-stant visual inspection for cleanliness,color, sedimentation.In the same way, Corning research forthree quarters of a century has licked suchglass problems as the bulb for Edison's firstlamp, cooking ware for housewives, andtiny glass springs for chemical equipment.And in these days of metal-conservation,Corning ability has reached a new high inusefulness as engineers and produc-tion men use glass to solve theirnew problems. Industrial Division,Corning Glass Works, Corning, N. Y.CORNINGmeansThe Nash Glass Centrifugal Pump (left — cross section; above—coupled with driving motor, glass pump pans in background)can handle up to 6000 gal. of corrosive acids and chemical fluidsper hour against a Oi-ft, head.Research in GlassHere's the Glass pump that couldn't be built...To adjust the position of the limit switches theoperator runs the head to its approximate positionand sets his first switch, the f:slow-down" switch, sothat it will be tripped in that position.This first limit switch changes the potentiometerconnections in the excitation circuit. This slows thedriving motor to one-fiftieth of its normal speed. Themechanic can then adjust the second limit switch toits proper position at this new speed. With this set-up an accuracy of stopping within two thousandths ofan inch in 14 feet is provided.Excitation for a 5000 horsepower synchronous motoris provided by dynamoelectric amplifier in a steel millinstallation.This control was designed to provide hightorque for peak loads. The motor is controlled tooperate with a leading current up to fifteen per centof the normal load. The necessary few watts of con-trol excitation are provided as functions of the linevoltage and the motor current.The Wheeling Steel Corporation had a d-c, dyna-moelectric amplifier-controlled installation for one oftheir mills completed in 60 hours which included dis-mantling time for the a-c. apparatus. With this newapparatus the operators are able to bring the tableup to speed in three and one-half seconds and stopthe table more quickly than with the old a-c. installa-tion which had frequency control.The Jones and Laughlin Steel Corporation installeddynamoelectric amplifier control on a 54 inch, fourstand, tandem mill producing tin plate. The originalproduction speed on the mill was 905 feet per minutebut with new gears and the new control system the speedwas raised to over 1650 feet per minute. The value ofsuch equipment at the present is obvious.The Irvin Works of the Carnegie-Illinois SteelCorporation provides another example of the versa-tility of the dynamoelectric amplifier. On a hot millshear high current acceleration for the motor andhigh acceleration and deceleration with precise stopsand smooth control for the shear are necessary. For-merly this was provided by an involved system ofcontactors, control relays, a counter e.m.f. set and acurrent regulator. Now an amplifier replaces all ofthese machines.Several installations of control generators for motorsoperating the rewinding and slitting of rolls of paperhave been made. To correct the deviation of theactual width of the slit paper from its optimum width(tolerance, 0.005 inch) a d-c. motor placed beneaththe machine is geared to move the main roll of papersideways to correct any error. A photoelectric scannerwhich registers on either a printed line or the edgeof the slit sheet sends its current to a vacuum tubeamplifier. The amplifier provides the excitation forthe control field of the dynamoelectric amplifier whichin turn, controls the d-c. motor for shifting the rolls.To eliminate any tendency for the equipment to lag orhunt, the paper is shifted under the photoelectric cellsimultaneous with the movement of the main roll.Strip grinders which require accurate control ofthe tension of the winding and unwinding reels areother successful applications of the control generator.Here compensation is also made for the changinginertia of the tension reels. Runout tables requirecurrent limit control for their acceleration and voltagecontrol at running speed. Tandem cold strip millsneed close interdependent speed regulation. Directcurrent machines mechanically parallelled demandspecial load divisions. In all of these situations thecontrol generator has proved itself. Other possibleuses for the dynamoelectric amplifier lie in the fieldsof transportation and combustion control.In general it may be said that apparatus requiringthe swiftly changing field excitation of some largemotors or with the highly variable armature demandsof some small motors, are well suited for dynamoelec-tric amplifier control. The control generator is nota panacea for all power control ills and should notbe thought of as such; it is a piece of machinery de-signed to supplement and not supplant any particularpiece of existing control equipment. However, inthe present situation in the heavy industries it can doa great deal of good in increasing speed and pro-duction.BIBLIOGRAPHY1. General Electric Review, March, 1940.2. Welding Journal, July, 1939.3. Power Plant Engineering, April, 1940.4. Iron and Steel Engineer, September, 1940.Page 26 The Ohio State Engineer

Control Generators

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Control Generators

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