speed governor fundamentals

7/29/2019 Speed Governor Fundamentals

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of Prime Mover Speedmanual25031.

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THE CONTROL OF PRIME MOVER SPEED is published as a series of four man-uals, each complete in itself, which together constitute a complete manual on gen-eral theory. The Table of Contents for all of the four parts is as follows:

PART I . . . . . .THE CONTROLLED SYSTEMFundamental MechanicsSpeed Control MethodsEffects of Prime Mover CharacteristicsEffects of Load Characteristics

PARTII . . . . .SPEED GOVERNOR FUNDAMENTALSSpeed Sensing DevicesCentrifugal BallheadHydraulic ServomotorSpeed Droop and RegulationTemporary Speed Droop (Compensation)lsochronous GovernorDerivative Control (Acceleration Sensitive Governor)Load Sensitive GovernorSummary of Woodward Governor Types

PART Ill. m.. .PARALLEL OPERATION OF ALTERNATORSFundamental Electricity and MagnetismElementary Generator and MotorAlternating CurrentThree-Phase Alternating CurrentInfinite BusSynchronizing TorqueLoad DivisionAmortisseur or Damping WindingsNatural Frequency of OscillationResonanceComputation of Natural Frequency

PART IV. . . . .MATHEMATICAL ANALYSISEquations of MotionApproximationsSolution of Second Order EquationCritical DampingFrequency ResponseAssumptions of LinearityUncompensated lsochronous Governor on Hydraulic TransmissionUncompensated lsochronous Governor on Diesel EngineStabilizing Effect of Speed DroopFirst Order Time LagTime ConstantDead Time or Transportation LagSpring Driven Oil Damped BallheadAbsolute DampingRelative Damping

0 Woodward Governor Company 1981

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MANUAL 2503 1THE CONTROL OF PRIME MOVER SPEED

PART IISPEED GOVERNOR FUNDAMENTALS

This manual replaces Manual 25011 Comparison of Hydraulic Speed Governor Princi-ples. This manual with Parts I and I II replaces our Manual 25010 Fu@amentals ofSpeed Governing.


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THE CONTROL OF PRIME MOVER SPEEDPART II: SPEED GOVERNOR FUNDAMENTALS

In Part I of this series we were concerned primarilywith the effect of prime mover and load character-istics on governing, rather than with the governorsthemselves, which is the subject of Part II.The use of the word precision as it is used in con-nection with speed governors implies that there aregovernors which are not precise, or that there arevarying degrees of precision in governor control.This is, of course, true. It is also true that for manypurposes, such as limiting controls in the automotivefield, relatively crude and insensitive governors areperfectly adequate for the job required of them. Anoutstanding example of this type of governor is thatused to control the throttle on small engines such asthose used to power lawn m owers. In this case a flatvane is placed in the blast from the cooling fan andso pivoted and connected to the throttle valve thatan increase in the air blast resulting from increasedspeed moves the vane against a light spring in thedirection to close the throttle (Figure 1).

r GOVERNOR SPRING

TLEFUEL

L VANE

Figure 1

This represents about the ultimate in inexpensivegoverning, yet it is completely adequate for the ap-plication. Our interest, however, lies with the jobwhich requires high accuracy of control and theavailability of considerable force to operate thethrottle. Such a problem does not yield to such asimple and inexpensive solution.

It is apparent from what has gone before that speed governor must include at least two compo-nents; a speed sensing element and a device to operate the throttle. In the simplest governors, as whave seen, these may be one and the same. Whereconsiderable force is required, however, an addition-al device called a servomotor is required. This servomotor, capable of exerting the required force, controlled by a speed sensing element which in itselfmay have very little energy available at its output.Anumber of means which are used for sensing speedchanges and actuating the gate or throttle will bmentioned.

SENSING DEVICE OUTPUT

5.5 PRESSUREFROM PUMPROTATION PROPORTIONAL TOPRIME MOVER SPEED

Figure 2

A speed sensor of long standing consists of a centrif-ugal pump in which the dead-end or zero flow pressure is taken as an indication of speed (Figure 2)

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SENSING DEVICE OUTPUT PERM. MAGNETROTATING FIELD

FIXEDORIFICE

ROTATION PROPORTIONAL TOPRIME MOVER SPEEDFigure 3

Another uses a constant displacement pump dis-charging through a fixed orifice, the pressure dropacross the orifice being indicative of speed (Figure3).

K PERMANENT MAGNET FIELD

CONTROLVALVE

Figure 4A number of electrical schemes have been used in-cluding a small d.c. generator with permanent mag-net field for which the output voltage is propor-tional to speed (Figure 4).

I RECTIFIERFigure 5

A variation of this scheme which eliminates com-mutator and brushes uses a small permanent magnetalternator, the output of which is rectified, the d.c.voltage being used as before (Figure 5).

PERM. MAGNETALTERNATOR

FREQUENCYSENSITIVENETWORK IIr 1\ II AMPLI FER I

I

Figure 6

A third electrical scheme uses the permanent magnetalternator, but feeds its output into a frequencysensitive network which provides a signal approxi-mately proportional to the deviation in speed froman equilibrium value (Figure 6).

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All of the above have their advantages and disad-vantages which determine their fields of applica-tion. W oodward has considered and, for specialpurposes, used most of them, but the speed sens-ing device preferred for its sensitivity, ruggedness,and the usefulness of its output is the centrifugalballhead.This mechanism is probably the oldest and certainlyone of the simplest of speed sensing devices. It de-pends for its operation upon the fact that a force isrequired to compel a mass to follow a circular path,as mentioned earlier. This force is proportional tothe square of the speed of rotation and to the firstpower of the distance of the mass from the axis ofrotation. In its best known form the ballhead con-sists of a pair of weights, usually spherical, at theends of two arms pivoted near the axis of rotationin such a way that the flyweights can move radiallyin a plane through the axis. Additional links are at-tached to the arms and a collar about the axis toform a parallelogram configuration. Thus, when the

Figure 7weights move outwardly, the collar moves up (Fig-ure 7). Since the centrifugal force always acts atright angles to the axis of rotation, it exerts a torqueabout its pivot equal to the product of the forcetimes the vertical distance of the ball below thepivot. This torque is opposed and, if no other forcesare present, must be balanced at equilibrium by thetorque of gravity, which is equal to the weight ofthe ball times the horizontal distance to the pivot(Figure 81. Thus as the speed increases, the centrifu-gal force increases and the ball moves outward, de-creasing the centrifugal force torque arm and in-creasing the gravity torque arm until equilibrium isreached. This results in a unique equilibrium posi-tion of flyball and collar for each speed of rotation.

AXISOFROTATI.ONCENT.FORCE t

WWEIGHT

CENT. FORCE TORQUE = FAGRAVITY TORQUE = WBAT BALANCE FA = WBFigure 8

In the direct mechanical governor this collar is conected to the throttle so as to close it as the fweights move outward. Early steam engine govenors were of this type. It should be noted that tunique relationship between speed and position ballhead and collar no longer exists if friction added to the system. This is for the reason that speed increases from an equilibrium condition tcentrifugal force torque must reach a value equal the gravity torque plus that due to friction befomovement results. Similarly on decreasing speed tcentrifugal torque must go down to a value equal the gravity torque minus that due to friction. Tresult is a so-called dead band or region in whicthe speed may wander without producing a corretive motion of the throttle. Efforts to minimize thdead band resulted in the huge cast iron flyweightscommon to early mechanical governors.From the above it is apparent that for greatest sentivity the force available from the ballhead upon small speed change should be large relative to tforce required for control. There are two approacheto this problem; the friction of the entire systeand the force requ ired for control may be minmized, or the force available from the ballhead upoa small speed change m ay be increased. The seconis accomplished when the size of the ballhead is creased or when a small ballhead is run at hispeed. The first implies careful construction to duce friction, and removal of the throttle load to


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servomotor controllable by a ballhead output oflow power level. Over the years the centrifugal ball-head has progressed through a great number of de-sign configurations to reach the form currently usedin Woodward governors (Figure 9). As used in thegovernors for internal combustion engines it usuallyconsists of two weights with their centers o f massapproximately the same distance from the axis ofrotation as the pivots about which they swing.So-called toes are arranged substantially at right

NON-ROTATINGSPEEDER SPRING

FLYWEIGHT

BALLHEAD

NON-ROTATING ASPEEDER ROD AXIS OFROTATION

Figure 9

angles to the body of the flyweight (they arentballs anymore) in such a way that as the weightmoves toward and away from the axis of rotation,

I the toes convert this motion to an axial movementof a PILOT VALVE or SPEEDER ROD through asuitable thrust bearing. The centrifugal force is op-posed and balanced by the force exerted by a com-pressed SPEEDER SPRING instead of gravity. Insome designs the toes are on the center line, andsince they move in an arc, some sliding with result-ant friction takes place. In other des igns the toes areoffset and contact the thrust bearing on a line atright angles to their plane of movement so that thearcuate movement is converted into a slight rotationof the thrust bearing with a minimum amount ofsliding friction (Figure 10). Friction is further re-duced in the more sensitive designs by the use of

CENTER OF ROTATION

THRUSTBEARINGFigure 10

antifriction bearings and cross spring pivots. Thesprecautions are taken to reduce the DEAD BANDwhich results from friction as described earlierIt might be noted here that complete elimination ofriction from the ballhead produces an undesirableresult. This is for the reason that the flyweight-speeder spring combination, forming a pendulumsystem, will tend to oscillate after a disturbance, giing a false indication of speed deviation. It is desiable to have sufficient damping in the system so thait will return to equilibrium with no more than onvery small overswing. It would appear that thimight best be done by utilizing a dashpot to provideviscous damping for a frictionless ballhead, inamuch as this would give the required damping without producing dead band. A new Woodward balhead for the cabinet actuator is, in fact, designedthis way, but primarily for different reasons. In thicase it was felt that this inherently more expensiveconstruction was justified in order to achieve maxmum performance over a long period without lubrication. In the smaller governors lubrication is inherent in the design, and this construction is unnecessary, since the amount of friction required for saisfactory damping is so small that the best we havbeen able to accomplish in the reduction of frictionin the pivoted ballhead still leaves enough for adequate damping and is at the same time sufficientlylow to provide an acceptably small dead bandAs stated above, the force necessary to restrain thflyweight, in this case by loading of the flyweighttoe, increases with the square of the speed andirectly with the radial distance of the flyweight

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center of gravity from the axis of rotation. Thus, ata given speed, the force will increase as the flyweightis permitted to move out. Wha t is called the fly-weight scale is the rate at which this force increaseswith movement, referred to the flyweight toe. Thus,if at some speed a movement of .OlO at the toeproduces a force change of 0.2 lb. the flyweight issaid to have a scale of 20 lb/in., which for the usualballhead with two flyweights is 40 lb/in. ballheadscale. For a given ballhead this is different at eachspeed, in fact, varying proportionally with thesquare of the speed.It should be noted that as the flyweights move out,the speeder spring is compressed. Since the speederspring also has a scale, this results in an increase inthe force opposing flyweight movement. If this in-crease is less than that of the ballhead for the samemovement, instability results. In other words, if acondition of equilibrium exists (which merely re-quires ballhead and spring forces to be equal), and aslight displacement of such a combination takesplace, the new forces of ballhead and spring are notequal and the net force is in the direction to con-tinue the motion. Such a ballhead and speederspring combination will, upon varying speed overthe necessary range, snap quickly from one extremeposition to the other. This characteristic may bedesigned into an overspeed trip, but it is useless in aregulating governor.If, on the other hand, the scale of the speeder springis appreciably greater than that of the ballhead, astable system results; one having a unique equili-brium position for each speed within the range ofmovement of the flyweights. Regulating governorballheads are so designed. By properly choosing theratio of speeder spring and ballhead scales a widerange of sensitivities or movement of pilot valve fora given speed change can be had. The best choice,however, depends upon the nature o f the systemwhich the governor is to control.Thus far no mention has been made of an addi-tional force factor which must be taken into ac-count in ballhead design, namely the hydraulicreaction force exerted on the piston type pilot valvedue to flow of oil through the ports. This force, forthe usual range of port openings, is in the directionto re-center the valve and varies approximatelylinearly with the valve opening. Thus this force alsohas a scale which, in the case of the directly connec-

ted valve, is additive to the spring scale, and if nottaken into account may produce a ballhead of muchlower sensitivity than anticipated. The magnitudeof this force is a function of the flow through thevalve and the pressure drop across it and is the prin-cipal limiting factor in the determination of the sizeof valve that can be satisfactorily actuated by a ball-head of given size.This implies that control of large oil flows, as required in water wheel governors, could not behandled directly by a ballhead of reasonable size.This problem is solved in cabinet actuators andwater wheel governors by the use of an additionalvalve. This valve, capable of controlling large oilflow, is arranged so that it takes a position propor-tional to the speeder rod position, but actuated bya hydraulic force many times greater than that avail-able from the ballhead (Figure 11). The dashpot illustrated will be discussed la ter.

I\ PI LOT VALVE

CLOSEGATES

t

1OPEN

GATES

1 RELAY VALVEFigure 11

As has been mentioned, the ballhead force and scalevary with the square o f the speed. This suggests thatto design a speeder spring for wide range operationmay be something of a problem, since it is desirableto have a nearly linear relation between speed andspeed setting and since for proper operation theratio between spring scale and ballhead scaleshould not vary too much. The problem has beenquite satisfactorily solved for speed ranges of sixto one and, in special cases, even higher, by the

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trumpet shaped or conical spring. This spring has anon-linear load deflection curve (Figure 12) since itis so wound that at light loads all turns are active,resulting in a low spring scale when the ballheadscale is low, while at high loads the larger turnsclose out, providing a stiffer or higher scale springfor the higher speeds.

LOAD

ORDINARY

SPEEDER SPRING

DEFLECTION

Figure 12

Despite the obvious changes which have takenplace during the evolution of the ballhead from itscrude beginnings, the qualities which recommend itremain substantially the same: it is rugged andsimple mechanically, has a useful output at a reason-ably high power level, is relatively insensitive totemperature changes, and is simply adjusted as tospeed setting. For these and other reasons this typeof speed sensing device is widely used in our gov-ernors in conjunction with a servomoto r to providea high power level output.Various types of servomoto rs are worthy of consid-,eration. In the case of hydraulic turbine gates it ispbvious that the energy required for their operationmakes direct control by the speed sensing elementimpractical. In the early water wheel governors thepower required was taken mechanically from thecontrolled water wheel through clutches selectivelyoperated by the ballhead. As control power require-ments became greater and higher speeds of gatemovement were made necessary by the demand forbetter regulation, the practical limit of the simpleclutch system was reached. Even for power require-ments well within its range, this mechanism has cer-tain disadvantages as a control servo, such as thedifficulty of securing and maintaining a smalldead band, and, most important, the fact that its

speed is proportional to prime m over speed regard-less of magnitude of speed error. The latter charac-teristic makes stable operation difficult because ofthe tendency to overcorrect for small errors. It isdesirable to have the speed of servo movement pro-portional to speed error at least for small errors, andan approximation of this can be secured by inter-mittent operation of the clutch, decreasing the per-cent of time engaged as the unit speed approachesthe proper value. However, this means added com-plication and more clutch wear.With electric motors so common, their use as servounits is an obvious possibility. As in the case of theclutch controlled servo, the power required is a vitalfactor in determining the suitability of electricmotors as servos. For water wheel gates where largeforces and rapid movements (that is, considerablepower) are involved, the size and cost of the electricservo becomes objectionably large. Furthermore,control of such a motor, continually started,stopped and reversed, is a serious problem. A fur-ther objection is that, the inertia being relativelyhigh, the speed of response is slower than desired.For relatively low power requirements, or wherehigh speed of response is not essential, the electricservomoto r finds entirely satisfactory application;but for our purposes it does not compare favorablywith the hydraulic servo.The hydraulic servo has a great many desirable char-acteristics to recomm end it and comparatively fewobjectionable features.Without storage accumulators, the combination ofhydraulic servo and pumqdriven by the controlledprime mover offers a relatively inexpensive, light-weight and compact means of providing power forcontrol. With accumulators, power can be suppliedfor a short time, say long enough to make one ortwo full servo strokes , at a very high level withoutthe necessity for increasing pump size (Figure 13).This is important where pumps may be electricmotor driven or where a limited volume of oil makesheating a problem. Control of a hydraulic servo isvery readily accomplished with little expenditure ofenergy by means of a piston type valve. If relativerotation between plunger and sleeve of such a valveis used, frictional opposition to motion is substan-tially eliminated so that speed sensitive elements oflow energy output may actuate directly a valve con-trolling a fairly large and fast servo. Furthermore

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CONTROLVALVE

ISERVO

ACCUMULATOR(OPTIONAL)

Figure 13

it is inherent in such a control that the speed ofservo movement decreases as the valve approachesits equilibrium position. The mass of the movingparts is so low compared with the force available,that the response of the hydraulic servo is extremelyrapid. If oil is used as the hydraulic fluid, lubrica-tion is inherent. True, the possibility of oil leaks,difficulties with flow at extremely low tempera-tures, and the necessity for maintaining the oilclean may be considered disadvantages, but they areat most no greater than those inherent in other sys-tems and are for our purpose far outweighed by theadvantages outlined above.Having given consideration to the relative meritsof the various governor com ponents discussed aboveand the accumulated experience of ourselves andothers, we have chosen to build our governorsaround the hydraulic servo of the reciprocatingpiston type and the centrifugal ballhead actuating apiston type pilot valve. Other types of hydraulicservo such as the vane type unit capable of move-ment of less than one revolution, and the hydraulicmotor capable of continuous rotation have foundspecial application, but the simplicity and dependa-bility of the reciprocating piston have thus far notbeen surpassed for general use. Three types of recip-rocating piston servos are used.

These are the double acting piston, requiring a pilotvalve having two control lands simultaneously regulating flow of oil to and from opposite sides of thepiston and connected to its load by a relatively smalldiameter piston rod projecting through a seal at oneend (Figure 13); the single acting spring loaded piston, in which hydraulic force under control of single land pilot valve overcomes spring and externalload in one direction and the spring discharges theoil from the cylinder and overcomes external load

SUPPLY

DRAINFigure 14

in the other direction (Figure 14); and the differen-tial piston, in which in effect the piston rod becomes large enough to reduce the effective area oits side of the piston to one-half that of the otherside. In this case oil at supply pressure is maintained

SUPPLY

DRAIN

Figure 15

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in the small area end of the cylinder and oil control-led by a single land valve is caused to flow in andout of the o ther side (Figure 15). In all cases the de-sign is such that substantially identical forces areavailable in both directions of motion. All three ofthe above servo types are used in Woodward gov-ernors, the choice being determined by the circum-stances surrounding each governor requirement.The basic principle of operation of the centrifugalballhead and the general form in which it is used inmost Woodward governors has been describedabove (Figure 9). However, ballheads appear in anumber of variations, the design being dictated indetail by the specific requirement. The same basicprinciples apply to all. The servomotor is so simplein principle as to require no further description thanhas already been given.Let us now consider the simplest form of hydraulicgovernor in which the ballhead and pilot valve de-scribed control a simple reciprocating piston servo.An adequate oil supply is assumed to be available.For the sake of discussion let us assume that thegovernor is to control a diesel engine. The ballheadis driven at a speed proportional to that of the en-gine and the servo is connected to operate the fuelracks (Figure 16). It should be noted that the sim-

For a given setting of the top end of the speederspring, the ballhead and valve will take this positionat only one speed; in other words, such a speedsensitive device is inherently isochronous. Unfor-tunately, such a system is also inherently unstable.This is because the engine speed does not instantlyassume a value proportional to the rack position dueto the inertia of the rotating mass. Therefore, if theengine is below the governor speed setting, the pilotvalve is positioned to move the servo to increase thefuel. By the time the speed has increased to the set-ting so that the valve is centered and the servo stop-ped, the fuel has already been increased too muchand the engine continues to speed up. This opensthe pilot valve the other way and fuel begins to de-crease. As before, when the speed gets to the rightvalue, the fuel control has traveled too far, the en-gine underspeeds, and the whole cycle continuesto repeat. Some means for stabilizing such a systemmust obviously be added to the two componentswe have described to secure a satisfactory governor.The simplest method of securing stability in the sys-tem described is to add means which will provideSPEED DROOP in the governor which in turnresults in REGULATION in the governed system.The distinction between these terms will appear aswe proceed.

OILSUPPL

INCREASEBALLHEAD SPEED

tFUI

PROPORTIONAL TOENGINE SPEED r-s- -THROTTLE

61RAIN

Figure 16ple combination of ballhead and directly connectedpilot valve has only one equilibrium position: theposition in which the valve is closed, neither admit-ting oil to nor discharging oil from the servo cyl-inder.

SPEED DROOP GOVERNORFor some obscure reason, the understanding ofspeed droop seems to give difficulty out of all pro-portion to its complexity. It is nothing more thanthe governor characteristic which requires a de-crease in speed to producen increase in throttle orgate. Since an increase in throttle or gate is requiredif the prime mover is to carry more load, it followsthat increased load means decreased speed. Why theaddition of speed droop should stabilize an other-wise unstable system is readily seen from a consid-eration of the differential equations of the two sys-tems. However, since this explanation is not univer-sally satisfactory, let us consider an analogy withinthe experience of all of us.Assume that the machine which is to be controlledis an ordinary automobile with foot throttle. Thedriver of the car is to function as servo in operatingthis throttle. For speed indication we will use thespeedometer, slightly modified; that is, we will re-place the speedometer needle with a solid disc, half

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red and half green, with the dividing line where theneedle pointer formerly was. On the face of thespeedometer we will place an opaque disc havinga very narrow slot near the edge through which wecan see the indicating disc. This slot may be posi-tioned at will as an adjustable speed setting, but wewill assume it to be at the location corresponding to50 mph, the speed which we wish to maintain. Thedriver is to attempt to maintain this speed by react-ing to the speed indication as follows: whenever theslot appears red, indicating an overspeed, the throt-tle is to be completely closed; if green, completelyopened. It is assumed that someone else will watchthe road. Suppose our car to be rolling along atequilibrium at 50 mph and to come to a slight up-grade. The speed will drop, the slot will show green,and the driver will tramp the throttle to the floor.The car will then speed up, the indication will turnquickly from green to red as it goes through 50mph, and the driver, his reaction time being what itis, will, an instant later and having overshot, com-pletely close the throttle. The speed will then dropquickly through 50 mph and the cycle will be re-peated indefinitely. This is a system without speeddroop.Now let us asume that the slot is increased in widthso that its lower edge is at 45 mph and its higheredge at 55 mph. Our instructions to the driver arethat when the slot shows all red, which will occur at55 mph, the throttle is to be completely closed;when all green, or at 45 mph, it is to be fully open.For intermediate speeds or ratios of green to totalslot width, the throttle must take a proportionalposition. With such an arrangement, starting againfrom equilibrium, the slight decrease in speed causedby the upgrade does not change the indication to allgreen, but only increases the amount of green whichresults in a small increase in fuel, not a completethrottle opening. The car will take the grade at aslightly reduced but stable speed, and the throttlewill not be completely opened unless the grade issufficiently severe to require full power at 45 mph.Such a system has speed droop. In effect it gives thedriver (or servo) a chance to start correction beforethe desired operating point is reached.Your attention is called to the fact that the simplemechanical flyball governor in which the throttleis operated directly by the ballhead has speed droopinherently, since the only way the throttle openingcan be increased, for instance, is by the flyweights

moving inwardly, which requires a decrease in speedSpeed droop in a simple governor can be providedby a mechanical interconnection between servo (an

INCREASE

Figure 17

therefore throttle) movement and governor speesetting such that as fuel is increased the speesetting is decreased. Such a device may consist simply of a lever of suitable ratio between servo anspeeder spring (Figure 17). The equilibrium relation-ship between speed setting and servo position fosuch a system may be represented by a line slopingor drooping downward to indicate decreasespeed setting with movement of the servo in the increase fuel direction. For each position of the manual speed adjustment there will correspond a slopingline such as that shown in Figure 18. In this e

SPEEDSETTING I INCREASE FUEL00 100%SERVO POSITIONFigure 18

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__ ._ -..

ample, the speed adjustment is assumed to be suchthat if the servo is at its extreme position in theincrease fuel direction, the governor speed settingis 1000 rpm. As the servo is moved to the oppositelimit of its motion, the speed setting is increased to1040 rpm. The speed droop is usually expressed asthe percent change in speed setting for full servostroke referred to the speed setting at the maxi-mum fuel position. In this case the governor is ad-justed for 4% speed droop. This characteristic is afunction of governor design and adjustment only.Regulation, however, is dependent not only uponthe speed droop setting but also the percentage ofgovernor servo stroke required to move the throttlebetween no load and rated load positions. It is thesteady speed rise resulting from decrease in loadfrom rated value to zero expressed as a percentageof rated speed. If the governor discussed above wereso connected to the throttle of its engine that only50% of the servo travel were required to move thethrottle between no load and rated load, the regu-lation would be 2% although the speed droop is 4%.

INCREASEFUEL

DRAIN

Figure 19

Speed droop in a hydraulic governor is not alwaysattained by operation on the speeder spring asdescribed above. It can also be secured by changingthe position of the flyweights (and therefore theirspeed for a given speeder spring setting) required tocenter the control valve at its equilibrium position.This might be done by using a floating lever connec-tion between speeder rod, servo and pilot valve (Fig-

INCREASE

@y Furp

Figure 20

ure 191, or by havingservo (Figure 20).ISOCHRONOUS

the bushing movable by the

GOVERNORIt is sometimes desirable to have an isolated primemover run ISOCHRONOUSLY, (speed constantregardless of load within the capacity of the primemover) or perhaps the allowable regulation is notsufficient to provide stability. In such cases, we re-sort to transient speed droop, or, as it has come tobe generally called, COMPENSATION. This callsfor the introduction of a temporary readjustment ofspeed setting with servo movement to produce thestabilizing speed droop characteristic, followed by arelatively slow return of speed setting to its originalvalue.This can be accomplishecf-in a number of ways.Present day hydraulic governors use a variety ofschemes, perhaps the most common (Figure 21) in-volving a floating fever connecting speeder rod, pilotvalve and receiving piston which is urged by springforce toward an equilibrium position. So long as thereceiving piston is in this equilibrium position, cen-tering the valve requires that the flyweights be al-ways in the same position. For a fixed speederspring setting this means that the ballhead must, forfinal equilibrium, be running always at the samespeed. The receiving piston is displaced from itsequilibrium position by a flow of oil initiated bymovement of a transmitting piston which moveswith the throttle- or gate-actuating servo. Thus ifthere were no leakage from this compensating hy-

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CENTERING INCREASESPRING \ 1 FUEL

RECEIVINGPISTON TRANSMITTINGPISTON

Figure 21draulic system the receiving piston would move asthough rigidly connected to the servo and perma-nent speed droop as described above would result,the centering spring being ineffective. However, if anadjustable leak in the form of a needle valve is pro-vided between the compensating hydraulic systemand an oil sump, the centering spring is permittedto return the receiving piston slowly to its initialposition after a disturbance by forcing oil out ordrawing oil in through the needle valve as required.As this occurs the speed setting of the governorslowly returns to its original value, although theservo and throttle are permitted to remain at thenew, different load position. Such a governor isisochronous although provided with the necessarytransient droop for stability. This scheme is used incurrent water wheel, type IC and type UG gov-ernors.Another method of producing the same result in-volves the direct (Figure 22) application of pressureto the pilot valve plunger, adding to or subtractingfrom the speeder spring force in order to effect achange in speed setting. In the adaptation of thismethod used in the PG governor the oil actuatingthe servo is required to deflect a buffer pistonagainst a centering spring load which produces apressure differential across a receiving piston rigidlyattached to the pilot valve plunger. A needle valve

permits equalization of pressure across the pilotvalve receiving piston to restore the initial speedsetting. In operation, as oil flows to the servo, thebuffer piston is moved against the force of its cen-tering spring, resulting in a higher pressure on thelower side of the receiving piston which produces anupward force on the pilot valve. This in effect de-creases the force which the flyweights must balance,resulting in centering of the pilot valve at a lowerspeed, thus providing speed droop. As the displacedoil is permitted to leak through the needle valve, thebuffer piston returns to its equilibrium position, thedifferential pressure disappears, and the speed set-ting reverts to its original value. This method, al-though requiring a more expensive pilot valve con-struction, has several operating advantages, not theleast of which is that exact return of the bufferpiston to center is not essential to exact return ofthe system to normal speed, equalization of pressureonly being required.

- PISTON

Figure 22

It is obviously possible to combine temporary andpermanent droop in a single governor, (Figure 23)and this is done in many s tandard units where par-allel operation requiring proper division of load isanticipated.Other methods of securing stability are known andhave been used. Among these is the so-called deri-vative control. This means a device sensitive to thefirst (or higher) time derivative or rate of change of

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CENTERINGSPRING \INCREASEFUEL

RECEIVING\f- TRANSMITTING

PISTON PISTON

Figure 23the controlled variable. In the case of a speed con-trol a first derivative system means simply a devicesensitive not only to speed, but sensitive also to therate of change (first derivative) of speed which isacceleration. Acceleration sensitive governors wereused long before they were called derivative con-trols, and we have made an extensive mathematicalanalysis of them in various applications. Accelera-tion sensitivity alone is unsatisfactory, since it willnot accurately maintain speed, although its correc-tive actions are in the directions to tend to do so.

, The minimum requirement in an isochronous con-trol so stabilized is therefore a speed sensitive element and an acceleration sensitive element. Prop-erly designed, it is possible to secure stability in sucha mechanical device, but we have not been favor-ably inclined toward it for the following reasons.If wander over a narrow band is to be avoided, thestabilizing effect of the acceleration sensitive ele-ment must be present for very small accelerations ordecelerations. This is very difficult to accomplishmechanically without the use of objectionably largemass or very expensive and delicate construction.Furthermore, mathematical analysis, confirmed bytests, reveals that with rare exceptions, use of thederivative control can accomplish nothing in the

way of regulation about an on-speed condition thatcannot be secured as well and at much lower cost byproper design of the compensating systems de-scribed above. If the disturbance consists of a largechange in speed setting rather than a change in loadthe acceleration sensitive element o ffers some advan-tage, but even here it is doubtful if the improvementobtained justifies the complication and cost in-volved.What has been said above applies to mechanical-hydraulic governors; it does not preclude the possi-bility of accelerometric stabilization in an electricgovernor, and we have in fact such a control sys-tem for hydraulic turbines.LOAD-SENSITIVE GOVERNORAnother approach to the improvement of governorperformance upon load change requires that thegovernor be made directly responsive to loadchanges as well as to speed, with the purpose of se-curing a corrective change in throttle position beforethe speed change, which is necessary to make thespeed governor function, occurs. This can be madequite effective in some applications if properly en-gineered and designed, and a governor of this typehas been developed by Woodward. The schemewhich we have used in this design positions thethrottle proportionately to the load change bymeans of an extremely fast hydraulic servo, leavingthe speed governor only the job of trimming or cor-recting for relatively small errors in throttle posi-tion. It has the unique advantage of being a verygood speed governor even if the load sensing func-tion were to become completely inoperative.However, load-sensing has certain limitations, whichshould be noted. For instance, the speed of responseand sensitivity of a good speed governor is such thatany load responsive device must be extremely fastto gain significant time advantage. This includes, ofcourse, the load-sensing means which in generalwould not be considered a part of the governorproper. Because of the practical difficulty and costof sensing mechanical loads (although this is by nomeans impossible) applications to date have beenlimited to generator drives. Careful considerationmust be given to securing extremely high rates ofresponse in obtaining the load signal which is sup-plied to the governor in order that the advantages ofthe high load-governor speed (0.01 sec. full strokein the LX-l) may not be lost.

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If such speeds of response are maintained, the limit-ing factor in transient performance is the response

f the engine itself. The less the total lag in the en-ine response, the smaller the speed deviation for a

given disturbance. However, although increased lagmeans increased speed deviation for the load-gover-nor, it also requires greater compensation for sta-bility and therefore increased speed deviation forthe usual speed-governor also; therefore, even forhigh lag systems appreciable improvement can bemade with a properly designed load-sensing speedgovernor although not as spectacular as that obtain-able on an engine with small lag. In the case of

turbo-supercharged engines an additional limitationis placed on the improvement obtainable with theload-sensing governor. Because the air available alow loads, and therefore low turbocharger speedsis inadequate to burn full load fuel, the rapid movement of throttle to the full load position by theload-sensing governor does not produce the desiredresult of full load torque. Thus on large load addition the performance would not be expected to bsubstantially different from that obtained with thespeed governor alone, which, as has been noted earlier, is quite capable of increasing fuel faster thansuch an engine can accept it.

SUMMARY OF WOODWARD HYDRAULIC GOVERNOR TYPESFor convenience of reference we review here themost commonly used types of Woodward mechani-Cal-hydraulic governors.1. Uncompensated lsochronous Governor

Figure 16This is the simplest hydraulic governor, but its appli-cation is very limited because it includes no stabiliz-ing components and if applied to throttle control ofan ordinary prime mover an unstable system resultsdue to throttle overtravel permitted by inherent de-lays in response. However, there are two applica-tions in which it is satisfactory and used in largequantities. Best known is probably the control of aconstant speed aircraft propeller. In this case it isstable because it is controlling torque required, orload torque which follows almost instantaneouslychanges in blade angle; and because, as explained inPart I of this series, the propeller is a very stableload with a high degree of self-regulation. Theother production application is the control of theoutput speed of a hydraulic transmission used todrive an alternator from the propulsion engine of anaircraft. In such a transmission, in which the pumpdischarge rate is controlled by the position of awobble-plate which in turn determines the pistonstroke, response to servo-movement is again ex-tremely rapid and stable operation is obtainable.Another possible application which may be accept-able but is nevertheless basically unstable is one in

which response is extremely slow. In such a casspeed will be hunting or oscillating about itequilibrium value, but at such a slow rate that theover- and under-shoot is so small as to be negligible.2. Speed Droop Governor

Figures 17, 19, 20The introduction of a floating lever as in Figure 1introduces speed droop and a stabilizing effect adescribed earlier. This is exemplified in the TypeSG governor, and where the application is capable ostabilization by an acceptable amount of permanentregulation, the resulting governor is obviously simpler and cheaper; especially so if, as in the case othe SG, engine oil is used and no separate sump oaccumulators are required. As built by Woodward,the amount of droop is conveniently adjustable ovea substantial range. If it is desired that a given speebe maintained under different load conditions, it necessary, of course, that such a governor be readjusted as to speed setting following each loadchange.3. Compensated lsochronous Governor

Figures 21, 22This governor combines the stabilizing effect speed droop with the zero regulation characteristicof the uncompensated isochronous governor. It not limited in its application to systems having verspecial characteristics, as is the latter. Figure 2shows schematically the method of compensation

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..-._ _ _.._ - .- __ _ - _..-.- -.... _ .- ..__ _, ___-_-_ _~ ._..a.- A- .I- _--- -.--1---m_..-...-. -_-_

utilized in Woodward water wheel governors and ac-tuators and in Type UG and IC diesel governors. Thescheme of Figure 22 is used in the PG, LSG and air-craft gas turbine governors and, with some modifi-cation, in the PSG. In all cases, droop is introducedby the compensating system and its effect allowedto dissipate through a needle valve, gradually restor-ing the speed setting to its original value.The magnitude of the stabilizing effect (tempor-ary speed droop) can be adjusted by varying theratio of the lever between servo and transmittingpiston in the first schem e, and by selecting differentbuffer spring scales in the second; in both cases, theneedle valve provides further adjustment.

4. Compensated Governor with Permanent SpeedDroop - Figure 23

Particularly in parallel operation of alternators, itis necessary that a small amount of speed droop beused in order to properly divide loads between units.

The amount for load division is usually far less thanthat required for stable speed governing of the unitwhen operating independently. As an example, itmay take 6% speed droop on some generating sets to 0give stable governing but only 1% speed droop togive good load division. With a simple speed droopgovernor, the higher value has to be accepted. Witha compensated isochronous governor with perma-nent speed droop, the governor may be set for 1%permanent droop to give good load division and anyamount of temporary droop which m ay be requiredfor stable speed governing.Although shown in the schematic diagram of Fig-ure 23 as a fixed lever ratio, the speed droop mech-anism is actually conveniently adjustable in the gov-ernors as manufactured by Woodward. Therefore,a further advantage is obtained in that the properamount of droop can easily be provided to matchparallel units, and if a given unit is to run isolated,the governor can be made isochronous by simplysetting the permanent droop to zero.

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at the heartof the system

since 1870Corporate HeadquartersRockford, Illinois, U.S.A.Branch PlantsFort Collins, Colorado, U.S.A.Stevens Point, W,isconsin, U.S.A.Rockton, Illinois, U.S.A. ,-Sydney, N.S.W., AustraliaSubsidiary OperationsWoodward Governor Nederland B.V.Hoofddorp, The NetherlandsWoodward Governor (U.K.) Ltd.Slough, Berks., England

Woodward Governor GmbHLucerne, Switzerland andHoofddorp, The NetherlandsWoodward Governor (Quebec) Ltd.Montreal, Quebec, CanadaWoodward Governor (Japan) Ltd.Tomisato, Chiba, Japan

Woodward Governor (Reguladores) LimitadaCampinas, S.P., BrazilWOODWARD GOVERNOR COMPANYHydraulic Turbine Controls Division2301 Country Club DriveF.O.Box 287Stevens Point, Wisconsin 54481, U.S.A.

Phone 7151344-2350

speed governor fundamentals

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