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Speed Control GovernorsFor GME Cadets
ajit / Tr Sup (Engg) 29/06/12
A governor maintains the average speedof an engine whereas the flywheelcontrols the cyclic fluctuations in speed.
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Contents
ajit / Tr Sup (Engg) 29/06/12
Governor classificationU Tube Video on Diesel Engine GovernorsA. Classification of Speed Governors as per the control media used
UG 8 Governor Operation
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1. Mechanical or Direct acting type
When the movement of the flyweights directly operate the engine fuel control rack, it iscalled a direct acting governor.
They need huge fly weights so that enough power can be obtained for moving the racks.
2. Mechanical / Hydraulic, Relay or Indirect acting type
This type has the movement of the flyweights operating a pilot valve plunger only.
The operation of the rack being through a power piston operated by hydraulic pressurecontrolled by the pilot valve.
3. Electronic governor
The speed governor can considered to be of three main parts,
1. Speed sensing
2. Control function
3. Actuator.
An electronic governor may have its speed sensing and the control functionselectronically carried out.
The actuating of the rack may be done by an electric motor or by a pneumaticactuator.
A. Classification of Speed Governors as per the control media used.
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1. Regulating governors
They are constantly positioning the fuel rack at all speeds.
Two kinds of regulating governors are possible,
Constant speed Where the set speed of the governor cannot be altered.
Variable speed Where the set speed can be varied to control load.
2. Speed limiting governors
Which act only to keep speed within the normal range, not allowing over speeding or underspeeding.
They are not acting when speed is in normal range.
These type of governors are found in vehicle engines.
3. Overspeed governors (overspeed trip / shutdown)
They act only in case of an overspeed.
An overspeed governor shuts down or trips the engine as compared to the others whichregulate the fuel rack.
B. Classification of Speed Governors as per the functions they carry out.
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1. Lever type
Lever type governor is one in which the speed setting is carried out by a lever which
adjusts the speeder spring tension.
2. Dial type
Dial type governor is one in which the speed setting is done by turning a dial on thegovernor.
C. Classification of Speed Governors as per the speed setting arrangement
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Further classification of Speed Governors based upon their operating principles.
Basic classification of mechanical speed governors based on operating principle are
1.Inertia governors
2.Centrifugal governors
a. Gravity Controlled / Dead Weight type
1. Watt
2. Porter
2. Proell
b. Hartnell (Spring controlled)
In the former type the centrifugal force is relied on for operation. The flyballs or flyweights responds tochange in rpm.
In the latter type the inertia force is relied on for operation. The inertia of rotating weights causes a shiftingof weights when acceleration or deceleration occurs. This rate of change in rpm is detected by inertia
governors and hence they are more sensitive that centrifugal governors. It should be noted that it onlydetects rate of change of speed and hence cannot be used independently for speed control.
A centrifugal governor can be dead weight type also called gravity controlled, in which the gravity force iswhat acts against the centrifugal force. Watt governors fall in this category, Porter and Proell are of sametype with some modifications. These governors are inherently stable.
The centrifugal governor that is used today is of Hartnell type where the gravity force is replaced by springforce. The spring rate can be varied as necessary and the spring rate decides the stability of the governor.Linear or non linear springs (trumpet / conical) can be used.
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Dead Weight type basic centrifugal governor
Draw Backs
Torque output low, depends on flyweight size
Large flyweights required to reduce static friction effects
Large flyweights leads to
inertia effects
low rpm, less sensitive
Speed droop inherent
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Mechanical / Hydraulic, Relay or Indirect acting type Governor
Advantages
Output no longer linked to fly weight size
Smaller flyweight size lead to
no inertia effects
higher rpm, more sensitive
Drawbacks
Over sensitive
Will hunt
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A mechanical negative feed back arrangement is provided
Feed back is having the movement of the power piston transmitted to the speeder spring
Negative feed back is so as to reduce its tension as the fuel in increased
Negative Feed back introduces stability
But introduces a permanent speed droop.
ajit / Tr Sup (Engg) 29/06/12
Speed droop (Permanent) , Feed back
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A hydraulic negative feed back arrangement is provided by the compensation system
The compensation system consists of the Compensating spring, actuating piston and receiving piston.
A compensating needle valve setting decides the rate of oil flow in or out of the compensation system
Provides a transient speed droop
As the fuel is being increased, the speed setting is reduced temporarily. which is set back to normalslowly as the engine rpm recovers.
Compensation gives stability without introducing droop, it enables isochronous operation.
ajit / Tr Sup (Engg) 29/06/12
Speed droop (Transient) , Compensation
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SG - Simple Governor Simple Proportional governor, without compensation. Has adjustable droop, cannotoperate isochronously. Pressure oil has to be supplied from the engine, no attached pump or sump.
UG - Universal Governor Governors with Adjustable compensation & Adjustable droop. Can operateisochronously or with droop. Has attached pump and sump.
PG - Pressure compensating hydraulic Governor No adjustment of droop or compensation. Has limitedcompensation.
PGA - PG + Air speed setting PGA/EG - PGA + Electric Actuator PGA/TL - PGA + Torque Limiter
The number "8/40/200" indicates the maximum torque that the terminal shaft can apply, called the "stallingwork capacity". It is the torque specified in "foot pounds".
Woodward UG8 Specs Rated for drive from 375 to 1500 rpm on drive shaft. Ratio of 1 : 2.2.
Woodward governor nomenclature SG, UG8, UG40, PG200, PGA200 etc.
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Controls on Woodward UG8
Speed droop Knob
Synchronizer Knob
Load limit Knob
Synchronizer indicator Knob
Speed setting motor and friction drive
Shut down solenoid
ajit / Tr Sup (Engg) 29/06/12
Controls on Woodward UG8
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Speed droop Knob
The graduations are from 0 to 100, this does not indicate percentage droop.
For stand alone engines, speed droop can be set to zero. For engines interconnected electrically (ACbusbar) or mechanically, the speed droop is to be set at the lowest setting which gives stable loaddivision. Normally 30 to 50 on the dial.
If speed droop is set to zero, the governor operates isochronously.
Synchronizer Knob
This knob is free to turn for many rotations. This changes the speeder spring tension, thus changing the setspeed of the governor.
Load limit Knob
This scale is graduated from 0 to 10. A pointer below the knob indicates the power piston position over thisscale. The knob can be put to any postion and the rack movement is limited to that position. This limitshydraulically the maximum possible movement of the terminal shaft. The load limit can be used to, limit fuelduring a start, Shut down a running engine, Limit the maximum load on the engine.
Synchronizer indicator Knob
This knob is coupled by gears to synchronizer knob. The gear ratio is in such a way that the indicatorturns one division for every one revolution of the synchronozer knob. This indicates the position of thesynchronizer knob over its range of movement.
Speed droop Knob, Synchronizer Knob, Load limit Knob, Synchronizer indicator Knob
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1.Load is decreased and speedincreases
2.As speed increases, FLY BALLS moveout raising SPEEDER ROD and innerend of FLOATING LEVER, thus raisingPILOT PLUNGER and uncoveringREGULATING PORT in PILOT VALVEBUSHING
3.Uncovering of REGULATING PORTopens bottom of POWER CYLINDERto sump and will allow oil pressure intop of POWER CYLINDER to movePOWER PISTON down
Load Reduction 1
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1.Oil pressure moves POWERPISTON down rotating TERMINALSHAFT in the direction to decreasefuel
2.As the POWER PISTON movesdown, ACTUATINGCOMPENSATING PISTON movesup and draws RECEIVINGCOMPENSATING PISTON downcompressing COMPENSATINGSPRING and lowering outer end ofFLOATING LEVER and PILOTVALVE PLUNGER
3.Movement of POWER PISTON,ACTUATING COMPENSATINGPISTON, RECEIVINGCOMPENSATING PISTON andPILOT VALVE PLUNGER continuesuntil REGULATING PORT inBUSHING is covered by land onPLUNGER
4.As soon as REGULATING PORT iscovered, POWER PISTON andTERMINAL SHAFT are stopped ata position corresponding todecreased fuel needed to runengine at normal speed underdecreased load.
Load Reduction 2
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1.As speed decreases to normal,FLYBALLS return to normal positionlowering the SPEEDER ROD tonormal position.
2.RECEIVING COMPENSATINGPISTON is returned to normalposition by COMPENSATINGSPRING at the same rate asSPEEDER ROD thus keepingREGULATING PORT in PILOTVALVE BUSHING covered by landon PILOT VALVE PLUNGER. Flowof oil through COMPENSATINGNEEDLE VALVE determines rate atwhich RECEIVINGCOMPENSATING PISTON isreturned to normal.
3.At completion of cycle, FLYBALLS,SPEEDER ROD, PILOT VALVEPLUNGER, and RECEIVINGCOMPENSATING PISTON are innormal position. POWER PISTONand TERMINAL SHAFT arestationary at a positioncorresponding to decreased fuelnecessary to run engine at normalspeed under decreased load.
Load Reduction 3
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1.Load is increased and speeddecreases.
2.As speed decreases, FLYBALLSmove in lowering the SPEEDERROD and inner end of FLOATINGLEVER, thus lowering the PILOTVALVE PLUNGER and uncoveringthe regulating port of the PILOTVALVE BUSHING.
3.Uncovering of REGULATINGPORT admits pressure oil to thebottom of the POWERCYLINDER. Since bottom area ofPOWER PISTON is greater thatthe top area, oil pressure will movePISTON up
Load Increase 1
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1.Oil pressure moves POWER PISTON upand rotates the TERMINAL SHAFT indirection to increase fuel.
2.As POWER PISTON moves up,ACTUATING COMPENSATING PISTONmoves down and forces the RECEIVINGCOMPENSATING PISTON upcompressing COMPENSATINGSPRING and raising outer end ofFLOATING LEVER and PILOT VALVEPLUNGER
3.Movement of POWER PISTON,ACTUATING COMPENSATINGPISTON, RECEIVINGCOMPENSATING PISTON and PILOTVALVE PLUNGER continues untilREGULATING PORT in PILOT VALVEBUSHING is covered by land onPLUNGER
4.As soon as REGULATING PORT iscovered, POWER PISTON andTERMINAL SHAFT are stopped at apoint corresponding to increased fuelneeded to run engine at normal speedunder increased load
Load Increase 2
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1.As speed increases to normal,FLYBALLS return to normal positionraising SPEEDER ROD to normalposition.
2.RECEIVING COMPENSATINGPISTON is returned to normal positionby COMPENSATING SPRING at thesame rate as SPEEDER ROD, thuskeeping REGULATING SHAFT inPILOT VALVE BUSHING covered byland on PILOT VALVE PLUNGER.Flow of oil through COMPENSATINGNEEDLE VALVE determines rate atwhich RECEIVING COMPENSATINGPISTON is returned to normal
3.At completion of cycle, FLYBALLS,SPEEDER ROD, PILOT VALVEPLUNGER and RECEIVINGCOMPENSATING PISTON are innormal positions, POWER PISTONand TERMINAL SHAFT are stationaryat a position corresponding to increasefuel necessary to run engine at normalspeed under increased load.
Load Increase 3
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ajit / Tr Sup (Engg) 29/06/12
Initial set up & Adjustments of Governor
Initial adjustments after setting up, or after overhauling, oil change (air bleeding).
Procedure of adjustments to the compensating system.
Compensation lever adjustment
Compensation needle adjustment
Hunting, high overspeeds and underspeeds and slow return to normal speed following loadchanges are results of incorrect compensation adjustment.
Adjustment should be done when the engine has been operating for some time and the governorand its oil has attained operating temperatures. The engine is to be off load, at the normal operatingrpm.
The compensating lever is put to maximum upward position and the needle valve opened four tofive turns outward. This will cause the engine to hunt which will bleed trapped air form passages.
Then the lever is put to minimum and the needle valve closed gradually till hunting stops. If huntingdoesn't stop at all till full closing of the needle valve, the compensating lever can be raised by twograduations and then procedure repeated. The needle valve should be ideally open from closedposition only by about 3/4 th turn, maximum.
If the needle valve is closed more than needed, slow return to normal speed will result following aload change. If the compensation lever is moved more towards maximum than needed, excessivespeed change upon load change will result.
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Speed droop is the drop in rpm that occurs as with full travel of the power piston, expressed as apercentage of full load rpm.
Steady state speed regulation is percentage change in speed from no load to full load expressed as apercentage of the full load rpm. If N1 (720, 60 Hz for 10 poles) is the no load rpm and N2 (690, 57.5Hz for 10 poles) the full load rpm, then speed droop is (N1 - N2)/ N2 * 100. (4.3%).
The term steady state speed regulation is mostly used to mean the same as speed droop. It doesmean the same as speed droop, when the fuel rack and governor linkage is such that, full travel ofgovernor power piston occurs as the engine goes from no load to full load.
Supposing only 50 % of the governor terminal shaft movement is utilized for the engine to change fromno load to full load and the governor is set to speed droop of 4 %, then in this case the steady statespeed regulation is only 2 %.
It is generally reccommeded that only about 2/3 rds of the power piston travel is utilized for full load /speed range of the engine. In such a case speed droop and steady state regulation would be different.
ajit / Tr Sup (Engg) 29/06/12
Speed droop & Steady state speed regulation
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A governor which maintains a steady speed at the set value irrespective of load changes is called anisochronous governor.
A constant speed or variable speed governor can be isochronous.
Isochronous operation deals with the ability of the governor to maintain the set speed whereas constant& variable speed deals with the provision in the governor to change the set speed.
Isochronous operation is obtained if speed droop is set to zero.
Isochronous Governor
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Stable load division between two engines feeding power into a common system
Supposing the system is running initially with load L1 on both machines and at an rpm of r1. Whenadditional load in imposed, the system rpm has to drop since droop is there in both machines. If thesystem rpm drops to r2, the corresponding load changes will have to follow the curves. It can be seenthat Engine 1 having lower droop will take up more load than the other.
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Droop and its relation to load division
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Manual turn of knob which adjusts speeder spring directly.(A/E governor).
Turning the speed set mechanism by an electric reversible motor from remote. (A/E governor).
Remote speed set pneumatic signal, through a pneumatic positioner controlling the speed set lever.(lever type governor)
Remote speed set pneumatic signal acting on the speeder spring directly through the pneumatic /hydraulic positioner built into the governor.
Methods of speed setting
(Adjustment of SPEEDER SPRING tension)
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It is a constant speed governor, within a narrow range, speed variation is possible for synchronizing ofalternators.
Speed droop setting is needed since generators have to run in parallel. If stable division of load is to beachieved, isochronous operation is not possible.
The remote speed setting is by an electric motor driving the speeder spring setting mechanism.
The power output of the governor is less since the engine is small.
The generator governor should be able to maintain engine speed during load changes as laid out byrules. (see below)
Requirements of A/E governor
With a load change of 25 % of the full rated load, the speed change is not greater than 2.5 %. Within twosecs the speed should remain within one % of the final steady state speed and within four secs, thespeed should be within half % of the final steady state speed.
The A/E governor
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The main engine governor is a variable speed governor which allows speed to be controlled formlowest to the maximum limit.
Speed droop setting is not needed other than for twin engined vessels which feed into a common shaftfor driving the propeller. Twin engined vessels where there is no mechanical coupling between enginedo not have to have speed droop setting.
The remote speed setting is pneumatic, either through a positioner with a lever type governor or bypneumatic / hydraulic built in arrangement. Being used on large engines, the power output is higher andan amplifier is used in case the power output is not sufficient to move the rack.
Scavenge air pressure fuel limiter
This system senses the scavenge air pressure from the scavenge air box and releases fuel as afunction of the scavenge air pressure. This ensures no more fuel than that which can burn fully isreleased, which ensures proper combustion during load increase.
Speed setting fuel limiter or Torque limiter
This is a system which releases fuel as a function of the speed set air signal to governor. This preventsengine overloading and also lower fuel admission at start.
The reversible attached oil pump The attached oil pump of the governor has to be having anarrangement to deliver oil irrespective of the running direction, for reversible main engines.
The M/E governor
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Balance of forces in a governor
Two opposing forces are at balance in equilibrium position
1. The centrifugal force of flyweights
2. The Gravity OR Spring force
The centrifugal force, FC = m ω2 r (mv 2 /r as v = r ω)
The spring force , FS = spring constant × deflection
Deflection is proportional to radius r
Therefore, both FC & FS are functions of the radius ‘r’ offlyweights for a given rpm
Both forces FC & FS (centrifugal & gravity / spring) can beplotted against radius ‘r’
For a given rpm, the centrifugal force is a straight linepassing through the origin
Considering a linear rate spring, the spring force line toowould be straight.
The point of intersection of the two lines will be the radius offlyweights at equilibrium.
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For a governor to be stable, the rate of spring force should be higher that that of the centrifugal force atthe point of intersection of the two lines.
Speeder Spring rate & governor stability
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In the left graph, the spring rate is higher at the point of intersection
in case of drop in rpm, the centrifugal force and spring force both drop and find a newequilibrium position at a reduced radius
in case of rise in rpm, the centrifugal force and spring force both rise and find a newequilibrium position at an increased radius
In the right graph, the spring rate is lower at the point of intersection
in case of drop in rpm, the centrifugal force reduces, radius also reduces, the increaseingspring force (over centrifugal force) takes the flyweights to the inner extreme position
In case of rise in rpm, the centrifugal force increases, radius also increases, the risingcentrifugal force (over spring force takes the flyweights to outer extreme position
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Over speed trip governor’s spring rate is chosen to be in theunstable range. There is only one equilibrium rpm and theflyweights takes extreme positions when rpm changes.
When the equilibrium rpm is reached, the flyweights startmoving out, increasing the radius and corresponding increasein centrifugal force over the spring force.
Since the spring rate is lower, this moving out of flyweights isvery positive and with increasing force.
Regulating governor & Over speed governor, spring rate selection
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Regulating governor spring rate is as per graph shown on left.For every rpm there is a different equilibrium position.
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Selection of conical or trumphet shaped spring as the SPEEDER SPRING in a governor
Graph on right shows How a non linearconical speeder spring makes the governormore sensitive. Percentage change in loadis plotted against spring deflection. It canbe seen that the deflection for a smallchange in load is high in case of a nonlinear spring and the deflection at a highload change is not correspondingly higher.
Graph on right shows how a non linear speederspring (conical OR trumphet shaped spring) makesthe governor more stable at a wide range of rpms.
ajit / Tr Sup (Engg) 29/06/12
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1. Speed droop (Permanent) , Feed back
2. Speed droop (Transient) , Compensation
3. Steady state speed regulation.
4. Isochronous
5. Boost air to governor (Booster)
6. Pre set air to governor
7. False scavenge air pressure to governor
(Cancellation of scavenge air pressure fuel limiter )
8. Hydraulic Amplifier
In a mechanical hydraulic governor, the dead band effects are reduced by,
1. A conical speeder spring
2. Having the flyweights rotate at a higher rpm than the engine, which increases sensitivity.
3. Reducing static friction effects by rotating the pilot valve bushing.
ajit / Tr Sup (Engg) 29/06/12
Some important features & terms in governors
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