A device used to control the speed of a prime mover. A governor protects the prime mover from overspeeding and keeps the prime mover speed at or near the desired revolutions per minute. When a prime mover drives an alternator supplying electrical power at a given frequency, such as 60 Hz, a governor must be used to hold the prime mover at a speed that will yield this frequency. An unloaded engine will fly to pieces unless its speed is under governor control. See also: Prime mover
A governor regulates the speed of a prime mover by properly varying the flow of energy to or from it. In the case of gas and steam turbines and internal combustion engines, the fuel furnishes the energy to the prime mover. For such applications, the governor usually controls the speed of the unit by regulating the rate at which fuel, and hence energy, is furnished to the prime mover. The governor controls the fuel flow so that the speed of the prime mover remains constant regardless of load and other disturbances, or changes in accordance with such operating conditions as changes in speed setting.
For a diesel engine the governor is connected to the rack which controls the amount of fuel injected. A governor on a gas or gasoline engine is attached to the engine throttle. A steam turbine governor strokes the steam valve or valves which regulate the steam flow to the turbine. See also: Diesel engine; Internal combustion engine; Steam turbine
The output mechanism of a gas turbine governor is connected to the fuel valve with the stroke normally limited in each direction by the allowable combustion chamber temperature and other factors. If the fuel rate is too low, the fire will go out. Compressor stall must be avoided. To give rapid control, combustion chamber temperature is often computed in the governor by measuring other variables and applying the laws of thermodynamics. See also: Gas turbine
A hydraulic turbine governor regulates the flow of water to the turbine by varying the openings of gates or other components. See also: Hydraulic turbine
An aircraft propeller governor varies the pitch of the propeller to keep constant the speed of the engine attached to the propeller. This type of governor varies the load on the engine and thus controls the speed by regulating the energy flow from the prime mover. See also: Propeller (aircraft)
The speed of a prime mover is usually measured by a ballhead that contains flyweights driven at a speed proportional to the speed of the prime mover. The force from the flyweights is balanced, at least in part, by the force of compression of a speeder spring (Fig. 1). The upper end of this spring is positioned according to the speed setting of the governor.
The ballhead toes press against one end of a plunger. In the simplest version of a governor, the plunger position is a function of the engine speed as a result of the balance between the centrifugal and spring forces. The plunger is directly connected to the throttle or other energy controlling means of the prime mover. Because the governor output power is drawn directly from the speed measuring means, the output power and the precision of such a governor are severely limited.
In automatic control theory, the input to the governor is taken to be the difference between the speed setting (reference) and the actual prime mover speed. This difference is the speed error. For the simplest governor, the position of the plunger is the output. The action of the governor is proportional since its output is proportional to its input. In equilibrium, where the engine is running in steady state, the speed of the prime mover depends on both the load and the speed setting. With the mechanical governor and a fixed speed setting, the equilibrium speed decreases as the load increases. A governor with this property is referred to as a droop governor, or a governor operating on droop. The governor–prime mover unit is then said to be running on droop.
To increase the power output of a governor, a hydraulic amplifier is often employed. A governor that keeps the speed of a prime mover constant is said to be isochronous. In a simple isochronous governor, the ballhead senses the speed and strokes a pilot valve plunger that regulates the flow of fluid to a servomotor (Fig. 2). Normally, the fluid is oil. This governor is intended to bring the prime mover speed back to the speed setting after any change in load. See also: Servomechanism
A hydraulic droop governor is obtained from the simple isochronous governor by introducing feedback from the governor output to the pilot valve (Fig. 3). This governor behaves like a simple mechanical governor except that a smaller ballhead is generally employed, greatly improving the precision of the governor by reducing hysteresis, dead band, and friction in the ballhead. The power output of the governor is much larger so that the effect of the load on governor performance incurred in moving the throttle or equivalent mechanism, is considerably diminished. A hydraulic governor may be sensitive to speed changes of as little as 1/1000 of 1%. For normal disturbances, prime mover speed error may be kept to 0.1% or better.
Use of dashpot
The performance of the simple isochronous governor is often greatly improved by the introduction of a dashpot in the feedback path from the output to the ballhead. If there is little damping in the prime mover, instability often occurs when the simple isochronous governor is used, whereas this instability is removed when the dashpot is incorporated. A system is stable when for each disturbance that dies out, the response of the system settles to an equilibrium condition. When instability occurs, unless some protection means is provided, the prime mover speed oscillates continuously or increases indefinitely until the unit breaks up.
When a dashpot is incorporated into a governor (Fig. 4), the governor output becomes a function of the speed error and the integral of this error. Such a governor action is referred to as a proportional plus integral controller. The velocity of the servomotor then depends on prime mover speed and prime mover acceleration. The time lag in the dashpot makes the governor sensitive to prime mover acceleration. As this lag is increased, the response of the governor to prime mover acceleration tends to increase. This increase in lag is accomplished by moving the dashpot needle valve, which controls the orifice area, toward the closed position.
Instead of mechanical feedback from the governor, force feedback is generally preferred (Fig. 5). An isochronous dashpot governor is turned into a droop governor by adding direct mechanical feedback from the servomotor to the ballhead.
Use of flywheel
Acceleration governors are sometimes used in place of governors with dashpots. In such governors a flywheel is employed instead of a dashpot. The prime mover drives the flywheel through a spring. This combination yields a motion proportional, except for a time lag, to the acceleration of the prime mover. See also: Flywheel
Governors for large hydraulic turbines require a second stage of hydraulic amplification. The governor servomotor piston is connected to the plunger of a relay valve that regulates the flow of oil to the turbine servomotors. The governor servomotor is then termed the controller. Turbine servomotors often require 10 hp (7500 W) or more.
The speed setting of a governor is often adjusted by an electric, hydraulic, or pneumatic motor as a function of auxiliary variables. Thus in an electrical power system the deviation of the frequency of the system from the desired value is used to position the governor speed setting so as to bring the frequency to the right value. Adjustment of governor speed settings is required to keep electric clocks on time. Oil and gas pipeline governors control pressure rather than speed. The pressure in the line is measured and the speed setting of the governor is adjusted accordingly. This affects the speed of the prime mover, which drives a compressor, raising or lowering the pressure.
Traditional mechanical governor systems are being displaced by computer speed control systems. The desired controls are programmed directly into a computer, which reads a speed signal and provides for proportional/integral/differential (PID) control of speed by varying energy input to the prime mover. The computer-controlled governor can be programmed to provide functions similar to traditional mechanical governing systems but without the mechanical failures inherent in such systems. See also: Computer; Digital computer; Digital control
Paralleled prime movers
Prime movers may be paralleled to supply power to the same load. In an electrical power system, prime movers drive alternators electrically connected to the system. At most, one of the governors of paralleled prime movers can be isochronous; the rest must be on droop. The prime movers may all be on droop.
When two or more identical prime mover-generator units are paralleled, and one is controlled by an isochronous governor while the others are on droop, electrical coupling will force all units to run at the same steady-state speed. The alternators then supply electrical power to the electric grid at the same frequency.
Speed control by governor droop is an integral part of frequency control on an electric power system. Governors provide frequency control, and control how increases or decreases in load on the generators (generation pick-up) is shared among the remaining generators in the event of a loss of a generator (or how generation reduction is shared among generators in the event of a loss of load). System coordination of governor droop settings provides for equitable division of replacement generation until system automatic generation control computers assign a new generation dispatch. Plant control computers also participate in controlling the output of individual generators in a plant. In the North American power system, governor droop settings range between 4 and 7%. The droop setting means that for the stated percent frequency deviation the governor calls for the generator's maximum power. For example, on a 60-Hz system, if the droop setting is 5%, during a 5% (3-Hz) frequency deviation governors call for maximum power from the generators. Governors do not return the power system to 60 Hz; automatic generation control computers perform this function. See also: Electric power systems
Aircraft engines are synchronized and synchrophased by using one engine as the master, and adjusting the speed settings of the governors on the other engines to make them follow the master.
Proper design of a governor involves making the governor characteristics fit those of the prime mover and load so as to give acceptable overall performance. Desirable and undesirable nonlinearities occur in governors to complicate design calculations. Thus centrifugal force varies as the square of speed. To compensate for this, nonlinear speeder springs are often employed. Bypass is often used in dashpots to limit the range of the dashpot output.
The output of a generator driven by the prime mover is sometimes used to obtain a voltage proportional to prime mover speed. This voltage, properly filtered, is fed to an electronic, magnetic, or other circuit, and eventually to a transducer to move the pilot valve. A gear with permanent magnet teeth is often employed with a pickup coil, where the gear is driven at a speed proportional to that of the prime mover. The output pressure of a hydraulic pump is sometimes employed as a measure of prime mover speed. Other speed-measuring means are utilized.
Limit or topping governors are often employed to meet fail-safe specifications. The limit governor is usually a mechanical governor that takes over control from the main governor to shut the prime mover down if its speed reaches a fixed overspeed above its rated speed. The flyweight force increases with an increase in the distance of the flyweights from the axis of rotation of the ballhead; this flyweight force opposes the speeder spring force. The flyweight force increases as the square of prime mover speed. When it is sufficiently large, the governor gain becomes infinite, so that a small input to the governor causes an unlimited output. This causes a snap action of the governor at the overspeed for which the governor is set.
In the interests of economy the four-way valve servomotor (Fig. 2) is often replaced by a two-way valve-differential servomotor (Fig. 6). The area on one side of the servomotor is half the area on the other side. The supply pressure on the small area is sometimes replaced by a spring.
A load is termed isolated if the power supplying this load comes from one prime mover only. In the design of a governor for a prime mover supplying power to an isolated load, a prime mover differential equation is obtained relating the input to the prime mover, such as throttle position, to the speed of the prime mover, which is taken to be its output. This equation is obtained by mathematically balancing the torques on the prime mover shaft. The equation also involves a delay between a change in the input to the prime mover and the resulting change in the driving output torque. Part of the delay is a dead time. This dead time is normally 0.01–0.5 s. The rest of the delay is usually in the order of 0.1 s.
The nature of a prime mover may introduce other terms in the equation. For example, with hydraulic turbines the equation is complicated because of water hammer; a sudden closing of the gates to decrease the speed of the turbine causes the turbine to speed up initially, resulting in a correction opposite to that desired. As the gates are moved, pressure waves travel up and down the hydraulic conduit from the source of the water to the discharge from the turbine. These waves tend to destabilize the unit. See also: Water hammer
The equation of the prime mover is combined with the equation of the governor to yield an equation for overall system response from which the governor response is determined. When full load on a diesel engine is rejected, an overspeed of 3% is often considered acceptable with the response dying out in 1 s or less. For a hydraulic turbine this overspeed may be 30–40% and the response may endure as much as 10–20 s. Performance for other prime movers under governor control falls between these extremes.