 Electronic governor or speed limiter | |
| Component type | Switch |
|---|---|
| Electronic symbol | |
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Agovernor, orspeed limiter orcontroller, is adevice used to measure and regulate thespeed of amachine, such as anengine.
A classic example is thecentrifugal governor, also known as theWatt or fly-ball governor on a reciprocating steam engine, which uses the effect ofinertial force on rotating weights driven by the machine output shaft to regulate its speed by altering the input flow of steam.
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Centrifugal governors were used to regulate the distance and pressure betweenmillstones inwindmills since the 17th century. Earlysteam engines employed a purely reciprocating motion, and were used for pumping water – an application that could tolerate variations in the working speed.
It was not until the Scottish engineerJames Watt introduced therotative steam engine, for driving factory machinery, that a constant operating speed became necessary. Between the years 1775 and 1800, Watt, in partnership with industrialistMatthew Boulton, produced some 500 rotativebeam engines. At the heart of these engines was Watt's self-designed "conical pendulum" governor: a set of revolving steel balls attached to a vertical spindle by link arms, where the controlling force consists of the weight of the balls.
The theoretical basis for the operation of governors was described byJames Clerk Maxwell in 1868 in his seminal paper 'On Governors'.[1]
Building on Watt's design was American engineerWillard Gibbs who in 1872 theoretically analyzed Watt's conical pendulum governor from a mathematical energy balance perspective. During his Graduate school years atYale University, Gibbs observed that the operation of the device in practice was beset with the disadvantages of sluggishness and a tendency to over-correct for the changes in speed it was supposed to control.[2]
Gibbs theorized that, analogous to the equilibrium of the simple Watt governor (which depends on the balancing of two torques: one due to the weight of the "balls" and the other due to their rotation),thermodynamic equilibrium for any work producingthermodynamic system depends on the balance of two entities. The first is theheat energy supplied to the intermediate substance, and the second is thework energy performed by the intermediate substance. In this case, the intermediate substance is steam.
These sorts of theoretical investigations culminated in the 1876 publication of Gibbs' famous workOn the Equilibrium of Heterogeneous Substances and in the construction of the Gibbs’ governor. These formulations are ubiquitous today in the natural sciences in the form of the Gibbs'free energy equation, which is used to determine the equilibrium of chemical reactions; also known asGibbs equilibrium.[3]
Governors were also to be found on early motor vehicles (such as the 1900Wilson-Pilcher), where they were an alternative to a hand throttle. They were used to set the required engine speed, and the vehicle's throttle and timing were adjusted by the governor to hold the speed constant, similar to a moderncruise control. Governors were also optional on utility vehicles with engine-driven accessories such as winches or hydraulic pumps (such asLand Rovers), again to keep the engine at the required speed regardless of variations of the load being driven.
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Governors can be used to limit the top speed for vehicles, and for some classes of vehicle such devices are a legal requirement. They can more generally be used to limit the rotational speed of theinternal combustion engine or protect the engine from damage due to excessive rotational speed.
Today,BMW,Audi,Volkswagen andMercedes-Benz limit their production cars to 250 km/h (155 mph). CertainAudi Sport GmbH andAMG cars, and theMercedes/McLaren SLR are exceptions. The BMW Rolls-Royces are limited to 240 km/h (149 mph).Jaguars, although British, also have a limiter, as do the SwedishSaab andVolvo on cars where it is necessary.
German manufacturers initially started the "gentlemen's agreement", electronically limiting their vehicles to a top speed of 250 km/h (155 mph),[4][5] since such high speeds are more likely on theAutobahn. This was done to reduce the political desire to introduce a legal speed limit.
In European markets,General Motors Europe sometimes choose to discount the agreement, meaning that certain high-poweredOpel orVauxhall cars can exceed the 250 km/h (155 mph) mark, whereas theirCadillacs do not.Ferrari,Lamborghini,Maserati,Porsche,Aston Martin andBentley also do not limit their cars, at least not to 250 km/h (155 mph). The Chrysler300C SRT8 is limited to 270 km/h (168 mph). Most Japanese domestic market vehicles are limited to only 180 km/h (112 mph) or 190 km/h (118 mph).[6] The top speed is a strong sales argument, though speeds above about 300 km/h (190 mph) are not likely reachable on public roads.
Many performance cars are limited to a speed of 250 km/h (155 mph)[7] to limit insurance costs of the vehicle, and reduce the risk oftires failing.
Mopeds in the United Kingdom have required a 30 mph (48 km/h) speed limiter since 1977.[8] Most other European countries have similar rules (see the main article).
Public service vehicles often have a legislated top speed.Scheduled coach services in the United kingdom (and alsobus services) are limited to 65 mph.[9]
Urban public buses often have speed governors which are typically set to between 65 km/h (40 mph) and 100 km/h (62 mph).[citation needed]
Allheavy vehicles inEurope andNew Zealand have law/by-law governors that limits their speeds to 90 km/h (56 mph) or 100 km/h (62 mph).[citation needed]Fire engines and other emergency vehicles are exempt from this requirement.
Aircraft propellers are another application. The governor senses shaft RPM, and adjusts or controls the angle of the blades to vary the torque load on the engine. Thus as the aircraft speeds up (as in a dive) or slows (in climb) the RPM is held constant.
Small engines, used to powerlawn mowers, portablegenerators, andlawn andgardentractors, are equipped with a governor to limit fuel to the engine to a maximum safe speed when unloaded and to maintain a relatively constant speed despite changes in loading. In the case of generator applications, the engine speed must be closely controlled so the outputfrequency of the generator will remain reasonably constant.
Small engine governors are typically one of three types:[10]
Insteam turbines, thesteam turbine governing is the procedure of monitoring and controlling the flow rate of steam into the turbine with the objective of maintaining its speed of rotation as constant. The flow rate of steam is monitored and controlled by interposing valves between the boiler and the turbine.[11]
Inwater turbines, governors have been used since the mid-19th century to control their speed. A typical system would use a Flyball governor acting directly on the turbine input valve or thewicket gate to control the amount of water entering the turbine. By 1930, mechanical governors started to usePID controllers for more precise control. In the later part of the twentieth century, electronic governors and digital systems started to replace mechanical governors.[12]
For electrical generation onsynchronous electrical grids, prime movers driveelectrical generators which are electrically coupled to any other generators on the grid. With droop speed control, the frequency of the entire grid determines the fuel delivered to each generator, so that if the grid runs faster, the fuel is reduced to each generator by its governor to limit the speed.
Governors are used inelevators. It acts as a stopping mechanism in case the elevator runs beyond its tripping speed (which is usually a factor of the maximum speed of the lift and is preset by the manufacturer as per the international lift safety guidelines). This device must be installed in traction elevators and roped hydraulic elevators.
Governors are used in some wind-up music boxes to keep the music playing at a somewhat constant speed while the tension on the spring is decreasing.
