Governor

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Governor

  1. 1. Governors: • Types of governors • force analysis of Porter and Hartnell governors • Controlling force • Stability • Sensitiveness • Isochronisms • effort and power 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 1
  2. 2. Introduction • The function of a governor is to regulate the mean speed of an engine, when there are variations in the load • For example, when the load on an engine increases, its speed decreases, therefore it becomes necessary to increase the supply of working fluid. • On the other hand, when the load on the engine decreases, its speed increases and thus less working fluid is required. 2
  3. 3. • The governor automatically controls the supply of working fluid to the engine with the varying load conditions and keeps the mean speed within certain limits. • A little consideration will show that, when the load increases, the configuration of the governor changes and a valve is moved to increase the supply of the working fluid ; conversely, when the load decreases, the engine speed increases and the governor decreases the supply of working fluid. 3
  4. 4. Note : • The function of a flywheel in an engine is entirely different from that of a governor. • It controls the speed variation caused by the fluctuations of the engine turning moment during each cycle of operation. • It does not control the speed variations caused by a varying load. • The varying demand for power is met by the governor regulating the supply of working fluid. 4
  5. 5. Types of Governors • The governors may, broadly, be classified as 1. Centrifugal governors 2. Inertia governors. 5
  6. 6. Centrifugal Governors • The centrifugal governors are based on the balancing of centrifugal force on the rotating balls by an equal and opposite radial force, known as the controlling force. 6
  7. 7. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 7 • It consists of two balls of equal mass, which are attached to the arms as shown in Fig. • These balls are known as governor balls or fly balls. The balls revolve with a spindle, which is driven by the engine through bevel gears.
  8. 8. Centrifugal Governors • The upper ends of the arms are pivoted to the spindle, so that the balls may rise up or fall down as they revolve about the vertical axis. • The arms are connected by the links to a sleeve, which is keyed to the spindle. This sleeve revolves with the spindle ; but can slide up and down. • The balls and the sleeve rises when the spindle speed increases, and falls when the speed decreases. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 8 In order to limit the travel of the sleeve in upward and downward directions, two stops S, S are provided on the spindle.
  9. 9. • The sleeve is connected by a bell crank lever to a throttle valve. • The supply of the working fluid decreases when the sleeve rises and increases when it falls. • When the load on the engine increases, the engine and the governor speed decreases. • This results in the decrease of centrifugal force on the balls. Hence the balls move inwards and the sleeve moves downwards. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 9 The downward movement of the sleeve operates a throttle valve at the other end of the bell crank lever to increase the supply of working fluid and thus the engine speed is increased.
  10. 10. • When the load on the engine decreases, the engine and the governor speed increases, which results in the increase of centrifugal force on the balls. • Thus the balls move outwards and the sleeve rises upwards. • This upward movement of the sleeve reduces the supply of the working fluid and hence the speed is decreased. In this case, the power output is reduced. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 10
  11. 11. Terms Used in Governors • The following terms used in governors are important from the subject point of view ; 1. Height of a governor. It is the vertical distance from the centre of the ball to a point where the axes of the arms (or arms produced) intersect on the spindle axis. It is usually denoted by h. 2. Equilibrium speed. It is the speed at which the governor balls, arms etc., are in complete equilibrium and the sleeve does not tend to move upwards or downwards. 3. Mean equilibrium speed. It is the speed at the mean position of the balls or the sleeve. 4. Maximum and minimum equilibrium speeds. The speeds at the maximum and minimum radius of rotation of the balls, without tending to move either way are known as maximum and minimum equilibrium speeds respectively. 5. Sleeve lift. It is the vertical distance which the sleeve travels due to change in equilibrium speed. • Note : There can be many equilibrium speeds between the mean and the maximum and the mean and the minimum equilibrium speeds. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 11
  12. 12. Governor Effort and Power 12 Governor effort and power can be used to compare the effectiveness of different type of governors. Governor Effort : It is defined as the mean force exerted on the sleeve during a given change in speed. When governor speed is constant the net force at the sleeve is zero. When governor speed increases, there will be a net force on the sleeve to move it upwards and sleeve starts moving to the new equilibrium position where net force becomes zero. It is defined as the work done at the sleeve for a given change in speed. Therefore, Power of governor = Governor effort x Displacement of sleeve
  13. 13. Characteristics of Governors Different governors can be compared on the basis of following characteristics : 1.Stability : A governor is said to be stable when there is one radius of rotation of the balls for each speed which is within the speed range of the governor. 2. Sensitiveness The smaller the change in speed from no load to the full load, the more sensitive the governor will be. According to this definition, the sensitiveness of the governor shall be determined by the ratio of speed range to the mean speed. The smaller the ratio more sensitive the governor will be where N2 – N1 = Speed range from no load to full load.
  14. 14. 14 3.Isochronism A governor is said to be isochronous if equilibrium speed is constant for all the radii of rotation in the working range. Therefore, for an isochronous governor the speed range is zero and this type of governor shall maintain constant speed. 4.Hunting Whenever there is change in speed due to the change in load on the engine, the sleeve moves towards the new position but because of inertia if overshoots the desired position. Sleeve then moves back but again overshoots the desired position due to inertia. This results in setting up of oscillations in engine speed. If the frequency of fluctuations in engine speed coincides with the natural frequency of oscillations of the governor, this results in increase of amplitude of oscillations due to resonance. The governor, then, tends to intensity the speed variation instead of controlling it. This phenomenon is known as hunting of the governor. Higher the sensitiveness of the governor, the problem of hunting becomes more acute.
  15. 15. Porter Governor • In Porter governor central load is attached to the sleeve as shown in Fig . • The load moves up and down the central spindle. This additional downward force increases the speed of revolution required to enable the balls to rise to any predetermined level. • Consider the forces acting on one-half of the governor as shown in Fig. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 15
  16. 16. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 16 Though there are several ways of determining the relation between the height of the governor (h) and the angular speed of the balls (ω), yet the following two methods are important from the subject point of view : 1. Method of resolution of forces ; and 2. Instantaneous centre method.
  17. 17. Hartnell Governor • A Hartwell governor is a spring loaded governor as shown in Fig. It consists of two bell crank levers pivoted at the points O,O to the frame. • The frame is attached to the governor spindle and therefore rotates with it. 17
  18. 18. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 18 • Each lever carries a ball at the end of the vertical arm OB and a roller at the end of the horizontal arm OR. • A helical spring in compression provides equal downward forces on the two rollers through a collar on the sleeve. • The spring force may be adjusted by screwing a nut up or down on the sleeve.
  19. 19. 3/12/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 19

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