Lecture 07

LESSON 7: GOVERNORS

UNIT II GOVERNORS

THEORY OF MACHINES-II

In this lecture we will start our next chapter governors. You might have listened the word flywheel, governor is also like this but it has also some difference in function. So let us discuss governors and its types.

Introduction The function of the governor, as applied to the engines, is to adjust the supply of fuel according to the load requirements so as to keep the speeds at various, loads, as close to the mean speed as possible, over long range of working of the engines. Its function is distinct from that of a flywheel, which acts as a reservoir and keeps the speed within certain limits of the mean speed during the thermodynamic cycles. The function of a flywheel is continuous from cycle to cycle, nut that of governor it is more or less intermittent i.e. it reacts only whenever there is variation of load. In brief governor takes care of the change of speed due to load variation over periods of the engine’s running and tends to keep it as close to the mean speed as possible, where as the flywheel is responsible only in keeping the speed fluctuations, during each cycle within certain permissible limits of the mean speed. As such, one cannot be replaced by the other.

Chapter Learning Objectives After studying this chapter you should understand • Function of governor • Working of centrifugal governors • Effect of friction on functioning of governor. • Difference between a porter and a hartnell governor • Sensitiveness, stability, isochronism, hunting in connection

with governors • Effort and power in governors

Comparison between a flywheel and governor: S.no 1

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3. 4. 5.

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Flywheel It is provided on the engine and fabricating machines viz., rolling mills; punching machines; shear machines, presses etc. Its function is to store available mechanical energy when it is in excess of the load requirements and to part with the same when the available energy is less than that required by the load. In engines it takes care of fluctuations of speed during thermodynamic cycle. It works continuously from cycle to cycle. In fabrication machines it is very economical to use it as its use reduces capital investment on prime movers and their running expenses.

Governor It is provided on prime movers such as engines and turbines Its function is to regulate the supply of driving fluid producing energy, according to the load requirements so that at different loads almost a constant speed is maintained. It take care of fluctuation of speed due to variation of load over range of working of engines and turbines. It works intermittently, i.e. only when there is change in the load. But for governor, there would have been unnecessarily more consumption of driving fluid thus it economizes its consumption

Principle of Operation When the load on an engine increases or decreases, obviously its speed will respectively decrease or increase to the extent of variation of load. This variation of speed has to be controlled by the governor, within small limits of the mean speed. This necessities that when the load increases and consequently the speed decreases, the supply of fuel to one engine has to be increased accordingly, to compensate for the loss of the speed, so as to bring back the speed close to the mean speed. Conversely when the load decreases, and the speed increase, the supply of fuel has to be reduced. This implies that the governor should have its mechanism working in such a way, that the supply of fuel is automatically regulated according to the load requirement for maintaining approximately a constant speed. There are some general terms used in governors that describe qualities of governor. These terms are as: 1. Height of governor: It is the vertical distance between the center of the governor halls and the point of intersection between the upper arms on the axis of spindle is known as governor height. It is generally denoted by h. 2. Equilibrium speeds-the speeds at which the governor balls, the arms etc. are in complete equilibrium and the sleeve doest not tend to move upward or downward are called the equilibrium speeds. The speed at the mean position of the balls or the sleeve is the mean equilibrium speed and at the maximum and minimum radius of rotation of the balls without tending to move either way are termed as maximum and minimum equilibrium speeds respectively. There can be many equilibrium speeds between the mean and maximum and the mean and the minimum equilibrium speeds. 3. Sleeve lift: the vertical distance the sleeve travels due to change in the equilibrium speed is called the sleeve lift. The vertical downward travel may be termed as negative lift. 4. Effort and power of a governor-a governor running at a constant speed is in equilibrium and the resultant force acting on the sleeve is zero. if the speed of the governor increases there is a force on the sleeve which tends to lift it. This force will gradually go on decreasing till the governor starts rotating in equilibrium at the new position of rotation. The mean force acting on the sleeve for a given change of speed or lift of the sleeve is known as the governor effort. 5. Controlling force: the force acting radially upon the rotating balls to counteract its centrifugal force, is called the controlling force. It is provided by wt of the sleeve, central load on the sleeve, compressed spring and the last one i.e. the wt of the balls. 6. Isochronism: this is an extreme case of sensitiveness. When the equilibrium speed is constant for all radii of rotation of the balls within the working range, the governor is said to be in isochronism. This means that the difference between the

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THEORY OF MACHINES-II

maximum and minimum equilibrium speeds is zero and the sensitiveness shall be infinite. 7. Hunting: The phenomenon of continuous fluctuation of the engine speed above and below the mean speed is termed as hunting. This occurs in over-sensitive or isochronous governors. Suppose an isochronous governor is fitted to an engine running at a steady load. With a slight increase of load, the speed will fall and the sleeve will immediately fall to its lowest position. This shall open the control valve wide and excess supply of energy will be given, with the result that the speed will rapidly increase and the sleeve will rise to its higher position. As a result of this movement of the sleeve, the control valve will be cut off; the supply to the engine and the speed will again fall, the cycle being repeated indefinitely. Such a governor would admit either more or less amount of fuel and so effect would be, that the engine would hunt. 8. Sensitiveness: A governor is said to be sensitive, if its change of speed s from no load to full load may be as small a fraction of the mean equilibrium speed as possible and the corresponding sleeve lift may be as large as possible. Suppose ω1 = max. equilibrium speed ω2 = min. equilibrium speed ω = mean equilibrium speed =(ω1+ω2)/2 Therefore sensitiveness = (ω1-ω2)/2

Classification of Governors

When the loads 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 move downwards. 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. Characteristics Of Centrifugal Governors

For satisfactory performance and working a centrifugal governor should possess the following qualities. a. On the sudden removal of load its sleeve should reach at the top most position at once. b. Its response to the change of speed should be fast.

Governors are classified based upon two different principles. These are:

c. Its sleeve should float at some intermediate position under normal operating conditions.

1. Centrifugal governors

d. At the lowest position of sleeve the engine should develop maximum power.

2. Inertia governors

e. It should have sufficient power, so that it may be able to exert the required force at the sleeve to operate the control & mechanism.

Centrifugal Governors Principle of Working

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. 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. 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 & down. The balls and the sleeve rises when the spindle speed increases and falls when the speed decreases. In order to limit the travel of the sleeve in upward and downward directions, two stops S, S are provided on the spindle. 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.

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Inertia Governors Introduction

The inertia type governors are fitted to the crankshaft or flywheel of an engine and so radically in appearance from the centrifugal governors. The balls are so arranged that the inertia forces caused by an angular acceleration or retardation of the shaft tend to alter their positions. The amount of displacement of governor balls is controlled by the suitable springs and through the governor mechanism, alters the fuel supply to the engine. This governor is more sensitive than the centrifugal, but it becomes difficult to completely balance the revolving parts. For this reason centrifugal governors are more frequently used. Let us continue with centrifugal governors again as it can be further classified as

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THEORY OF MACHINES-II

So we have seen that there are various types of centrifugal governors depending upon the type of the main controlling force provided on the balls. 1. Watt’s governor 2. The Porter governor 3. The Proell governor 4. The Spring loaded governor Notes

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