24-08-2012, 04:59 PM
GOVERNORS
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INTRODUCTION
In the last unit, you studied flywheel which minimises fluctuations of speed within the cycle but it cannot minimise fluctuations due to load variation. This means flywheel does not exercise any control over mean speed of the engine. To minimise fluctuations in the mean speed which may occur due to load variation, governor is used. The governor has no influence over cyclic speed fluctuations but it controls the mean speed over a long period during which load on the engine may vary.
When there is change in load, variation in speed also takes place then governor operates a regulatory control and adjusts the fuel supply to maintain the mean speed nearly constant. Therefore, the governor automatically regulates through linkages, the energy supply to the engine as demanded by variation of load so that the engine speed is maintained nearly constant.
Figure 5.1 shows an illustrative sketch of a governor along with linkages which regulates the supply to the engine. The governor shaft is rotated by the engine. If load on the engine increases the engine speed tends to reduce, as a result of which governor balls move inwards. This causes sleeve to move downwards and this movement is transmitted to the valve through linkages to increase the opening and, thereby, to increase the supply.
On the other hand, reduction in the load increases engine speed. As a result of which the governor balls try to fly outwards. This causes an upward movement of the sleeve and it reduces the supply. Thus, the energy input (fuel supply in IC engines, steam in steam turbines, water in hydraulic turbines) is adjusted to the new load on the engine. Thus the governor senses the change in speed and then regulates the supply. Due to this type of action it is simple example of a mechanical feedback control system which senses the output and regulates input accordingly.
Centrifugal Governors
In these governors, the change in centrifugal forces of the rotating masses due to change in the speed of the engine is utilised for movement of the governor sleeve. One of this type of governors is shown in Figure 5.1. These governors are commonly used because of simplicity in operation.
Inertia and Flywheel Governors
In these governors, the inertia forces caused by the angular acceleration of the engine shaft or flywheel by change in speed are utilised for the movement of the balls. The movement of the balls is due to the rate of change of speed in stead of change in speed itself as in case of centrifugal governors. Thus, these governors are more sensitive than centrifugal governors.
Pickering Governors
This type of governor is used for driving a gramophone. As compared to the centrifugal governors, the sleeve movement is very small. It controls the speed by dissipating the excess kinetic energy. It is very simple in construction and can be used for a small machine.
Hartnell Governor
The Hartnell governor is shown in Figure 5.5. The two bell crank levers have been
provided which can have rotating motion about fulcrums O and O. One end of each bell
crank lever carries a ball and a roller at the end of other arm. The rollers make contact
with the sleeve. The frame is connected to the spindle. A helical spring is mounted
around the spindle between frame and sleeve. With the rotation of the spindle, all these
parts rotate.
With the increase of speed, the radius of rotation of the balls increases and the rollers lift
the sleeve against the spring force. With the decrease in speed, the sleeve moves
downwards. The movement of the sleeve are transferred to the throttle of the engine
through linkages.
SUMMARY
The governors are control mechanisms and they work on the principle of feedback
control. Their basic function is to control the speed within limits when the load on the
prime mover changes. They have no control over the change is speed within the cycle.
The speed control within the cycle is done by the flywheel.
The governors are classified in three main categories that is centrifugal governors,
inertial governor and pickering governor. The use of the two later governors is very
limited and in most of the cases centrifugal governors are used. The centrifugal
governors are classified into two main categories, gravity controlled type and spring
loaded type.
The gravity controlled type of governors are larger in size and require more space as
compared to the spring controlled governors. This type of governors are two, i.e. Porter
governor and Proell governor. The spring controlled governors are : Hartnel governor,
Wilson-Hartnell governor and Hartung governor.