Governors

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.

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.

CLASSIFICATION OF GOVERNORS

The broad classification of governor can be made depending on their operation.

(a) Centrifugal governors

(b) Inertia and flywheel governors

(c) Pickering governors.

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. 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.

Types of Centrifugal Governors

Depending on the construction these governors are of two types :

(a) Gravity controlled centrifugal governors, and

(b) Spring controlled centrifugal governors.

Gravity Controlled Centrifugal Governors

In this type of governors there is gravity force due to weight on the sleeve or weight of sleeve itself which controls movement of the sleeve. These governors are comparatively larger in size.

Spring Controlled Centrifugal Governors

In these governors, a helical spring or several springs are utilised to control the movement of sleeve or balls. These governors are comparatively smaller in size.

GRAVITY CONTROLLED CENTRIFUGAL GOVERNORS

There are three commonly used gravity controlled centrifugal governors :

(a) Watt governor

(b) Porter governor

(c) Proell governor

Watt governor does not carry dead weight at the sleeve. Porter governor and proell governor have heavy dead weight at the sleeve. In porter governor balls are placed at the junction of upper and lower arms. In case of proell governor the balls are placed at the extension of lower arms. The sensitiveness of watt governor is poor at high speed and this limits its field of application. Porter governor is more sensitive than watt governor. The proell governor is most sensitive out of these three.

 Watt Governor

This governor was used by James Watt in his steam engine. The spindle is driven by the output shaft of the prime mover. The balls are mounted at the junction of the two arms. The upper arms are connected to the spindle and lower arms are connected to the sleeve .

Porter Governor

A schematic diagram of the porter governor is shown in figure .There are two sets

of arms. The top arms OA and OB connect balls to the hinge O. The hinge may be on the

spindle or slightly away. The lower arms support dead weight and connect balls also. All

of them rotate with the spindle. We can consider one-half of governor for equilibrium.

Let w be the weight of the ball, Governors

T1 and T2 be tension in upper and lower arms, respectively,

Fc be the centrifugal force,

r be the radius of rotation of the ball from axis, and

I is the instantaneous centre of the lower arm.

Taking moment of all forces acting on the ball about I and neglecting friction at the

sleeve, we get

If friction at the sleeve is f, the force at the sleeve should be replaced by W + f for rising

and by (W – f) for falling speed as friction apposes the motion of sleeve. Therefore, if the

friction at the sleeve is to be considered, W should be replaced by (W  f). The

expression in

SPRING CONTROLLED CENTRIFUGAL

GOVERNORS

In these governors springs are used to counteract the centrifugal force. They can be

designed to operate at high speeds. They are comparatively smaller in size. Their speed

range can be changed by changing the initial setting of the spring. They can work with

inclined axis of rotation also. These governors may be very suitable for IC engines, etc.

The most commonly used spring controlled centrifugal governors are :

(a) Hartnell governor

(b) Wilson-Hartnell governor

(c) Hartung governor

 Hartnell Governor

The Hartnell governor is shown in figure. 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.

Let r Governors 1 = Minimum radius of rotation of ball centre from spindle axis, in m,

r2 = Maximum radius of rotation of ball centre from spindle axis, in m,

S1 = Spring force exerted on sleeve at minimum radius, in N,

S2 = Spring force exerted on sleeve at maximum radius, in N,

m = Mass of each ball, in kg,

M = Mass of sleeve, in kg,

N1 = Minimum speed of governor at minimum radius, in rpm,

N2 = Maximum speed of governor at maximum radius, in rpm,

s = Stiffness of spring or the force required to compress the spring by one m,

r = Distance of fulcrum O from the governor axis or radius of rotation,

a = Length of ball arm of bell-crank lever, i.e. distance OA, and

b = Length of sleeve arm of bell-crank lever, i.e. distance OC.

GOVERNOR EFFORT AND POWER

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.

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Governor Power Governors

It is defined as the work done at the sleeve for a given change in speed. Therefore,

Power of governor = Governor effort  Displacement of sleeve

 CHARACTERISTICS OF GOVERNORS

Different governors can be compared on the basis of following characteristics :

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.

Sensitiveness Governors

The sensitiveness can be defined under the two situations :

(a) When the governor is considered as a single entity.

(b) When the governor is fitted in the prime mover and it is treated as

part of prime mover.

(a) A governor is said to be sensitive when there is larger displacement of the

sleeve due to a fractional change in speed. Smaller the change in speed of

the governor for a given displacement of the sleeve, the governor will be

more sensitive.

 (b) 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

 Sensitivenes.

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.

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.

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.

For comparing different type of governors, effort and power is used. They determine

whether a particular type of governor is suitable for a given situation or not. To

categorise a governor the characteristics can be used. It can be determined whether a

governor is stable or isochronous or it is prone to hunting. The friction at the sleeve

gives rise to the insensitiveness in the governor. At any particular radius, there shall be

two speeds due to the friction. Therefore, it is most desirable that the friction should be

as low as possible.

The stability of a spring controlled governor can be determined by drawing controlling

force diagram which should have intercept on the negative side of Y-axis.

KEY WORDS

Watt Governor : It is a type of governor which does not have load

on the sleeve.

Porter Governor : This is a type of governor which has dead weight

at the sleeve and balls are mounted at the hinge.

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Theory of Machines

Hartnell Governor : It is a spring controlled governor in which balls are mounted on the bell crank lever and sleeve is loaded by spring force.

Governor Effort : It is the mean force exerted on the sleeve during a given change of speed.

Governor Power : It is defined as the work done at the sleeve for a given change in speed.

Hunting of Governor : It can occur in governor when the fluctuations in engine speed coincides the natural frequency of oscillations of the governor. In that case governor intensifies the speed variation instead of controlling it.

Controlling Force : It is the resultant external force which controls the movement of the ball and acts along the radial line towards the axis