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.
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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
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.
145
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.
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