Answer EKM Question 10

Q10. With reference to mechanical/hydraulic governors:

A. Why flyweights are driven at a higher rotational speed than the engine;

B. How dead band effects are reduced;

C. How hunting is reduced;

D. How the output torque is increased.
Answer: A. Flyweights driven at a higher rotational speed than the engine:
In a flyweight type governor, the speed sensing is by means of the centrifugal force due to the rotation of the flyweights. If W is the weight of the flyweight, of the angular velocity and r the radius of rotation, then the centrifugal force $\displaystyle \small \mathrm{F=\frac{W}{g}\times \omega ^2\times r}$ i.e. F varies as the square of the governor rpm.
The flyweights are driven by the engine and by the use of step up gear these are usually rotated at a higher speed to make the governor more sensitive in detecting small speed changes.
Where the governor is required to operate the fuel control mechanism directly (i.e without a hydraulic servo amplifier) a higher speed of rotation of the governor increases the power to operate the fuel control system.

B. How dead band effects are reduced:
Dead band is the amount of speed or rpm change that must take place at any point of time before the governor linkage system will move to respond.
Dead band is due to frictional force that must be overcome for the governor to act and it can be reduced by designing the components such that they offer minimum friction. Increasing rotational speed of the governor by step up gear drive, thereby increasing the centrifugal force, will also help in reducing the dead band.

C. How hunting is reduced:
Hunting can take place if the governor is over-sensitive which can be the case if the droop is too small. A compensation arrangement is provided within the governor by which it is possible to adjust the droop increasing it slightly to reduce hunting. Ideally, sensitivity should be adjusted as high as possible without causing hunting of the governor.

D. How the output torque is increased:
In a mechanical centrifugal governor the output torque is increased by either increasing the mass of the flyweights or the speed of rotation of the flyweight by a suitable gear ratio or both.
Where a hydraulic servo amplifier is provided, the higher output torque may be obtained by increased hydraulic oil pressure or the size of the servo piston.