Governors

Posted by: MarinerGalaxy

Governors are used to control the speed of the engine or output automatically. The purpose of any prime mover is to produce power. The prime mover should be controlled and directed as desired. It detects the fluctuations in the turbine speed and keeps the speed constant by controlling the steam flow into the turbine. The hydraulic governor system consists of the wood ward UG-8 speed governor, the hydraulic servomotor and the governor valve. The governor is incorporated with a hydraulic shut down device that shuts down the governor valve when control oil pressure falls below 1 kg/cm2 G.

A Governor is a device that controls the engine speed or output automatically. The purpose of any prime mover is to produce power. The prime mover should be controlled and directed as desired.

In a propulsion plant the operator regulates the power output of the plant to match propeller thrust at a desired rpm of the engine. The rate of power supply is constantly controlled while the ship is under way. It is impossible for even the most skilled operator to do this job effectively and efficiently. Hence, the governor is used for automatic control.

Speed Governor

The primary function of the governor is to measure speed. The secondary function is to move the governor terminal or output shaft according to the measured speed. This shaft is connected to the control rod of the fuel injection system.

Governor Terms, Concepts and Operation

Speed Governor

Types

Depending upon how the centrifugal force is used in moving the throttle, governors may be divided into two groups:

Mechanical governor:

Flyweight balls through mechanical linkages directly operate control mechanism, which regulates the fuel supply.

Mechanical hydraulic governor:

Here flyweights actuate a relay or a supply of external power (oil under pressure).

Electronic governor:

Compared with mechanical and hydraulic governors, electronic governors use DC/AC signals to maintain speed. They do not have any mechanical linkages. Henceforth, they are accurate in speed measurement and control.

Hydraulic and Digital Governors:

Governors are used to control the speed of the engine or output automatically. A mechanical governor overcomes friction in the linkages and exerts a controlling force. The electronic speed sensor consists of a set of gear teeth that rotate at speed of engine and a Magnetic Pick Up (MPU) that has a slight air gap. The MPU has a permanent magnet and pole piece. A coil surrounds the pole piece. The permanent magnet creates its own magnetic field.

Mechanical Governor:

A mechanical governor overcomes friction in the linkages and exerts a controlling force. These forces act in different directions depending upon whether the load is increasing or decreasing.

A simple mechanical governor with flyweights is shown. The ball head or ball arm consists of two eccentrically pivoted, flyweights mounted on opposite sides of a rotating sleeve or a spindle. Speed of rotation of the ball arms is proportional to engine speed. It may be a direct drive or through a step up gear. When ball arm rotates it causes the flyweights to rotate. This results in a centrifugal force to be set up. The centrifugal force is used as a feed back in controlling the engine speed. At equilibrium condition the speeder spring force matches the centrifugal force generated by the flyweights.

The spindle is connected to a linkage. A pivot supports the linkage. The other end of the linkage is connected to the fuel control valve through a fuel rod.

Increase in load

When load is applied to the engine, the speed of engine starts to decrease. Since the governor is connected directly to the engine or through a gear mechanism, the rotational speed of the ball arms also decreases. Reduced speed of flyweights results in reduced centrifugal force.

The speeder spring force exceeds the centrifugal force. This causes the flyweights to move in. The flyweights rotate close to the axis of rotation. This results in spindle or rotating sleeve to move down. As the spindle moves down, the linkage at the other end moves up, allowing more amount of fuel. When quantity of fuel increases the speed of engine also increases and reaches the set value.

Decrease in load

When load on the engine decreases, the speed of engine starts to increase. Since the governor is connected to the engine directly or through a gear mechanism, the rotational speed of the ball arms also increases. Increased speed of flyweights results in increased centrifugal force.

The centrifugal force exceeds the speeder spring force. This causes the flyweights to move out. The flyweights rotate away from the axis of rotation. This causes the spindle or rotating sleeve to move up. As the spindle moves up, the linkage at the other end moves down, reducing the amount of fuel. When quantity of fuel decreases the speed of engine also decreases and reaches the set value.

Mechanical Hydraulic Governor

A simple mechanical hydraulic governor with flyweights is shown in sketch. The ball head or ball arm consists of two flyweights which rotate eccentrically.

They are mounted on opposite sides of a rotating sleeve or a spindle. Speed of rotation of the ball arms is proportional to speed of engine. It may be a direct drive or through a step up gear. When ball arm rotates it causes the flyweights to rotate. This results in a centrifugal force which is used as a feedback in controlling the speed of engine.

Hydraulic oil pressure is from a separate gear pump operated by a governor drive.

A rod connects the sleeve to the hydraulic piston. There is also a compensating piston which is attached to this rod. In equilibrium condition the oil pressure on both sides of compensating piston are balanced. If oil pressures on each of the piston are unbalanced then this compensating piston is subjected to an unbalanced force. This unbalanced force will then get added up or subtracted to from the speeder spring compression force at the ball head.

If the engine speed decreases, the centrifugal force decreases and allows the sleeve to be lowered by the spring. Conversely if the engine speed increases, the centrifugal force increases and fly weights raise the sleeve.

As the engine speed slows down, the ball head moves the piston valve down, uncovers the port and allows oil under pressure to pass into the system. The pressure acts on the buffer piston, forcing it to the right. This compresses the right hand buffer spring which displaces oil on the right hand side.

This movement causes the power piston to move up, which increases the fuel pump settings.

Due to the compression of the buffer springs, the oil pressure on the right of the buffer piston will be less than that on its left. These pressures act on the under side of the compensation piston. This results in an upward force. The piston valve returns to its shut position even though the speed of engine has not returned to its set value.

The pressure difference across the compensating piston is temporary because the needle valve allows oil to pass slowly to balance both sides. The buffer piston spring returns to its mid position.

The ball head returns to its original setting. The controls are at a higher setting matching with the demand for greater power.

If the engine speed increases, the ball head raises the piston valve, releasing oil from the servo system to oil return. The buffer piston moves to the left. This causes the power piston to move down reducing the settings to slow the engine. There is an unbalanced force on the compensating piston. This together with the speeder spring forces the piston valve to move down and close the oil passages. These changes are temporary until the pressure equalise through the needle valve.

The flow rate past the needle valve is adjusted to match the engine response characteristic. This is the time taken to respond to a change in fuel setting.

Electronic Governor

The block diagram shows the general arrangement of an electronic governor. The speed sensor consists of a set of gear teeth that rotate at speed of engine and a Magnetic Pick Up (MPU) that has a slight air gap. The MPU has a permanent magnet and pole piece. A coil surrounds the pole piece. The permanent magnet creates its own magnetic field.

During running as each ferrous gear tooth passes the core, the reluctance path decreases and the flux lines increase. The change in flux lines produces an AC sine wave voltage in the coil and its frequency represents the speed of engine. The AC voltage is amplified and rectified to a DC voltage which is proportional to the engine rpm. This DC voltage is compared with the desired set voltage at the controller (corresponding to the desired rpm). An appropriate electric signal is sent to an electro hydraulic converter. The electro hydraulic converter processes the signal and operates an actuator (hydraulic cylinder and piston) to increase or reduce the fuel supply as required. An actuator position feedback is provided to the controller as show

Governing Systems

Speed Governing System

The hydraulic governor system consists of the wood ward UG-8 speed governor, the hydraulic servomotor and the governor valve.

The governor detects the fluctuation of the turbine speed and keeps it constant by controlling the steam flow into the turbine by means of the governor valve through the lever mechanism and the hydraulic servomotor.

In case of load increasing, the turbine speed tends to fall, causing a response in the governor output linkage. The control valve in the servomotor moves up, allowing oil pressure build up on top of the servo piston pulling the output link from servomotor down, thereby taking the nozzle control valves towards opening direction, allowing more steam admission and stabilize turbine speed, within 10% of rated speed instantaneously. The opposite happens when 'load' tends to fall.

The governor is incorporated with a hydraulic shut down device which shuts down

the governor valve when control oil pressure falls below 1 kg/cm2 G.

Power Sharing

Parallel operation of generators enables better reliability of power supply systems to be reached.It is used when the power capacity of electrical loads is large. These procedures assume that one generator set is on line (operating and connected to the distribution feeder lines through the switchgear). The set that is to be paralleled is designated the incoming set. When you are operating generator sets in parallel, they must have the same output voltage, frequency, phase relation, and phase sequence before they can be connected to a common distribution bus. Severe damage may occur to the generator sets if these requirements are not met.