Types
A Caterpillar Engines Electric Set is generally furnished with one of the following types of governors:
- Mechanical approximately 3% speed droop (fixed)
- Woodward UG8 Hydraulic Adjustable from 0% to approximately 4% speed droop
- Woodward PSG Hydraulic Adjustable from 0% to approximately 4% speed droop
Two types of governors that are being used for special applications are the electric-hydraulic and electronic governors. These are adjustable from 0% to approximately 4% speed droop.
Characteristics
The governors listed above include in their description a value of speed droop. Speed droop is defined as the percent change in engine speed between no load (high idle) and full load. Speed droop can be calculated using the equation given below.
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| Illustration 1 | g01058906 |
A schematic of a typical mechanical governor is shown in Illustration 2. The basic components are a governor spring, a set of weights, and linkage connecting these components to the engine fuel rack.
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| Illustration 2 | g01058910 |
Schematic of a typical Mechanical Governor | |
The weights are driven by the engine through gears of an appropriate ratio.
The engine speed control is set for the desired engine speed. This, in turn, compresses the governor spring and the fuel rack moves to a position to supply fuel to the engine. When the engine starts, the weights rotate. Since they are constructed to move under the action of centrifugal force, the weights move out as the engine speed increases. This action causes the toes of the weights to compress the governor spring further. This movement continues until the centrifugal force of the weights is in balance with the force of the compressed spring. When the weights move out against the force of the governor spring, the fuel rack setting is reduced. The engine speed will stabilize at a value where the spring force, weight centrifugal force, and engine speed are in balance.
An electric set engine is customarily set at "High Idle" speed when there is no load on the generator. Generators rated 1200 rpm, 60 cycles will be used in the examples that follow.
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| Illustration 3 | g01058916 |
Typical speed droop characteristics of a 3% mechanical governor. Line A represents an ideal 3% speed droop characteristic: Line B an actual speed droop characteristic and Line C the rated speed or frequency | |
Illustration 3 shows the speed droop characteristics of a 3% mechanical governor. In this case, high idle speed is 1236 rpm. Line "A" represents an ideal 3% speed droop characteristic. Line "B" represents an actual speed droop characteristic. Because of friction losses and limitations of materials, a governor speed droop characteristic is never a straight line. This is one reason why the descriptions of speed droop percentage include the word "approximately". The ideal speed droop characteristic figure of 3% will be used in this discussion.
At 0% load, the engine speed or generator frequency is 103% of the rated value. This represents 61.8 cycles for a 60 cycle system. At 100% load the engine speed or generator frequency is 100% of rated value. This represents 60.0 cycles for a 60 cycle system. Speed characteristics will be given in per cent of rated frequency of the electrical system, or in cycles. Line "C" of Illustration 3 represents rated speed or frequency.
A simple mechanical governor cannot maintain rated speed over the load range. This can only be accomplished with the use of additional governor components which will be mentioned in the description of hydraulic governors. The simple mechanical governor can maintain constant speed if the load does not vary. When load is applied, however, engine speed decreases and the centrifugal force of the governor weights decreases. Therefore, the governor spring is opposed by a smaller force and moves the fuel rack in the direction to give the engine more fuel to carry the load. The engine speed will then increase until the force of the governor weights again balances the force of the governor spring. Illustration 4 shows typical changes in engine speed, or generator frequency when 1/3 load is added and then dropped for an engine electric set with 3% mechanical governor.
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| Illustration 4 | g01058931 |
Typical plot of changes in engine speed or generator frequency on an engine electric set with 3% mechanical governor when 1/3 load is added then dropped | |
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| Illustration 5 | g01058948 |
Typical plot of changes in engine speed or generator frequency on an engine electric set with isochronous governor when 1/3 load is added and then dropped | |
A hydraulic governor is capable of being adjusted to maintain constant speed over the load range. This is called "Isochronous" operation. Illustration 5 shows a typical speed characteristic curve when load is added and dropped from an engine electric set with isochronous governor.
An isochronous governor is more complicated than a simple mechanical governor. A detailed description of the principles of isochronous governing is available from such companies as the Woodward Governor Company.
Two other types of governors being used for special applications are the electric-hydraulic and the electronic. The electric-hydraulic governor has basic hydraulic governor has basic hydraulic governor components with electrical components added. When a load change occurs, the electrical circuit senses the electrical change and acts through the governor to change the fuel setting to maintain constant speed.
Mechanical and hydraulic governors sense a change in speed as a signal that load has been added or removed and that a change in fuel setting is required (Illustration 4 and 5). The electric-hydraulic and electronic governors sense load changes directly and make a corresponding change in fuel setting, thus keeping the speed changes to a minimum. The above description of the electric-hydraulic governor pertains to isochronous governing. These governors can be adjusted for speed droop. However, they are capable of being operated in parallel when both are adjusted for isochronous operation. This is not the case with the regular hydraulic governors. A bulletin from the governor manufacturer describing in detail the operation of the electric-hydraulic governor is considered essential before any governor of this type is placed into operation.
The electronic governor usually consists of the following components:
- Hydraulic Pump
- Oil Reservoir
- Hydraulic Servo-motor
- Electric Control Cabinet
The hydraulic pump is driven by the engine. It draws oil from the reservoir and delivers it to the hydraulic servo-motor. Servo-motor action is initiated by a solenoid valve which is operated by power from the electric control cabinet. The servo-motor moves the fuel rack to increase or decrease fuel to the engine as required to maintain the engine settings. The cabinet receives signals from the voltage, current, and frequency output of the generator. Electronic governors can be adjusted for speed droop or isochronous operation. A bulletin from the governor manufacturer describing the operation of the electronic governor is essential before any governor of this type is placed into operation.