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| Illustration 1 | g00851777 |
Generator Components (1) Shaft (2) Exciter Armature (Rotor) (3) Main Armature (Stator) (4) Main Field (Rotor) | |
An engine supplies the power in order to turn rotor shaft (1). Exciter armature (2) and main field (4) attach to rotor shaft (1). As rotor shaft (1) turns, exciter armature (2) and related components generate DC current. This DC current is supplied to main field (4). A magnetic field is created around the poles of main field (4). As main field (4) turns with rotor shaft (1), the magnetic field also rotates. The revolving magnetic field induces an AC voltage into stationary main armature (3). Main armature (3) is a coil with many turns of wire. The current that flows through the main armature also flows to the load.
The exciter supplies DC current to main field (4). The load voltage is controlled by varying the current that goes to exciter armature (2). There are two methods for excitation that are used on SR4 generators:
- Permanent magnet pilot excited (PMPE)
- Self-excited (SE)
The permanent magnet pilot excited (PMPE) generator better sustains an overload for a short duration. The PMPE generator provides excitation power to the voltage regulator that is isolated from regulator sensing.
Self-Excited (SE) Generators
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| Illustration 2 | g00874825 |
SE Generator Wiring Diagram (CR1-CR6) Diodes (CR7) Varistor (L1) Exciter Field (Stator) (L2) Exciter Armature (Rotor) (L3) Main Field (Rotor) (L4) Main Armature (Stator) (RFA) Rotating Field Assembly (CT1) Optional Voltage Droop Transformer (T0, T1, T2, T3, T7, T8, T9) Generator Terminals and/or Generator Leads | |
Self-excited generators receive the power for excitation from the generator armature (the generator output).
When the engine starts turning rotating field assembly (RFA), the residual magnetism in exciter field (L1) causes a small amount of AC voltage to be generated in exciter armature (L2). Induced voltage causes AC current to flow. AC current flows through three-phase full wave bridge rectifier circuit (CR1-CR6). DC current then flows through main field (L3). The flow of DC current through main field (L3) creates a magnetic field which adds to the existing residual magnetism of main field (L3). As main field (L3) rotates, an AC voltage is induced into main armature (L4) which appears as a three-phase AC voltage at output terminals (T0), (T1), (T2), and (T3). The voltage regulator taps the AC output through wires 20, 22, and 24. During start-up, this tapped output is sensed by the voltage regulator. The voltage regulator senses the output as a low-voltage output condition. Therefore, the voltage regulator output to exciter field (L1) is increased so that the generator output will continue to increase up to the rated voltage.
The amount of current that flows through the exciter directly affects the generator output voltage. The voltage regulator maintains a constant generator output voltage with changing loads. The voltage regulator output is a DC voltage and a DC current to exciter field (L1). The voltage regulator senses the generator output voltage at wires 20, 22, and 24. The voltage regulator then supplies the controlled DC voltage and DC current to the exciter through wires F1 and F2.
Note: For more information on voltage regulation, see the appropriate voltage regulator service manual.
When the voltage regulator senses a decrease in output voltage, the voltage regulator will increase the DC voltage and the DC current. The DC voltage and DC current is sent through the exciter through wires F1 and F2. The exciter field's magnetic field increases. As the magnetic field in exciter field (L1) is increased, the AC voltage that is induced in exciter armature (L2) is increased. This increased AC voltage from exciter armature (L2) causes more AC current to flow. The AC current is then rectified by a three-phase full wave bridge rectifier circuit. This rectifier circuit consists of the following diodes: (CR1-CR6). The increased DC output from the bridge rectifier is carried to main field coils (L3) by conductors. These conductors are routed through a passage in the generator shaft. Increased current through main field coils (L3) increases the magnetic field of the generator. The increased magnetic field induces a larger AC voltage into main armature (L4). The three-phase AC voltage increases until the voltage regulator no longer senses a decreased output voltage.
When the voltage regulator senses an increase in the output voltage, the voltage regulator will decrease the DC voltage to the exciter. This will result in a decrease in the generator output voltage.
Residual magnetism is necessary for start-up of the self-excited generator. The main field coils are wound on magnetic steel that retains a small amount of magnetism after shutdown. After time and certain conditions, the residual magnetism may decrease. The residual magnetism will then be insufficient to start the generating process. If this occurs see Testing And Adjusting Section, "Exciter Field - Flash".
Permanent Magnet Pilot Excited (PMPE) Generator
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| Illustration 3 | g00874841 |
PMPE Generator Wiring Diagram (CR1-CR6) Diodes (CR7) Varistor (L1) Exciter Field (Stator) (L2) Exciter Armature (Rotor) (L3) Main Field (Rotor) (L4) Main Armature (Stator) (L5) Pilot Exciter Armature (PM) Permanent Magnet (R5) Bleed Resistor (RFA) Rotating Field Assembly (CT1) Optional Voltage Droop Transformer (T0, T1, T2, T3, T7, T8, T9) Generator Terminals and/or Generator Leads | |
Permanent magnet pilot excited (PMPE) generators receive power for the voltage regulator from a pilot exciter. The pilot exciter consists of permanent magnet (PM) and PM armature (L5). The pilot exciter operates independently from the generator output voltage. Constant excitation during a large load application is possible. Constant excitation is possible because the irregularities that occur in the generator output voltage are not fed back into the exciter. The irregularities that occur in the generator output voltage are caused by load conditions. The independent operation also allows the generator to sustain excessive currents for short periods of time.
When the engine starts turning rotating field assembly (RFA), the permanent magnet (PM) induces an AC voltage in PM armature (L5). PM armature (L5) has three coils of wire. PM armature (L5) generates three-phase AC current (AC). The resulting AC current flows through wires 11, 12 and 13 to the voltage regulator. At terminals 26, 28, and 30 within the voltage regulator, the three-phase AC current is rectified to DC current. A controlled amount of DC current is fed to exciter field (L1) through terminals F1 and F2.
DC current now flows to exciter field (L1) which creates a magnetic field. Exciter armature (L2) rotates in this magnetic field. Exciter armature (L2) generates three-phase AC current. The AC current is then rectified by a three-phase full wave bridge rectifier circuit (CR1-CR6). The DC output from the bridge rectifier is carried to main field (L3) by conductors which are routed through a passage in the generator shaft. Current through main field (L3) creates the magnetic field of the generator. As main field (L3) rotates, main field (L3) induces a three-phase AC voltage in main armature (L4). This voltage is sent to terminals (T0), (T1), (T2) and (T3). These terminals are connections for the load.
To keep the output voltage constant with changing loads, it is necessary to control the exciter current and the voltage. This control is the function of the voltage regulator. The voltage regulator senses the generator output voltage at wires 20, 22, and 24. The voltage regulator sends current to the exciter through wires F1 and F2. The amount of current is dependent on the sensed voltage. The current for excitation is drawn from permanent magnet (PM) and PM armature (L5) at wires 11, 12, and 13. Regardless of the generator's type (PMPE generator or self-excited generator), changing the exciter current and the voltage has the same effect on the generator's operation. See "Self-Excited (SE) Generators" in the Systems Operation Section, "Generator Operation" for a description of the generator operation when the output voltage changes.
Note: For more information on voltage regulation, see the appropriate voltage regulator service manual.
PMPE generators provide the magnetism for start-up of the generator. Permanent magnet (PM) supplies the initial magnetism that is required at start-up. Flashing the field is not required for start-up of the PMPE type generators.
Rectifier Circuits
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| Illustration 4 | g00610163 |
Rectifier Circuit (CR1 - CR6) Diodes (CR7) Varistor (L2) Exciter Armature (Rotor) (L3) Main Field (Rotor) (R5) Bleed Resistor | |
The following diodes form a bridge rectifier circuit: CR1, CR2, CR3, CR4, CR5 and CR6. The bridge rectifier circuit receives three-phase alternating current from exciter armature (L2). The bridge rectifier circuit rectifies the alternating current into direct current. The DC power is then routed to main field (L3).
Diodes CR1 through CR6 are contained in rotating rectifier blocks. Two different rotating rectifier blocks are currently used on SR4 generators. The type of generator and the size of the generator determine the rotating rectifier block that is used.
There are two types of rotating rectifier blocks:
Two-diode rectifier block - The two-diode rectifier block contains two diodes. Three identical blocks are required.
Six-diode rectifier block - The six-diode rectifier block contains six diodes. One block is required.
Rectifying the current creates heat. The rotating rectifier blocks are fastened to heat sinks. These heat sinks spread the heat. These heat sinks also allow the rotating rectifier blocks to operate at a cooler temperature.
Two-Diode Rectifier Block
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| Illustration 5 | g00610186 |
Two-diode rectifier block | |
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| Illustration 6 | g00610197 |
The locations of the three two-diode rectifier blocks | |
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| Illustration 7 | g00610202 |
The wiring of the three two-diode rectifier blocks (1) L2 (wire passage) (2) Heat sink assembly (3) Three two-diode rectifier blocks (4) L3 (wire passage) (5) R5 (Bleed Resistor) (6) CR7 | |
Three identical two-diode rectifier blocks (3) are interconnected in order to form a bridge rectifier circuit. The two-diode rectifier blocks contain a set of two diodes. Each rectifier block contains one of the following sets of diodes:
- CR1 and CR4
- CR2 and CR5
- CR3 and CR6
Two-diode rectifier blocks must be wired correctly. Refer to Illustration 7. Each AC input terminal connects to an exciter armature wire (1). The "+" terminals connect together. The "+" terminals also connect to one L3 wire (4) of the main field. The "-" terminals connect together. The "-" terminals also connect to the other L3 wire (4) of the main field.
The two-diode rectifier blocks (3) are mounted to heat sink assembly (2). The heat sink assembly is on the end of the generator shaft. Heat sink assembly (2) also contains varistor (6) and bleed resistor (5). The varistor and the resistor are used to protect the generator circuit. Refer to "Generator Circuit Protection" in the Systems Operation Section, "Generator Operation".
Six-Diode Rectifier Block
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| Illustration 8 | g00874861 |
Six-Diode Rectifier Block | |
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| Illustration 9 | g00610330 |
Location of the Six-Diode Rectifier Block (Inboard Bearing) | |
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| Illustration 10 | g00610335 |
Location of the Six-Diode Rectifier Block (Outboard Bearing) (1) Exciter (2) Six-Diode Rectifier Block (3) Disc (4) Main Field | |
The six-diode rectifier block contains the six diodes of the bridge rectifier circuit. Each AC input terminal connects to an exciter armature wire (L2). The "+" terminal and the "-" terminal connect to main field (L3) .
On inboard bearing type generators, the six-diode rectifier block is on the end of the generator shaft. On outboard bearing type generators, six-diode rectifier block (2) is mounted on disc (3). Disc (3) is between exciter (1) and main field (4) .
The six-diode rectifier block also contains varistor (CR7) which is used to protect the generator circuit. Bleed resistor (R5) is not used on generators with the six-diode rectifier block. Refer to "Generator Circuit Protection" in the Systems Operation Section, "Generator Operation".
Generator Circuit Protection
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| Illustration 11 | g00874841 |
PMPE Generator Wiring Diagram (CR1 - CR6) Diodes (CR7) Varistor (L1) Exciter Field (Stator) (L2) Exciter Armature (Rotor) (L3) Main Field (Rotor) (L4) Main Armature (Stator) (L5) Pilot Exciter Armature (PM) Permanent Magnet (R5) Bleed Resistor (RFA) Rotating Field Assembly (CT1) Optional Voltage Droop transformer (T0, T1, T2, T3, T7, T8, T9) Generator Terminals and/or Generator Leads | |
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| Illustration 12 | g00610379 |
Varistor (CR7) | |
Varistor (CR7) protects the following diodes by suppressing any abnormal transient peak voltages: CR1, CR2, CR3, CR4, CR5 and CR6. On generators that use the two-diode rectifier blocks, varistor (CR7) is a separate component and mounts on the heat sink assembly. On generators that use the six-diode rectifier block, varistor (CR7) is contained within the six-diode rectifier block.
Note: Some generators are provided with another varistor (CR8) for additional protection.
Bleed resistor (R5) is a separate component and mounts on the heat sink assembly. Bleed resistor (R5) is used only on some of the larger generators. Bleed resistor (R5) provides a low resistance circuit from the insulated windings to the shaft of the rotating field assembly (RFA). Bleed resistor (R5) is a 27000 ohm resistor. Air friction on the windings can cause an electrostatic charge. If this resistor is not installed, these charges can cause voltages to become high enough to damage the winding insulation. Bleed resistor (R5) allows charges to dissipate as the charges are generated. This resistor also prevents any buildup of electrostatic voltage. Because of the resistance value and the power rating of bleed resistor (R5), a ground failure at any point on the rotating field assembly (RFA) will not prevent the generator from operating normally. A ground failure will not damage bleed resistor (R5) .
The voltage regulator and related components also protect the generator. All VR3, VR3F, and VR4voltage regulators have fuses, which will stop the current flow to the exciter. When no voltage is applied to the exciter, the generator output voltage is reduced to a very low level. These fuses open very rapidly. This protects against secondary damage that is caused by another component failure. If any fuse is replaced, use only a fuse of the same type and amperage rating. A larger amperage rating or a fuse which does not open rapidly will not prevent damage to other components.
All voltage regulators have excessive overload current protection circuits that also open the excitation circuit.
Note: For more information on voltage regulation, see the appropriate voltage regulator service manual.
Space Heaters
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| Illustration 13 | g00851810 |
Location of the Space Heater (1) Space Heaters | |
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| Illustration 14 | g00851811 |
Space Heater Terminal Strip | |
The space heater for small frame generators is located within the housing of the main stator. The space heater is located in the end of the generator with the exciter.
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| Illustration 15 | g00610396 |
Space Heater Connection Diagram | |
The SR4 generator can operate in high humidity conditions without problems. However, problems can occur when the generator is idle and the surrounding air is warmer than the generator windings. Moisture can form on the windings. This moisture will result in poor performance or even damage to the windings. Whenever the generator is not active, the space heaters should be operated.
An external source is required to operate the space heaters. This source can be either 115 VAC or 230 VAC. Both of these sources must be single-phase. When the external source is 50 hertz, 200 VAC must be used. If the 115 VAC source is available, connect both heaters in parallel across the source (L1-L2). If the 200 VAC is available, connect both heaters in series across the source (L1-L2). Refer to Illustration 15.