This DC current is supplied to the main rotor windings. A magnetic field is created around the poles of the main rotor windings. As the main rotor turns with the rotor shaft, the magnetic field also rotates.
The magnetic field induces an AC voltage into stationary main stator. The main stator is a coil with multiple turns of wire. The current that flows through the main stator flows to the load.
The rectifier assembly will supply DC current to main stator. The load voltage is controlled by varying the current that goes to the exciter field (or stator).
The Series LC generator uses a field excitation system to regulate the output voltage of the generator. The automatic voltage regulator (AVR) will monitor the output voltage of the generator. The AVR automatically adjusts the current in the field excitation system in order to maintain the correct output voltage.
There are three methods for excitation that are used on LC5000, LC6100 and LC7000 generators:
- Shunt
- Auxiliary winding regulation excitation principle (AREP)
- PMG
Shunt Generator
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| Illustration 1 | g02175074 |
Shunt generator wiring example | |
AVR Shunt Field Excitation
Shunt field excitation uses the AVR for the exciter field.
Shunt field excitation uses power from the main windings of the generator to power the AVR. The generator with shunt field excitation is self-excited. The AVR monitors the excitation current of the exciter as a function of the generator output voltage. The generator with Shunt field excitation is simple in design. The generator with Shunt field excitation is not capable of sustaining a short circuit.
Permanent Magnet Pilot Excited (PMPE) Generator
The PMPE, also known as PMG, excited generators use the AVR for regulation of the exciter field.
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| Illustration 2 | g02175065 |
PMPE generator wiring diagram example (CR1 - CR6) Diodes (CR7) Varistor (L1) Exciter Stator (L2) Exciter Rotor (L3) Main Rotor (L4) Main Stator (L5) PMG Stator (PM) Permanent Magnet (R5) Resistor (RFA) Rotating Field Assembly (TR1) STD Voltage Droop Transformer (optional) (T0, T1, T2, T3) Generator Terminals and/or Generator Leads | |
Permanent magnet pilot excited (PMPE) generators receive power for the voltage regulator from a pilot exciter. Internal excitation generators receive power for the voltage regulator from auxiliary windings that are embedded in the stator slots. The PMG components consist of a permanent magnet (PM) and a PMG stator (L5) .
The pilot exciter operates independently from the generator output voltage. Constant excitation during a large load application is possible because the irregularities that occur in the generator output voltage do not affect the exciter.
The irregularities that occur in the generator output voltage are caused by load conditions. The independent exciter operation also allows the generator to sustain excessive currents for short periods of time.
When the engine starts turning the "Rotating Field Assembly "(RFA), the permanent magnet (PM) induces an AC voltage in the PMG stator (L5) .
The PMG stator has three coils of wire. The PMG stator generates three phase alternating current (AC). The resulting AC powers the voltage regulator.
Within the voltage regulator, the three phase alternating current is rectified to direct current (DC). A controlled amount of the DC current is supplied to the exciter stator (L1) through terminals F1 and F2. Refer to the following note about terminal designations for the exciter.
Note: The designations for the (L1) terminals will vary depending on the type of AVR. The VR6 uses "F1" and "F2" terminal labels. The R448 uses "E+" and "E-" terminal labels. The R450 has "F+" and "F-" terminal labels.
Direct current flows to the exciter stator (L1) which creates a magnetic field. The exciter rotor (L2) rotates in this magnetic field.
The AC current is then rectified by a three phase full wave bridge rectifier circuit.
The DC output from the bridge rectifier is carried to the main rotor (L3) by conductors. The conductors are routed through a groove that is in the generator shaft. Current through the main rotor windings creates the magnetic field of the generator.
As the main rotor windings rotate, the main field induces a three phase AC voltage in main stator (L4) . The voltage is developed and is present across the following terminals: T0 (N), T1 (U), T2 (V) and T3 (W). These terminals are connections for the load.
In order to keep the output voltage constant with changing loads, the exciter current must be controlled. The function of the voltage regulator is to control the exciter current.
The voltage regulator senses the generator output voltage directly at the T1, T2, T3, and theT0 terminals, or at the sensing transformers that are on the generators that are over 600 V.
The regulator sends current to the exciter through wires 5 and 6. The amount of current is dependent on the sensed voltage. Refer to the following note about terminal designations for the exciter.
Note: The designations for the (L1) terminals will vary depending on the type of AVR. The VR6 uses "F1" and "F2" terminal labels. The R448 uses "E+" and "E-" terminal labels. The R450 has "F+" and "F-" terminal labels.
Regardless of the type of generator (PMPE generator or internal excited generator), changing the exciter current has the same effect on generator operation.
Note: For more information on voltage regulation, see the appropriate voltage regulator service manual.
PMPE generators provide the magnetism for startup of the generator. A Permanent Magnet (PM) supplies the initial magnetism that is required at startup. Flashing the field is not required for startup of the generator.
Auxiliary Winding Regulation Excitation Principle ( AREP)
The AREP excited generators use the AVR for regulation of the exciter field.
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| Illustration 3 | g02999698 |
AREP Generator Wiring Diagram Example (CR1 - CR6) Diodes (CR7) Varistor (L1) Exciter Stator (L2) Exciter Rotor (L3) Main Rotor (L4) Main Stator (R5) Resistor (RFA) Rotating Field Assembly (TR1) STD Voltage Droop Transformer (T0, T1, T2, T3) Generator Terminals and/or Generator Leads (Auxiliary Windings) Internal Excitation Windings | |
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| Illustration 4 | g01321903 |
(10) Main stator windings (11) Auxiliary windings (12) Core stack | |
AREP excitation consists of two special sets of coils that are wound to fit in carefully selected slots of the main stator. The wire coils provide total separation and isolation from the stator main winding. The coils can only be fitted to the main stator while the main stator is being wound.
The two auxiliary windings are designed in order to provide power to the voltage regulator. The two coils are connected in series and the two coils are connected to the three phase power input of the voltage regulator.
One auxiliary winding produces a voltage proportional to the output voltage of the generator. The other auxiliary winding acts like a current transformer and produces a voltage that is proportional to the output current of the generator.
The outputs from the two coils are combined inside of the voltage regulator and provide a constant power source.
When the engine starts turning the "Rotating Field Assembly" (RFA), the residual magnetism that is in the exciter stator (L1) and the embedded permanent magnets that are in the exciter cause a small amount of AC voltage to be generated in the exciter rotor (L2) . Induced voltage causes current to flow. The current that is caused by the induced voltage is present in the exciter rotor.
The AC voltage is then rectified by a three-phase full-wave bridge rectifier circuit. Like the PMPE generators, flashing of the field is not required in order to start the generator.
The rectified direct current then flows through the main rotor windings (L3) . The flow of DC through the main rotor windings creates a magnetic field. This magnetic field adds to the existing residual magnetism of the main rotor windings.
As the main rotor windings rotate, an AC voltage is induced into main stator (L4) which appears as a three phase AC voltage at the following output terminals: T0, T1, T2 and T3.
The voltage regulator senses the output as a low voltage output condition. Therefore, the voltage regulator output to the exciter stator is increased so that the generator output will continue to increase up to the rated voltage.
The amount of current which 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 controls the DC voltage and the DC current. The DC voltage and the DC current are supplied to the exciter which produces the generator output voltage.
The voltage regulator senses the generator output voltage at wires that are connected to the generator phase leads.
The voltage regulator then supplies a controlled DC voltage and DC current to the exciter through wires 5 and 6. Refer to the following note about terminal designations for the exciter.
Note: The designations for the (L1) terminals will vary depending on the type of AVR. The VR6 uses "F1" and "F2" terminal labels. The R448 uses "E+" and "E-" terminal labels. The R450 has "F+" and "F-" terminal labels.
Note: For more information on voltage regulation, refer to the Service Manual, "Voltage Regulator" section.
When the voltage regulator senses a decrease in output voltage, the voltage regulator will increase the DC voltage and the DC current that is sent to the exciter through wires 5 and 6. Refer to the following note about terminal designations for the exciter.
Note: The designations for the (L1) terminals will vary depending on the type of AVR. The VR6 uses "F1" and "F2" terminal labels. The R448 uses "E+" and "E-" terminal labels. The R450 has "F+" and "F-" terminal labels.
The exciter stator magnetic field increases. As the magnetic field in the exciter stator is increased, the AC voltage that is induced in the exciter rotor is increased. This increased AC voltage from exciter rotor (L2) causes more AC current to flow.
Direct current flows to exciter stator (L1) which creates a magnetic field. The exciter rotor (L2) rotates in this magnetic field. The exciter stator and the exciter rotor generate three phase alternating current. The exciter stator and the exciter rotor generate six phase alternating current for the generator.
The AC current is rectified by a full-wave bridge rectifier circuit.
The DC output from the bridge rectifier is carried to the main rotor windings (L3) by conductors which are routed through a groove that is in the generator shaft. Current through the main rotor windings creates the magnetic field of the generator.
Increased current through main rotor windings increases the magnetic field of the generator. The increased magnetic field induces a higher AC voltage into main stator (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 output voltage, the voltage regulator will decrease the DC voltage to the exciter. The decreased DC voltage to the exciter will result in a decrease in generator output voltage.
AREP Power Curves
The curves that are below provide additional information about the output voltage of the auxiliary windings. The curves are related to the load that is applied to the generator.
The shunt winding provides most of the normal load excitation current required for the generator. The shunt winding output is constant even at 150% to 200% of the rated load.
If the load is increased above these levels, the regulator does not get enough power to maintain the rated output voltage of the generator and the voltage collapses.
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| Illustration 5 | g01321904 |
The series boost auxiliary winding provides little if any excitation at no load. However, the series boost auxiliary winding output increases with the load and saturates at 300% of the rated current. This output voltage of the series boost auxiliary winding is sufficient for the regulator to provide the 300% short circuit current capability.
The curve that is below shows the combination of the two auxiliary windings that provide a constant power source for the voltage regulator. The generator output voltage curve, that is below and to the right, shows the output voltage of the generator collapsing under the overload. The combined output of the two auxiliary windings provide power for the voltage regulator for sustaining the short circuit current.
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| Illustration 6 | g01321906 |
The constant power supply that is provided by the auxiliary windings also gives the generator excellent transient performance with load fluctuations.
The immunity to non-linear loads, motor starting, and the short circuit current capability are standard AREP features that are not available on self excited generators. The AREP generator provides the features with no additional components. The AREP generator provides the features inside the same physical envelope that is used by self excited generators.