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WEG’s Alternator Excitation Systems - Auxiliary Winding and Exciter equipped with Permanent Magnets
Reference: Technical Article Luís Fábio Menoncin WEG Brazil
Summary - This article is intended to provide broader and more uniform information about the excitation system used by WEG in all brushless generators of the G and AG lines (low voltage), The complete excitation system is composed of the main exciter equipped with permanent magnets and auxiliary coil.
1. PRESENTATION The alternator dynamic behavior which includes the voltage drop and response time for recovery The auxiliary coil, also called auxiliary winding or after the sudden application of load when in auxiliary phase, is installed in the slots of the main stabilized condition with no load, depends on stator together with the main winding. It is totally several factors, such as: independent and isolated from the main winding . Load power factor (the smaller it is, the greater and works as if it were the auxiliary exciter the voltage drop and recovery time). (permanent magnet generator). The permanent . Current load level magnets are mounted on the polar foot of the . For instantaneous application of 100% of rated main exciter stator, strengthening the magnetic load, with cos φ 0.8 and 440 V, the response flux produced there. time for the voltage to reach the range of +/- 3% is approximately 500ms, this also varies 2. OPERATION PRINCIPLE AND per the individual machine. CHARACTERISTICS . Voltage regulator type (rectification in full wave or half wave, analog or digital processing, etc.); In order to maintain constant voltage at the . Utilization voltage, in case of multivoltage terminals, WEG alternators are equipped with a machine; voltage regulator, which controls the generator . Settings of the voltage regulator: excitation based on the comparison of the -Stability (P.I.D.) measured voltage and a predetermined reference -U/F, which provides a faster recovery of value. In short, the auxiliary coil works as the the frequency of the prime mover speed voltage source for the voltage regulator. (Diesel engine, turbines etc.); The voltage generation on the auxiliary coil . Characteristics of the diesel engine: torque depends on the voltage and frequency at the curve, type and settings of the diesel motor terminals of the alternator main stator, and on the speed regulator (electronic or mechanical, variations and level of current flowing on the etc.). stator. WEG voltage regulators can operate with both The alternator has the excitation for the conditions auxiliary coil and shunt excitation designs. As well with and without load guaranteed and it does not as with PMG if required. The auxiliary coil is a lose this excitation capacity, even with the main standard feature for all G and AG low voltage stator in short circuit, that is, with null generated alternators. However, a PMG can be supplied voltage. The G and AG lines were developed to from the factory, on the customer’s request, or ensure the maintenance of the short circuit current even retrofit installed in the field. above the rated current. This capacity is not guaranteed in the shunt excitation, in which the The system used by WEG performs the function source for the voltage regulator is the main stator. of maintaining short circuit provided by the PMG A greater voltage drop will also be observed when with the advantage of having a dynamic behavior load is applied suddenly to a shunt excited of the voltage generated on the auxiliary coil generator. without increasing the final length of the machine. In the case of the PMG, the excitation or magnetic
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flux provided by the rotor with permanent magnets magnetism requiring an energization or flashing is fixed, which results in a passive behavior in from an external power source after periods of relation to the current flow on the main stator at inactivity. the moment of load application. Furthermore, the This is an important factor for the reliability of the excitation increase imposed by the voltage system, especially in standby applications, in regulator will cause a voltage drop on the PMG, which the generator set remains long periods on seeing that the project of the PMG must consider stand-by mode and must be able to operate at the minimum voltage required for the proper any moment. operation of the voltage regulator in the desired working condition. Also, the PMG wave form 3. ILLUSTRATIONS under load may present, depending on the project, great distortions, and thus generate 3.1 BRUSHLESS SYSTEM – AUXILLARY COIL instability on the voltage regulator.
When the alternator is applied to nonlinear load applications, it is necessary to calculate an 5 oversizing factor to define the generator output. 1 This is necessary because of the increase in the 2 temperature of the alternator because of the high 3 frequency of the load current harmonics, which 6 arise because of the electronic switching 4
(transistors, thiristors, IGBT, etc.). In terms of 8 performance of the auxiliary coil with nonlinear 9 loads, it is fully satisfactory in meeting the load requirements as established by NEMA MG1 7 10 KIT standard Part 32, for alternator operation: “the maximum difference of any current harmonic in relation to the fundamental harmonic (also of current) is of 5%”. Figure 1: Brushless System AG10
Legend of Error! Reference source not found.: In tests monitoring the stability of the generated 1. Main coil 6. Fuse voltage (absence of oscillations), it was observed 2. Auxiliary coil 7. Rotating diodes that the G and AG lines with auxiliary coil 3. Main stator 8. Main exciter stator presented excellent performance with loads that 4. Main rotor 9. Magnet were twice the standard specification, operating in 5. Regulator 10. Auxiliary exciter (PMG a stable condition with a total harmonic distortion Kit) of 26% of current.
The permanent magnets, which are mounted on the stator of the main exciter (position 9 in figure 1), generate a magnetic flux, in addition to the one generated by the exciting current coming from the voltage regulator to ensure the build-up process on the voltage regulator (initial excitation) by means of the generation of a residual voltage of approximately 100 V on the main stator and 50 V on the auxiliary coil, without any exciting current. WEG's auxiliary winding also ensures 300% of short circuit current up to 10 seconds.
The electrical design of the G and AG Lines having the magnets imbedded in the main exciter stator guarantee a generation of approximately 100V, which also insures the build-up process or initial excitation. This solves the commonly known Figure 2: Diagram issue found in typical auxiliary winding designs in which the excitation system loses residual
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3.3 TEST RESULT
RECORD OF SUDDEN LOAD APPLICATION Alternator model GTA200 equipped with auxiliary coil (standard) and permanent magnet exciter (single-phase PMG). Tests were performed using the same regulator, since it was a single-phase PMG (WEG AVR-A- OPT-06), applying one load step and simultaneously recording the current and voltage Auxiliary of the stator, auxiliary coil and PMG. Coil
Figure 3: Picture of the winding
3.2 PERMANENT MAGNETS ON THE EXCITER
WEG Alternators are equipped with a robust exciter designed to provide very fast response to load application or rejection. Some characteristics of the WEG Exciter include . 100% wound in copper. . With permanent magnets providing very fast response during motor starting. . Guaranteed residual voltage (no need of external source for flashing). . Injected ABS support spool providing extra protection on the exciter winding - Avoid loose coils and corrosion by the contact between coils and lamination.
Figure 4: Permanent Magnets in the Exciter Stator (right) Figure 4: Record of sudden load application Exciter Rotor (left)
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4. CONCLUSIONS
. The auxiliary coil provides a constant power source to the voltage regulator even under short circuit condition in main phases. . The dynamic response (alternator + AVR) is identical when considering auxiliary coil and external PMG system. The small difference in the stator voltage drop between these two systems does not affect the performance of the alternator due to the short time response by the AVR and very fast voltage recovery of the auxiliary winding. . The maintenance of the exciting stator current under constant load is identical and for both excitation system. . Short circuit capabilities for both the PMG and auxiliary winding are the same – 300% of short circuit current up to 10 seconds. . The maintenance of the exciting current under constant load is similar and satisfactory for PMG and auxiliary coil.
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