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Abstract:
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Salient pole brushless alternators coupled to IC engines are
extensively used as stand-by power supply units for meeting in-
dustrial power demands. Design of such generators demands high
power to weight ratio, high e ciency and low cost per KVA out-
put. Moreover, the performance characteristics of such machines
like voltage regulation and short circuit ratio (SCR) are critical
when these machines are put into parallel operation and alterna-
tors for critical applications like defence and aerospace demand
very low harmonic content in the output voltage. While designing
such alternators, accurate prediction of machine characteristics,
including total harmonic distortion (THD) is essential to mini-
mize development cost and time.
Total harmonic distortion in the output voltage of alternators
should be as low as possible especially when powering very sophis-
ticated and critical applications. The output voltage waveform
of a practical AC generator is replica of the space distribution of
the
ux density in the air gap and several factors such as shape
of the rotor pole face, core saturation, slotting and style of coil
disposition make the realization of a sinusoidal air gap
ux wave
impossible. These
ux harmonics introduce undesirable e ects on
the alternator performance like high neutral current due to triplen
harmonics, voltage distortion, noise, vibration, excessive heating
and also extra losses resulting in poor e ciency, which in turn
necessitate de-rating of the machine especially when connected
to non-linear loads. As an important control unit of brushless
alternator, the excitation system and its dynamic performance
has a direct impact on alternator's stability and reliability.
The thesis explores design and implementation of an excitation
i
system utilizing third harmonic
ux in the air gap of brushless al-
ternators, using an additional auxiliary winding, wound for 1=3rd
pole pitch, embedded into the stator slots and electrically iso-
lated from the main winding. In the third harmonic excitation
system, the combined e ect of two auxiliary windings, one with
2=3rd pitch and another third harmonic winding with 1=3rd pitch,
are used to ensure good voltage regulation without an electronic
automatic voltage regulator (AVR) and also reduces the total
harmonic content in the output voltage, cost e ectively.
The design of the third harmonic winding by analytic methods
demands accurate calculation of third harmonic
ux density in
the air gap of the machine. However, precise estimation of the
amplitude of third harmonic
ux in the air gap of a machine by
conventional design procedures is di cult due to complex geome-
try of the machine and non-linear characteristics of the magnetic
materials. As such, prediction of the eld parameters by conven-
tional design methods is unreliable and hence virtual prototyping
of the machine is done to enable accurate design of the third har-
monic excitation system.
In the design and development cycle of electrical machines, it is
recognized that the use of analytical and experimental methods
followed by expensive and in
exible prototyping is time consum-
ing and no longer cost e ective. Due to advancements in com-
putational capabilities over recent years, nite element method
(FEM) based virtual prototyping has become an attractive al-
ternative to well established semi-analytical and empirical design
methods as well as to the still popular trial and error approach
followed by the costly and time consuming prototyping. Hence,
by virtually prototyping the alternator using FEM, the important
performance characteristics of the machine are predicted.
Design of third harmonic excitation system is done with the help
of results obtained from virtual prototype of the machine. Third
harmonic excitation (THE) system is implemented in a 45 KVA
ii
experimental machine and experiments are conducted to validate
the simulation results. Simulation and experimental results show
that by utilizing third harmonic
ux in the air gap of the ma-
chine for excitation purposes during loaded conditions, triplen
harmonic content in the output phase voltage is signi cantly re-
duced. The prototype machine with third harmonic excitation
system designed and developed based on FEM analysis proved
to be economical due to its simplicity and has the added advan-
tage of reduced harmonics in the output phase voltage. |