19-11-2012, 05:36 PM
ELECTROMECHANICAL ENERGY CONVERSION
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Open Circuit Characteristic (O.C.C.)
This curve shows the relation between the generated e.m.f. at no-load
(E0) and the field current (If) at constant speed. It is also known as
magnetic characteristic or no-load saturation curve. Its shape is
practically the same for all generators whether separately or selfexcited.
The data for O.C.C. curve are obtained experimentally by
operating the generator at no load and constant speed and recording
the change in terminal voltage as the field current is varied.
. Internal or Total characteristic (E/Ia)
This curve shows the relation between the generated e.m.f. on load (E)
and the armature current (Ia). The e.m.f. E is less than E0 due to the
demagnetizing effect of armature reaction. Therefore, this curve will lie
below the open circuit characteristic (O.C.C.). The internal
characteristic is of interest chiefly to the designer. It cannot be obtained
directly by experiment. It is because a voltmeter cannot read the e.m.f.
generated on load due to the voltage drop in armature resistance. The
internal characteristic can be obtained from external characteristic if
winding resistances are known because armature reaction effect is
included in both characteristics
External characteristic (V/IL)
This curve shows the relation between the terminal voltage (V) and load
current (IL). The terminal voltage V will be less than E due to voltage
drop in the armature circuit. Therefore, this curve will lie below the
internal characteristic. This characteristic is very important in
determining the suitability of a generator for a given purpose. It can be
obtained by making simultaneous measurements of terminal voltage
and load current (with voltmeter and ammeter) of a loaded generator.
Critical External Resistance for Shunt Generator
If the load resistance across the terminals of a shunt generator is
decreased, then load current increase? However, there is a limit to
the increase in load current with the decrease of load resistance. Any
decrease of load resistance beyond this point, instead of increasing
the current, ultimately results in reduced current. Consequently, the
external characteristic turns back (dotted curve) as shown in Fig.
(3.10). The tangent OA to the curve represents the minimum external
resistance required to excite the shunt generator on load and is called
critical external resistance. If the resistance of the external circuit is
less than the critical external resistance (represented by tangent OA
in Fig. 3.10), the machine will refuse to excite or will de-excite if
already running This means that external resistance is so low as
virtually to short circuit the machine and so doing away with its
excitation.
CONNECTING SHUNT GENERATORS IN PARALLEL
The generators in a power plant are connected in parallel through busbars.
The bus-bars are heavy thick copper bars and they act as +ve
and -ve terminals. The positive terminals of the generators are
.connected to the +ve side of bus-bars and negative terminals to the
negative side of bus-bars. Fig. (3.15) shows shunt generator 1
connected to the bus-bars and supplying
load. When the load on the power plant increases beyond the capacity
of this generator, the second shunt generator 2 is connected in
parallel wish the first to meet the increased load demand.