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Synchronous GeneratorTests
To obtain the parameters of asynchronous generator, we performthree simple tests as described below.
The Resistance Test
The Open-Circuit Test
The Short-Circuit Test
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The Resistance Test
This test is conducted to measure-windingresistance of a synchronous generatorwhen it is at rest and the eld winding isopen.
The resistance is measured between twolines at a time and the average of thethree resistance readings is taken to bethe measured value of the resistance, RL,from line to line. If the generator is Y-connected, the per-phase resistance is :
R A = 0,5 R L
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The Open-Circuit Test
The open-circuit test, or the no-loadtest ,is performed by :1) enerator is rotated at the rated
speed.!) "o load is connected at the
terminals.#) $ield current is increased from % to
ma&imum.') (ecord values of the terminal
voltage and eld current value.
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Circuit diagram to perform open-circuittest
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ith the terminals open, * + %, so + / . *t is thus possible to construct a plot
of +
or T
vs * $
graph. This plot is calledopen-circuit characteristic 0 22) of agenerator. ith this characteristic, it ispossible to nd the internal generated
voltage of the generator for any giveneld current.
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Open-circuit characteristic (OCC of agenerator
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The 22 follows a straight-linerelation as long as the magneticcircuit of the synchronous generatordoes not saturate.3ince, in the linear region, most ofthe applied mmf is consumed by theair-gap, the straight line isappropriately called the air-gapline.
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The Short-Circuit Test
The short-circuit test providesinformation about the currentcapabilities of a synchronousgenerator. *t is performed by1) enerator is rotated at rated speed.!) +d4ust eld current to %.#) 3hort circuit the terminals.') 5easure armature current or line
current as the eld current isincreased.
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3ince 6 3 77 ( + , the e8uation reducesto:
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322 is essentially a straight line. To understand why this characteristic is a straight line, look at the e8uivalent circuit belowwhen the terminals are short circuited.
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hen the terminals are short circuited, the armature current * + is:
!rom both tests, here we can "nd the internal machine impedance (# $ from OCC, % $ from SCC :
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3hort 2ircuit (atio
Ratio of the eld current re uired for therated !oltage at open circuit to the eldcurrent re uired for rated armaturecurrent at short circuit.
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