From the previous article,
it is clear that if excitation is varied from very low (under
excitation) to very high (over excitation) value, then current Ia decreases,
becomes minimum at unity p.f. and then again increases. But initial
lagging current becomes unity and then becomes leading in nature. This
can be shown as in the Fig. 1.
be increased by increasing the field current passing through the field
winding of synchronous motor. If graph of armature current drawn by the
motor (Ia) against field current (If) is plotted,
then its shape looks like an english alphabet V. If such graphs are
obtained at various load conditions we get family of curves, all looking
like V. Such curves are called V-curves of synchronous motor. These are
shown in the Fig. 2a).
|Fig. 2 V-curves and Inverted V-curves|
Stator is connected top three phase supply through wattmeters and
ammeter. The two wattmeter method is used to measure input power of
motor. The ammeter is reading line current which is same as armature
(stator) current. Voltmeter is reading line voltage.
|Fig. 3 Experimental setup for V-curves|
potential divider arrangement is used in the field circuit. By
controlling the voltage by rheostat, the field current can be changed.
Hence motor can be subjected to variable excitation condition to note
down the readings.
The entire procedure can be repeated for various load conditions to obtain family of V-curves and Inverted V-curves.