Page 780 - Basic Electrical Engineering

In the synchronous impedance method, we are required to perform no-load
test, i.e., the open-circuit test and short-circuit test on the generator.
For the open-circuit test, the generator is driven at its rated speed by the

prime mover. It’s field current I is gradually increased using a field
f
regulator. The values of I and induced EMF, E are recorded and plotted. This
f
will give us the open-circuit characteristic, i.e., OCC.

The short-circuit test is conducted by running the generator at rated speed,
keeping the output terminals short circuited through an ammeter. Reduced

excitation current I is allowed to flow through the field winding so that the
f
short-circuit current, I does not exceed the rated current of the generator.
sc
The short-circuit characteristic, SCC is plotted as I as a function of I by
sc
f
taking a few readings. Both OCC and SCC are drawn as a function of I as
f
shown in Fig. 10.13.

At a particular value of field current, say I = OP, the open-circuit voltage
f
is PQ Volts. When the output terminals are kept short circuited under the
short-circuit test, the voltage E will cause a short-circuit current I to flow.
sc1
The EMF E on open circuit is regarded as being responsible for circulating a
1
short-circuit current of I through the synchronous impedance Z . Thus, Z s
s
sc1
can be calculated as

Figure 10.13 Open-circuit and short-circuit characteristics of a synchronous generator