Open Circuit Test and Short
Circuit Test are performed on a Synchronous Machine to find out the parameters
of Synchronous Machine and hence to have an idea of their performance. Open Circuit Test of Synchronous Machine is
also called No Load, Saturation or Magnetizing Characteristics for the reason
which will be clear after going through the post.

For getting the Open Circuit
Characteristics of Synchronous Machine, the alternator is first driven at its
rated speed and the open terminal voltage i.e. voltage across the armature
terminal is noted by varying the field current. Thus Open Circuit
Characteristic or OCC is basically the plot between the armature terminal
voltage E_{f} versus field current I_{f} while keeping the
speed of rotor at rated value. It shall be noted that for OCC, the final value
of E_{f} shall be 125% of the rated voltage.

Figure below shows the
connection diagram for performing the Open Circuit Test of Alternator.

As clear from the figure
above, an Ammeter is connected in series with the field circuit to measure the field
current and a Voltmeter is connected across the armature terminals to note down
the voltage generated. Figure (b) shows the plot between I_{f} and E_{f}.
It can be seen from the graph that the relationship between the field current I_{f}
and no load generated voltage E_{f} is linear up to certain value of
field current but as the the field current increases the relationship no longer
remains linear. The linear part of the relationship is because, at small value
of filed current the whole mmf is required by the air gap to create magnetic
flux but as the value of mmf exceeds some certain value, the iron parts get
saturated and hence the relationship between the flux (No load generated emf is
proportional to flux) and field current no longer remain linear.

Next assume that if there
were no saturation (assuming no iron part is present rather only air gap is
present), the relationship between the field current and no load voltage would
have been a straight line and that is why the straight line ob in the figure is
called Air Gap Line.

Thus we observe that because
of saturation in iron parts of machine, the no load generated voltage E_{f}
does not increase in the same proportion as the increase in field current.

###
**Short
Circuit Test of Synchronous Machine:**

For performing Short Circuit
Test on an Alternator, the machine is driven at rated synchronous speed and the
armature terminals are short circuited through an Ammeter as shown in figure
below.

Now the field current If is
gradually increased from zero until the armature short circuit current reaches
its maximum safe value i.e. 125 to 150% of its rated current value. Readings of
field current If and short circuit current are noted and plotted.

If you see the above plot of
Short Circuit Test, you notice that the short circuit characteristics of a
synchronous machine is a straight line.

**Why
Short Circuit Characteristics of Synchronous Machine is Straight Line?**

For short circuit test, as
the armature terminals are shorted, therefore terminal voltage V_{t} =
0. Therefore the air gap emf E_{r} shall only be enough to provide the
leakage impedance drop in the armature i.e.

E_{r} = Ia(Ra + jXal)
where Xal = Armature Leakage Reactance

As we know that, for a
Synchronous machine the value of Xal is of the order of 0.1 to 0.2 per unit and
Ra (Armature Resistance) is negligible thus we can write as

Xal = 0.15 (Taking average
value of 0.1 and 0.2)

Ra = 0

then E_{r} = Ia (Ra
+jXal) = 0.15Ia

Taking rated current of
armature, Ia = 1 pu

Therefore, E_{r} =
0.15 pu

Thus we observe that during
short circuit test, the air gap generated emf Er is only 0.15 pu which mean
that air gap flux must also be 0.15 pu. As the resultant air gap flux is only
0.15 of its rated value under normal voltage condition, such a low value of air
gap flux does not saturate the iron parts of synchronous machine and hence the
short circuit characteristics is a straight line. It shall also be noted here
that, in case of short circuit test the armature mmf is almost entirely demagnetizing
in nature which results in very low value of air gap flux.