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 Ef versus field current If while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of Ef 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 If and Ef. It can be seen from the graph that the relationship between the field current Ifand no load generated voltage Ef 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 Efdoes 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.
For short circuit test, as the armature terminals are shorted, therefore terminal voltage Vt = 0. Therefore the air gap emf Er shall only be enough to provide the leakage impedance drop in the armature i.e.
Er = 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 Er = Ia (Ra +jXal) = 0.15Ia
Taking rated current of armature, Ia = 1 pu
Therefore, Er = 0.15 pu
You must read Per Unit System in Electrical Engineering
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.