Short Circuit and Open Circuit Test of Synchronous Machine

Short Circuit and Open Circuit Test of Synchronous Machine

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. The reason why the Open Circuit Test of Synchronous Machine is also known as No Load, Saturation, or Magnetizing Characteristics will become clear after reading the post.

The alternator is initially driven at its rated speed in order to obtain the open terminal voltage,. Or voltage across the armature terminal, which 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_fand 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.

The relationship between the field current and no load voltage would have been a straight line. If there hadn’t been saturation, which is why the straight line in the figure is called the “Air Gap Line” (assuming there isn’t any iron part present and only an air gap).

As a result, we see that the no load generated voltage does not increase in the same manner. As the increase in field current due to saturation in the iron parts of the machine.

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, as the armature short circuit current approaches its maximum safe value,. Which is 125 to 150% of its rated current value, the field current If is gradually increased from zero. Short circuit current and field current readings are recorded 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 = I_a(R_a + jX_al) where X_al = Armature Leakage Reactance

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

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

R_a = 0

then E_r = I_a (R_a +jX_al) = 0.15I_a

Taking rated current of armature, I_a = 1 pu

Therefore, E_r = 0.15 pu

Thus, we see that the air gap generated emf Er during the short circuit test is only 0.15 pu, indicating that the air gap flux must also be 0.15 pu. A straight line represents the short circuit characteristics. Because the resultant air gap flux,. Which is only 0.15 of its rated value under normal voltage conditions, does not saturate the iron parts of the synchronous machine. It should be noted that the armature mmf demagnetizes almost entirely during short circuit tests,. Leading to very low air gap flux values.