Electrical Engineering ⇒ Topic : Open-Circuit Characteristic (O.C.C.) Test

The circuit diagram for conducting the O.C. test is given in Figure (a). In this, the alternator under test is coupled to a dc shunt motor.The electrical connections are made as shown in the figure. First, switch S₃triple-pole single-throw switch is opened and the alternator is started with the help of the dc motor. The speed of the set is then adjusted to the rated value of the alternator. Next, the excitation of the alternator field winding is provided by connecting it to the dc source through switch S₂. To start with,the potentiometer AB across the field winding is adjusted such that the field current I_f is zero. The reading of the voltmeter V, connected across the stator winding of the alternator is noted. Then, I_f is increased in steps with the help of the potential divider AB, and the corresponding readings of the voltmeter 

  FIGURE (a) Circuit diagram for pre-determining the regulation of an alternator by the synchronous impedance method

V and I are noted down. This is continued till the voltage reaches a value which is 50% more than the rated value of the alternator and both the field current (I) and alternator voltage (V) readings thus obtained are tabulated as shown in Table. Throughout this test, the speed of the alternator is kept constant. These observations (Table ) are plotted as a graph as shown in Figure (b). This graph, which gives the relationship between the generated emf and the field current is called the open-circuit characteristic (0.C.C.) of the alternator.

                       TABLE  Observations from the open circuit test on an alternator. The speed of the alternator is maintained constant at the rated speed.

                                  FIGURE (b) Open-circuit characteristic, synchronous impedance (Z_s) of an alternator

Open-Circuit Characteristic (0.C.C.) Test

The alternator is run at the rated synchronous speed. The terminals are kept open, i.e., all loads are disconnected, as shown in Figure 1 (a). The field current is set to zero. The field current is gradually increased so that the final value of E_f should be about 125% of the rated voltage.Figure 1 (b) shows the 0.C.C. with the field current along abscissa and e.m.f. along ordinate. The 0.C.C. will not be a straight line because of saturation in the iron part of the magnetic circuit.

                                 Figure 1 (a) Open-circuit test of an alternator, (b) 0.C.C. of an alternator