Electrical Engineering ⇒ Topic : Short-Circuit Characteristic (S.C.C.) Test

Short-circuit test The circuit diagram shown in Figure (a) is also useful to determine the short-circuit characteristic of the alternator. The short circuit characteristic is the relation between the armature current and the field current when the terminals of the alternator are short circuited. Before determining the short-circuit characteristic, the field current (/_f) is reduced to zero. Then, the switch S₃ is closed and the field current (/_f) is gradually increased. As a result, the armature current /_a also increases. When /_a read by the ammeter A, has increased to its full-load value, the corresponding value of /_f is recorded. Only one reading is sufficient for this test. The results are tabulated as shown in Table. The resulting short-circuit characteristic (S.C.C.) is shown in Fig (b). along with the O.C.C. The S.C.C. is usually a straight line as shown in the figure, for moderate values of field current.

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

                            TABLE  Observations from the short-circuit test on an alternator

FIGURE  (b) short-circuit characteristic and synchronous impedance (Z_s) of an alternator

Short-Circuit Characteristic (S.C.C.) Test

To obtain the short-circuit characteristic (S.C.C.), the machine is driven at rated speed. The armature terminals are short-circuited by an ammeter as illustrated in Figure (a). The field current is gradually increased from zero until the short-circuit armature current has reached its maximum safe value equal to about 125% to 150% of the rated current. The reading should be taken in a short time to avoid armature overheating. Figure (b) shows the S.C.C. of an alternator.

                    Figure (a) Short-circuit test of an alternator.          Figure (b) S.C.C. of an alternator.

Synchronous impedance Z_s can be defined with the above two test results as follows:

                 .... (1)