Theory and calculation of alternating current phenomena . ing current exists between two ranges of lag-ging currents. In the case, Ei > Eo (Fig. 152), the current cannot equal zeroeither, but begins at a finite value, Il, corresponding to the mini-mum value of OE, /i = At this value, however, the alternator, Ei, is still generator and changes to a motor, its powerpassing through zero, at the point corresponding to the verticaltangent, upon Ci, with a very large lead of the impressed the current. At H the lead changes to lag. 310 ALTERNATING-CURRENT PHENOMENA The minimum and ma
Theory and calculation of alternating current phenomena . ing current exists between two ranges of lag-ging currents. In the case, Ei > Eo (Fig. 152), the current cannot equal zeroeither, but begins at a finite value, Il, corresponding to the mini-mum value of OE, /i = At this value, however, the alternator, Ei, is still generator and changes to a motor, its powerpassing through zero, at the point corresponding to the verticaltangent, upon Ci, with a very large lead of the impressed the current. At H the lead changes to lag. 310 ALTERNATING-CURRENT PHENOMENA The minimum and maximum values of current in the threeconditions are given by: Minimum1st. / = 0, Eq — El 2d. / 3d. / = El — Eo Maximu7?i 2Eo I - / = Eo + El Eo -f El Since the current in the Hne at Ei = 0, that is, when the motorstands still, is 7o = —, we see that in such a synchronous motor-plant, when running at synchronism, the current can rise far be-3^ond the value it has at standstill of the motor, to twice thisvalue at 1, somewhat less at 2, but more at Fig. 153. 217. C. Eo = constant, Ex varied so that the efficiency is a7naximum for all currents. (Fig. 153.) Since we have seen that the output at a given current strength,that is, a given loss, is a maximum, and therefore the efhciencya maximum, when the current is in phase with the , Eo, of the generator, we have as the locus of £0 the point,Eo (Fig. 153), and when E with increasing current varies on e,El must vary on the straight line, ei, parallel to e. SYNCHRONOUS MOTOR 311 Hence, at no-load or zero current, Ei = Eo, decreases withincreasing load, reaches a minimum at OEi^ perpendicular to ei,and then increases again, reaches once more Ei = Eq at Ei^, andthen increases beyond Eq. The current is always ahead of thegenerated , Ei, of the motor, and by its lead compensatesfor the self-induction of the system, making the total circuit non-inductive. The power is a iiiaxinmm at Ei^, where OEi* = Ei^Eo = O.
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