Electric Power Generation, Transmission, and Distribution

or generators are determined as shown, on the rated voltage line, as ‘‘RatedLoad.’’ F o r
synchronous condensers, the field current is at the crossing of the zero P.F. saturation line at 1.0 V.
As an approximate magnetic figure of merit, the no-load saturation curve should not exceed its
extrapolated straight line by more than 25%, unless of a special design. From these criteria, and the
knowledge of the stator current and cooling system effectiveness, the manufacturer can project
the motor component heating, and thus insulation life, and the efficiency of the machine at
different loads.
Vee curves (Fig. 5.3) show overall loading performance of a synchronous machine for different loads
and power factors, but more importantly show how heating and stability limit loads. For increased
hydrogen pressures in a generator frame, the load capability increases markedly.
The characteristics of all synchronous machines when their stator terminals are short-circuited are
similar (see Fig. 5.4). There is an initial subtransient period of current increase of 8 to 10 times rated,
with one phase offsetting an equal amount. These decay in a matter of milliseconds to a transient value
of 3 to 5 times rated, decaying in tenths of a second to a relatively steady value. Coincident with this, the

PER UNIT FIELD AMPERES

0.95 PF LEAD

1.0 PF

0.0 PF LEAD

PER UNIT kVA

0

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.0 2.0 3.0 4.0

LIMITED BY

FIELD HEATING

LIMITED BY

ARMATURE

HEATING

LIMITED BY

END HEATING

30 PSIG H

2

0.9 PF LAG

0.0 PF LAG

60 PSIG H

2

45 PSIG H

2

FIGURE 5.3 Vee curves.