SYNCHRONOUS INDUCTION GENERATOR 203 W^ ]
j'" !
and since the total effect must be the exciting current: |^7
Io = *'o-f ;/"o, . |/
it follows that: !$r/
fc'i — ^2 = i'o and t"i + t"2 = i"o. L*;'
Hence, the stator power current and rotor power current, |f;'i.'
zv! and i'z, are equal to each other (when neglecting the small $ ' <
hysteresis power current). The synchronous exciter of the [Vc
machine must supply in addition to the magnetizing current, * \\ ^
the total reactive current of the load. Or in other words, such l7 '''
a machine requires a synchronous exciter of a volt-ampere tfy *
capacity equal to the volt-ampere excitation plus the reactive k*!
volt-amperes of the load, that is, with an inductive load, a large \£ ^(
exciter machine. In this respect, the double-synchronous [1? j,
generator is analogous to the induction generator, and is there- $'/ {;
fore suited mainly to a load with leading current, as over- ffU '
excited converters and synchronous motors, in which the reactive £ f
component of the load is negative and so compensates for the
reactive component of excitation, and thereby reduces the size
of the exciter. j$\
This means that the double-synchronous alternator has zero
armature reaction for non-inductive load, but a demagnetizing
armature reaction for inductive, a magnetizing armature reac-
tion for anti-inductive load, and the excitation, by alternating-
reactive current, so has to be varied with the character of the
load, in general in a far higher degree than with the synchronous
alternator.
120. 2. Synchronous-induction Generator with Low-frequency
Excitation.
Here two cases exist:
(a) If the magnetic field of excitation revolves in opposite
direction to the mechanical rotation.
(6) If it revolves in the same direction.
In the first case (a) the exciter is a low-frequency generator
and the machine a frequency converter, calculated by the same
equations.
Its voltage regulation is essentially that of a synchronous
alternator: with increasing load, at constant voltage impressed
upon the rotor or exciter circuit, the voltage drops moderately
at non-inductive load, greatly at inductive load, and rises at