n

n

\\i\\

!

436

ELECTRICAL APPARATUS

half intensity, to zero, reversing it to half intensity, to full in-
tensity, and to such intensity that the fundamental disappears,
then are given by:

ter pa
of pole
density

(1)

(2)
13)

full, e =

0.5, e = 6i-0.5e2

o,

c =

-0.5 c =

(5) - full, e = ei-2

(6) -1.17,

= 0.960 A {cos 0+0.077 cos 3 8

+0.0065 cos 5 6 - 0.0084 cos 7 6 .
=0.755 A {cos 0+0.168 cos 3 0

- 0.0144 cos 5 0-0.0085 cos 7 Q.
= 0.549 A {cos 0+0.328 cos 3 0

-0.053 cos 5 0-0.084 cos 70. .
= 0.344 A {cos 0 +0.680 cos 3 0

-0.131 cos 5 0-0.0084 cos 70. .
= 0.138 A {cos 0+2.07 cos 3 0

-0.45 cos 5 0-0.008 cos 70. . .
= 61 -2.34 e2 = 0.322 A {cos 3 0-0.227 cos 5 0.

•1.5 e->

It is interesting to note that in the last case the fundamental
frequency disappears and the machine is a generator of triple
frequency, that is, produces or consumes a frequency equal to
three times synchronous frequency. In this case the seventh
harmonic also disappears, and only the fifth is appreciable, but
could be greatly reduced by a different kind of pole arc. From
above table follows:

(1) (2) (3) (4) (5) (6) normal

Maximum funda-
mental alter-

0.960 0.755 0.549 0.344 0.138 0

0.960

nating volts ...
Direct volts ...... 1.033 0.883 0.743 0.578 0.423 0.322 0.960
237. It is seen that a considerable increase of direct voltage
beyond the normal ratio involves a sacrifice of output, due to
the decrease or reversal of a part of the magnetic flux, whereby
the air-gap section is not fully utilized. Thus it is not advisable
to go too far in this direction.
By the superposition of the third harmonic upon the funda-
mental wave of the Y voltage, in a converter with three sections
per pole, thus an increase of direct voltage over its normal
voltage can be produced by lowering the excitation of the middle
section and raising that of the outside sections of the field pole,
and also inversely a decrease of the direct voltage below its
normal value by raising the excitation of the middle section