AI/TERNATING MOTORS FOR RAILWAY WORK. 1389
of the compound section of the transformer system to pro-
duce the requisite go deg. phase difference, 2 and /, the
parts of the compound transformer, and g % 7 j £ the
segments of the secondary windings. Once given the two
phase current, the shifting over to three phase 1is easy.
The coil, 7, furnishes one phase, the resultant of & and £ a
second, and the resultant of % and 7 the third, all of which
are connected in the ordinary way to the motor, M. The
result of this very ingenious combination is a very close
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?IMPLE SECTION OF TRANSFORMER
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FIG. 104.
approximation to a true three phase relation throughout a
considerable range of load, both in starting and running.
The use of three resultant phases tends to preserve a more
uniform phase relation than would be obtained by utilizing
the original two derived phases.
The employment of a condenser, while it adds to the
complication, tends to annul the inductance of the main
circuit. At all events it can be made to give a very high
power factor, better than that given by ordinary poly-
phase motors.
On the other hand, the condenser is an element of
weakness in that it is of somewhat uncertain life, and un-
less exposed to high voltage and used at rather high fre-
quency, is both bulky and expensive. Its use involves