SINGLE-PHASE COMMUTATOE MOTORS 337

secondary. The first case gives the conduetively compensated
scries motor, the second case the inductively compensated series
motor, the third case the repulsion motor.

In the first case, by giving the compensating winding more
turns than the armature, overcompensation, by giving it less
turns, -undercompensation, is produced. In the second case
always complete (or practically complete) compensation results,
irrespective of the number of turns of the winding, as primary
and secondary currents of a transformer always are opposite in
direction, and of the same m.m.f. (approximately), and in the
third case a somewhat less complete compensation.

With a compensating winding, C, of equal and opposite m.rn.f .
to the armature "winding, A, the resultant armature reaction is
zero, and the field distortion, therefore, disappears; that is, the
ratio of the armature turns to field turns has no direct effect on
the commutation, but high armature turns and low field turns
can be used. The armature self-inductance is reduced from that
corresponding to the armature magnetic flux, $1, in Fig. 155 to
that corresponding to the magnetic leakage flux, that is, the
magnetic flux passing between armature turns and compensating
turns, or the ".slot inductance/7 which is small, especially if rela-
tively shallow armature slots and compensating slots are used.

The compensating winding, or the "cross field," thus fulfils
the twofold purpose of reducing the armature self-inductance to
that of the leakage flux, and of neutralizing the armature reac-
tion and thereby permitting the use of very high armature
ampere-turns.

The main purpose of the compensating winding thus is to de-
crease the armature self-inductance; that is, increase the effect-
ive armature reluctance and thereby its ratio to the field reluc-

tance, b, and thus permit the use of a much higher ratio, q = ^ ••-,

before maximum power-factor is reached, and thereby a higher
power-factor.

Even with compensating winding, with increasing q, ultimately
a point is reached where the armature self-inductance equals
the field self-inductance, and beyond this the power-factor again
decreases, It becomes possible, however, hy the use of the com-
pensating winding, to reach, with a mechanically good design,
values of 6 as high as 16 to 20.

Assuming 6 = 16 gives, substituted in (11) and (12):

ff = 4;

t#*

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