SINGLE-PHASE COMMUTATOR MOTORS 369

that a phase displacement exists between the secondary and the
primary current. The secondary current, Ji, of the transformer
lags behind the primary current, Jo, slightly less than 180°; that is,
considered in opposite direction, the secondary current leads the
primary by a small angle, 0o, and in the motors with secondary
excitation the field flux, <£, being in phase with the field current,
/i (or lagging by angle a behind it), thus leads the primary
current, J0, by angle 60 (or angle 60 — a). As a lag of the mag-
netic flux $ increases, and a lead thus decreases the power-factor,
motors with secondary field excitation usually have a slightly

FIG. ISO.—Single-phase commutator motor with secondary excitation
power-factor improved by shunting field winding with non-inductive
circuit.

P
I

lower power-factor than motors with primary field excitation,
and therefore, where desired, the power-factor may be improved
by shunting the field with a non-inductive resistance, r0. Thus
for instance, if, in Fig. 179, OJo = primary current, 011 = sec-
ondary current, OEit in phase with OJj, is the e.m.f. of rotation,
in the case of the secondary field excitation, and OEQ, in quadra-
ture ahead of 011, is the e.m.f. of alternation, while OE- is the
impedance voltage, and OEi, OE0 and OE' combined give the
supply voltage, OE, and EOI = 6 the angle of lag.
Shunting the field by a non-inductive resistance, r0, and thus
resolving the secondary current 01\ into the components OP\ in
the field and 01" i in the non-inductive resistance, gives the dia-
gram Fig. 180, where a = J'i0<£ = angle of lag of magnetic
field.