364

ELECTRICAL APPARATUS

203. The behavior of the motor at different speeds is best
shown by plotting i, p = cos 0, P and D as ordinates with the
speed, Sj as abscissa, as shown in Fig. 173.

In railway practice, by a survival of the practice of former
times, usually the constants are plotted with the current, /, as
abscissae, as shown in Fig. 174, though obviously this arrange-
ment does not as well illustrate the behavior of the motor.

Graphically, by starting with the current, I, as zero axis, 07, the
motor diagram is plotted in Fig. 175.

0.6 0.7 0,8 09 10 11 12 13 1.4 1.5 1.8 1-7 1.8 1,9 2,0 2,1

PIG. 173.—Single-phase commutator-motor speed characteristics.
The voltage consumed by the resistance, r, is OEr = ir, in phase
with 01 j the voltage consumed by the reactance, x, is OEX = ix,
and 90° ahead of 07. OEr and OE* combine to the voltage con-
sumed by the motor impedance, OE' = iz.
Combining OE' = iz, OEi = ei} and OEQ = e0 thus gives the
terminal voltage, OE = e, of the motor, and the phase angle,
EOI = e.
In this diagram, and in the preceding approximate calculation,
the magnetic flux, $, has been assumed in phase with the current, J.
In reality, however, the equivalent sine wave of magnetic
flux, <£, lags behind the equivalent sine wave of exciting current, /,
by the angle of hysteresis lag, and still further by the power