We begin with an excess of weight which holds the
motor fast, and then a maximum current will flow through it without
producing any external work. When we remove the brake altogether, the
motor will run with a maximum speed, and again produce no external work,
but in this case very little current will pass; this maximum speed is om
on the diagram. Between these two extremes external work will be done,
and there is a speed at which this is a maximum. To find these speeds we
load the brake to different weights, and plot the resulting speeds and
horse powers as abscissae and ordinates producing the curve, BB. Another
curve,
e = B/E
made with an arbitrary scale, gives the commercial efficiency; the speed
for a maximum external horse power is o a, and the speed for the highest
efficiency is represented by o b. In practice it is not necessary to
test a motor to the whole limits of this diagram; it will be sufficient
to commence with a speed at which the efficiency becomes appreciable,
and to leave off with that speed which renders the desired power.
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