Before fixing a motor in its final position, it should also be tested
for power with a dynamometer, and for this purpose a Prony brake answers
very well.
An ammeter inserted in the circuit will show at a glance what current is
passing at any particular speed, and voltmeter readings are taken at the
terminals of the machine, when the same is standing still as well as
when the armature is running, because the E.M.F. indicated when the
armature is at rest alone determines the commercial efficiency of the
motor, whereas the E M.F. developed during motion varies with the speed
until it nearly reaches the E.M.F. in the leads; at that point the
theoretical efficiency will be highest.
Calculations are greatly facilitated, and the value of tests can be
ascertained quickly, if the constant of the brake is ascertained; then
it will be simply necessary to multiply the number of revolutions and
the weight at the end of the lever by such a constant, and the product
gives the horse power, because, with a given Prony brake, the only
variable quantities are the weight and the speed.
Pages:
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50