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202
FIAT· 850 PASSENGER CARS
v
G
51
67
Field winding ohmic resistance may be deter-
mined with assembled generator, by simply measur-
ing the resistance between terminal 67 and ground.
The recommended measurement method is that
of the Volt/Ampere ratio in which a sufficiently
high voltage is applied and measured, together
with
the current drawn by the winding. The ratio
of applied voltage to drawn current gives the ohmic
resistance, according to the formula:
~
=
R.
I
Alternatively, a Wheatstone bridge may be used.
provided
it
is sufficiently accurate.
Determining armature winding resistance is more
difficult on account of the low value involved and,
therefore, must be made only in exceptional cases,
provided proper equipment is available.
Ohmic Resistance Test.
R
Check that temperatures of frame and commutator
r--:f:---,----{A
do not exceed respectively 86° F (30
0
C) and 149
0
F
(65
0
C).
Fig.
320 -
Wiring diagram for plotting the ampere.revolution
output curve at a steady 12 V voltage (680 F - 200 C).
G. Generator•• V. Voltmeter, 15 V scale. - A. Ammeter, 25 A
scale. - R. Plate type rheostat. 100 A capacity, variable resistance
of 0.20 to 20
n.
Disconnect load rheostat.
Start generator and speed up gradually until
voltmeter reading is 12 Volts; at this point, deter-
mine generator speed
by
a revolution counter.
the voltage remaining steady.
This represents the • cut-in speed .. at 12 Volt
output (origin of curve on abscissa axis).
Stop generator and connect load rheostat.
While driving generator at steady speed in
dif-
ferent increments, adjust load rheostat to ensure a
constant 12 V output at each given r.p.m. rate; at
the same time, read the value of current supplied.
Each reading will give a point of the output curve.
Note that readings must be taken in a very short
while, since the curve extends beyond generator
nominal power and currents higher than those of
the nominal output range impose an overload that
generator cannot withstand for long without over-
heating, with consequent failure of insulations.
The curve plotted by points must lie within the
limits of the shaded area shown in fig. 321.
Heating Test.
Fig. 321 - Output curve of warm D 90/12/16/3 CS generator.
Steady 12 V Voltage.
GIRl/MIN _ R. P. M.
Fig. 322 • Wiring diagram for the generator heating test.
G. Generator. - GR. Regulator. - V. Voltmeter, 15 V scale. - A. Am-
meter, 20 A scale. - R. Carbon plate type rheostat, 100 A capacity,
0.2 to 20
n
variable resistance.
In
this case proceed as follows:
- solder two wire lengths on the faces of two
commutator segments 1800 apart;
- apply 2 to 2.5 Volts and measure with accu-
racy the current drawn (Volt and Ampere readings).
.
till·
b
V
Resistance R
(in
Oluns) 15 s·
gIven
y - .
I
Mechanical Characteristics Check
Data.
Brush hold-down springs load must be 1.3 10
1.6 lbs (0.60 to 0.72 kg).
Maximum commutator out-of-round (on rubbing
surface): not more than .0004" (0.01 mm).
Commutator mica must be undercut at least ,04"
(1 mm) throughout its entire length and width.
l500
"./M.
4000
3000
1500
1700 1000
4
~
0
~
?
6
.
I
. t-
.;yt>'
8
4
0
Wire up the generator complete with fan pulley.
according to diagram fig. 322.
Run generator for one hour and 45 minutes at
4,500 r.p.m., delivering 15.5 to 16.5 Amperes at
14 Volts to a resistor.
A
I
I
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