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TM 10-3930-671-24
FIELD CURRENT DRAW TEST - KEY OFF:
ENGINE NOT RUNNING (22/30 Amp Systems)- This
test requires the temporary application of a controlled
load across the battery in order to measure field current
draw at a common reference voltage point, 10.0 volts
(Figure No.11).
Turn control knob of carbon pile off, connect load leads
across battery terminals. Remove lead from alternator
field terminal. Turn control knob of 50 ohm rheostat off,
connect leads to field terminal and output terminal of the
alternator. Switch test ammeter to low scale 0-5
amoeres.
Connect voltmeter as follows: negative lead to alternator
ground terminal, positive lead to alternator output
terminal. At this point, test voltmeter should indicate
battery voltage. Test ammeter should indicate zero
amperes.
Slowly reduce the resistance of the 50 ohm rheostat,
observe the action of the test ammeter. If reduction of
rheostat resistance can be fully completed without
causing test ammeter to indicate more than 1.5
amperes, there are no shorts or grounds, in the rotor
(field) circuit.
If test ammeter tends to read in excess of 3.0 amperes,
it is necessary to discontinue the test until the condition
of the rotor winding, brush system and slip rings, has
been determined. Possible sources of difficulty may be;
shorted rotor turns, accidental grounding of the brush
shunt leads, shorted rotor rings, perhaps from foreign
material being embedded between the rings. Check for
missing or broken insulators. Repair as necessary,
continue test.
With test rheostat at minimum resistance, note the
voltmeter and test ammeter readings. Slowly increase
load of carbon pile until voltmeter reads 10.0 volts.
Note the test ammeter reading, it should be: .55 to 1.15
amperes 0 10V (700 to 800F). Proceed with next step if
correct reading is noted.
Reverse carbon pile load knob immediately after test to
avoid
discharging
battery,
but
leave
load
leads
connected to the battery.
FIGURE NO. 11
F-279
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