(1) Remember, the ammeter is always connected in series with the
circuit. An ammeter offers a very small amount of resistance to current flow, so
current can be measured accurately.
(2) Also, the voltmeter is always connected in parallel. Normally, the
voltmeter offers a very great amount of resistance to current flow, so it will not
cause loading of the circuit.
(3) If an ammeter is connected across the battery, the battery will
effectively be short circuited because of the very small amount of resistance of
the meter. This will cause a great amount of current to flow and will damage the
meter or the battery external connections.
(4) Since the voltmeter offers a very large amount of resistance, it may
be connected in series without damage, providing polarities have been observed.
(5) It is important to observe polarity. If polarity is reversed, this
will pin the needle against its peg and bend the needle, thereby requiring it to be
recalibrated in order to certify accuracy.
d. The voltmeter is connected in the testing circuit across the generator
to monitor its voltage. Connections are made between the BAT terminal and GRD,
which is the case of the generator.
e. The battery and carbon pile (which is a variable load) are connected in
parallel to form the load for the generator. They are connected to the generator
through the current meter.
f. The generator is operated at specified speed and the variable load is
adjusted to obtain specified voltage.
(1) Variation in the generator load should have minimum effect on the
output voltage as observed on the voltmeter.
(2) Variation in the generator load should have a great effect on the
output current as observed on the ammeter.
the specifications, should match the observed meter indications.
g. If the output does not meet specifications, disassemble the generator
for checks of the rotor, stator, and diodes.
4. ROTOR CHECKS. The rotor windings may be checked by connecting a battery,
ammeter, and voltmeter to the edge of the sliprings. If the current draw is above
specifications, the windings are shorted; if the current draw is low, excessive
resistance is indicated (fig 2).
specified resistance may be calculated by dividing the voltage by the current
listed in the specifications booklet. A low resistance indicates shorted windings,
and a high resistance an open or poor connection (fig 3).
b. An ohmmeter connected from either slipring to the shaft should show a
high resistance. A low resistance indicates the field windings are grounded.
c. A test light may be used in place of an ohmmeter to check for opens and
grounds, but the test light will not check for shorts. When connected across the
sliprings, failure to light indicates an open. The windings are grounded if the
lamp lights when connected from either slipring to the shaft.