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Section 4
MEASURING ELECTRICITY
METERS
Devices used to measure electrical properties are
called meters. Meters are available that allow one
to measure (a) AC voltage, (b) DC voltage, (c) AC
frequency, and (d) resistance in ohms. The following
apply:
❏ To measure AC voltage, use an AC voltmeter.
❏ To measure DC voltage, use a DC voltmeter.
❏ Use a frequency meter to measure AC frequency In
“Hertz” or “cycles per second”..
❏ Use an ohmmeter to read circuit resistance, in
“ohms”.
THE VOM
A meter that will permit both voltage and resistance to
be read is the “volt-ohm-milliammeter” or “VOM”.
Some VOM’s are of the “analog” type (not shown).
These meters display the value being measured by
physically deflecting a needle across a graduated
scale. The scale used must be interpreted by the user.
“Digital” VOM’s (Figure 4-1) are also available and
are generally very accurate. Digital meters display the
measured values directly by converting the values to
numbers.
NOTE: Standard AC voltmeters react to the
AVERAGE value of alternating current. When
working with AC, the effective value is used. For
that reason a different scale is used on an AC
voltmeter. The scale is marked with the effective or
“rms” value even though the meter actually reacts
to the average value. That is why the AC voltmeter
will give an incorrect reading if used to measure
direct current (DC).
Figure 4-1. – Digital VOM
MEASURING AC VOLTAGE
An accurate AC voltmeter or a VOM may be used to
read the generator’s AC output voltage. The following
apply:
1.
Always read the generator’s AC output voltage
only at the unit’s rated operating speed and AC
frequency.
2. The generator’s voltage regulator can be adjusted
for correct output voltage only while the unit is
operating at its correct rated speed and frequen-
cy.
3. Only an AC voltmeter may be used to measure
AC voltage. DO NOT USE A DC VOLTMETER
FOR THIS PURPOSE.
*
DANGER!: RV GENERATORS PRODUCE HIGH
AND DANGEROUS VOLTAGES. CONTACT
WITH HIGH VOLTAGE TERMINALS WILL
RESULT IN DANGEROUS AND POSSIBLY
LETHAL ELECTRICAL SHOCK.
MEASURING DC VOLTAGE
A DC voltmeter or a VOM may be used to measure
DC voltages. Always observe the following rules:
1. Always observe correct DC polarity.
a. Some VOM’s may be equipped with a polarity
switch.
b. On meters that do not have a polarity switch,
DC polarity must be reversed by reversing the
test leads.
2. Before reading a DC voltage, always set the
meter to a higher voltage scale than the anticipat-
ed reading. If in doubt, start at the highest scale
and adjust the scale downward until correct read-
ings are obtained.
3. The design of some meters is based on the “cur-
rent flow” theory while others are based on the
“electron flow” theory.
a. The “current flow” theory assumes that direct
current flows from the positive (+) to the nega-
tive (-).
b. The “electron flow” theory assumes that cur-
rent flows from negative (-) to positive (+).
NOTE: When testing generators, the “current flow”
theory is applied. That is, current is assumed to
flow from positive (+) to negative (-).
MEASURING AC FREQUENCY
The generator’s AC output frequency is proportional
to Rotor speed. Generators equipped with a 2-pole
Rotor must operate at 3600 rpm to supply a frequency
of 60 Hertz. Units with 4-pole Rotor must run at 1800
rpm to deliver 60 Hertz.
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