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 fre- quency, 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 allows both voltage and resistance to be read is the
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
NOTE: Standard AC voltmeters react to the AVER- AGE value of alternating current. When working with AC, the effective value is used. For that rea- son 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. This 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 can 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 frequency.
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. CON- TACT WITH HIGH VOLTAGE TERMINALS WILL RESULT IN DANGEROUS AND POSSI- BLY LETHAL ELECTRICAL SHOCK.
MEASURING DC VOLTAGE
A DC voltmeter or a VOM can 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 polar- ity 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 anticipated reading. If in doubt, start at the highest scale and adjust the scale downward until correct readings are obtained.
3.The design of some meters is based on the “current flow” theo- ry while others are based on the “electron flow” theory.
a.The “current flow” theory assumes that direct current flows from the positive (+) to the negative
b.The “electron flow” theory assumes that current flows from negative
(+).
NOTE: When testing generators, the “current flow” theory is applied. That is, current is assumed to flow from positive (+) to negative
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