Measuring Electricity; Section 2: Measuring Electricity; Meters; The Vom - Generac Power Systems 4451 & 4986 Repair Manual

section 2

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 VOMs 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 2-1) 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 2-1. – Digital VOM
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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 frequency.
3. Only an AC voltmeter may be used to measure AC voltage. DO
NOT USE A DC VOLTMETER FOR THIS PURPOSE.
* DangeR!: 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 polar-
ity switch.
b. On meters that do not have a polar-
ity 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"
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 negative (-).
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.
Correct engine and Rotor speed is maintained by an