Section 1.4 - Measuring Electricity; Meters; The Vom; Measuring Ac Voltage - Generac Power Systems 7 kW NG Repair Manual

GENERAL INFORMATION

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 14) 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 14. Digital VOM
PART 1

MEASURING AC VOLTAGE

An accurate AC voltmeter or a VOM may be used to read the
generator's AC output voltage. The following guidelines apply:
1. Always read the generator's AC output voltage at the
unit's rated operating speed and AC frequency.
2. The generator's rated AC output voltage is 250 to 254
VAC and is not adjustable.
3. Only an AC voltmeter may be used to measure AC voltage.
DO NOT USE A DC VOLTMETER FOR THIS PURPOSE.
 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 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 current 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 Rotors must run at 1800 rpm to deliver a 60 Hertz
output.
Section 1.4
Measuring Electricity
Page 19