Generac Power Systems 5410 Section MEASURING ELECTRICITY, Meters, The VOM, Measuring AC Voltage

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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