Guardian Technologies 4760 Measuring Ac Frequency, Measuring Current, Measuring Resistance, Part

SECTION 1.4

TESTING, CLEANING AND DRYING

PART 1

GENERAL INFORMATION

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 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 engine speed governor. For models rated 60 Hertz, the governor is generally set to maintain a no-load frequency of about 62 Hertz with a corresponding output voltage of about 124 volts AC line-to-neutral. Engine speed and frequency at no-load are set slightly high to prevent excessive rpm and frequency droop under heavy electrical loading.

MEASURING CURRENT

To read the current flow, in AMPERES, a clamp-on ammeter may be used. This type of meter indicates current flow through a conductor by measuring the strength of the magnetic field around that conductor. The meter consists essentially of a current transformer with a split core and a rectifier type instrument connected to the secondary. The primary of the current transformer is the conductor through which the current to be measured flows. The split core allows the Instrument to be clamped around the conductor without disconnecting it.

Current flowing through a conductor may be measured safely and easily. A line-splitter can be used to measure current in a cord without separating the conductors.

Figure 2. Clamp-On Ammeter

Figure 3. A Line-Splitter

NOTE: If the physical size of the conductor or ammeter capacity does not permit all lines to be measured simultaneously, measure current flow in each individual line. Then, add the Individual readings.

MEASURING RESISTANCE

The volt-ohm-milliammeter may be used to measure the resistance in a circuit. Resistance values can be very valuable when testing coils or windings, such as the stator and rotor windings.

When testing stator windings, keep in mind that the resistance of these windings is very low. Some meters are not capable of reading such a low resistance and will simply read CONTINUITY.

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