CHAPTER 5. FUNCTIONING OF EQUIPMENT

TM 11-6625-298-14

FUNCTIONING OF EQUIPMENT

5-1. Function

tween two carbon brushes. When the armature is

rotated in the magnetic field, an armature voltage

Ohmmeter ZM-21(*) /U measures high values of

is generated that is led out to the ohmmeter

insulation resistance of all types of electrical

terminals through the carbon brushes. With the

equipment and wiring. The resistance across the

generator rotating at the proper speed, the

input terminals is in series with the generator lead

predominant ripple frequency is approximately

to the meter, thus affecting the current flow

1,200 hertz (Hz) with a peak-to-peak ripple voltage

through the meter. A hand crank on the ohmmeter

of approximately 6 volts. Because of the low ac

drives the dc generator with voltage being

component, no capacitor is required across the

maintained at a constant level by an electronic

output of the generator.

controller circuit. The resistance range of the ohm-

b. The generator drive assembly of ohmmeter is

meter is from O through 1,000 megohms. The

shown in figure 5-1. The generator is driven by a

resistance is indicated by a meter of special design,

hand-operated crank through a gear train and

and the indication given is as near as possible to a

clutch assembly. The clutch assembly is adjusted

true representation of the insulation resistance of

so that it will not slip. On, earlier models of the

the equipment, wire, or cable under test.

ohmmeter (fig. 5-2) when the crank reaches an

5-2. Hand Generator

operating speed of approximately 160 rpm, cen-

a. The dc hand generator consists essentially of

trifugal force disengages the clutch shoes from the

a rotating armature in a fixed magnetic field. The

hub in order to maintain constant voltage.

armature has a 39-bar commutator running be-

Figure 5-1. Generator drive assembly, Ohmmeter ZM-21A/U and ZM-21B/U

5-1