Figure 3-1. Connections for testing a.c. or d.c. rotating machines.

TM 11-6625-298-14

Figure 3-1. Connections for testing a.c. or d.c. rotating machines.

3-7. Insulation Test of Ac and Dc Rotating

b. Connect another test lead from the GROUND

Machines Rated Over 800 Volts

terminal to the shaft or frame of the equipment

a. Connect the equipment and test as directed

under test. Be sure that the surface to which the

for machines at 600 volts or less (para 3-6 above).

connection is made is free of paint or any other

b. Turn the hand generator crank at operating

substance that is a nonconductor.

speed. If the machine is cold, readings should

c. Turn the hand generator crank at operating

range between 10 and 100 megohms. Readings will

speed. If the machine is cold, the indication should

be lower if the equipment is warm.

be between 1 and 10 megohms. If the machine is

c. Direct current (dc) fields on ac machines

warm, the indication will be lower. Also, a large

should indicate between 1 and 10 megohms.

machine will have a lower insulation resistance

than a small machine.

3-8. Insulation Test of Transformers

d. If all the circuits in the machine are not

a. Connect a test lead from the GROUND

interconnected as in separately excited generators,

terminal of the ohmmeter to the core or case of the

measure each circuit individually.

transformer (fig. 3-2).

3-3