Since we seldom know the inductance value of a machine winding accurately,
we need a more fl exible yet accurate method to analyze a signal trace on the
oscilloscope. To do this, we use a dual trace scope and two identical capacitance
discharge circuits to charge two different machine windings that should
match at the same time. For example, in the testing of three-phase machine
stator we know that each of the three phases should have the same inductance.
Consequently, by comparing one phase against another we have two
complete, and hopefully, identical L-C circuits for our test to work. A simplifi
ed schematic of the surge comparison tester is shown in Figure 10.43.
Detection of one-turn shorts or grounded coil is possible in all windings of
few parallel circuits. Often only a small trace separation may be detectable
with a one-turn short in a very large motor that has several parallel paths per
phase. However, the winding con nections can be broken to reduce the parallel
paths in order to obtain a larger trace