Megger Baker DX - Armature Span Tests Using the DX Host; Selecting a Test Voltage for the Span Test

Procedures for Testing with the DX Host
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Armature Span Tests Using the DX Host
DC motor armature windings can be tested for turn insulation integrity. Armature windings are
connected to the commutator in such a manner that all the windings are connected to each
other. As a result, the bar-to-bar inductance measurements can be very low—in the micro-henry
range.
If the inductance is so low that the standard Surge test is unable to reach the required test
voltage when testing between adjacent bars, a “span” of several bars can be tested instead. The
inductance between every 5–10 bars is often high enough to allow for a conventional Surge test
using the DX’s high-voltage leads.
Selecting a Test Voltage for the Span Test
According to Paschen’s law, the minimum breakdown voltage for air is approximately 325 volts,
which would suggest that at least 325 volts per turn of the coil being tested. If, for example,
three bars are spanned with three turns per coil then the test voltage would be: 3 x 3 x 325V =
2925V
Unfortunately, such a high voltage may exceed the dielectric strength of the commutator-to-
shaft insulation or the ground wall insulation of the coil. Therefore, a test voltage that
adequately tests the coils without overstressing perfectly good insulation would be a better
choice.
Due to variations in DC motor complexity and current standards, consult the motor’s original
equipment manufacturer, or follow company guidelines to determine optimal test voltages when
conducting span testing.
NOTICE: Review all safety information provided with the equipment and presented in chapter 1 —“General
Operating Information” before setting up or operating.
Confirm that the Baker DX setup procedure has been followed. Refer to chapter 5 —“Setting up the Baker
DX Tester” as needed.
Refer to chapter 7 —“Setting up Fixtures, Test Accessories” for information needed on this topic.