Baker D30R, User Manual

Page: 54 / 82

Surge Test Applications
12/3/2009 | 71-022 V6
D30R 54
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Check for open test leads at the clip end. With heavy use, check test leads weekly to
ensure there is no breakage. Check test leads by firmly grasping the boot and clip in one
hand while pulling on the lead with the other. A broken lead will stretch. A good lead will
not stretch.
Form coils
The surge test is a recommended for form coils. It can generate the turn-to-turn voltage
needed to test these low impedance coils.
Notes and tips for form coils
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IEEE-522-1992 recommends a test voltage for vacuum pressure impregnation coils,
before they are cured, of 60-80 percent of the test voltage of fully cured coils.
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Currents required to test form coils often limit the maximum surge voltage. Placement of
the coils into the stator iron or spare laminations has the effect of enabling the tester to
produce a higher voltage drop across the coil for a given current level.
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Caution should be exercised when testing form coils, since the laminations or stator core
have induced voltage on them, which can provide a voltage path or ground.
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Many formulas are used in calculating a test voltage for AC form-wound coils. These are
generally based on experience and theoretical arguments about the distribution of
voltage in a coil and the entire winding. Some of these formulas are difficult to apply
because of the great diversity of coil specifications and characteristics. One popular
formula (based on Paschen’s Law) states a minimum and maximum test voltage range:
Minimum = Number of turns x 500 Volts
Maximum = Winding operating voltage x 1.5
The minimum voltage would be necessary to show a void in the turn insulation that would
result in arcing. The maximum voltage value is based on the worst case distribution of a
surge in the winding. Studies (IEEE-522-1992 and IEEE-587-1980) have shown that a very
rapid surge from a lightning strike or contactor closing/opening may be distributed across the
first coil of a winding.
Three phase motors
Wave patterns for three phase windings are compared in pairs. The storage capabilities of the
digital tester, allows all three phases to be compared after reconnecting the test leads. The
operator simply recalls any one of the previously tested leads. For each test, check the display
for a wave pattern, saving each waveform. Recall the waveform Summary from the digital
tester’s memory for the wave patterns to be compared. If the three wave comparisons seen
Fig 8-3: Open phase in a wye-connected motor