SKF AWAIV-12, User Manual

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Surge testing principles and theory
Surge tests detect insulation damage between turns within a motor’s winding; there is no
other test or way to determine if this type of insulation problem exists. A surge test applies a
high-voltage high-current impulse to a winding using a fast rise time, which will induce—via
Lenz’s Law—a voltage difference between adjacent loops of wire within the winding. If the
insulation between the two loops of wire is damaged or somehow weakened, and if the
voltage difference between the wires is high enough, it will produce an arc between the wires.
You can detect the arc by observing a shift in the surge waveform.
The surge test is performed with an impulse generator and an oscilloscope type display to
observe the “surge waveform” in progress. The surge waveform is a representation of the
voltage present across the test leads of the tester during a test. The indication of a turn-to-
turn fault is a shift to the left and/or a decrease in amplitude of the surge test waveform as
the test voltage increases.
As mentioned above, very short high-current pulses are applied to a coil during a surge test
to create a voltage gradient (or potential) across the length of the wire in the winding. This
gradient produces a momentary voltage stress between turns.
The coil will respond to the surge pulse with a ringing or damped sinusoidal waveform
pattern. Each coil has its own unique signature ringing or wave pattern, which can be
presented on a test display screen as shown below.
Figure 118. Ringing wave pattern resulting from surge testing.
The wave pattern observed during a surge test directly relates to the coil’s inductance. (Other
factors can influence the wave pattern, but inductance is the primary.) The coil becomes one
of two elements in what is known as a “tank circuit,” which is an LC-type circuit made up of
the coil’s inductance (L) and the surge tester’s internal capacitance (C).
Inductance of a coil is basically set by the number of turns in a winding and the type of iron
core in which it rests. The wave pattern’s frequency is determined by the formula:
This formula implies that when the inductance decreases, the frequency will increase.
A surge test can detect a fault between turns due to weak insulation. If the voltage potential
is greater than the dielectric strength of a turn’s insulation, one or more turns may short out
of the circuit. In effect, the number of turns in the coil is reduced. Fewer working turns
reduce the inductance of the coil and increase the frequency of the ringing pattern from the
surge.
SKF Static Motor Analyzer—Baker AWA-IV User Manual 125
Motor testing theory and reference