Baker D30R User Manual - page 40
Principles of Surge Testing
12/3/2009 | 71-022 V6
D30R
40
This formula implies that when the inductance decreases, the frequency will increase.
A surge test can detect a fault between turns that is due to weak insulation. If the voltage
potential is greater than the dielectric strength of the turn insulation, one or more turns may
be shorted 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 increased the frequency of the ringing
pattern from the surge.
The voltage or amplitude of the surge wave pattern is also reduced due to the decrease in
inductance of a coil with a fault between turns. It is determined by the formula:
Where the current (i) varies according to time (t)
When the insulation between turns is weak, the result is a low energy arc-over and a change
in inductance. When this happens the wave pattern becomes unstable – it may shift rapidly to
the left and right, and back to the original position.
A reduction in inductance occurs due to turn-to-turn faults, phase-to-phase faults, miss-
connections, open connections, etc. Partial ground wall testing is also performed in a surge
test when there is a ground line to the machine frame.
The Surge test is most often used to test turn-to-turn insulation of coils or single windings.
Form coils, start and run windings, and multi-tapped windings are a few examples. Surge
tests are also used to compare new windings to a standard winding to assure they conform.
Determination of a fault
If a fault exists in a motor, the wave pattern on the display will collapse in amplitude and a
distinct shift to the left will occur, signifying an increase in frequency (a decrease in
inductance). When inductance decreases, the frequency of the wave pattern will increase
according to the formula above.
This is illustrated in the figure below. This type of fault is generally one that indicates a failure
of the turn-to-turn short.
If any wave pattern becomes erratic and/or flickers during testing, intermittent shorting or
arcing is probably occurring in the windings under the voltage stress. Arcing is often
accompanied by audible sounds. It may be desirable to store the wave pattern by this arcing
for reference if the operator can release the TEST (this freezes the wave pattern) at the
moment when the wave pattern appears the most affected by the fault (reduced amplitude
and increased frequency or shift to the left).
dt
di
L
Voltage
=