temperature during testing to minimize
temperature change between test points.
Experience indicates that tests at the lower
temperature yield more sensitive warnings
of weakness. Consequently, there is less
danger of unexpected puncture during
test at ambient temperature than at
e l e v a t e d t e m p e r a t u r e . S l i g h t
humidification of the winding, similarly, has
been found to improve sensitivity of the
test. This is believed to be caused by more
rapid absorption of moisture into any
weakened or porous areas of insulation
than into good intact insulation.
temperature change between test points.
Experience indicates that tests at the lower
temperature yield more sensitive warnings
of weakness. Consequently, there is less
danger of unexpected puncture during
test at ambient temperature than at
e l e v a t e d t e m p e r a t u r e . S l i g h t
humidification of the winding, similarly, has
been found to improve sensitivity of the
test. This is believed to be caused by more
rapid absorption of moisture into any
weakened or porous areas of insulation
than into good intact insulation.
38. Interpretation of Curves.
-The
quality of the insulation may be judged by
the position of any curvature or knee in the
plot of insulation current versus test
voltage. If the plot is a straight line, the
knee may be assumed to be above the
maximum test voltage. The higher the test
voltage at which the knee appears in the
curve, the better the insulation quality is. If
the knee appears below the maximum peak
ac voltage which could be applied to the
winding in service, the insulation may be in
danger of an in-service failure. If the
leakage current increases to the extent that
its plotted curve becomes almost vertical,
the winding is approaching failure on test
and the test should be discontinued. This
point is the most difficult to judge with this
type of graph. Considerable experience is
necessary to judge the maximum safe test
voltage from plotted insulation current.
the position of any curvature or knee in the
plot of insulation current versus test
voltage. If the plot is a straight line, the
knee may be assumed to be above the
maximum test voltage. The higher the test
voltage at which the knee appears in the
curve, the better the insulation quality is. If
the knee appears below the maximum peak
ac voltage which could be applied to the
winding in service, the insulation may be in
danger of an in-service failure. If the
leakage current increases to the extent that
its plotted curve becomes almost vertical,
the winding is approaching failure on test
and the test should be discontinued. This
point is the most difficult to judge with this
type of graph. Considerable experience is
necessary to judge the maximum safe test
voltage from plotted insulation current.
Another approximate, though useful,
indication of insulation quality is given by
the position of a major downward bend of
the resistance versus voltage curve (see
indication of insulation quality is given by
the position of a major downward bend of
the resistance versus voltage curve (see
). Any major downward bend should
occur at a voltage above the peak
operating voltage (
operating voltage (
x V rms). Progress
2
of deterioration is shown by a shift of the
bend to a lower voltage over a period of
time and by a lowering of the overall
insulation resistance. A sharp bend is most
indicative of a single area of weakness,
while a gradual bend is most often
associated with numerous areas of
weakness. While the breakdown voltage
occasionally has been predicted by a
projection of the resistance curve to zero,
this prediction is rather inaccurate and is
not the useful purpose of the test.
bend to a lower voltage over a period of
time and by a lowering of the overall
insulation resistance. A sharp bend is most
indicative of a single area of weakness,
while a gradual bend is most often
associated with numerous areas of
weakness. While the breakdown voltage
occasionally has been predicted by a
projection of the resistance curve to zero,
this prediction is rather inaccurate and is
not the useful purpose of the test.
39. Progressive Condition of Insula-
tion.
tion.
- The dc, high-voltage test is believed to
give a practical indication of the electrical
quality of the insulation at the time of the test.
Although a rough estimate of the puncture
strength of the winding at the time of test may
be made, of more concern is an estimate of
the years of service remaining in a winding.
Such an estimate is aided by comparing the
insulation tests over a period of time, as
shown in
quality of the insulation at the time of the test.
Although a rough estimate of the puncture
strength of the winding at the time of test may
be made, of more concern is an estimate of
the years of service remaining in a winding.
Such an estimate is aided by comparing the
insulation tests over a period of time, as
shown in
benchmark data on a machine have been
established, the interval between tests may
be extended to from 3 to 5 years unless an
abnormality is indicated or suspected. Often
when the insulation quality drops, it can be
restored by cleaning and varnishing the
winding. A drop in quality, which cannot be
recovered, may be caused by deterioration
within the slots and must be regarded as per-
manent. Replacement of one or more coils
may then be necessary. It is not unusual that
the revarnishing of winding end turns
produces a drop in insulation resistance at
the low and intermediate voltages. This may
occur when a winding has become very dry.
The insulation quality at the higher voltages is
more important; and, if this is improved, the
overall quality is considered improved. This is
illustrated in
established, the interval between tests may
be extended to from 3 to 5 years unless an
abnormality is indicated or suspected. Often
when the insulation quality drops, it can be
restored by cleaning and varnishing the
winding. A drop in quality, which cannot be
recovered, may be caused by deterioration
within the slots and must be regarded as per-
manent. Replacement of one or more coils
may then be necessary. It is not unusual that
the revarnishing of winding end turns
produces a drop in insulation resistance at
the low and intermediate voltages. This may
occur when a winding has become very dry.
The insulation quality at the higher voltages is
more important; and, if this is improved, the
overall quality is considered improved. This is
illustrated in
13 (FIST 3-1 12/91)