average machine insulation. See
figure 11
for a comparison of test curves between the
10- and 30-minute schedules for the same
machine.
10- and 30-minute schedules for the same
machine.
33. Maximum Voltage for Test.
-If the
insulation microampere versus voltage
plots
are straight lines, the test may be co ntinued
to the maximum test voltages shown in
are straight lines, the test may be co ntinued
to the maximum test voltages shown in
table
A
. If curvature appears, the test should be
stopped when the resistance has dropped to
approximately one-third the maximum value
unless it is desired to locate the weakness
by puncture so that it can be repaired.
Cases of abrupt breakdown before the
resistance curve approached zero have
occurred in insulation where mechanical
abrasion, cracking, or acute mica migration
existed. Hence, as a general practice, but
particularly where mechanical defects are
known to exist, tests should only be
performed when time could be taken for
bypassing a coil or other maintenance if a
weak point should be punctured. This allows
taking best advantage of the "proof" feature
of the test. For the most conservative,
nondestructive use of the test, a very abrupt
increase in insulation current (excluding the
random variations of absorption current
often found) is also warning that the test
should be stopped. An abrupt rise in
insulation resistance (drop in leakage
current) is rarely found, but when it occurs
above the peak operating voltage for the
winding, it has usually preceded breakdown
of the insulation. The increase of resistance
is thought to be caused by vaporization of
the moisture trails in the insulation, which
would immediately precede puncture.
approximately one-third the maximum value
unless it is desired to locate the weakness
by puncture so that it can be repaired.
Cases of abrupt breakdown before the
resistance curve approached zero have
occurred in insulation where mechanical
abrasion, cracking, or acute mica migration
existed. Hence, as a general practice, but
particularly where mechanical defects are
known to exist, tests should only be
performed when time could be taken for
bypassing a coil or other maintenance if a
weak point should be punctured. This allows
taking best advantage of the "proof" feature
of the test. For the most conservative,
nondestructive use of the test, a very abrupt
increase in insulation current (excluding the
random variations of absorption current
often found) is also warning that the test
should be stopped. An abrupt rise in
insulation resistance (drop in leakage
current) is rarely found, but when it occurs
above the peak operating voltage for the
winding, it has usually preceded breakdown
of the insulation. The increase of resistance
is thought to be caused by vaporization of
the moisture trails in the insulation, which
would immediately precede puncture.
34. Importance of Regulated Power
Supply.
Supply.
- It is important that voltage
variations in the power supply to the test set
be avoided. Because of the capacitance of
the winding under test, even minor
fluctuations in supply voltage to the test set
will cause wide fluctuations of the
microammeter and contribute to a random
scattering of test points. A voltage stabilizer
or electronic regulator in the power supply is
recommended. Magnetic voltage stabilizers
usually perform better when working into a
resistance load than when driving a rectifier
only. A 500-volt ampere regulator is usually
sufficient so that about 200 volt amperes of
resistance load can be used in addition to
be avoided. Because of the capacitance of
the winding under test, even minor
fluctuations in supply voltage to the test set
will cause wide fluctuations of the
microammeter and contribute to a random
scattering of test points. A voltage stabilizer
or electronic regulator in the power supply is
recommended. Magnetic voltage stabilizers
usually perform better when working into a
resistance load than when driving a rectifier
only. A 500-volt ampere regulator is usually
sufficient so that about 200 volt amperes of
resistance load can be used in addition to
the test set. If random variations still persist,
the currents may be averaged graphically. If
absorption currents are plotted against time
on log-log paper, the mean will be a smooth
curve and almost a straight line from which
the mean reading of the appropriate time
may be selected. For this plotting, the time is
reckoned from zero when voltage is raised
from the preceding step.
the currents may be averaged graphically. If
absorption currents are plotted against time
on log-log paper, the mean will be a smooth
curve and almost a straight line from which
the mean reading of the appropriate time
may be selected. For this plotting, the time is
reckoned from zero when voltage is raised
from the preceding step.
35. Discharge of Winding After Test.
-
Upon completion of the dc, high- voltage
test, the winding should be discharged
through the special discharge resistor
ordinarily provided with the test set. The
discharge resistor consisting of several
kilohms is for the purpose of retarding the
discharge current so no destructive surge is
produced. The winding may be solidly
grounded when the voltage has dropped to
zero or after a few minutes of discharge
have occurred.
test, the winding should be discharged
through the special discharge resistor
ordinarily provided with the test set. The
discharge resistor consisting of several
kilohms is for the purpose of retarding the
discharge current so no destructive surge is
produced. The winding may be solidly
grounded when the voltage has dropped to
zero or after a few minutes of discharge
have occurred.
36. Minimum Period of Grounding
After Test Before Returning to Ser-
vice.
After Test Before Returning to Ser-
vice.
- A winding should remain solidly
grounded long enough after test for the ab-
sorbed charge to be completely drained off
before restoring the machine to service.
Windings vary greatly in the length of time
an absorbed charge will remain, but in
absence of actual checks, it is
recommended the winding be grounded for
at least 1 hour. If the dc, high-voltage test
precedes a maintenance ac overpotential
test, it is advisable to double the minimum
ground time to assure that absorbed charge
does not contribute to puncture. Also, the
time spent in making a discharge test can be
credited to the total grounding time.
sorbed charge to be completely drained off
before restoring the machine to service.
Windings vary greatly in the length of time
an absorbed charge will remain, but in
absence of actual checks, it is
recommended the winding be grounded for
at least 1 hour. If the dc, high-voltage test
precedes a maintenance ac overpotential
test, it is advisable to double the minimum
ground time to assure that absorbed charge
does not contribute to puncture. Also, the
time spent in making a discharge test can be
credited to the total grounding time.
37. Effects of Temperature and Hu-
midity.
midity.
- The influence of temperature upon
leakage current of winding insulation under
dc, high-voltage test is similar to the
influence of temperature on insulation
resistance meter readings. However, the
most significant factor of the dc, high voltage
test, the position of the curvature, is not
appreciably affected by temperature; con-
sequently, correction of individual test points
to the standard temperature of 40 °C is not
necessary. Because the dc, high-voltage
test does require appreciable time, it is
desirable that the insulation be near ambient
dc, high-voltage test is similar to the
influence of temperature on insulation
resistance meter readings. However, the
most significant factor of the dc, high voltage
test, the position of the curvature, is not
appreciably affected by temperature; con-
sequently, correction of individual test points
to the standard temperature of 40 °C is not
necessary. Because the dc, high-voltage
test does require appreciable time, it is
desirable that the insulation be near ambient
(FIST 3-1 12/92) 12