MIL-DTL-3124E(SH)
4.6.10 Insulation resistance. This test shall be made before and after the dielectric tests (see 4.6.11). Prior to
application of the test voltage, the winding of the machine shall be thoroughly discharged. Separate measurements
shall be made on the stator and rotor windings. Circuits of equal voltage above ground shall be connected together.
Circuits or groups of circuits of different voltage above ground shall be separated. Insulation resistance shall be
measured with an insulation-resistance-indicating meter. The time of test voltage application shall be not less than
60 seconds. The temperature of the generator windings at the time of the test shall be measured. Insulation
resistance measurements shall be corrected to 77 °F (25 °C). Correction shall be made on the basis of insulation
resistance doubling for each 18 °F (10 °C) decrease in temperature. The insulation resistance test may be conducted
at any convenient ambient temperature. The relative humidity at the time of the test shall be measured.
4.6.11 Dielectric. The dielectric test shall be made after all other tests have been completed. If the insulation
resistance of the windings is lower than specified, because of dirt or moisture or damage to windings, the condition
shall be remedied before application of the dielectric test voltage. The dielectric test shall be made on the
completely assembled generator and not upon individual parts unless: (1) it can be demonstrated that the tests on the
individual parts are equivalent to a test on the assembled generator; (2) the generator is not assembled when all other
routine tests have been completed and the generator is to be shipped in a disassembled condition. If the generator is
to be shipped elsewhere for assembly and combined testing, the dielectric test may be conducted there after
assembly. An exception is made in the case of maintenance parts, such as coils and rotating elements with insulated
windings, which require dielectric testing.
4.6.11.1 Armature windings. The dielectric test voltage shall be equal to twice the rated value of the rms
terminal voltage plus 1,000 volts. The frequency of the testing voltage shall be not less than 60 Hertz. The voltage
wave shall approximate a sine wave. The testing voltage shall be applied continuously for not less than 60 seconds.
Generators built in large quantities for which the test voltage is 2,500 volts or less may be tested for 1 second, with a
test voltage 20 percent higher than the 60-second test voltage.
4.6.11.2 Field coils. The dielectric test voltage for field coils shall be equal to 10 times the exciter voltage, but
shall be not less than 1,500 volts, nor more than 3,500 volts. The frequency of the testing voltage shall be not less
than 60 Hertz. The voltage wave shall approximate a sine wave applied continuously for not less than 60 seconds.
4.6.11.3 Measurements of test voltage. The voltage used in dielectric tests shall be measured by a voltmeter
which derives voltage directly from the high voltage circuit, either by a voltmeter coil placed in the testing
transformer, or through an auxiliary potential transformer.
4.6.11.4 Points of application. The test voltage shall be applied between each electric circuit and the frame,
with all other electric circuits and metal parts grounded. Interconnected polyphase windings shall be one circuit.
The test voltage shall be applied in a manner as to prevent pitting the bearings in case of insulation failure. Voltage
need not be applied between stationary and rotating windings.
4.6.12 Enclosure. The enclosure shall be tested in accordance with MIL-E-2036.
4.6.13 Heating and overload. Heating and overload tests shall be as specified in 4.6.13.1 through 4.6.13.4.4.
4.6.13.1 Test conditions. The heating and overload tests shall be conducted with the generator assembled to its
prime mover in accordance with MIL-G-3087, MIL-G-21296, or MIL-G-22077, as applicable.
4.6.13.2 Duration. Heat runs on continuous-duty (see 6.5.4) generators shall be continued until the steady final
temperatures have been attained in all parts of the generator. Steady final temperatures have been reached when at
least four consecutive readings taken at 15-minute intervals show no change in the temperature greater than 3.6 °F
(2 °C) of any part of the generator. Immediately following all other loads and overloads, the generator shall be run
at 150 percent of rated current for 2 minutes at rated voltage and frequency, and at a power factor of 0 .5 lagging or
less. Temperature measurements are not required. The exciter which is supplied as part of the complete generator
set shall be used to supply excitation during the tests.
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