MIL-DTL-917F(SH)
4.6.1.2.1 Procedure. The temperature rise (Tr) of a winding of an inductor or motor for a specific power flow,
usually rated power, shall be determined. In the application of Method 2, accuracy is essential in the measurement
of all resistance and the temperature of the windings at which the cold and hot resistance is measured.
Thermometers and thermocouples shall be used for temperature measurement. To measure the temperature by
Method 2, a bench mark of the cold resistance shall be determined by measuring the resistance of the winding at
room temperature (Rc). Temporary heating or cooling should be provided to maintain ambient temperature in the
room within a band of ±9 °F (5 °C) during the 24-hour stabilization period. To achieve this end, the winding being
measured must be stored in the unenergized condition for at least 24 hours at the ambient temperature level. The
ambient temperature (Ta) may be measured with a thermometer, and compared to thermocouple measurements at
some point on the unenergized windings. The measurements should be equal unless the windings have been given
insufficient time to achieve ambient temperature. Operating temperature above ambient is determined by making
winding resistance readings as a function of time after the power is removed; ohmmeter or bridge readings are taken
with a stopwatch used for timing purposes. Readings are taken periodically for sufficient time to extrapolate back to
zero to yield the operating winding resistance (Rh) at zero time after power was removed. From the initial winding
resistance (Rc-cold resistance), and the operating resistance at zero time (Rh-hot resistance), winding temperature
rise (Tr) is calculated from the following:
Tr (Temperature rise) °C = (234.5 + Tc)Rh/Rc (234.5 + Ta)
Where:
Rc = Cold resistance of winding in ohms.
Rh = Hot resistance of winding in ohms.
Tc = Temperature (°C) of winding when cold resistance was measured.
Ta = Ambient temperature (°C) during the time that the hot resistance reading was taken.
NOTE 1: All readings of resistance should be made with the same equipment during the testing period for identical
resistance measurement equipment calibration. Cold resistance of windings from prior measurement periods shall
not be used.
NOTE 2: When the final resistance measurements can be performed within 1 minute of power removal, the initial
resistance reading can be used as Rh.
NOTE 3: When performing temperature rise tests of equipment with multiple windings, such as transformers, the
winding closest to the winding core shall be used as the basis for the test. When the winding physical orientation is
not known, the winding with the highest resistance shall be used as the test basis.
4.6.1.3 Method 3. The "embedded detector" method consists of the determination of temperature by
thermocouples or resistance temperature detectors built into the equipment either permanently or for test purposes in
specified locations inaccessible to thermometers by Method 1 as specified (see 6.2). This method is suitable for
interior measurements at designated locations. It is used in those cases where a high degree of accuracy is desired,
where other methods are not suitable or practical, and usually for large rotating equipment.
4.6.1.3.1 Procedure. This method shall be used only where specified in the individual equipment specification
or contract (see 6.2). A temperature distribution analysis shall be made which will indicate the relationship of
normal temperature distribution throughout the areas of interest to the temperature at the measured points.
4.6.2 Inaccessible parts. In the case of inaccessible parts, such as certain rotating parts, the temperature
readings shall be taken as soon as possible after shutdown. A curve shall be plotted with temperature readings as
the ordinate and time as the abscissa. That portion of the linear curve starting where successive readings show
decreasing temperatures shall be extrapolated back to the instant of shutdown. The temperature at the instant of
shutdown as determined in this manner shall be considered the shutdown temperature. Where thermometers are
used, they shall be preheated to approximate the temperature of the part to be measured.
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