MIL-PRF-27G
6.3 Qualification. With respect to products requiring qualification, awards will be made only for products which
are, at the time set for opening of bids of award of contract, qualified for inclusion in the applicable qualified products
list whether or not such products have Qualified Products List QPL No. 27 whether or not such products have actually
been listed by that date. The attention of the contractors is called to these requirements, and manufacturers are urged
to arrange to have the products that they propose to offer to the Federal Government tested for qualification in order
that they may be eligible to be awarded contracts or orders for the products covered by this specification. Information
pertaining to qualification of products may be obtained from the DLA Land and Maritime, ATTN: DLA Land and
Maritime-VQP, PO Box 3990, Columbus, OH 43218-3990, or by e-mail to vqp.chief@dla.mil. An online listing of
products qualified to this specification may be found in the Qualified Products Database (QPD) at
6.4 Dielectric withstanding voltage. Users of transformers and inductors should note that the units have been
previously tested at 100-percent dielectric withstanding voltages at least one or more times and, therefore, should be
tested only at 75 percent test voltage during subsequent inspections such as during incoming inspection by a
purchaser. For units with a working voltage in excess of 10 kV, corona measurements should be considered instead of
dielectric withstanding voltage.
6.5 Magnetic shielding. The approximate magnetic field strength for the Helmholtz structure described in
6.6 Notes for airborne application.
6.6.1 Temperature rise (method III). Figure 10 is an example of two units placed in a duct for method III of the
temperature rise test. "A" indicates the longer side, and "B" indicates the shorter side (see 4.7.13.3).
6.7 Ambient temperature increase. Ambient temperature increase for use under 4.7.21 will be derived on an
assumed linear extrapolation. For example, a class R unit operated under 4.7.13 at an ambient temperature of 70°C
with a 20°C temperature rise would be operated under 4.7.21 at an ambient temperature of 85°C (105°C - 20°C =
85°C).
6.8 Test circuits for electrical characteristics. The actual circuit may be used for group B electrical inspection
testing in lieu of the test circuits specified herein (see 4.7.12).
6.9 Reduction of dielectric withstanding voltage testing. When the dielectric withstanding voltage potential
required between windings is greater than that required between windings and ground, and where there is sufficient
insulation used in the construction of the transformer or inductor, a reduction of testing may be accomplished by
specifying a sufficiently high winding-to-ground potential so that it includes the required test potential between
windings.
6.10 Notes regarding general applications for equipment designers. The equipment designer, by proper
application of the information contained in the following paragraphs, can communicate to the transformer or inductor
designer of the supplier a more complete understanding of his requirements and thus realize better equipment
reliability. This will also prevent the costly overspecification of requirements not needed for the intended use.
6.10.1 Specification sheet transformers and inductors. For any transformer or inductor requirement, the
specification sheets listed in supplement 1 should first be reviewed and if usable for the requirement, should be
specified.
6.10.2 Temperature. The class designation in table II refers solely to maximum operating temperature and has no
relation to types of insulation material. Any insulation material may be used in any class of transformer or inductor,
depending entirely upon the maximum operating temperature and its associated life expectancy. The maximum
operating temperature refers to the maximum ambient temperature specified for equipment operation, plus the internal
temperature rise at the time that thermal stability is reached; however, since the temperature rise of audio transformers
is negligible, the maximum operating temperature, in most cases, is the same as the maximum ambient temperature.
It should be noted that where the total of the specified maximum ambient temperature and the specified allowable
temperature rise exceeds, by any amount, the maximum operating temperature for any given class, the unit must be
described by the letter designating the next higher class designation and meet the requirements thereof.
6.10.2.1 Maximum operating temperature. The maximum operating temperature is the same as the allowable
temperature rise plus the maximum ambient temperature for the class. Accordingly, temperature rise is the allowable
temperature differential between the ambient and maximum operating temperature for a given insulation for a specified
life. For example, class R has a maximum operating temperature designation of 105°C; this is normally at 65°C
ambient plus a 40°C rise. If the temperature rise was determined to be 30°C, this same transformer could operate in
an ambient as high as 75°C (75°C +30°C = 105°C).
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