MIL-PRF-27G
4.7.12.18 Phase shift. The phase shift in electrical degrees attributable to a transformer shall be measured by a
phase shift meter or other suitable methods.
4.7.12.19 Short-circuit test (for qualification only). With the secondary windings shorted in turn, the voltage applied
to the primary shall be adjusted until the secondary is carrying rated current. The primary power in watts shall then be
measured.
4.7.12.20 Capacitance (when specified (see 3.1). Transformers and inductors shall be tested in accordance with
method 305 of MIL-STD-202 for distributive or winding to winding capacitance as specified (see 3.1).
a.
Test frequency - As specified (see 3.1).
b.
Magnitude of polarizing voltage - As specified (see 3.1).
4.7.13 Temperature rise (see 3.15). Unless otherwise specified (see 3.1), the temperature-rise test shall be
performed on transformers rated at more than 0.8-watt average output, and on inductors for which the product of the
dc resistance and the square of the rated current is more than 0.2 watt. The temperature rise of each winding shall be
based on the change-in-resistance method and shall be computed by the following formula:
R-r
ĆT =
r (t + 234.5) - (T-t)
Where:
ĆT = Temperature rise (in °C) above specified maximum ambient temperature
(see 3.1).
R = Resistance of winding (in ohms) at temperature (T + ĆT).
r = Resistance of winding (in ohms) at temperature (t).
t = Specified initial ambient temperature (in °C).
T = Maximum ambient temperature (in °C) at time of power shutoff. (T) shall not differ from (t) by more
than 5°C.
The transformers shall be conditioned for at least 8 hours at temperature (t) in a location free from drafts before
resistance (r) is measured. For transformers, rated voltage shall be applied to the primary with the specified loads
across the secondaries (see 3.1). For inductors, rated dc and ac current shall be applied to the windings.
Transformers or inductors shall be operated until two consecutive resistance readings on the highest resistance
winding, taken 30 minutes apart, are the same. If the power is required to be shut off, the resistance measurements
(R) shall be made as soon as possible. The transformers and inductors shall then be examined for evidence of
physical damage. At the option of the supplier, the test may be performed at 60 Hz for transformers rated at 50/60 Hz
provided that the primary voltage is increased to 1.2 times the rated voltage and the secondary currents are maintained
at rated current. Unless otherwise specified (see 3.1) method I shall be used.
4.7.13.1 Method I.
4.7.13.1.1 For small units (units that will clear each wall of the cabinet by a minimum of 8 inches when supported 3
inches above the base). The transformers or inductors shall be supported in the test cabinet by means of
low-thermal-conductivity cleats 3 inches in height. The samples shall be shielded from forced convection air-currents
and from direct thermal radiation. The cabinet dimensions, insulation, air circulation, and all other factors influencing
the thermal capacity shall be such that the ambient temperature does not increase more than 5 degrees during the
determination.
4.7.13.1.2 For large units. Transformers or inductors with dimensions in excess of 8 x 8 x 13 inches shall be
supported by means of low-thermal-conductivity cleats 3 inches in height, and shall be tested under standard test
conditions (see 4.3) in still air.
4.7.13.2 Method II. Transformers and inductors shall be tested as specified in 4.7.13.1, except that the unit
shall be mounted on brass spacers (according to the mounting-screw size) as specified in table XV. The brass
spacers shall be connected to a steel plate .125 inch thick and large enough to cover the bottom of the test enclosure
within 2.00 inches on each side. For tests on large units, this plate shall be 2-feet square. When the mounting surface
area is greater than 1 square foot, a 4-foot square plate shall be used. The brass spacers shall be tapped and shall be
in intimate contact by threaded connections with the steel plate and that part of the case, which normally rests on the
mounting surface.
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