MIL-DTL-3124E(SH)
3.4.5 Short circuit requirements. Generators and associated excitation systems shall withstand, without injury
to any part, three-phase and single-phase (line-to-line) short circuits at the generator terminals for a period of time as
determined by I2t = 180, where I is the sustained value of line current in per-unit and t is the time in seconds. Unless
otherwise specified (see 6.2), the sustained value of current during a three-phase or single-phase short circuit shall
be not less than 3.2 times rated current.
3.4.6 Voltage rating. Unless otherwise specified (see 6.2), the generators shall be rated at 450 volts.
3.4.6.1 Duty. Generators shall deliver continuous rated current at rated voltage, frequency, and power factor.
3.4.7.1 Phase unbalance. Under a condition of a single-phase load of 15 percent of rated current at any power
factor between rated and unity, and with no other load on the generator, the variation in voltage between phases
line-to-line shall not exceed 5 percent of rated voltage. The temperature rises shall be not greater than those
specified in table III (see 4.6.26).
TABLE III. Maximum temperature rise. 1/
Temperature rises for class of insulation 2/
Method of temperature
°F (°C)
Machine part
determination
F
H
N
R
S
Armature winding
a. All kVA 3/ ratings
Resistance
203
239
275
302
338
(95)
(115)
(135)
(150)
(170)
b. 1563 kVA and less
Embedded detector
221
266
302
356
392
(105)
(130)
(150)
(180)
(200)
c. Over 1563 kVA
Embedded detector
212
257
293
329
365
(100)
(125)
(145)
(165)
(185)
Field windings
a. Salient-pole
Resistance
203
239
275
302
338
(95)
(115)
(135)
(150)
(170)
b. Cylindrical rotor
Resistance
203
239
275
302
338
(95)
(115)
(135)
(150)
(170)
NOTES:
1/
The temperatures attained by cores, amortisseur windings, and mechanical parts shall not injure the
machine in any respect.
2/
Temperature rises are based on an ambient temperature of 233 °F (50 °C). For 104 °F (40 °C)
ambient, the temperature rises may be increased 50 °F (10 °C).
3/
kVA stands for kilovolt-ampere.
3.4.7.2 Negative-sequence impedance. Negative-sequence impedance (see 6.5.10) shall be determined as
3.4.8 Frequency and revolutions per minute. Generators shall have a frequency rated at 60 Hertz and shall be
of the r/min rating specified (see 6.2).
3.4.9 Power factor. The rated power factor shall be 0.80, lagging.
3.4.10 Ambient temperature. Unless otherwise specified (see 6.2), generators shall meet the performance
requirements as specified herein at an ambient temperature of 122 °F (50 °C).
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