(c) For Class II Sets:
3. Compare these results with the re-
1. Alternately increase the load, volt-
UNDER FREQUENCY PROTECTIVE
age and frequency in small increments until the fuel
system controls are in the maximum fuel position as
permitted by the governor control linkage and the
General. For generators that power certain
voltage and frequency are within 1 percent of their
type; of equipment, it is extremely important that
the circuit interrupter open when the frequency fails
appreciably below rated value. Severe damage may
otherwise result to the powered equipment. To insure
that the circuit interrupter will open at or before the
Small increments should be taken to avoid
critical frequency value, the generators are equipped
passing the maximum power at the rated
with an under frequency protective device. The de-
voltage and frequency point and to avoid
vice on this generator operates electrically. Although
racing or bogging the engine.
the under frequency protective device must be cap-
able of functioning at any voltage throughout the speci-
2. Hold the conditions in step (1) above
for two minutes. However, if the voltage and fre-
fied voltage operating range, it is necessary only to
perform the test at the specified maximum, at rated,
quency cannot be maintained within 1 percent of their
and at minimum voltage limits.
rated values, the load must be adjusted to the point
at which the voltage and frequency can be maintained
within 1 percent of the rated value for two minutes.
load conditions shall be as described and illustrated
in method 205.1, paragraph 205.1.10 of MIL-HDBK-
It may be necessary to reduce the load to a
value below the rated kilowatt load for a
(1) Preparation for test. Connect a volt-
short period of time to prevent serious over-
heating or damage to the generator set if the
minals, ahead of the circuit interrupter, and the re-
conditions can not be readily attained.
mainder of the apparatus as shown in the applicable
figure of paragraph 205. 1. 10 of MIL-HDBK-705.
3. At the end of the two minute interval
record all instrument readings including thermal in-
(a) Operate the generator at rated speed
and voltage, and at no load since the generator is
4. Reduce the load to approximately
equipped with an electrical-type underfrequency pro-
rated kilowatt load and allow the generator set to
tective device, this test shall be repeated with the
cool for approximately 5 to 10 minutes.
voltage adjusted to maximum and minimum voltage
for the specified voltage operating range.
5. Repeat steps (1) through (4) above
until three valid sets of maximum power data are ob-
(b) With the circuit interrupter closed,
slowly decrease the operating speed until the protec-
(d) For Class I Sets:
tive device causes the circuit interrupter to open. The
electric governor must be deactivated to sufficiently
Repeat step (c) above but do not ad-
decrease the speed on Class 1, precise sets.
just the freqqency.
(c) Record the speed.
2. Repeat steps (a) through (c) above as
applicable for all other voltage connections and fre-
(d) Repeat the test while rapidly decreas-
ing the operating speed. Again record the speed.
(e) Operate the generator at rated speed
and voltage, and at rated load.
1. Average the three valid power read-
ings. This average is the observed maximum power
(f) Repeat steps (b), (c) and (d).
2. Correct the observed maximum
power value to standard conditions using the procedure
d. Results. Compare the test value of frequency
in MIL-HDBK-705, Method 220.2, paragraph 220.2.3.
This is the corrected maximum power value.
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