TM 5-685/NAVFAC MO-912
installed between the compressor and the storage
tanks will prevent depletion of compressed air
should the plant system fail. Air starter motors are
suitable on diesel engine driven generators ranging
from 85 kW up to the largest diesel engine genera-
3-8. Governor/speed control.
A diesel engine used in an auxiliary generator must
have a governor to regulate and control engine
speed. Since an automatic governor functions only
with a change in speed, constant engine speed may
not be totally possible and "hunting" can occur due
to over-correction. The governor's sensitivity is de-
termined by the minimum change in speed of the
prime mover which will cause a change in governor
setting; its speed regulation is the difference in gen-
erator speeds at full-load and no-load divided by the
arithmetical mean of the two speeds. Refer to the
glossary for descriptions of governor characteristics.
a. Usually, this ratio is stated as a percentage,
with synchronous speed considered rather than
m e a n speed. For example, a generator with a syn-
chronous speed of 1,200 rpm, operated at 1,190 rpm
when fully loaded and 1,220 rpm with no load, has
2.5 percent speed regulation.
b. The governor must be capable of speed adjust-
PER CENT, LOAD
ment so the proper governed speed can be selected.
In most governors, this adjustment is made by
SPEED VS LOAD-MECHANICAL GOVERNOR
changing the tension of the main governor spring.
6% DROOP-RATED SPEED AT
The governor should also be adjustable for speed
8 6% DROOP- RATED SPEED AT 0% L O A D
regulation so the droop of the speed-load curve can
DROOP INTERMEDIATE SETTINGS
be altered as required to suit operating conditions.
4% DROOP-RATED SPEED AT 50% LOAD
Determine the curve by observing the generator
speed or frequency at various loads and plotting
Figure 3-11. Chart of speed droop characteristics.
them as abscissa against the loads (from no-load to
full-load) as ordinates. The curve droops at the full-
the speed. Speed droop is usually adjusted by
load end (hence, the expression "speed droop" of the
lengthening or shortening the governor operating
levers, changing the ratio between governor move-
c. An example of speed droop characteristics is
ment and throttle or gate movement.
shown in figure 3-11. The characteristics are for a
e. Alternating Current (AC) Generators. Gover-
mechanical governor but the same principles can be
nors of prime movers driving AC generators which
used for other engine/governor applications. The
operate in parallel with other generators must have
chart is based on a six percent speed droop governor
enough speed regulation or speed droop to prevent
on an engine running at rated speed at no load.
surging of the load from one generator to another.
When full load is applied, engine speed drops to 94
Ordinarily, three to five percent speed regulation is
percent (94%) of rated value (line B). The engine
adequate. Some governors have antisurging devices
can be brought to rated speed at full load by reset-
to damp out the surges. Speed regulation should be
ting the governor (line A). However, with the load
increased if the surges continue. Speed regulation of
removed, engine speed would increase beyond its
rated limit. Intermediate speed settings are shown
governors controlling AC generators affects the fre-
quency and the load division between generators
by lines C and D. Line E shows speed droop at 50
but has almost no effect upon voltage.
percent (50%) load.
f. Direct C urrent (DC) Generators. Regulation of
d. Speed droop can be determined quickly by
DC generators affects voltage regulation and the
loading the generator to full-load, observing the
division of load between generators. In general, the
speed, unloading the generator, and again observing