NOSE HOLES
STAGE
Figure 3-26. Turbine vane cooling air f l o w .
(4) Scavenging. Scavenging is accomplished by
b. Fuel. As the engine is accelerated by the
a multi-element lubrication and scavenge pump.
starter, fuel is supplied when a specified rotational
One element is used for pumping. The other ele-
speed is attained. When this speed is attained, the
ments are used for forward and aft scavenging of
compressor and engine-driven fuel pump will de-
the B-sump and C-sump. Oil in the A-sump drains
liver sufficient air and fuel, respectively to the com-
by gravity into the accessory gearbox.
bustion chamber to sustain satisfactory combustion.
(5) Venting. S ome lubrication systems are
c. Ignition system. An ignition system, consisting
vented. To maintain high differential pressure
of an ignition exciter, igniter plug lead assemblies,
across the carbon seals to prevent oil leakage, a
and igniter plugs, is required. Fuel ignition is en-
high sump vent capacity is required. The A and C
sured by one or two igniter plugs connected to the
sumps vent through the engine output shaft and
exciter by the separate igniter leads. The plugs are
vent collector to ambient. The B-sump vents to the
located in the combustion chamber. Each plug con-
turbine exhaust gas stream.
sists of center and outer electrodes with a semicon-
ductor surface coating at the tip between the
3-19. Starting system.
two
electrodes. The semiconductor ma.terial is used as a
Gas turbine engine starters must be capable of ro-
shunt to aid in ionizing the air gap between the two
tating an engine up to a speed-at which it becomes
electrodes so that the plugs will fire. An air shroud
self-sustaining. The starter must provide sufficient
covers the end of the plug immersed in the air
torque to accelerate the engine from a standstill to a
stream for cooling.
self-sustaining speed within a specified time. Al-
d. Specialized system. Starting systems are
though it must continue to assist the engine in ac-
highly specialized and are usually applicable to a
celerating up to a predetermined speed.
given installation or site. Refer to supplier’s on-site
a. Electric motor. An electric starter motor is usu-
technical literature for details.
ally used for a gas turbine engine in service as an
auxiliary generator prime mover. The starter ro-
3-20. Governor/speed control.
tates the engine compressor shaft via the gear train
a. Engine operation. The engine is started by an
in the accessory gearbox. In most installations the
external power source. Once the engine reaches idle
starter can be energized either automatically or
speed, it is self-sustaining. All it needs is adequate
manually.
supplies of air and fuel. Combustion gas drives the
3-35
TM 5-685/NAVFAC MO-912
Figure 3-27. Lubrication system for gas turbine.
3-36
TM 5-685/NAVFAC MO-912
turbine which is mounted on a common shaft with
nance program that is built around records and
the compressor. The compressor draws in the air for
observation. The program is described in the manu-
combustion and also drives the gearbox gear train.
facturer’s literature furnished with each engine. It
About two-thirds of the power derived from combus-
includes appropriate analysis of these records.
tion is required to sustain combustion. The remain-
b. Record keeping. Engine log sheets are an im-
ing power is available for work purposes and drives
portant part of record keeping. The sheets must be
the output shaft.
developed to suit individual applications (i.e., auxil-
b. Speed signal. An engine speed signal, gener-
iary use) and related instrumentation.
ated by magnetic pickups (speed transducers) in the
c. Log sheet data. Log sheets should include en-gearbox, provides electrical signals that are propor-
gine starts and stops, fuel and lubrication oil con-
tional to engine speed. The signal causes a dc volt-
sumption, and a record of the following:
age to be generated.
(1) Hours since last oil change.
c. Thermocouples. Thermocouples sense the tur-
(2) Hours since first put in service or last over-
bine discharge/inlet total temperature. The electri-
haul.
cal temperature sensing signal is an average of the
(3) Total ho urs on engine.
operating temperature profile.
d. Oil analysis program. Use of a Spectrometric
d. Pressure sensing. Sensing of compressor dis-
Oil Analysis Program is recommended to determine
charge static pressure and turbine discharge pres-
the internal condition of the engine’s oil-wetted
sure is also required for engine speed control. These
(wear metal) components, such as bearings, gears,
pressures are combined to produce an electrical sig-
and lubrication pump.
nal equal to pressure ratio.
(1) The program should be used as a supple-
e. Computer. The three signals (speed, tempera-ment to the regular maintenance procedure of chip
ture, and pressure ratio) are summed in an
detection and filter inspection. Normal wear causes
acceleration/deceleration computer. Computer out-
microscopic metal particles (smaller than one mi-
put functions with a governor to meter fuel required
cron) to mix with the lubricating oil and remain in
for engine operation. If required, a signal derived
suspension. Samples of oil taken from the engine
from a tachometer can be used to determine a rate-
after a shutdown will contain varying amounts of
of-change feedback signal.
wear-metal particles.
(2) Oil samples should be removed from the
3-21. Compressor.
engine at the time intervals specified by the engine
The function of the compressor is to raise the pres-
manufacturer. A sample should always be taken
sure and reduce the volume of the air as it pumps
from the same location on the engine (this may vary
it through the engine. An axial flow or centrifugal
from each engine). Refer to manufacturer’s litera-
flow compressor is used. Most engines use a multi-
ture. See appendix C paragraph C-le(2).
stage, axial flow compressor such as described
(a) Metal content. Evaluation of the oil’s herein. The axial flow consists of two major sub-wear-metal content is very important. The quantity
assemblies: the rotor assembly and the stator as-
of wear-metal in the sample as well as type (iron or
sembly. Axial flow compressor efficiency is better
steel, silver, chromium, nickel, etc.) must be evalu-
than centrifugal flow compressor efficiency. Cen-
ated and recorded.
trifugal flow compressors were first used in early
(b) failure forecast. Evaluation records are
design gas turbine engines. The main component is
intended as an aid in forecasting what components
an impeller which is mounted on a common shaft
are in danger of failing. Contamination of the oil
with the turbine. These compressors are generally
sample must be prevented to avoid false indication
used with smaller engines and have a fairly low
of engine internal conditions.
,
pressure ratio. The design has lower efficiency than
e. Industrial practices. Use recognized industrial
the axial-flow design but is less expensive to manu-
practices as the general guide for engine servicing.
facture.
Service information is provided in manufacturer’s
literature and appendixes B through G.
3-22. Gas turbine service practices.
f. Reference Literature. The engine user should re-
a. Maintenance program. Service practices for fer to manufacturer’s literature for specific informa-gas turbine engines consist of a complete mainte-
tion on individual units.
3-37
TM 5-685/NAVFAC MO-912
GENERATORS AND EXCITERS
4-1. Electrical energy.
needed to direct the flow of current in one direction.
Mechanical energy provided by a prime mover is
The generator rotating commutator provides the
converted into electrical energy by the generator
rectifying action.
(see fig 4-l). The prime mover rotates the generator
4-4. AC generators.
rotor causing magnetic lines of force to be cut by
electrical conductors. Electrical energy is thereby
a. AC generators are considered either brush or
produced by electromagnetic induction. The ratio of
brushless, based on the method used to transfer DC
exciting current to the generator field. In addition,
output energy generated by input energy is ex-
AC generators are classified as salient-pole or
pressed as a percentage and always shows a loss in
nonsalient-pole depending on the configuration of
efficiency.
the field poles. Projecting field poles are salient-pole
4-2. Generator operation.
units and turbo-type (slotted) field poles are
nonsalient-pole units. Typical AC generator
a. A generator consists of a number of conducting
armatures are shown in figures 4-3 and 4-4.
coils and a magnetic field. The coils are called the
b. Damper windings on the rotor stabilize the
armature. Relative motion between the coils and
speed of the AC generator to reduce hinting under
magnetic field induces voltage in the coils. This
changing loads. If the speed tends to increase,
action is called electromotive force (emf). A sche-
induction-generator action occurs in the damper
matic for a typical generator system is shown in
windings. This action places a load on the rotor,
figure 4-2.
tending to slow the machine down. If the speed
b. An alternating current (AC) generator needs a
tends to decrease, induction-motor action occurs in
separate direct current (DC) source to feed the mag-
the damper winding, tending to speed the machine
netic field. The required DC is provided by an exter-
up. The windings are copper bars located in the
nal source called an exciter. Usually, the exciter is a
faces of the rotor pole pieces. Mounted parallel to
small DC generator that is driven by the generator
the rotor axis, the bars are connected at each end by
rotor. The exciter may be mounted on the rotor shaft
a copper ring.
or rotated by belt-drive. Some generating systems
c. AC generators that operate at a speed that is
use a static, solid-state exciter to provide DC.
exactly proportional to the frequency of the output
c. A voltage regulator controls the induced volt-
voltage are synchronous generators. Synchronous
age by regulating the strength of the electromag-
generators are usually called alternators.
netic field established by the exciter. Frequency is
controlled by the speed at which the prime mover
4-5. Alternator types.
rotates the rotor.
Alternators are single-phase or polyphase. Varia-
tions include three-phase alternators used as
4-3. Types of generators.
single-phase units by insulating and not using one
Depending on the type of generating equipment em-
phase lead. Since the lead is unused, it is not
ployed, the electrical energy produced is either di-
brought out to a terminal. The kilowatt rating is
rect current ( D C ) or alternating current (AC).
reduced from that of the three-phase unit as limited
a. AC generators. AC generators are classified as by the amount of current carried by a coil. An alter-single-phase or polyphase. A single-phase generator
nator designed only for single-phase operation usu-
is usually limited to 25 kW or less and generates AC
ally does not have coils in all of the armature slots
power at a specific utilization voltage. Polyphase
because end coils contribute little to the output volt-
generators produce two or more alternating volt-
age and increase the coil impedance in the same
ages (usually two, three, or six phases).
proportion as any other coil.
b. DC generators. DC generators are classified as (a) Single-phase alternators are usually used in either shunt, series, or compound-wound. Most DC smaller systems (limited to 25kW or less) and pro-generators are the compound-wound type. Shunt duce AC power at utilization voltages.
generators are usually used as battery chargers and (1) Terminal voltage is usually 120 volts. The as exciters for AC generators. Series generators are electric load is connected across the terminals with sometimes used for street lights. The emf induced in protective fuses. One voltmeter and one ammeter a DC generator coil is alternating. Rectification is measure the output in volts and amperes, respec-TM 5-685/NAVFAC MO-9122
Figure 4-l. Typical alternating current generator.
EXCITER
GENERATOR
WINDINGS
F E E D B A C K =
-
-
-
- - - - - - - - - - - - - - - - - - - - - - - - -
I
Figure 4-2. Brush-type excitation system, schematic.
TM 5-685/NAVFAC MO-912
DAMPER
SLIP
Figure 4-3. Brush-type AC generator field and rotor:
Figure 4-4. AC generator field and rotor with brushless-type excitation system.
tively The two-wire alternator has two power termi-
and 240 volts between the two ends. The load is
nals, one for each end of the armature coil (see fig
connected between the two outside wires or between
4-5).
either outside wire and neutral, depending upon the
(2) The three-wire, single-phase alternator has
voltage required by the load. Assuming alternator
three power terminals; one from each end of the
voltage to be 120/240 volts, load 1,0 and load 2,0
armature coil and one from the midpoint (neutral,
would consist of 120-volt lamps and 120-volt single-
see fig 4-6). Terminal voltage is usually 120 volts
phase power equipment. Load 1,2 would consist of
from the midpoint to either end of the armature coil
240-volt power equipment. Two voltmeters and two
TM 5-685/NAVFAC MO-912
TO E X C I T E R
AMMETER
L O A D
Figure 4-5. Two-wire, single-phase alternator.
1
I
l
TO
0
LOAD
AMMETER
Figure 4-6. Three-wire, single-phase alternator.
ammeters (or equivalent) are required to determine
of single-phase windings spaced 120 electrical de-
the load in kilovoltamperes (kVA).
grees apart around the armature. One electrical
(b) Polyphase alternators are two, three, or six
degree is equivalent to one degree of arc in a two-
phases. Two-phase power is used in only a few lo-
pole machine, 0.50 degree of arc in a four-pole ma-
calities. Six-phase is primarily used for operation of
chine, 0.33 degree of arc in a six-pole machine, and
rotary converters or large rectifiers. Three-phase
so on. The three single-phase windings are con-
alternators are the most widely used for power pro-
nected in series to form the delta connection, and
duction. Polyphase alternators have capacities from
the terminals are connected to the junction point of
3 kW to 250,000 kW and voltage from 110 V to
each pair of armature coils (see fig 4-7). The total
13,800 V. Two general types of three-phases alterna-
current in a delta-connected circuit is always equal
tor windings are the delta winding used in three-
to the vector sum of currents in two-phase wind-
wire, three-phase alternators, and the star or wye
ings. The instantaneous current flows out to the
winding used in four-wire, three-phase types.
load through two windings and returns from the
Three-wire, three-phase alternators have three sets
load through the third winding. Since the coils are
TM 5-685/NAVFAC MO-912
SINGLE PHASE
AMMETERS LOAD
.
.
I
VM AM. L O A D
GENERATOR
EXCITER
ROTOR
PHASE LOAD
VM AM L O A D
Figure 7.92 Three-wire, three-phase alternator.
similar physically and electrically, equal voltages
(d) The four-wire, three-phase alternator (see fig
are generated and applied to the terminals. Due to
4-8) has three sets of armature coils spaced 120
spacing of the coils about the armature, the maxi-
electrical degrees apart about the armature, the
mum voltage between the pairs of terminals does
same as the three-wire, three-phase alternator. One
not occur simultaneously. The characteristics of
end of each of the three coils is connected to a
three- wire, three-phase (or delta) alternators are:
common terminal (neutral). The other end of each
(1) The amount of current through the alterna-
coil is connected to separate terminals (phase ter-
tor terminals is the algebraic sum of current
minals). Thus, the four-wire alternator has four
through the alternator coils.
terminals which connect to the three-phase con-
(2) The currents are not equal in magnitude or
ductors and the neutral of the power-plant bus.
time.
When each end of each coil is brought out to sepa-
(3) Connection between coils can be made ei-
rate terminals, the connections between coils are
ther inside or outside the generator.
made outside of the alternator, enabling installation
(c) In a 60-Hertz machine, each coil experiences
of a more comprehensive protective relaying sys-
maximum instantaneous voltage, first positive and
tem.
then negative, 120 times each second. Disregarding
(e) The four-wire, three-phase alternator can be
voltage direction, the maximum instantaneous volt-
connected to a transformer instead of the power-
ages occur on successive coils 0.003 seconds apart.
plant bus by using a wye-wye transformation. Ir-
Due to time differences between the voltages and
regular (double or triple) harmonics, which may be
resulting currents, the amount of current through
produced, can be suppressed by using a core-type
the alternator terminals and the amount through
transformer. A third or tertiary winding with a delta
the alternator coils are not equal in magnitude or
connection may also be used as a suppressor. A
time. The current through the alternator is 73 per-
wye-delta transformer may be used if the power
cent greater than through the coils. Coil and termi-
plant bus is three wire and the alternator is four
nal voltages are the same magnitude. Three voltme-
wire wye connected.
ters and three ammeters (or equivalent) are
(f) Four-wire three-phase, dual voltage and
required to measure the load on the alternator. The
frequency alternators are also used. These are sup-
average value of the three currents times the aver-
plied in sizes from 15 to 1500 kW, 127-220 volts,
age value of the three voltages plus 73 percent gives
three-phase, 60 Hertz, or 230-400 volts, three-
a close approximation of the alternator load in
phase, 50 Hertz. Dual stator coils are used on each
kilovolt-amperes. Two single-phase or one two-
phase. Coil ends are brought out to a terminal board
\_
element polyphase kilowatt-hour meter is required
for making connections. Voltage and frequency com-
to measure the alternator output in kilowatt-hours.
binations are shown in figure 4-9.
4-5
TM 5-685/NAVFAC MO-912
VOLTMETERS
PHASE
LOAD
AMMETERS LINE TO
NEUTRAL
TO LINE
G E N E R A T O R
EXCITER ROTOR
LOAD
1.2
3
2 VM. AM. LOAD
LOAD
Q
2.3
. ,
I
l
VM.
Figure 4-8. Four-wire, three-phase alternator.
VOLTAGE AN0 FREQUENCY COM
ATIONS
ENGINE-GENERATOR SETS USED
ERSEAS
SERIES COIL CONNECTION
PARALLEL COIL CONNECTION
Figure 4-9. Dual voltage and frequency.
TM 5-685/NAVFAC MO-912
(g) Most parts of the world have standard-
end of each coil is connected to separate terminals.
ized on either 50 or 60 Hertz alternating current
Conductors attached to the four terminals carry the
power. Sixty Hertz power is commonly used in the
current to the system’s switchgear and on to the
United States. Fifty Hertz power is used in many
load.
countries outside the United States. The ratio be-
d. Collector slip rings. Slip rings are usually
tween the 60-50 Hertz frequencies is 6:5. Electrical
made of nonferrous metal (brass, bronze or copper);
energy received at one frequency can be converted
iron or steel is sometimes used. Slip rings usually
to a different frequency by using a frequency
do not require much servicing. The wearing of
changer. If a large power requirement exists, it may
grooves or ridges in the slip rings is retarded by
be more economical to use a special alternator to
designing the machine with limited endplay and by
produce power at the desired frequency The appli-
staggering the brushes. Surfaces of the slip rings
cable equation is:
should be bright and smooth, polishing can be per-