Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SHUTTLE/ELECPOWR.TXT

Text File | 1993-02-06 | 68KB | 1,176 lines

"6_2_3_13_7_2.TXT" (2926 bytes) was created on 12-13-88

ELECTRICAL POWER SYSTEM

The EPS consists of three subsystems: power reactant storage and
distribution, fuel cell power plants (electrical power generation) and
electrical power distribution and control.

The PRSD subsystem stores the reactants (cryogenic hydrogen and
oxygen) and supplies them to the three fuel cell power plants, which
generate all the electrical power for the vehicle during all mission
phases.  In addition, cryogenic oxygen is supplied to the
environmental control and life support system for crew cabin
pressurization.  The hydrogen and oxygen are stored in their
respective storage tanks at cryogenic temperatures and supercritical
pressures.  The storage temperature of liquid oxygen is minus 285 F
and minus 420 F for liquid hydrogen.

The three fuel cell power plants, through a chemical reaction,
generate all of the 28-volt direct-current electrical power for the
vehicle from launch through landing rollout.  Before launch,
electrical power is provided by ground power supplies and the onboard
fuel cell power plants until T minus three minutes and 30 seconds.
Each fuel cell power plant consists of a power section, where the
chemical reaction occurs, and a compact accessory section attached to
the power section, which controls and monitors the power section's
performance.  The three fuel cell power plants are individually
coupled to the reactant (hydrogen and oxygen) distribution subsystem,
the heat rejection subsystem, the potable water storage subsystem and
the EPDC subsystem.  The fuel cell power plants generate heat and
water as by-products of electrical power generation.  The excess heat
is directed to fuel cell heat exchangers, where the excess heat is
rejected to Freon coolant loops.  The water is directed to the potable
water storage subsystem.

The EPDC subsystem distributes the 28 volts dc generated by each of
the three fuel cell power plants to a three-bus system that
distributes dc power to the forward, mid-, and aft sections of the
orbiter for equipment in those areas.  The three main dc buses-MNA,
MNB and MNC-are the prime sources of power for the vehicle's dc loads.
Each of the three dc main buses supplies power to three solid-state
(static), single-phase inverters, which constitute one three-phase
alternating-current bus; thus, the nine inverters convert dc power to
115-volt, 400-hertz ac power for distribution to three ac buses-AC1,
AC2 and AC3-for the vehicle's ac loads.

The EPDC subsystem controls and distributes electrical power (ac and
dc) to the orbiter subsystems, the solid rocket boosters, the external
tank and payloads.  Power is controlled and distributed by assemblies.
Each assembly is a housing for electrical components, such as remote
switching devices, buses, resistors, diodes and fuses.  Each assembly
usually contains a power bus or buses and remote switching devices for
distributing bus power to subsystems located in its area.


"6_2_3_13_7_3.TXT" (14057 bytes) was created on 12-13-88

POWER REACTANT STORAGE AND DISTRIBUTION.

Cryogenic hydrogen and oxygen are stored in a supercritical condition
in double-walled, thermally insulated spherical tanks with a vacuum
annulus between the inner pressure vessel and outer shell of the tank.
Each tank has heaters to add energy to the reactants during depletion
to control pressure.  Each tank is capable of measuring quantity
remaining.

The tanks are grouped in sets consisting of one hydrogen and one
oxygen tank.  The number of tank sets installed depends on the
specific mission requirement.  Up to five tank sets can be installed.
The five tank sets are all installed in the midfuselage under the
payload bay liner.

The oxygen tanks are identical and consist of inner pressure vessels
of Inconel 718 and outer shells of aluminum 2219.  The inner vessel is
33.43 inches in diameter and the outer shell is 36.8 inches in
diameter.  Each tank has a volume of 11.2 cubic feet and stores 781
pounds of oxygen.  The dry weight of each tank is 201 pounds.  The
initial temperature of the stored oxygen is minus 285 F.  Maximum fill
time is 45 minutes.

The hydrogen tanks also are identical.  Both the inner pressure vessel
and the outer shell are constructed of aluminum 2219.  The inner
vessel's diameter is 41.51 inches and the outer shell's is 45.5
inches.  The volume of each tank is 21.39 cubic feet, and each stores
92 pounds of hydrogen.  Each tank weighs 216 pounds dry.  The initial
storage temperature is minus 420 F.  Maximum fill time is 45 minutes.

The inner pressure vessels are kept supercold by minimizing
conductive, convective and radiant heat transfer.  Twelve
low-conductive supports suspend the inner vessel within the outer
shell.  Radiant heat transfer is reduced by a shield between the inner
vessel and outer shell (hydrogen tanks only), and convective heat
transfer is minimized by maintaining a vacuum between the vessel and
shell.  A vacuum ion pump maintains the required vacuum level and is
also used as a vacuum gauge to determine the vacuum's integrity.

Each hydrogen tank has one heater probe with two elements, while each
oxygen tank has two heater probes with two elements on each probe.  As
the reactants are depleted, the heaters add heat energy to maintain a
constant pressure in the tanks.  The heaters operate in manual and
automatic modes.  The oxygen tank and hydrogen tank switches (auto,
on, off) for tanks 1, 2 and 3 are located on panel R1; switches for
the oxygen and hydrogen tank 4 heaters are on panel A11.  When a
heater switch is positioned to auto, the heater is controlled by a
tank heater controller.  Each heater controller receives a signal from
a tank pressure sensor.  If pressure in a tank is equal to or below a
specific pressure and the controller sends a low pressure signal to
the heater logic and the heater is powered on, the pressure bands are
200 to 206 psia; hydrogen tanks 3 and 4, 217 to 223 psia; oxygen tanks
1 and 2, 805 to 817 psia; and oxygen tanks 3 and 4, 834 to 846 psia.
When the pressure of hydrogen tanks 1 and 2 is 220 to 226 psia,
hydrogen tanks 3 and 4 is 237 to 243 psia, oxygen tanks 1 and 2 is 840
to 852 psia, and oxygen tanks 3 and 4 is 869 to 881 psia, the
respective controller sends a high pressure signal to the heater
logic, and the heater involved is turned off.

Dual-mode heater operation is available for pairs of oxygen and
hydrogen tanks.  If the heaters of both tanks 1 and 2 or tanks 3 and 4
are placed in the automatic mode, the tank heater logic is
interconnected.  In this case, the heater controllers of both tanks
must send a low pressure signal to the heater logic before the heaters
will turn on.  Once the heaters are on, a high pressure signal from
either tank will turn off the heaters in both tanks.

In the manual mode, the flight crew controls the heaters by using the
on/off positions for each heater switch on panel R1 or A11.  High or
low pressure in each tank is shown on the CRT display or the gauges on
panel O2.  The specific tank is selected by setting the rotary switch
on panel O2.

Before lift-off, the oxygen and hydrogen tank 1 and 2 heater switches
are set on auto.  After SRB separation, all the hydrogen and oxygen
tank 1 and 2 heater switches are positioned to auto, and the tank 3
and 4 heaters remain off.  On orbit, the tank 3 and 4 heater switches
are positioned to auto.  Because the tank 3 and 4 heater controller
pressure limits are higher than those of tanks 1 and 2, tanks 3 and 4
supply the reactants to the fuel cells.  For entry, the tank 3 and 4
heater switches are set to off, and tanks 1 and 2 supply the reactants
to the fuel cells.

The cryo oxygen htr assy temp meter on panel O2, in conjunction with
the rotary switch tk1 1-2, tk2 1-2, tk3 1-2, tk4 1-2, selects one of
the two heaters in each tank and permits the temperature of the heater
element to be displayed.  The range of the display is from minus 425 F
to plus 475 F.  The temperature sensor in each heater also is
hard-wired directly to the yellow O 2 heater temp caution and warning
light on panel F7.  This light is illuminated if the temperature is at
or above 349 F.  A signal also is sent to the computers, where
software checks the limit; and if the temperature is at or above 349
F, the backup C/W alarm light on panel F7 is illuminated.  This signal
also is transmitted to the CRT and telemetry.

Two current level detectors are built into the circuit of each oxygen
tank heater to interrupt power in case of electrical shorts.  The
second detector is redundant.  Each detector is divided into A and B
detectors.  One monitors the heater A current and the other monitors
the heater B current.  The detectors are powered by circuit breakers
on panels O14, O15, O16 and ML86B and are identified as cryo O2 htr
tk1, 2, 3, 4 snsr 1, 2.  The detectors monitor the current in and out
of a heater.  If the current difference is 0.9 amp or greater for 1.5
milliseconds, a trip signal is sent to the heater logic to remove
power from the heaters regardless of the heater switch position.  If
one element of a heater causes a ''trip-out,'' power to both elements
is removed.  The O 2 tk 1, 2, 3 heaters reset/test switches on panel
R1 and the O 2 tk 4/5 reset/test switch on panel A11 can be used to
reapply power to that heater by positioning them to reset.  The test
position will cause a 1.4-amp delta current to flow through all four
detectors of a specified oxygen tank, causing them to trip out.
During on-orbit operations, the flight crew will be alerted to a
current level detector trip-out by an SM alert on panel F7 and on the
CRT.

Each oxygen and hydrogen tank has a quantity sensor powered by a
circuit breaker.  These are identified on panel O13 as cryo qty O 2
(or H2) tk1 and tk2 and on panel ML86B as cryo qty O2 (or H2) tk3 and
tk4.  Data from the quantity sensors is sent to panel O2, where the
tk1, tk2, tk3, tk4 rotary switch is used to select the tank for
display on the cryo O2 (or H2) qty meters.  The range of the meters is
zero to 100 percent.  The data is also sent to the CRT.

There are two tank pressure sensors for each oxygen and hydrogen tank.
One sensor transmits its data to the tank heater controllers and to
the yellow O2 or H2 press C/W light on panel F7, which is illuminated
if oxygen tank pressure is below 540 psia or above 985 psia or if
hydrogen tank pressure is below 153 psia or above 293.8 psia.  The
signal also is transmitted to the CRT and to panel O2, where the tk1,
tk2, tk3, tk4 rotary switch is used to select the tank for display on
the cryo O 2 (or H 2 ) press meter.  The data also goes to the SM
alert, backup C/W alarm light on panel F7 and to telemetry.  The range
of the oxygen meter is zero to 1,200 psia.  The hydrogen meter's range
is zero to 400 psia.

The oxygen and hydrogen fluid temperature sensors transmit data to the
CRT and telemetry.

Each tank set (one hydrogen and one oxygen tank) has a hydrogen/oxygen
control box that contains the electrical logic for the hydrogen and
oxygen heaters and controllers.  The control box is located on cold
plates in the midbody under the payload bay envelope.

The reactants from the tanks flow through two relief valve/filter
package modules and valve modules and then to the fuel cells through a
common manifold.  Oxygen is supplied to the manifold from the tank at
a pressure of 815 to 881 psia, and hydrogen is supplied at a pressure
of 200 to 243 psia.  The pressure of the reactants will be essentially
the same at the fuel cell interface as it is in the tanks since only a
small decrease in pressure occurs in the manifolds.

The relief valve/filter package module contains the tank relief valve
and a 12-micron filter.  The filter removes contaminants that could
affect the performance of components within the power reactant storage
and distribution subsystem and fuel cells.  The valve relieves
excessive pressure that builds up in the tank, and a manifold valve
relieves pressure in the manifold lines.  The oxygen tank relief valve
relieves at 1,005 psia, and the hydrogen tank relief valve relieves at
310 psia.

The reactants flow from the relief valve/filter packages through four
reactant valve modules: two hydrogen (hydrogen valve modules 1 and 2)
and two oxygen (oxygen modules 1 and 2).  Each valve module contains a
check valve for each cryogenic tank line to prevent the reactants from
flowing from one tank to another tank in the event of a tank leak.
This prevents a total loss of reactants.  The oxygen valve modules
also contain the environmental control and life support system
atmosphere pressure control system 1 and 2 oxygen supply.  Each module
also contains a manifold valve and fuel cell reactant valves.

Each fuel cell reactant valve consists of two valves-one for hydrogen
and one for oxygen.  The valves are controlled by the fuel cell 1, 2,
3 reac open/close switches on panel R1.  When the switch is positioned
to open, the hydrogen and oxygen reactant valves for that fuel cell
are opened, and reactants are allowed to flow from the manifold into
the fuel cell.  When the switch is positioned to close, the hydrogen
and oxygen reactant valves for that fuel cell are closed, isolating
the reactants from the fuel cell and rendering that fuel cell
inoperative.  Each fuel cell reac switch on panel R1 also has a
talkback indicator.  The corresponding talkback indicator indicates op
when both valves are open and cl when either valve is closed.

Because it is critical to have reactants available to the fuel cells,
the red fuel cell reac light on panel F7 is illuminated when any fuel
cell reactant valve is closed, a caution/warning tone is sounded, and
the computers sense the closed valve, which causes the backup C/W
alarm light on panel F7 to be illuminated, an SM alert to occur, and
the data to be displayed on the CRT.  This alerts the flight crew that
the fuel cell will be inoperative within approximately 20 seconds for
a hydrogen valve closure and 130 seconds for an oxygen valve closure.

Each H2 and O2 manifold 1, 2 open/close switch on panel R1 controls
the respective hydrogen and oxygen manifold valve.  When the two
hydrogen and two oxygen manifold valves are in the close position,
fuel cell 1 receives reactants from cryogenic tank set 1, fuel cell 2
receives reactants from cryogenic tank set 2, and fuel cell 3 receives
reactants from cryogenic tank sets 3 and 4.  ECLSS atmosphere pressure
control system 1 receives oxygen from oxygen tank 1, and system 2
receives oxygen from oxygen tank 2.  When each H 2 and O 2 manifold 1,
2 open/close switch is positioned to close, the respective talkback
indicator associated with each switch indicates cl .

With the H 2 and/or O2 manifold 1 open/close switch positioned to
open, cryogenic tanks 1 and 2 supply hydrogen to fuel cells 1 and 3,
and oxygen cryogenic tanks 1 and 3 supply oxygen to fuel cells 1 and 3
as well as to ECLSS atmosphere pressure control system 1.  The
talkback indicator associated with each switch indicates op .

When the H 2 and/or O2 manifold 2 open/close switch is positioned to
open, hydrogen cryogenic tanks 2 and 3/4/5 supply hydrogen to fuel
cells 2 and 3, and oxygen cryogenic tanks 2 and 3/4/5 supply oxygen to
fuel cells 2 and 3 as well as to ECLSS atmosphere pressure control
system 2.  The talkback indicator associated with each switch
indicates op.

With the H 2 and O2 manifold 1 and 2 switches positioned to op, all
hydrogen cryogenic tanks are supplying hydrogen to all three fuel
cells, and all oxygen cryogenic tanks are supplying oxygen to all
three fuel cells as well as to ECLSS atmosphere pressure control
systems 1 and 2.

The manifold relief valves are a built-in safety device in the event a
manifold valve and fuel cell reactant valves are closed because of a
malfunction.  The reactants trapped in the manifold lines would be
warmed up by the internal heat of the orbiter and overpressurize.  The
manifold relief valve will open at 290 psi for hydrogen and 975 psi
for oxygen to relieve pressure and allow the trapped reactants to flow
back to their tanks.

Two pressure sensors located in the respective hydrogen and oxygen
valve modules transmit data to the CRT.  This data is also sent to the
systems management computer, where its lower limit is checked; and if
the respective hydrogen and oxygen manifold pressures are below 150
psia and 200 psia, respectively, an SM alert will occur.

If cryogenic tank set 5 is added to an orbiter, the displays and
controls associated with controlling the tank set will be added to
panel A15.

During prelaunch operations, the onboard fuel cell reactants (oxygen
and hydrogen) are supplied by ground support equipment to assure a
full load of onboard reactants before lift-off.  At T minus two
minutes 35 seconds, the GSE filling operation is terminated.  The GSE
supply pressure is 300 to 320 psia for hydrogen and 1,000 to 1,020
psia for oxygen, which is higher than the onboard PRSD pressures.  The
GSE supply valves close automatically to transfer to onboard
reactants.


"6_2_3_13_7_4.TXT" (24717 bytes) was created on 12-13-88

FUEL CELL POWER PLANTS.

Each of the three fuel cell power plants is reusable and restartable.
The fuel cells are located under the payload bay area in the forward
portion of the orbiter's midfuselage.

The three fuel cells operate as independent electrical power sources,
each supplying its own isolated, simultaneously operating 28-volt dc
bus.  The fuel cell consists of a power section, where the chemical
reaction occurs, and an accessory section that controls and monitors
the power section's performance.  The power section, where hydrogen
and oxygen are transformed into electrical power, water and heat,
consists of 96 cells contained in three substacks.  Manifolds run the
length of these substacks and distribute hydrogen, oxygen and coolant
to the cells.  The cells contain electrolyte consisting of potassium
hydroxide and water, an oxygen electrode (cathode) and a hydrogen
electrode (anode).

The accessory section monitors the reactant flow, removes waste heat
and water from the chemical reaction and controls the temperature of
the stack.  The accessory section consists of the hydrogen and oxygen
flow system, the coolant loop and the electrical control unit.

Oxygen is routed to the fuel cell's oxygen electrode, where it reacts
with the water and returning electrons to produce hydroxyl ions.  The
hydroxyl ions then migrate to the hydrogen electrode, where they enter
into the hydrogen reaction.  Hydrogen is routed to the fuel cell's
hydrogen electrode, where it reacts with the hydroxyl ions from the
electrolyte.  This electrochemical reaction produces electrons
(electrical power), water and heat.  The electrons are routed through
the orbiter's EPDC subsystem to perform electrical work.  The oxygen
and hydrogen are reacted (consumed) in proportion to the orbiter's
electrical power demand.

Excess water vapor is removed by an internal circulating hydrogen
system.  Hydrogen and water vapor from the reaction exits the cell
stack, is mixed with replenishing hydrogen from the storage and
distribution system, and enters a condenser, where waste heat from the
hydrogen and water vapor is transferred to the fuel cell coolant
system.  The resultant temperature decrease condenses some of the
water vapor to water droplets.  A centrifugal water separator extracts
the liquid water and pressure-feeds it to potable tanks in the lower
deck of the pressurized crew cabin.  Water from the potable water
storage tanks can be used for crew consumption and cooling the
Freon-21 coolant loops.  The remaining circulating hydrogen is
directed back to the fuel cell stack.

The fuel cell coolant system circulates a liquid fluorinated
hydrocarbon and transfers the waste heat from the cell stack through
the fuel cell heat exchanger of the fuel cell power plant to the
Freon-21 coolant loop system in the midfuselage.  Internal control of
the circulating fluid maintains the cell stack at a normal operating
temperature of approximately 200 F.

When the reactants enter the fuel cells, they flow through a preheater
(where they are warmed from a cryogenic temperature to 40 F or
greater); a 6-micron filter; and a two-stage, integrated dual gas
regulator module.  The first stage of the regulator reduces the
pressure of the hydrogen and oxygen to 135 to 150 psia.  The second
stage reduces the oxygen pressure to a range of 62 to 65 psia and
maintains the hydrogen pressure at 4.5 to 6 psia differential below
the oxygen pressure.  The regulated oxygen lines are connected to the
accumulator, which maintains an equalized pressure between the oxygen
and the fuel cell coolant.  If the oxygen's and hydrogen's pressure
decreases, the coolant's pressure is also decreased to prevent a large
differential pressure inside the stack that could deform the cell
stack structural elements.

Upon leaving the dual gas regulator module, the incoming hydrogen
mixes with the hydrogen-water vapor exhaust from the fuel cell stack.
This saturated gas mixture is routed through a condenser, where the
temperature of the mixture is reduced, condensing a portion of the
water vapor to form liquid water droplets.  The liquid water is then
separated from the hydrogen-water mixture by the hydrogen pump/water
separator.

The hydrogen pump circulates the hydrogen gas back to the fuel cell
stack, where some of the hydrogen is consumed in the reaction.  The
remainder flows through the fuel cell stack, removing the product
water vapor formed at the hydrogen electrode.  The hydrogen-water
vapor mixture then combines with the regulated hydrogen from the dual
gas generator module, and the loop begins again.

The oxygen from the dual gas regulator module flows directly through
two ports into a closed-end manifold in the fuel cell stack, achieving
optimum oxygen distribution in the cells.  All oxygen that flows into
the stack is consumed, except during purge operations.

Reactant consumption is directly related to the electrical current
produced: if there are no internal or external loads on the fuel cell,
no reactants will be used.  Because of this direct proportion, leaks
may be detected by comparing reactant consumption and current
produced.  An appreciable amount of excess reactants used indicates a
probable leak.

Water and electricity are the products of the chemical reaction of
oxygen and hydrogen that takes place in the fuel cells.  The water
must be removed or the cells will become saturated with water,
decreasing reaction efficiency.  With an operating load of about 7
kilowatts, it takes only a few minutes to flood the fuel cell with
produced water, thus effectively halting power generation.  Hydrogen
is pumped through the stack, reacting with oxygen and picking up and
removing water vapor on the way.  After being condensed, the liquid
water is separated from the hydrogen by the hydrogen pump/water
separator and discharged from the fuel cell to be stored in the ECLSS
potable water storage tanks.

If the water tanks are full or there is line blockage, the water
relief valves open at 45 psia to allow the water to vent overboard
through the water relief line and nozzle.  Check valves prevent water
tanks from discharging through an open relief valve.  An alternate
water delivery path is also available to deliver water to the ECLSS
tanks if the primary path is lost.

For redundancy, there are two thermostatically activated heaters
wrapped around the discharge and relief lines to prevent blockage
caused by the formation of ice in the lines.  Two switches on panel
R12, fuel cell H 2 O line htr and H2O relief htr , provide the flight
crew with the capability to select either auto A or auto B for the
fuel cell water discharge line heaters and the water relief line and
vent heaters, respectively.

Thermostatically controlled heaters will maintain the water line
temperature above 53 F, when required.  The normal temperature of
product water is approximately 140 to 150 F.  The thermostatically
controlled heaters maintain the water relief valve's temperature when
in use between 70 to 100 F.  Temperature sensors located on the fuel
cell water discharge line, relief valve, relief line and vent nozzle
are displayed on the CRT.

If the potassium hydroxide electrolyte in the fuel cell migrates into
the product water, a pH sensor located downstream of the hydrogen
pump/water separator will sense the presence of the electrolyte, and
the crew will be alerted by an SM alert and display on the CRT.

During normal fuel cell operation, the reactants are present in a
closed-loop system and are 100 percent consumed in the production of
electricity.  Any inert gases or other contaminants will accumulate in
and around the porous electrodes in the cells and reduce the reaction
efficiency and electrical load support capability.  Purging,
therefore, is required at least twice daily to cleanse the cells.
When a purge is initiated by opening the purge valves, the oxygen and
hydrogen systems become open-loop systems; and increased flows allow
the reactants to circulate through the stack, pick up the contaminants
and blow them out overboard through the purge lines and vents.
Electrical power is produced throughout the purge sequence, although
no more than 10 kilowatts should be required from a fuel cell being
purged because of the increased reactant flow and preheater
limitations.

Fuel cell purge can be activated automatically or manually by the use
of fuel cell switches on panel R12.  In the automatic mode, the fuel
cell purge heater switch is positioned to GPC .  The purge line
heaters are turned on to heat the purge lines to ensure that the
reactants will not freeze in the lines.  The hydrogen reactant is the
more likely to freeze because it is saturated with water vapor.
Depending on the orbit trajectory and vehicle orientation, the heaters
may require 27 minutes to heat the lines to the required temperatures.
The fuel cell current is checked to ensure a load of less than 350
amps, due to limitations on the hydrogen and oxygen preheaters in the
fuel cells.  As the current output of the fuel cell increases, the
reactant flow rates increase, and the preheaters raise the temperature
of the reactants to a minimum of minus 40 F in order to prevent the
seals in the dual gas regulator from freezing.

The purge lines from all three fuel cells are manifolded together
downstream of their purge valves and associated check valves.  The
line leading to the purge outlet is sized to permit unrestricted flow
from only one fuel cell at a time.  If purging of more than one cell
at a time is attempted, pressure could build in the purge outlet line
and cause a decrease in the flow rate through the individual cells,
which would result in an inefficient purge.

When the fuel cell purge valves 1, 2 and 3 switches are positioned to
GPC, the fuel cell GPC purge seq switch is positioned to start and
must be held until the GPC purge seq talkback indicator indicates gray
(in approximately three seconds).  The automatic purge sequence will
not begin if the indicator indicates barberpole.  The GPC turns the
purge line heaters on and monitors the temperature.  The one oxygen
line temperature sensor must register at least 69 F and the two
hydrogen line temperature sensors 79 and 40 F, respectively, and be
verified by the GPC before the purge sequence begins.  If the
temperatures are not up to minimum after 27 minutes, the GPC will
issue an SM alert and display the data on the CRT.  When the proper
temperatures have been attained, the GPC will open for two minutes and
then close the hydrogen and oxygen purge valves for fuel cells 1, 2
and 3 in that order.  Thirty minutes after the fuel cell 3 purge
valves have been closed (to ensure that the purge lines have been
totally evacuated), the GPC will turn off the purge line heaters.
This provides sufficient time and heat to bake out any remaining water
vapor.  If the heaters are turned off before 30 minutes have elapsed,
water vapor left in the lines may freeze.

The manual fuel cell purge would be initiated by the flight crew using
the switches on panel R12.  In the manual mode, the three fuel cells
must be purged separately.  The fuel cell purge heater switch is
positioned to on for the same purpose as in the automatic mode, and
the flight crew verifies that the temperatures of the oxygen line and
two hydrogen lines are at the same minimum temperatures as in the
automatic mode before the purge sequence is initiated.  The fuel cell
purge valves 1 switch is positioned to open for two minutes, and the
flight crew observes that the oxygen and hydrogen flow rates increase
on the CRT.  The fuel cell purge valves 1 switch is then positioned to
close , and a decrease in the oxygen and hydrogen flow rates is
observed on the CRT, indicating the purge valves are closed.  Fuel
cell 2 is purged in the same manner using the fuel cell purge valves 2
switch.  Fuel cell 3 is then purged in the same manner using the fuel
cell purge valves 3 switch.  After the 30-minute line bakeout period,
the fuel cell purge heater switch is positioned to off.

In order to cool the fuel cell stack during its operations, distribute
heat during fuel cell starting, and warm the cryogenic reactants
entering the stack, the fuel cell circulates a coolant-fluorinated
hydrocarbon-throughout the fuel cell.  The fuel cell coolant loop and
its interface with the ECLSS Freon-21 coolant loops are identical in
fuel cells 1, 2 and 3.

Where the coolant enters the fuel cell, the temperature of the F-40
coolant returning from the ECLSS Freon-21 coolant loops is sensed
before it passes through a 75-micron filter.  After the filter, two
temperature-controlled mixing valves allow some of the hot coolant to
mix with the cool returning coolant to prevent the condenser exit
control valve from oscillating.  The condenser exit control valve
adjusts the flow of the coolant through the condenser to maintain the
hydrogen-water vapor exiting the condenser at a temperature between
148 and 153 F.

The stack inlet control valve maintains the temperature of the coolant
entering the stack between 177 and 187 F.  The accumulator is the
interface with the oxygen cryogenic reactant to maintain an equalized
pressure between the oxygen and the coolant (the oxygen and hydrogen
pressures are controlled at the dual gas regulator) to preclude a
high-pressure differential in the stack.  The pressure in the coolant
loop is sensed before the coolant enters the stack.

The coolant is circulated through the fuel cell stack to absorb the
waste heat from the hydrogen/oxygen reaction occurring in the
individual cells.  After the coolant leaves the stack, its temperature
is sensed and the data transmitted to the GPC, to the fuel cell stack
temp meter through the fuel cell 1, 2, 3 switch located below the
meter on panel O2, and to the CRT display.  The yellow fuel stack temp
C/W and the backup C/W alarm lights on panel F7 and the SM alert light
will be illuminated if fuel cell and stack temperatures exceed certain
limits: below 172.5 F or above 243.7 F.  The hot coolant from the
stack flows through the oxygen and hydrogen preheaters, where it warms
the cryogenic reactants before they enter the stack.

The coolant pump utilizes three-phase ac power to circulate the
coolant through the loop.  The differential pressure sensor senses a
pressure differential across the pump to determine the status of the
pump.  The fuel cell pump C/W light on panel F7 will be illuminated if
fuel cell 1, 2 or 3 coolant pump delta pressure is lost.  The SM alert
light also will be illuminated, and a fault message will be sent to
the CRT.  If the coolant pump for fuel cell 1, 2 or 3 is off , the
backup C/W alarm light will be illuminated, and a fault message will
be sent to the CRT.  The temperature-actuated flow control valve
downstream from the pump adjusts the coolant flow to maintain the fuel
cell coolant exit temperature between 190 and 210 F.  The stack inlet
control valve and flow control valve have bypass orifices to allow
coolant flow through the coolant pump and to maintain some coolant
flow through the condenser for water condensation, even when the
valves are fully closed due to the requirements of thermal
conditioning.

The coolant (that which is not made to bypass) exits the fuel cells to
the fuel cell heat exchanger, where it transfers its excess heat to be
dissipated through the ECLSS Freon-21 coolant loop systems in the
midfuselage.

In addition to thermal conditioning by means of the coolant loop, the
fuel cell has internal startup and sustaining heaters.  The 2,400-watt
startup heater is used only during startup to warm the fuel cell to
its operational level.  The 1,100-watt sustaining heaters normally are
used during low power periods to maintain the fuel cells at their
operational temperature.

Two 160-watt end-cell electrical heaters on each fuel cell power plant
were used to maintain a uniform temperature throughout the fuel cell
power section.  As an operational improvement, the end-cell electrical
heaters on each fuel cell power plant were deleted due to potential
electrical failures and were replaced by fuel cell power plant coolant
(F-40) passages.  This permits waste heat from each fuel cell power
plant to be used to maintain a uniform temperature profile for each
fuel cell power plant.

The hydrogen pump and water separator of each fuel cell power plant
were also improved.  To minimize excessive hydrogen gas entrained in
each fuel cell power plant's product water, modifications were made to
the water pickup (pitot) system.  The centrifugal force of
high-velocity water flowing around the pitot tube's bends separates
the hydrogen gas and water.  Pitot pressure then expels the hydrogen
gas into the hydrogen pump's inlet housing though a bleed orifice.

A current measurement detection system was added to monitor the
hydrogen pump load for each fuel cell power plant.  Excessive load
could indicate improper water removal, which could lead to flooding of
the fuel cell power plant and eventually render that power plant
inoperative.

The start/sustaining heater system for each fuel cell power plant was
also modified.  The modification was required specifically for fuel
cell power plant No.  1, mounted on the port, or left, side.  The No.
1 fuel cell power plant start/sustaining heater system added heat to
that fuel cell power plant's F-40 coolant loop system during the
startup of the power plant.  Because of its orientation, any entrained
gas in the coolant could enter the heater and become trapped at the
heater elements.  This would result in overheating of the heater
elements, which could vaporize the F-40 coolant, causing heater
failure and extensive damage to the fuel cell power plant.  The F-40
coolant loop flow system within the start/sustaining heater of each
fuel cell power plant was modified to prevent a gas bubble from
developing or being trapped at the heater elements, preventing the
loss of the start/sustaining heater.

A stack inlet temperature measurement was added to each fuel cell
power plant.  The temperature measurement was added to the in-flight
system to provide full visibility of the thermal conditions of each
fuel cell power plant (similar to the existing stack exit and
condenser exit temperatures of each fuel cell power plant).

The product water from all three fuel cell power plants flows to a
single water relief control panel.  The water can be directed from the
single panel to the ECLSS potable water tank A or to the fuel cell
power plant water relief nozzle.  Normally, the water is directed to
water tank A.  In the event of a line rupture in the vicinity of the
single water relief panel, water could spray on all three water relief
panel lines, causing them to freeze and prevent fuel cell power plant
water discharge.

The product water lines from all three fuel cell power plants were
modified to incorporate a parallel (redundant) path of product water
to ECLSS potable water tank B in the event of a freeze-up of the
single water relief panel.  In the event of the single water relief
panel freeze-up, pressure would build up and relieve through the
redundant paths to water tank B.  Temperature sensors and a pressure
sensor installed on each of the redundant water line paths transmit
data via telemetry for ground monitoring.

A water purity sensor (pH) was added at the common product water
outlet of the water relief panel to provide a redundant measurement of
water purity.  A single measurement of water purity in each fuel cell
power plant was provided previously.  If the fuel cell power plant pH
sensor failed, the flight crew was required to sample the potable
water.

The electrical control unit located in each fuel cell power plant is
the brain of the power plants.  The ECU contains the start up logic,
heater thermostats, and 30-second timer and interfaces with the
controls and displays for fuel cell startup, operation and shutdown.
The ECU controls the supply of ac power to the coolant pump, hydrogen
pump/water separator, the pH sensor, and the dc power supplied to the
flow control bypass valve (open only during startup) and the internal
startup and sustaining heaters.  The ECU also controls the status of
the fuel cell 1, 2, 3 ready for load and coolant pump P talkback
indicators on panel R1.

The nine fuel cell circuit breakers that connect the three-phase ac
power to the three fuel cells are located on panel L4, and the fuel
cell ECU receives its power from an essential bus through the FC cntlr
switch on panel O14.

The fuel cell start/stop switch on panel R1 for each fuel cell is used
to initiate the start sequence or stop the fuel cell operation.  When
this switch is held in its momentary start position, the ECU connects
the three-phase ac power to the coolant pump and hydrogen pump/water
separator (allowing the coolant and the hydrogen-water vapor to
circulate through these loops) and connects the dc power to the
internal startup and sustaining heaters and the flow control bypass
valve.  The switch must be held in the start position until the
coolant pump P talkback shows gray in approximately three to four
seconds, which indicates that the coolant pump is functioning properly
by creating a differential pressure across the pump.  When the coolant
pump P talkback indicates barberpole, it indicates the coolant pump is
not running.

The ready for load talkback for each fuel cell will show gray after
the 30-second timer times out and the stack-out temperature is above
187 F (which can be monitored on panel O2 in conjunction with the 1,
2, 3 switch located beneath the fuel cell stack out temp meter).  This
indicates that the fuel cell is up to the proper operating temperature
and is ready for loads to be attached to it.  It should not take
longer than 25 minutes for the fuel cell to warm up and become fully
operational, the actual time depending on the fuel cell's initial
temperature.  The ready for load indicator remains gray until the fuel
cell start/stop switch for each fuel cell is placed to stop, the FC
cntlr switch is placed to off , or the essential bus power is lost to
the ECU.

The startup heater enable/inhibit switch on panel R12 for each fuel
cell provides the crew control of the off/on status of the startup
heaters during fuel cell startup.  The inhibit position allows the
startup heaters to remain off and would be used only when immediate
power is required from a shutdown fuel cell.

Fuel cell 1, 2 or 3 dc voltage and current (amps) can be monitored on
the dc volts and dc amps meters on panel F9, using the fuel cell
volts/amp rotary switch to select a specific fuel cell.

The fuel cells will be on when the crew boards the vehicle, and the
vehicle is powered by the fuel cells and load sharing with the ground
support equipment power supplies.  Just before lift-off (T minus three
minutes and 30 seconds), the GSE is powered off and the fuel cells
take over all of the vehicle's electrical loads.  Indication of the
switchover can be noted on the CRT display and the dc amps meter.  The
fuel cell current will increase to approximately 220 amps; the oxygen
and hydrogen flow will increase to approximately 4 and 0.6 pound per
hour, respectively; and the fuel cell stack temperature will increase
slightly.

Fuel cell standby consists of removing the electrical loads but
continuing operation of the fuel cell pumps, controls, instrumentation
and valves, with the electrical power being supplied by the remaining
fuel cells.  A small amount of reactants is used to generate power for
the fuel cell internal heaters.

Fuel cell shutdown, after standby, consists of stopping the coolant
pump and hydrogen pump/water separator by positioning that fuel cell
start/stop switch on panel R1 to the stop position.  If the
temperature in the fuel cell compartment beneath the payload bay is
lower than 40 F, the fuel cell should be left in standby instead of
being shut down to prevent it from freezing.

Each fuel cell power plant is 14 inches high, 15 inches wide and 40
inches long and weighs 255 pounds.

The voltage and current range of each is 2 kilowatts at 32.5 volts dc,
61.5 amps, to 12 kilowatts at 27.5 volts dc, 436 amps.  Each fuel cell
is capable of supplying 12 kilowatts peak and 7 kilowatts maximum
continuous power.  The three fuel cells are capable of a maximum
continuous output of 21,000 watts with 15-minute peaks of 36,000
watts.  The average power consumption of the orbiter is expected to be
approximately 14,000 watts, or 14 kilowatts, leaving 7 kilowatts
average available for payloads.  Each fuel cell will be serviced
between flights and reused until each accumulates 2,000 hours of
on-line service.


"6_2_3_13_7_5.TXT" (24830 bytes) was created on 12-13-88

ELECTRICAL POWER DISTRIBUTION AND CONTROL.

The EPDC subsystem distributes 28-volt dc electrical power and
generates and distributes 115-volt, three-phase, 400-hertz ac
electrical power to all of the space shuttle systems' electrical
equipment throughout all mission phases.  The EPDC subsystem consists
of a three-bus system that distributes electrical power to the
forward, mid-, and aft sections of the orbiter for equipment used in
those areas.  The three main dc buses are main A (MNA), main B (MNB)
and main C (MNC).  Three ac buses, AC1, AC2 and AC3, supply ac power
to the ac loads.  Three essential buses, ESS1BC, ESS2CA and ESS3AB,
supply control power to selected flight crew controls and operational
power to electrical loads that are deemed essential.  Nine control
buses, CNTL AB1, 2, 3; CNTL BC1, 2, 3; and CNTL CA1, 2, 3, are used
only to supply control power to flight crew controls.  Two preflight
buses, PREFLT 1 and PREFLT 2, are used only during ground operations.

Electrical power is controlled and distributed by assemblies.  Each
assembly-main distribution assembly, power controller assembly, load
controller assembly and motor controller assembly-is an electrical
equipment container or box.

The dc power generated by each of the fuel cell power plants is
supplied to a corresponding DA.  Fuel cell power plant 1 is supplied
to DA 1, FCP 2 to DA 2 and FCP 3 to DA 3.  Each DA contains remotely
controlled motor-driven switches called power contactors used for
loads larger than 125 amps.  The power contactors are rated at 500
amps and control and distribute dc power to a corresponding mid power
controller assembly, forward power controller assembly and aft power
controller assembly.  Power contactors are also located in the APCAs
to control and distribute GSE 28-volt dc power to the orbiter through
the T-0 umbilical before the fuel cell power plants take over the
supply of orbiter dc power.

Each of the mid, forward, and aft PCAs supplies and distributes dc
power to a corresponding mid motor controller assembly, forward motor
controller assembly, forward load controller assembly, aft load
controller assembly and aft motor controller assembly and dc power to
activate the corresponding ac power system.

Each PCA contains remote power controllers and relays.  The RPCs are
solid-state switching devices used for loads requiring current in a
range of 3 to 20 amps.  The RPCs are current protected by internal
fuses and also have the capability to limit the output current to a
maximum of 150 percent of rated value for two to three seconds.
Within three seconds the RPC will trip out, which removes the output
current.  To restore power to the load, the RPC must be reset, which
is accomplished by cycling a control switch.  If multiple control
inputs are required before an RPC is turned on, hybrid drivers are
usually used as a logic switch, which then drives the control input of
the RPC.

Each LCA contains hybrid drivers, which are solid-state switching
devices (no mechanical parts) used as logic switches and for low-power
electrical loads of less than 5 amps.  When the drivers are used as a
logic switch, several control inputs are required to turn on a load.
Hybrid drivers are also used in the MPCAs.  The hybrid drivers are
current protected by internal fuses.  Hybrid relays requiring multiple
control inputs are used to switch three-phase ac power to motors.

Relays are also used for loads between 20 amps and 135 amps in PCAs
and MCAs.

In the midbody there are no LCAs; therefore, the MPCAs contain RPCs,
relays and hybrid drivers.  Each MCA contains main dc buses, ac buses
and hybrid relays that are remotely controlled for control of the
application or removal of ac power to ac motors.  The main dc bus is
used only to supply control or logic power to the hybrid relays so the
ac power can be switched on or off.

The remotely controlled switching devices permit the location of major
electrical power distribution buses close to the major electrical
loads, which eliminates heavy electrical feeders to and from the
pressurized crew compartment display and control panels.  In addition,
this reduces the amount of spacecraft wiring, thus weight, and permits
more flexible electrical load management.

The No.  1 distribution assembly and all No.  1 controllers go with
fuel cell 1 and MNA bus, all No.  2 controllers and DA 2 go with fuel
cell 2 and MNB bus, and all No.  3 controllers and DA 3 go with fuel
cell 3 and MNC bus.  The FC main bus A switch on panel R1 positioned
to on connects fuel cell 1 to the MNA DA and controllers and
disconnects fuel cell 1 from the MNA DA and controllers when
positioned to off .  The talkback indicator associated with the FC
main bus A switch will indicate on when fuel cell 1 is connected to
main bus A DA and controllers and off when fuel cell 1 is disconnected
from main bus A DA and controllers.  The FC main bus B and C switches
and talkback indicators on panel R1 function in the same manner.

Main bus A can be connected to main bus B or main bus C through the
use of the main bus tie switches on panel R1 and power contactors in
the DAs.  For example, main bus A can be connected to main bus B by
positioning the main bus tie A switch to on and the main bus tie B
switch to on .  The talkback indicator associated with the main bus
tie A and B switches will indicate on when main bus A is connected to
main bus B.  To disconnect main bus A from main bus B, the main bus
tie A and B switches must be positioned to off; the talkback
indicators associated with the main bus tie A and B switches will then
indicate off .  Main bus A can be connected to main bus C in a similar
manner using the main bus tie A and C switches.  Main bus B can be
connected to main bus A or C in a similar manner using the main bus
tie B and A or C switches.  Similarly, main bus C can be connected to
main bus B or A using the main bus C and B or A switches.

Main bus A, B or C voltages can be displayed on the dc volts meter on
panel F9 through the main volts A, B or C rotary switch on panel F9.
The main bus undervolts red caution and warning light on panel F7 will
be illuminated if main bus A, B or C voltage is 26.4 volts dc,
informing the crew that the minimum equipment operating voltage limit
of 24 volts dc is being approached.  A backup caution and warning
light will also be illuminated at 26.4 volts dc.  An SM alert light
will be illuminated at 27 volts dc or less, alerting the flight crew
to the possibility of a future low-voltage problem.  A fault message
also is transmitted to the CRT.

The nominal fuel cell voltage is 27.5 to 32.5 volts dc, and the
nominal main bus voltage range is 27 to 32 volts dc, which corre spond
to 12- and 2-kilowatt loads, respectively.

Depending on the criticality of orbiter electrical equipment, some
electrical loads may receive redundant power from two or three main
buses.  If an electrical load receives power from two or three
sources, it is for redundancy only and not for total power
consumption.

Essential buses supply control power to switches that are necessary to
restore power to a failed main dc or ac bus and to essential
electrical power system electrical loads and switches.  In some cases,
essential buses are used to power switching discretes to
multiplexers/demultiplexers.  Examples of the selected flight crew
switches and loads are the EPS switches, GPC switches, tactical air
navigation mode switches, radar altimeter and microwave scan beam
landing system power switches, the caution and warning system,
emergency lighting, audio control panel, and master timing unit.

The three essential buses are ESS1BC, ESS2CA and ESS3AB.  ESS1BC
receives power from three redundant sources.  It receives dc power
from fuel cell 1 through the ESS bus source FC 1 switch on panel R1
when the switch is positioned to on and from main dc buses B and C
through RPCs when the ESS bus source MN B/C switch on panel R1 is
positioned to on .  Electrical power is then distributed from the
essential bus in DA 1 through fuses to the corresponding controller
assemblies and to the flight and middeck panels.  ESS2CA receives
power from fuel cell 2 through the ESS bus source FC 2 switch on panel
R1 when positioned to on and main dc buses C and A through RPCs when
the ESS bus source MN C/A switch on panel R1 is positioned to on.
Electrical power is then distributed from the essential bus in DA 2
through fuses to the corresponding controller assemblies and to the
flight and middeck panels.  ESS3AB receives power from fuel cell 3
through the ESS bus source FC 3 switch on panel R1 when positioned to
on and main dc buses A and B through RPCs when the ESS bus source MN
A/B switch on panel R1 is positioned to on.  Electrical power is then
distributed from the essential bus in DA 3 through fuses to the
corresponding controller assemblies and to the flight and middeck
panels.

The ESS bus voltage can be monitored on the volts meter on panel F9
through the ESS volts 1 BC, 2 CA, 3 AB rotary switch.  An SM alert
light will be illuminated to inform the flight crew if the essential
bus voltage is less than 25 volts dc.  A fault message also is
transmitted to the CRT.

Nine control buses are used to supply only control power to the
display and control panel switches on the flight deck and in the
middeck area.  A control bus does not supply operational power to any
system loads.  The control buses are enabled by the control bus power
MNA, B, C switches on panel R1 and the MNA control bus BC 1/2/3
circuit breaker on panel R15, the MNB control bus CA 1/2/3 circuit
breaker on panel R15 and the MNC control bus AB 1/2/3 circuit breaker
on panel R15.  The corresponding main bus is connected through RPCs
and diodes.  Each control bus receives power from three main dc buses
for redundancy.  MNA bus is supplied to three control buses, AB1/2/3,
BC1/2/3 and CA1/2/3.  (The numbers 1, 2 and 3 indicate the number of
the bus and not a fuel cell.) MNB bus is supplied to three control
buses, AB1/2/3, BC1/2/3 and CA1/2/3.  MNC bus is supplied to three
control buses, AB1/2/3, BC1/2/3 and CA1/2/3.  The RPCs are powered
continuously unless the control bus pwr MNA, MNB, MNC switch on panel
R1 is positioned to the momentary reset position, which turns the
corresponding RPC's power off and resets the RPC if it has been
tripped off.  An SM alert light is illuminated if the control bus
voltage is less than 24.5 volts dc, and a fault message is sent to the
CRT.  The Mission Control Center in Houston can monitor the status of
each RPC.

Until T minus three minutes and 30 seconds, power to the orbiter is
load shared with the fuel cells and GSE, even though the fuel cells
are on and capable of supplying power.  Main bus power is supplied
through the T-0 umbilicals, MNA through the left-side umbilical and
MNB and C through the right-side umbilical to aft power controllers 4,
5 and 6.  From APCs 4, 5 and 6, the GSE power is directed to the DA,
where the power is distributed throughout the vehicle.  The power for
the PREFLT 1 and PREFLT 2 test buses is also supplied through the T-0
umbilical.  These test buses are scattered throughout the orbiter and
are used to support launch processing system control of critical
orbiter loads, although they also power up the essential buses in the
APCs when on GSE.  As in the main bus distribution, essential bus
power from the APCs is directed to the DAs and then distributed
throughout the vehicle.  At T minus three minutes 30 seconds, the
ground turns off the GSE power to the main buses, and the fuel cells
automatically pick up the loads.  At T minus zero, the T-0 umbilical
is disconnected with the preflight test bus wires live.

Fuel cell 3 may be connected to the primary payload bus by positioning
the pri FC3 switch on panel R1 to the momentary on position.  The
talkback indicator next to this switch will indicate on when fuel cell
3 is connected to the PRI PL bus.  The PRI PL bus is the prime bus for
supplying power to the payloads.  Fuel cell 3 may be disconnected from
the payload bus by positioning the pri FC3 switch to the momentary off
position.  The talkback indicator will indicate off .

A second source of electrical power for the PRI PL bus may be supplied
from MNB bus by positioning the pri MN B switch on panel R1 to the
momentary on position.  The talkback indicator next to this switch
will indicate on.  MNB bus may be removed from the PRI PL bus by
positioning the switch momentarily to off .  The talkback indicator
will indicate off .  A third possible source of electrical power for
the PRI PL bus may be supplied from MNC bus through the pri MN C
switch on panel R1, positioned momentarily to the on position.  The
adjacent talkback indicator will indicate on.  MNC bus may be removed
from the PRI PL bus by positioning the switch momentarily to off .
The talkback indicator will indicate off.

There are two additional payload buses in the aft section of the
payload bay at the Yo 1307 aft bulkhead station.  The aft payload B
bus may be powered up by positioning the aft MN B switch on panel R1
to on .  The aft payload C bus may be powered up by positioning the
aft MN C switch on panel R1 to on .  The off position of each switch
removes power from the corresponding aft payload bus.

The payload aux switch on panel R1 permits main bus A and main bus B
power to be supplied to the AUX PL A and AUX PL B buses when the
switch is positioned to on.  The auxiliary payload buses provide power
for emergency equipment or controls associated with payloads.  The off
position removes power from the AUX PL A and PL B buses.  It is also
noted that the two auxiliary payload buses may be dioded together to
form one bus for redundancy.

Two or more feeders to the payload may be used simulta neously, but
two orbiter power sources may not be tied directly within the payload.
Any payload equipment requiring electrical power from two separate
orbiter sources is required to ensure isolation of these power sources
so that no single failure in a load, or succession or propagation of
failures in a load, will cause an out-of-limit condition to exist on
the orbiter system equipment on more than one bus.

The payload cabin switch on panel R1 provides MNA or MNB power to
patch panels located behind the payload specialist and mission
specialist stations located on the aft flight deck.  These patch
panels supply power to the payload-related equipment located on panels
at these stations.  Two three-phase circuit breakers, AC2 cabin PL3 J
and AC3 cabin PL3J, on panel MA73C provide ac power to the payload
patch panels.

Alternating-current power is generated and made available to system
loads by the EPDC subsystem, using three independent ac buses, AC1,
AC2 and AC3.  The ac power system includes the ac inverters for dc
conversion to ac and inverter distribution and control assemblies
containing the ac buses and the ac bus sensors.  The ac power is
distributed from the IDCAs to the flight and middeck display and
control panels and from the MCAs to the three-phase motor loads.

Each ac bus consists of three separate phases connected in a
three-phase array.  Static inverters, one for each phase, are located
in the forward avionics bays.  Each inverter has an output voltage of
116 to 120 volts root mean square at 400 hertz, plus or minus 7 hertz.

The inverters are controlled by the inv pwr 1, 2, 3 switches on panel
R1.  Inverter 1 receives power only from MNA, inverter 2 from MNB and
inverter 3 from MNC.  All three inverters of inverter 1 receive MNA
bus power when the switch is positioned to on , and all three must be
in operation before the adjacent talkback indicator indicates on .
The indicator will show off when main bus power is not connected to
the inverter.

The inv/ac bus 1, 2, 3 switches on panel R1 are used to apply each
inverter's output to its respective ac bus.  An indicator next to each
switch shows its status, and all three inverters must be connected to
their respective ac buses before the indicator shows on .  The
talkback indicator will show off when the three inverters are not
connected to their respective ac bus.

The inv pwr and inv/ac bus switches must have control power from the
ac contr circuit breakers on panel R1 in order to operate.  Once ac
power has been established, these circuit breakers are opened to
prevent any inadvertent disconnection, whether by switch failure or
accidental movement of the inv pwr or inv/ac bus switches.

Each ac bus has a sensor, switch and circuit breaker for flight crew
control.  The AC1, 2, 3 snsr circuit breakers located on panel O13
apply essential bus power to their respective ac bus snsr 1, 2, 3
switch on panel R1 and operational power to the respective inv/ac bus
switch indicator.  The ac bus snsr 1, 2, 3 switch selects the mode of
operation of the ac bus sensor: auto trip, monitor or off .  The ac
bus sensor monitors each ac phase bus for over- or under voltage and
each phase inverter for an overload signal.  The overvoltage limits
are bus voltages greater than 123 to 127 volts ac for 50 to 90
milliseconds.  The undervoltage limits are bus voltages less than 102
to 108 volts ac for 6.5 to 8.5 milliseconds.  An overload occurs when
any ac phase current is greater than 14.5 amps for 10 to 20 seconds or
is greater than 17.3 to 21.1 amps for four to six seconds.

When the respective ac bus snsr switch is positioned to the auto trip
position and an overload or overvoltage condition exists, the ac bus
sensor will illuminate the respective yellow ac voltage or ac overload
caution and warning light on panel F7 and trip out (disconnect) the
inverter from its respective phase bus for the bus/inverter causing
the problem.  There is only one ac voltage and one ac overload caution
and warning light; as a result, all nine inverters/ac phase buses can
illuminate the lights.  The ac volts meter and rotary switches ( AC1
JA, JB, JC; AC2 JA, JB, JC; AC3 JA, JB, JC) on panel F9 or the CRT
display would be used to determine which inverter or phase bus caused
the light to illuminate.  The phase bus causing the problem would show
zero volts.  Because of the various three-phase motors throughout the
vehicle, there will be a small induced voltage on the phase bus if
there is only one phase that has loss of power.

Before power can be restored to the tripped bus, the trip signal to
the inv/ac bus switch must be removed by positioning the ac bus snsr
switch to off , then back to the auto trip position, which
extinguishes the caution and warning light.  The inv/ac bus switch is
then positioned to on, restoring power to the failed bus.  If the
problem is still present, the sequence will be repeated.

If an undervoltage exists, the yellow ac voltage caution and warning
light on panel F7 will be illuminated, but the inverter will not be
tripped out from its phase bus.

When the ac bus snsr 1, 2, 3 switches are in the monitor position, the
ac bus sensor will monitor for an overload, overvoltage and
undervoltage and illuminate the applicable caution and warning light;
but it will not trip out the phase bus/inverter causing the problem.

When the ac bus snsr switches are off, the ac bus sensors are
non-operational, and all caution and warning and trip-out capabilities
are inhibited.

A backup caution and warning light will be illuminated for overload or
over- and undervoltage conditions.  The SM alert will occur for over-
and undervoltage conditions.  A fault message also is sent to the CRT.

There are 10 motor controller assemblies used on the orbiter: three
are in the forward area, four are in the midbody area, and three are
in the aft area.  Panel MA73C contains the controls for the MCAs.
Their only function is to supply ac power to non-continuous ac loads
for ac motors used for vent doors, air data doors, star tracker doors,
payload bay doors, payload bay latches and reaction control
system/orbital maneuvering system motor-actuated valves.  The MCAs
contain main buses, ac buses and hybrid relays, which are the remote
switching devices for switching the ac power to electrical loads.  The
main buses are used only to supply control or logic power to the
hybrid relays so that ac power can be switched on and off.  If a main
bus is lost, the hybrid relays using that main bus will not operate.
In some cases, the hybrid relays will use logic power from a switch
instead of the MCA bus.

The three forward motor controller assemblies (FMC 1, FMC 2 and FMC 3)
correspond to MNA/AC1, MNB/AC2 and MNC/AC3, respectively.  Each FMC
contains a main bus, an ac bus and an RCS ac bus.  The main bus
supplies control or logic power to the relays associated with both the
ac bus and RCS ac bus.  The ac bus supplies power to the forward left
and right vent doors, the star tracker Y and Z doors, and the air data
left and right doors.  The RCS ac bus supplies power to the forward
RCS manifold and tank isolation valves.

The aft motor controller assemblies (AMC 1, AMC 2 and AMC 3)
correspond to MNA/AC1, MNB/AC2, and MNC/AC3, respectively.  Each AMC
assembly contains a main bus and its corresponding ac bus and a main
RCS/OMS bus and its corresponding RCS/OMS ac bus.  Both main buses are
used for control or logic power for the hybrid relays.  The ac bus is
used by the aft RCS/OMS manifold and tank isolation and crossfeed
valves.

The mid motor controller assemblies (MMC 1, MMC 2, MMC 3 and MMC 4)
contain two main dc buses and two corre sponding ac buses.  MMC 1
contains main bus A and B and their corresponding buses, AC1 and 2.
MMC 2 contains MNB and AC2 and AC3 buses.  MMC 3 contains the same
buses as MMC 1, and MMC 4 the same buses as MMC 2.  Loads for the main
buses/ac buses are vent doors, payload bay doors and latches, radiator
panel deployment actuator and latches, and payload retention latches.

The electrical components in the midbody are mounted on cold plates
and cooled by the Freon-21 system coolant loops.  The PCAs, LCAs, MCAs
and inverters located in forward avionics bays 1, 2 and 3 are mounted
on cold plates and cooled by the water coolant loops.  The inverter
distribution assemblies in forward avionics bays 1, 2 and 3 are
air-cooled.  The LCAs, PCAs and MCAs located in the aft avionics bays
are mounted on cold plates and cooled by the Freon-21 system coolant
loops.

The contractors are Aerodyne Controls Corp., Farmingdale, N.Y.
(oxygen, hydrogen check valve and water pressure relief valve); Aiken
Industries, Jackson, Mich.  (thermal circuit breakers; three-phase
circuit breakers); American Aerospace, Farmingdale, N.Y.  (ac and dc
current sensors, current level detector); Applied Research, Fairfield,
N.J.  (rotary switch); Rockwell International Autonetics Group,
Anaheim, Calif.  (ac bus sensor, load controller assemblies); Beech
Aircraft Corp., Boulder, Colo.  (power reactant storage hydrogen and
oxygen tanks, gaseous oxygen and hydrogen ground support equipment);
Bell Industries, Gardena, Calif.  (modular terminal boards); Bendix
Corp., Sidney, N.Y., and Franklin, Ind.  (high-density connectors);
Bussman Division of McGraw Edison, St.  Louis, Mo.  (fuses, fuse
holders, fuse dc limiter high current); Brunswick-Circle Seal,
Anaheim, Calif.  (water check valve); Consolidated Controls, El
Segundo, Calif.  (hydrogen, oxygen solenoid valve,
undirectional/bidirectional shutoff valve); Cox and Co., New York,
N.Y.  (heaters); Deutsch, Banning, Calif.  (general-purpose
connector); Fairchild Stratos, Manhattan Beach, Calif.  (cryogenic
fluid and gas supply disconnects); G/H Technology Co., Santa Monica,
Calif.  (connector cryo); Hamilton Standard, Windsor Locks, Conn.
(fuel cell heat exchanger); Haveg Industries Inc., Winooski, Vt.
(general-purpose wire); ITT Cannon, Santa Ana, Calif.  (connectors,
bulkhead feedthrough); Labarge, Santa Ana, Calif.  (general-purpose
wire); Leach Relay, Los Angeles, Calif.  (relay); Malco Microdot
Corp., Pasadena, Calif.  (connector); International Fuel Cells
Division of United Technologies, South Windsor, Conn.  (fuel cell
power plants); R.V.  Weatherford, Glendale, Calif.  (shunt); Statham
Instruments, Oxnard, Calif.  (cryo pressure transducer); Tayco
Engineering, Long Beach, Calif.  (fuel cell water dump nozzle);
Teledyne Kinetics, Solana Beach, Calif.  (dc power contactor);
Teledyne Thermatics, Elm City, N.C.  (general-purpose wire); West
inghouse Electric Corp., Lima, Ohio (remote power controller,
electrical system inverters); Weston Instruments, Newark, N.J.
(electrical indicator meter); Brunswick-Wintec, El Segundo, Calif.
(reactant and coolant filters); Rockwell International Space
Transportation Systems Division, Downey, Calif.  (power controller
assemblies, motor controller assemblies, distribution assemblies and
inverter distribution control assemblies).