Electrical System

Overview

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.

 

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.

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.

 

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.