24 Electrical power

TTP	B1-L3	ATA24
Beech 90 Series	B2-L3	Electrical Power

24 Electrical Power

00   General

10   Generator Drive

20   AC Generation

30   DC Generation

40   External Power

50   Electrical Load Distribution

 Electrical power includes battery, electrical power supply, electrical equipment, system protection and distribution, electrical system schematic.

The  electrical  system  is  a  28-volt,  direct  current (DC), negative ground system. The power sources include one battery and two generators. Power from these sources is distributed to the individual electrical loads with a multibus system consisting of a hot battery bus, left generator bus, right  generator  bus,  center  bus,  and  triple-fed bus. Two redundant AC inverters provide power to the avionics and, on aircraft through LJ-1360, to the engine torquemeter. The power sources are connected to the distribution system through line contactors and relays. A voltmeter in the over- head instrument panel monitors battery, external power, and individual bus voltages. Left and right loadmeters indicate the percentage of output of each generator. A battery ammeter is installed on aircraft LJ-1534 and after.

Wiring Diagram Information

The Wiring Diagram Manual is prepared in accordance  with General Aviation  Manufacturers Association (GAMA) Specification No. 2 and meets the intent of the Air Transport Association (ATA) with respect to the arrangement and content.

A reference designator is a combination of letters and numbers, which identify a particular component in the Wiring Diagram Manual. A reference designator is assigned to each item in the Wiring Diagram Manual.  

CB101  •  CB—Identifies the component as a circuit breaker

        •  101—Identifies it as a specific circuit breaker

Assemblies are identified on the schematic  by the reference designator and the assembly name. The  accompanying equipment list displays the reference designator, part number, and the assembly name. The reference designator of a component that belongs to an assembly begins with the reference designator of the assembly.

e.g. •  A151—Overhead meter panel assembly

•  A151A1—Emergency light control assembly

Wire Identification

 

Each wire is identified by a combination of numbers and letters imprinted on the wire. Each wire should be identified within three inches of each junction. A code is not required for short wires or cables less than three inches in length. A nonmetallic sleeve carrying identification appears on wires where imprinted codes cannot be affixed. Circuit-function letters indicate a relationship to a particular system (noted in the list of circuit- function designations). Wire numbers are provided to differentiate between wires in a circuit. A wire segment  is  a  conductor  between  two  terminals or connections. Alphabetical sequence is usually followed. Wire segment “A” indicates the power source, “B” indicates the next segment, etc. A wire size is also used to identify wire or cable size. A suffix to the wire identification code (N) identifies any wire or cable that completes the circuit to the ground network. Phase letter “A,” “B,” or “C” is used as a suffix to identify three-phase wiring in AC circuits. The phase letter “V” is used as a suffix identifying an ungrounded wire or cable in a single-phase system. Circuit-function letters indicate a relationship to a particular system. Schematics in the Wiring Diagram Manual are shown with the aircraft power OFF, doors shut and locked, control surfaces streamlined, and weight on the landing gear. Any deviations to this should be noted on the individual sheets.

A listing of circuit functions follows:

•  C—Control surfaces, automatic pilot

 

•  D—Miscellaneous instruments other than flight or engine instruments

 

•  E—Engine  instruments,  fuel  flow,  fuel quantity, etc.

•  F—Flight instruments

 

•  G—Landing   gear,   actuator,   retraction, warning, etc.

•  H—Heating, ventilating, deicing

•  J—Ignition

•  K—Engine control, starter, prop sync, etc.

•  L—Lighting

 

•  M—Miscellaneous  electrical,  windshield wiper, etc.

•  P—DC power

 

•  Q—Fuel and oil, fuel valves, fuel pump motors, oil pumps, etc.

•  V—DC power for AC systems

•  W—Warning and emergency

 

•  X—AC power

DC ELECTRICAL SYSTEM The electrical system provides maximum protection against loss of electrical power due to ground fault.  High-current  sensors  hall  effect  device (HED) bus-tie relays, and current limiters isolate a ground fault from a power source. The arrangement of the electrical system buses affords multiple power sources for all circuits as follows:

•  Right and left generator bus

•  Triple-fed bus

•  Center bus

•  Hot battery bus

 

A white ring on the edge-lit panel around the switch identifies all switches receiving power from the triple-fed or center bus.

Battery

The battery is in the right wing center section forward of the main spar. On aircraft LJ-1534 and after, the battery is a 42-amp hours, recombinant gas (RG), lead-acid  battery. On aircraft prior to LJ-1534, the battery is a 34-amp hours, air-cooled, 20-cell nickel-cadmium battery. The NiCad battery is air cooled by an air inlet tube underneath the battery and extending underneath the wing. Cooling air is thermostatically controlled by a valve in the air inlet tube. The valve is fully closed at 30°F and fully opened at 80°F.

The battery  switch, in the pilot  outboard sub- panel, controls the battery relay and the battery bus tie. In the OFF position, the battery relay and the battery bus tie are open and the battery is disconnected from all buses except the hot battery bus. When the battery switch is in the ON position, the battery relay closes to apply power to the triple-fed bus and the battery bus tie closes to apply power to the center bus.

For a battery engine start the battery voltage should indicate at least 23 volts. Before using auxiliary power the battery voltage must indicate more than 20 volts.

Battery Monitoring

Monitor the battery voltage with the voltmeter in the overhead instrument panel on all aircraft.

On aircraft LJ-1534 and after equipped with a RG, lead-acid battery, monitor battery current with the battery ammeter in the overhead instrument panel.

On aircraft prior to LJ-1534 equipped with a NiCad battery, a battery monitor circuit continuously senses battery current and illuminates the BATTERY CHARGE annunciator when the battery charge rate exceeds 7 amps for a time period of longer than 6 seconds. The system indicates when the conditions exist for a possible thermal runaway of the nickel-cadmium battery. High charge rates  to an electrically-depleted  battery result in high temperatures and damage to the gas barrier between the battery plates. This reduces internal battery resistance and allows an increased charge rate and a further increase in temperature. Thermal runaway is the final result. The battery may be isolated from the aircraft system by turning the battery  switch  OFF, preventing further battery damage.

After an engine battery start when the generator is turned on, the BATTERY CHARGE annunciator illuminates  after and should extinguish within 5 minutes. If the BATTERY CHARGE annunciator remains illuminated for more than 5 minutes during ground operations, monitor the battery charge rate. If the annunciator illuminates in flight, the aircrew should follow the appropriate check list procedures.

The battery monitor system consists of a shunt connected to the negative lead of the battery, a battery charge current detector, and a BATTERY CHG light in the annunciator panel. The shunt is in the aft end of the battery compartment in the right center section, The detector module is on a printed circuit board (PCB) beneath the center aisle floor aft of the forward wing spar. The charge-current-detector circuit amplifies the signal from the shunt and provides a 28-volt output for the BATTERY CHG light when the charge rate exceeds 7 amps for a time period of longer than 6 seconds. The time delay prevents spurious BATTERY CHG lights during momentary recharges when heavy loads are switched on.

 

Following an engine start, the BATTERY CHG light  illuminates and remains on for approximately 5  minutes  until  the  charge  current  decreases below the 7-amps rate as the battery approaches full charge. The light remains on in excess of 5 minutes if the battery is in a low state of charge, is at a low temperature, has a low-charge voltage per cell, or has gas barrier damage.

 

After the BATTERY CHG light extinguishes, it should remain off unless either a thermal runaway has begun or the battery idle current increases in response to an increase  in electrical-system load. A current increase nominally results from poor generator paralleling or load switching. The BATTERY CHG light may illuminate for short intervals as the battery recharges when the generator speed is increased above cut-in speed.

The hot battery bus connects directly to the battery and provides power for the operation of certain systems considered essential for aircraft operation. The bus is under the battery compartment in the right wing center section. Access to the hot battery bus by removing a panel on the bottom of the wing. Five-amp fuses supply power to the boost pumps and the fuel crossfeed system. Five-amp circuit breakers supply power to all other items on the bus. The firewall shutoff valves are NOT powered from the hot battery bus. All appliances powered through the hot bus are also powered through one of the other aircraft buses for normal operation; diodes are provided to prevent any cross flow of electricity between the hot bus and the aircraft systems buses. These hot bus diodes are located along the backside of the top edge of the pilot fuel panel and are commonly referred to as “fuel panel diodes.”

The triple-fed  bus, in the lower forward cabin under the copilot floor, is triple-fed from the battery and from each generator bus through 60-amp limiters  and  diodes  which  provide  fault  isolation between the power sources. A white ring on the edge-lit panel around the switch identifies all switches receiving power from the triple-fed or center bus.

Left/Right Generator Buses Each  generator  bus  panel  assembly  is  aft  of the firewall in the inboard side of its respective nacelle. The bus connects to its generator through a line contactor  relay on the DC power panel assembly which is just forward of the bus panel assembly. The bus can be connected to the center bus through its respective generator tie contactor. Circuits from the generator buses are protected by current limiters on the bus panel assembly. Additional down line current protection is usually provided to these circuits. Excessive current draw protection out of the generator buses is provided by a 60-amp current limiter in the feed line to the triple-fed bus and a 250-amp current limiter in the line to the center bus. Excessive current draw protection into the generator buses is provided by the GCU, controlled line contactor and the 275- amp, HED-controlled, generator tie contactor in the line to the center bus.

GEN BUS TIEs do not close with the BAT relay (such as occurs with the BAT BUS TIE) because items they power are not considered essential to aircraft operation and deplete the battery more rapidly if allowed to power up.

The center  bus is under the pilot floor and is powered from each generator through a 250-amp limiter and generator bus-tie relay. The generator buses, triple-fed bus, and battery are all tied together by the center bus, which also supplies power to the landing gear, electric heat, and air conditioning systems.

Power Panel

The DC power panels are on the left and right nacelles and contain:

•  Line contactor         •  Cross-start relay    •  Start relay

•  Loadmeter shunt     •  Circuit breakers    •  Generator field and sense circuit breaker

•  Generator control circuit breaker              •  Bus-tie limiter

•  Generator limiters

 

The line contactor relays on the power panels are equipped with auxiliary contacts, which are used for various control and logic functions.

Relay Panel Assemblies

Many of the aircraft relays have been centrally located on the four relay panel assemblies under the cabin center-aisle floor behind the main spar and aft of the GCU/printed circuit boards. Each relay panel assembly has provisions for five plug-in relays. Test points are provided on each assembly to permit the in- circuit testing of the relays and associated circuit components. The  relays  may  be  removed  and replaced without soldering. Remove and replace the relay panel assemblies as assemblies.

The  four  relay  panel  assemblies  are  identi- fied by the reference designators A118 through A121. The individual relays are designated as K1 through K5, numbered from top to bottom, on each assembly. This identification also corresponds the Wiring Diagram Manual circuit diagrams.

Bus System Operation

In normal operation, the buses are automatically tied into a single-loop system in which all sources collectively supply power through individual protective devices. When the battery switch is ON, the battery relay and the battery bus-tie relay close. Battery power is routed through the battery relay to the triple-fed bus and the battery bus-tie relay to the center bus and both starter relays. Battery power is then available to start either engine. After either engine has been started and the generator system has been activated, the GCU closes the generator line contactor and the generator bus-tie relays automatically close. The generator output is then routed through the center bus and the battery bus-tie relay to permit battery charging and to supply generator power to all aircraft systems. As each generator bus is energized, power is routed to the opposite cross-start system, and is ready for use in starting the opposite engine. The cur- rent supplied by the operating generator during a generator-assisted start bypasses the 250-amp current limiter of the operating generator to prevent opening the limiter.

Starter-Generators There are  two engine-driven 30-volt,  250-amp starter-generators on the aircraft.

The starter is controlled by the left and right starter switches on the pilot outboard subpanel. The switch can be moved up and lever locked to the IGNITION AND ENGINE START position to energize both the starter and the engine ignition system. Or, it can be held down to the spring-loaded STARTER ONLY position to energize only the starter. Do not exceed the starter motor operating time limits.

Starter-Generator Brushes

Four sets of carbon brushes carry the starter (two each) and generator (two each) electrical loads of each of the starter-generator assemblies. Each brush is manufactured from soft carbon infused with hard, cylindrical carbon cores. The difference in hardness enables the brushes to cut grooves in the face of the commutator, increasing total con- tact  surface  area  anytime  the  starter-generator is spinning. Each brush is manufactured with a diagonal groove on its side, beginning from the brush contact face to the brush cable end. The length of the groove remaining on the brush provides an indication of remaining brush life. If the wear grooves indicate any brush is at or near the final quarter of its remaining life, replace the brush.

 

Starter Operating Limits

40 seconds ON ..................60 seconds OFF

 

40 seconds ON ..................60 seconds OFF

40 seconds ON ..................30 minutes OFF For   generator   operation,   individual   control

switches are in the pilot outboard subpanel and are placarded GEN 1 and GEN 2 (OFF–ON–GEN RESET). 

The generators are self-exciting and do not require battery power for operation. To bring a

generator on line, the generator switch should be placed in the GEN RESET position, held in that position for approximately one second, and then released to ON. In the GEN RESET position, the generator voltage builds up to 28 volts and the line contactor is open. When the generator switch is released to ON, the line contactor closes.

 

Generator-control units provide voltage regulation, differential voltage, reverse current protection, paralleling, and overvoltage protection control for the generators. The voltage regulation circuit controls the generator to maintain a constant 28.25, ± .25-volt output. When the generator switch is released from RESET to ON, the differential voltage circuit compares the generator output and the center bus voltages, then closes the line contactor if the generator is within acceptable limits of the center bus voltage.

Reverse  current  protection  circuitry  opens  the line contactor and disconnects the generator if a reverse current condition occurs. If the condition corrects itself, the line contactor  automatically closes again.

The paralleling circuit provides load equalization between both generators.

The overvoltage protection circuit senses the generator output voltage and de-excites the generator and opens the line contactor if an overvoltage occurs. If the generator is disconnected for overvoltage, select GEN RESET, then, ON to reset the generator.

L DC GEN and R DC GEN annunciators indicate the line contactor is open and the generator is off line. Load meters in the overhead instrument panel indicate the load applied to each generator as a percent of generator rating.

The bus-tie systems consists of the left generator bus tie, the right generator bus tie, the battery bus tie, and their associated control components.

The battery bus tie connects the battery to the center bus. When the battery switch ON, the battery relay closes and (1) provides power to the triple-fed bus, and (2) closes the battery bus tie which connects the battery to the center bus.

The generator buses connect to the center bus with the left and right bus-tie relays. The three- position, generator, bus-tie control switch on the pilot outboard subpanel is placarded GEN TIES– OPEN–NORM– MAN CLOSE. When the battery switch is first turned on and the GEN TIES switch is in the NORM position, the left and right genera- tor buses are not powered. Momentarily selecting the MAN CLOSE position closes both generator bus ties, powers the generator buses from the battery, and illuminates the MAN TIES CLOSED annunciator. In the NORM position, when either generator or the external power comes on line, the left and right bus-tie relays automatically close. If the generator ties are manually closed, the MAN TIES CLOSED annunciator extinguishes. The OPEN position causes both the left and right bus- tie relays to open, isolating the generator buses from the center bus and illuminating  LEFT & RIGHT GEN TIE OPEN annunciators. With both generator bus ties open, the battery is also isolated from the left and right generator bus.

 Modes of closing the generator bus ties.

    MANUALLY—WITH GEN-TIE SWITCH TO THE CLOSE POSITION

    AUTOMATICALLY—WITH AT LEAST ONE GENERATOR ON THE LINE

    AUTOMATICALLY—WITH GPU POWER ON THE AIRCRAFT

Bus current sensors are installed which sense current to each generator bus from the center bus and current from the battery to the center bus. If either generator bus sensor detects a high-current condition, it opens the corresponding bus tie to isolate the bus. If the battery bus sensor detects a high battery discharge current, it opens the battery bus tie to isolate the battery from the center bus. The bus current sensors are controlled with the three position bus sense control switch on the pilot outboard subpanel and placarded BUS SENSE–TEST– NORM–RESET. Momentarily selecting the TEST position tests the bus current sensors causing the generator bus ties and battery tie to open. Momentarily selecting the RESET position resets the bus current sensors if they have been tripped as a result of a test or an actual high- current condition. L GEN TIE OPEN, R GEN TIE OPEN, and BAT TIE OPEN annunciators indicate the positions of the bus ties. Illumination of  an  annunciator  indicates  the  corresponding bus tie is open. The MAN TIES CLOSE annunciator indicates the generator bus ties have been manually  closed  during  battery  operation  and the battery is providing any electrical load to the generator buses.

Bus-Sense System

The electrical  system is protected  from excessively high currents by the bus-tie system. Three, unidirectional, current sensors are utilized; one for each generator bus and a third for the battery. Each current sensor consists of a HED and solid state circuitry encapsulated into a module assembly. It senses one way overcurrent in the monitored circuit.

Within the sensor, a current of 275, ± 5 amps or greater activates a solid state switch and places a ground on the coil of a control relay on the bus-tie control PCB. This causes it to energize and open the coil circuit of the associated generator/battery bus-tie relay. The affected generator/battery bus- tie relay opens and the appropriate GEN–BAT TIE OPEN light on the annunciator illuminates. The coil circuit of the control relay is latched in to prevent the bus-tie relay from reclosing. All three current sensors are deenergized during engine starts and the BAT TIE sensor is deenergized during landing gear operation.

Placing the BUS SENSE control switch, on the pilot outboard subpanel, in the TEST position momentarily connects bus voltage to the test circuit built in to each of the three current sensors. This voltage simulates a high current through each bus-tie relay. The solid state switch of the sensor activates to energize the control relay, opening the coil circuit of the bus-tie relays, and illuminating the annunciator lights. The reaction time of the sensor modules is approximately 0.010 seconds for the generator current sensors and 0.120 sec- onds for the battery current sensor. Since reaction time for the system is limited to reaction time for the relays, only momentary actuation of the TEST switch is required. If the switch is held in the TEST position, voltage is continuously applied to the test circuit. Prolonged application of this voltage can possibly damage or destroy the sensor module.

Momentary actuation of the switch to the RESET position removes power from the coil of the control relays permitting them to deenergize. Voltage then switches  from the  annunciator lights  and is supplied to the bus-tie relay coils, closing the bus-tie relays.

 

The generator bus-tie  relays  may  be manually opened or closed with the switch on the left outboard subpanel placarded BUS-TIE–OPEN– MAN CLOSE. Manually closing the generator bus-tie  relays  connects  the  generator  buses  to the center bus and permits the application of battery and/or external power to the entire system. Momentarily placing the switch in the MAN CLOSE position applies bus voltage to the coil of a control relay, completes a latching circuit, energizes a bus-tie close relay, and activates the MAN TIES CLOSE annunciator light. The latching circuit is completed through the normally closed contacts  of the control relay for the generator bus-tie contactors. The generator bus-tie relays cannot be manually closed if either generator is in operation.

The DC generation system makes use of DC starter-generators,  power  panels,  generator control panels, battery bus-tie relay, current sensor, control switches, a voltmeter, load meters, annunciator lights, and the bus-tie and cross-start printed circuit boards.

 

Proper operation of each of these components is essential for satisfactory generator operation. Many  different component  failures  can  result in similar generator system response, making it extremely difficult to isolate a faulty component merely by analyzing the system responses. The generator provides electrical power to recharge the aircraft battery and operate the aircraft constant-voltage  source  for  proper  operation. The generator output is maintained at a constant level by controlling the shunt field excitation. The interpole and compensating windings of the generator are in series with the armature and provide a voltage proportional to the generator current.

 

The generator is controlled through the generator control switches under the master switches gang bar with the battery switch. If the generator is taken  off line  while  the  generator  is  running, the generator control switch must be placed in RESET to bring the generator back on line.

 

Components

 

Generator Control Unit

The generator control units (GCUs) are mounted in the center aisle subfloor just aft of the main spar. The GCUs control all generator operations. In addition, the GCUs provide the following:

•  Voltage regulation

•  Generator load sharing

 

•  Differential  voltage  and  reverse  current sensing and control

•  Overvoltage and overexcitation protection

•  The field flash circuit

•  Cross-start overload protection.

Operation

 

Voltage Regulation

The generator output voltage is sensed at the generator side of the generator line contactor and powers the voltage regulator circuit of the control unit at pin J. It, also, senses input to the control unit at pin B. The regulator circuit of the control unit supplies the generator field excitation current required to supply current for the electrical loads and to maintain a bus voltage of 28.25, ± 0.25 VDC.

PTP  Beech 90 Series	B1	LOC	FOT	SGH	R/I	MEL	TS
	B2	LOC	FOT	SGH	R/I	MEL	TS