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ATA 24
ELECTRICAL POWER
© Jazz Aviation LP, 2011. All rights reserved. No part of these materials may be reproduced, stored in any material
form (including photocopying or storing on any medium by electronic means) without the prior written permission of the
copyright holder, except in accordance with the provisions of the Copyright Act. The doing of any unauthorized act in
relation to a copyright work may result in a civil claim for damages and/or criminal prosecution.
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Page 2© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
DC EXTERNAL POWER SYSTEM . . . . . . . . .48
Description . . . . . . . . . . . . . . . . . . . . 48
DC External Power Receptacle . . . . . . . . .50
Controls and Indications . . . . . . . . . . . . .50
EIS DC External Power Indications . . . . . . .52DC ELECTRICAL LOAD DISTRIBUTION . . . . . .54
Description . . . . . . . . . . . . . . . . . . . .54
28 VDC Circuit Breaker Panels . . . . . . . . .56
28 VDC Avionics Circuit Breaker Panel . . . . .58
AC VARIABLE FREQUENCY SYSTEM . . . . . . .60
Description . . . . . . . . . . . . . . . . . . . . 60
AC Generators . . . . . . . . . . . . . . . . . .64
AC Generator Adapter Plate . . . . . . . . .66
AC Generator Control Units (GCU) . . . . . . .66Voltage Regulation . . . . . . . . . . . . . .66
Control Of The AC Generator
Line Contactor . . . . . . . . . . . . . . . . 66
Protection. . . . . . . . . . . . . . . . . . .68
AC Control Panel . . . . . . . . . . . . . . . .70
AC Contactor Boxes . . . . . . . . . . . . . . .72
AC Contactor Box Contactors . . . . . . . . . .74
AC Generator Line Contactors . . . . . . . .74
External Power Contactors . . . . . . . . . .74
Galley Load Shed Contactors . . . . . . . .74
Controls And Indication . . . . . . . . . . . . .76
AC Voltage . . . . . . . . . . . . . . . . . .76
AC Load . . . . . . . . . . . . . . . . . . .76
AC GEN Caution Lights . . . . . . . . . . .76
AC BUS Caution Lights . . . . . . . . . . .78
AC GEN HOT Caution Lights . . . . . . . .78
External Power Receptacle . . . . . . . . . . .80
Table of Contents
EPGDS . . . . . . . . . . . . . . . . . . . . . . . 6
Description . . . . . . . . . . . . . . . . . . . . 6
BATTERY SYSTEM . . . . . . . . . . . . . . . . . 8Description . . . . . . . . . . . . . . . . . . . . 8
Main and Auxiliary Batteries . . . . . . . . . . .10
Battery Containment Vessel . . . . . . . . . . .12
Standby Battery . . . . . . . . . . . . . . . . . 14
Controls and Indications . . . . . . . . . . . . .16
Battery Load Indication . . . . . . . . . . . . 16
Battery Temperature Indication . . . . . . . .16
STBY BATTERY, AUX BATTERY,
MAIN BATTERY Caution Lights . . . . . . .18
STBY BATT HOT, AUX BATT HOT,MAIN BATT HOT Warning Lights: . . . . . .18
MAIN 28 VDC GENERATION SYSTEM . . . . . . .20
Description . . . . . . . . . . . . . . . . . . . . 20
DC Starter/Generator . . . . . . . . . . . . . .22
DC Generator Control Units (GCU) . . . . . . .24
Electrical Power Control Unit (EPCU) . . . . . .26
DC Control Panel . . . . . . . . . . . . . . . . 28
DC Transformer Rectier Units (TRU) . . . . . .30
DC Contactor Box . . . . . . . . . . . . . . . .32Standby Contactor Box . . . . . . . . . . . . .36
Controls and Indications . . . . . . . . . . . . .38
Caution Lights . . . . . . . . . . . . . . . . 40
APU 28VDC GENERATION SYSTEM . . . . . . .42
Description . . . . . . . . . . . . . . . . . . . .42
APU DC Generator . . . . . . . . . . . . . . .44
Controls and Indications . . . . . . . . . . . . .46
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Page 3© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
External AC Power Protection Unit . . . . . . .80
Controls And Indications . . . . . . . . . . . . .82
ELECTRICAL LOAD DISTRIBUTION . . . . . . . .84
Description . . . . . . . . . . . . . . . . . . . . 84
115 VAC Variable FrequencyCircuit Breaker Panel . . . . . . . . . . . . .84
Operation . . . . . . . . . . . . . . . . . . . . 84
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Page 4© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Figure 23 – DC EXT. Power -
Control & Receptacle . . . . . . . . . . . . . .51
Figure 24 – EIS Indication - DC External Power .53
DC ELECTRICAL LOAD DISTRIBUTION
Figure 25 – DC Electrical Load Distribution . . .55Figure 26 – Left & Right DC Circuit
Breaker Panels. . . . . . . . . . . . . . . . . . 57
Figure 27 – 28 VDC Avionics C/B Panel . . . . .59
AC VARIABLE FREQUENCY SYSTEM
Figure 28 – AC Variable Frequency System . .61
Figure 29 – AC Generation - Normal Operation .63
Figure 30 – AC Generator . . . . . . . . . . . .65
Figure 31 – AC GCU Location . . . . . . . . . .67
Figure 32 – AC GCU . . . . . . . . . . . . . . .69Figure 33 – AC Control Panel . . . . . . . . . .71
Figure 34 – AC Contactor Box (ACCB) . . . . .73
Figure 35 – ACCB Contactors . . . . . . . . . .75
Figure 36 – AC Control and Indications . . . . .77
Figure 37 – Caution Lights . . . . . . . . . . . .79
Figure 38 – AC External Power
Receptacle and EPPU . . . . . . . . . . . . . . 81
Figure 39 – AC External Power - Controls and
Indications . . . . . . . . . . . . . . . . . . . . 83
ELECTRICAL LOAD DISTRIBUTION
Figure 40 – AC Distribution . . . . . . . . . . .85
List of Figures
EPGDS
Figure 1 – EPGDS Block Diagram . . . . . . . . 7
BATTERY SYSTEMFigure 2 – Battery System . . . . . . . . . . . . 9
Figure 3 – Main and Aux Batteries . . . . . . . . 11
Figure 4 – Battery Containment Vessel . . . . .13
Figure 5 – Standby Battery . . . . . . . . . . .15
Figure 6 – Controls and Indications . . . . . . .17
Figure 7 – Caution & Warning Lights - Batteries .19
MAIN 28 VDC GENERATION SYSTEM
Figure 8 – Main DC Generation System . . . . .21
Figure 9 – DC Starter Generator . . . . . . . . .23Figure 10 – DC Generator Control Unit (GCU) .25
Figure 11 – Electrical Power
Control Unit (EPCU) . . . . . . . . . . . . . . . 27
Figure 12 – DC Control Panel . . . . . . . . . .29
Figure 13 – Transformer Rectier Unit (TRU) . .31
Figure 14 – DC Contactor Box . . . . . . . . . .33
Figure 15 – DC Contactors . . . . . . . . . . .35
Figure 16 – STBY Contactor Box . . . . . . . .37
Figure 17 – Controls and Indications . . . . . .39
Figure 18 – Caution Lights . . . . . . . . . . . .41
APU 28VDC GENERATION SYSTEM
Figure 19 – APU Operation . . . . . . . . . . .43
Figure 20 – APU Starter/Generator . . . . . . .45
Figure 21 – Controls and Indications . . . . . .47
DC EXTERNAL POWER SYSTEM
Figure 22 – DC External - Schematic . . . . . .49
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Page 5© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
THIS PAGE INTENTIONALLY LEFT BLANK
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Page 6© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
The AC variable frequency generators make 115 VAC
three phase electrical power for AC systems that are
not frequency sensitive, such as deicing heaters,
fuel auxiliary pumps, the Standby Power Unit (SPU)
hydraulic pump and the galleys. They also energize theTransformer Rectier Units (TRU), which are part of the
Main 28 Vdc Generation System to supply 28 Vdc.
The Main 28 Vdc Generation System is energized by
theses sources:
Two engine driven starter/generators●
Two Transformer Rectier Units (TRU)●
Three NiCad batteries●
Auxiliary Power Unit (APU).●
EPGDS
The Electrical Power Generation and Distribution System
(EPGDS) supplies electrical energy to all electrical
equipment through an AC Variable Frequency System
and Main 28 Vdc Generation System.
The EPGDS does energy conversion, distribution,
storage, control, protection, monitoring, and indication.
Description
The EPGDS has these sub-systems:
Alternating current variable frequency●
Main 28 VDC generation●
Battery●
Auxiliary Power Unit (APU) 28 Vdc●
External AC ground power ●
External DC ground power ●
AC electrical load distribution●
DC electrical load distribution.●
The EPGDS has connections AC for and DC external
power while on the ground.
Note: The AC external power source does not supply
electrical power for engine starting.
The power is distributed by an electrical bus system.
It recongures when an electrical power source or bus
malfunction, by the automatic closing and opening of bus
tie contactors.
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Page 7© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
EPGDS Block DiagramFigure 1 –
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Page 8© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
The Battery system has these components:
Battery and Box, Main●
Vent and Drains, Main Battery●
Battery and Box, Auxiliary●
Vent and Drains; Auxiliary Battery●
Battery and Box, Standby●
Vent and Drains, Standby Battery.●
The contactors to connect the main and auxiliary
batteries are located in the DC Contactor Box (DCCB)
and the contactors that control the standby battery
operation are located in the Standby Battery Contactor
Box (SBCB)
The Multi-Function Display (MFD) electrical system page
and the caution and warning lights system show battery
system indications.
The Electrical Power Control Unit (EPCU) supplies
indication data through the two Input/Output Processors
(IOP1, IOP2) located in the Integrated Flight Cabinets
(IFC) to the Electronic Instruments System (EIS).
BATTERY SYSTEM
The battery system supplies aircraft engine start power,
emergency 28 Vdc electrical power to the aircraft DC
essential buses and to the No. 3 Standby Hydraulic
Pump.
Description
The battery power system of the aircraft has three
batteries and switching devices to supply electrical power
to the essential buses during emergency ight conditions.
The main, auxiliary, and standby batteries are connected
to the main feeder buses to receive a charging current.
The main and the auxiliary batteries also connected
to the main feeder buses to supply electrical power tostart the engines. During an engine start condition, the
standby battery is isolated from the main feeder bus,
but stays connected to the essential buses to maintain a
minimum voltage level on the essential bus.
The standby battery has a related battery power bus in
the right DC circuit breaker panel that is energized at all
times. The auxiliary and main batteries have a related
battery power bus in the left DC circuit breaker panel that
is energized when the AUX BATT or MAIN BATT toggleswitch is set to on.
The Engine and System Integrated Display (ESID)
system shows main 28 Vdc generation system electrical
indications
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Page 9© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Battery SystemFigure 2 –
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Page 10© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
After a long discharge period, the initial charge current
can peak at 200 A. The current rapidly decreases as the
battery starts to charge.
During the discharge mode, the battery supplies electrical
power. The battery output voltage decreases as thecurrent load increases.
Note: The voltage at the battery terminals cannot be
used as an indication of the battery state of
charge.
The battery has nickel and cadmium (NiCad) plates with
potassium hydroxide electrolyte. The NiCad battery has
relatively constant voltage for most of the discharge
condition.
A main power positive and negative electrical connection
to the battery terminals is made with a power plug
connector attached to the case. The batteries have
two independent internal sensors that sense battery
temperature. A Resistive Temperature Device (RTD)
thermal sensor is used to measure the battery
temperature for an EIS indication and a bimetallic
temperature switch is used to sense a temperature
exceedance for a warning indication. The internal sensor
wires are routed through a circular connector attached tothe battery case.
The left lower nose compartment door must be opened
to access the main and auxiliary batteries. The auxiliary
battery is located aft of the main battery. It is secured to
the airframe in the nose compartment with tabs that are
part of the battery case cover.
Main and Auxiliary Batteries
The 40 A hour batteries have a rated discharge of 40
A for one hour. The discharge rate is not linear. The
capacity is usually less than the rated value when the
discharge current is more than the nominal value.
The left lower nose compartment door must be opened to
access the main and auxiliary batteries. The main battery
is located forward of the auxiliary battery. The batteries
are secured to the airframe in the nose compartment with
tabs that are part of the battery case cover. The case is
a steel box with a removable cover to give access for
inspection.
It also has a vent port that attaches to a tube that routes
the gas overboard through a containment vessel.
The main and auxiliary batteries are 40 A hour batteries
with an assembly of individual cells contained in a steel
case. Each cell is connected in series to make 24 Vdc at
the terminals. Solid copper bus bars are used to connect
the cells. They weigh 76.51b (34.7 kg) and are 10.25 in.
(260.4 mm) wide, 10.5 in. (266.7 mm) long and 9.7 in.
(246.4 mm) high.
The main and auxiliary batteries are self contained powersources that operate in the charge and discharge modes.
The main 28 Vdc generation system supplies electrical
power to the battery during the charge mode. The
amount of charge current is determined by the state of
charge and the internal resistance of the battery.
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Page 11© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Main and Aux BatteriesFigure 3 –
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Page 12© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Battery Containment Vessel
The left lower nose compartment door must be opened
to access the containment system. It is secured to the
battery shelf with three mounting screws.
The overow containment system protects the aircraft
structure and other parts from the corrosive battery
gases and uid.
Each battery has a vent port that attaches to a tube to
route its gases through a containment vessel overboard.
The electrolyte in the battery is potassium hydroxide and
water. The containment system has a bottle with a boric
acid and water wetted sponge to neutralize the battery
electrolyte.
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Page 13© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Battery Containment VesselFigure 4 –
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Page 14© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
rated value when the discharge current is more than the
nominal value.
Note: The voltage at the battery terminals cannot be
used as an indication of the battery state of
charge.
The battery has nickel and cadmium (NiCad) plates with
potassium hydroxide electrolyte. The NiCad battery has
relatively constant voltage for most of the discharge
condition.
A main power positive and negative electrical
connection to the battery terminals is made with a
power plug connector attached to the case. The battery
has two independent internal sensors that sense
battery temperature. A Resistive Temperature Device
(RTD) thermal sensor is used to measure the battery
temperature for an EIS indication and a bimetallic
temperature switch is used to sense a temperature
exceedance for a warning indication. The internal sensor
wires are routed through a circular connector attached to
the battery case.
Standby Battery
The left upper nose compartment door must be opened
to access the standby battery. The battery is secured to
the airframe in the nose compartment with tabs that are
part of the battery case cover. The case is a steel boxwith a removable cover to give access for inspection.
It also has a vent port that attaches to a tube that routes
the gases overboard through a containment vessel. The
battery is a 17 or 40 A hour battery with an assembly
of individual cells contained in a steel case. Each cell
is connected in series to make 24 Vdc at the terminals.
Solid copper bus bars are used to connect the cells.
The 17 ampere hour standby battery weighs 38.5 Ib
(17.46 kg). It is 7.7 in. (195.6 mm) wide, 8.5 in. (215.9
mm) long and 9.75 in. (247.7 mm) high.
The Standby Battery is a self contained power source
that operates in the charge and discharge modes. The
main 28 Vdc generation system supplies electrical power
to the battery during the charge mode. The amount of
charge current is determined by the state of charge
and the internal resistance of the battery. After a long
discharge period, the initial charge current can peak at
200 A. The current rapidly decreases as the battery startsto charge.
During the discharge mode, the battery supplies electrical
power. The battery output voltage decreases as the
current load increases. The 17 A hour batteries have
a rated discharge of 17 A for one hour. The discharge
rate is not linear. The capacity is usually less than the
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Page 15© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Standby BatteryFigure 5 –
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Page 16© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
symbol before the numbers is used to show an overload
condition of the battery. The symbol is out of view
when the load is less than 100 0/0. A - symbol shows a
discharge condition.
Battery Temperature Indication
The battery temperature indication shows the battery
temperature in degrees Celsius in white numbers. The
indication changes to yellow when temperature is more
than 50 and less than 65°C. If the temperature is more
than 65 °c, the indication will change to red.
Note: The Resistive Thermal Device (RTD) supplies
temperature data through the EPCU and
FDPS to the EIS.
Note: When the voltage or load data malfunctions, the
digits are replaced by white dashes
Controls and Indications
The battery toggle switch located on the DC control panel
is set to BATTERY MASTER position to let the standby,
auxiliary and main batteries energize the essential buses
if no other DC power source is available.
The battery toggle switches are set to STBY BATT, AUX
BATT, and MAIN BATT positions to let the standby,
auxiliary and main batteries connect to the main feeder
buses to receive a charging current.
Note: The BATTERY MASTER, AUX BATT, or MAIN
BATT toggle switch is set to on to energize the
battery power bus in the left DC circuit breaker
panel.
The MFD1 and MFD2 reversion switches located on
the ESID Control Panel (ESCP) are used to select the
electrical page on the multi-functional displays.
The Multi-Function Display (MFD) electrical system page
shows these battery system indications:
Load●
Temperature.●
The battery indication has a BATT title in white letters.The indication has MAIN, AUX and STBY titles in white
fonts to show the load and temperature of the main,
auxiliary and standby batteries.
Battery Load Indication
The battery load indication shows the battery load in
white numbers. The numbers show the battery output
load as a percentage of the total rated output. A +
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Page 17© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Controls and IndicationsFigure 6 –
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Page 18© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
The battery system malfunctions are shown with these
caution lights:
STBY BATTERY●
AUX BATTERY●
MAIN BATTERY.●
STBY BATTERY, AUX BATTERY, MAIN BATTERY
Caution Lights
The STBY BATTERY, AUX BATTERY or MAIN BATTERY
caution lights come on to show that the battery is not
connected to its main feeder bus for charging.
The caution lights receive their signals from the related
contactor K25, K7 and K8.
The battery system malfunctions are shown with these
warning lights:
STBY BATT HOT●
AUX BATT HOT●
MAIN BATT HOT.●
STBY BATT HOT, AUX BATT HOT, MAIN BATT HOT
Warning Lights:
The STBY BATT HOT, AUX BATT HOT or MAIN BATTHOT warning lights come on to show that the battery
temperature is more than 71 °c. It goes out when the
temperature is less than 63°C.
The inputs to these warning lights come directly from the
bi-metal temperature switch located in each battery.
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Page 19© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Caution & Warning Lights - BatteriesFigure 7 –
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Page 20© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
DCCB Contactors●
Standby Battery Contactor Box (8BCB)●
DC Current Transformers (DC CT)●
Current shunts.●
The Multi-Function Display (MFD) electrical system page
and the caution and warning lights system show main 28
Vdc generation system indications.
The direct current (DC) system operates in these modes:
Normal●
Protection●
Fault Tolerant●
Emergency●Diagnostics and Monitoring.●
MAIN 28 VDC GENERATION SYSTEM
The main 28 Vdc generation system makes and supplies
28 Vdc electrical power to the aircraft DC buses.
DescriptionThe main 28 Vdc generation system has two DC starter/
generators, one on each engine, and two Transformer
Rectier Units (TRU) to supply electrical power
independently to the left and right DC bus systems.
When a DC starter/generator or TRU malfunctions, the
main 28 Vdc generation system automatically connects
its bus to another serviceable electrical source for
continuous operation.
A DC external power receptacle and control circuitry
allows the system to be powered from an external power
source while the aircraft is on the ground.
Toggle switches located on the DC control panel in
the ight compartment give manual control of the main
28 Vdc generation system. The Engine and System
Integrated Display (ESID) system shows main 28 Vdc
generation system electrical indications.
The Main 28 Vdc Generation System has these units:DC Starter/Generators (GEN)●
DC Generator Control Units (DC GCU)●
Electrical Power Control Units (EPCU)●
DC control panel●
DC Transformer Rectier Units (TRU)●
DC Contactor Boxes (DCCB)●
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Page 21© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
Main DC Generation SystemFigure 8 –
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Page 22© Jazz Aviation LP
Rev. 1.0 - Jan/2011
FOR TRAINING PURPOSES ONLY
ATA 24 - ELECTRICAL POWER
the DC starter/generator with its output voltage until the
DC GCU starts regulation. When regulating, the DC GCU
supplies a modulated current to the DC starter/generator
to maintain a 28.5 Vdc output. The DC starter/generator
has stator and rotor assemblies contained in a cylindrical
shaped housing. The housing is made of a high iron
content material. It is part of the magnetic circuit for the
starter assembly.
The rotor assembly has a rotor winding that connects to
the xed input terminals on the machine housing through
a brush/commutator assembly. The machine housing
has 4 poles with a four brush block assembly to switch
the input or output current to the correct rotor winding.
The rotor is held at each end by ball bearings that are
mounted in the housing. The ball bearings are prelubricated with grease when installed and service is not
necessary until overhaul.
The DC starter/generator has a fan to supply a forced
air ow axially through it to cool the rotor and starter
windings, the commutator assembly, and the bearings.
Air is drawn from outside the nacelle through NACA vents
to keep the operating temperatures satisfactory. The
brushes that are used in the DC starter/generator have
optimum life when blast cooling is supplied by propellerthrust to lower their temperature.
The DC starter/generator electrical power circuit has a
4 pole shunt that operates at 12,000 rpm. The electrical
circuit of the DC starter/generator has interpole windings
to help the ux path between the stator and rotor to
increase generator efciency. The DC GCU supplies
a control current to the shunt eld winding of the DC
DC Starter/Generator
The DC starter/generator attaches to the accessory
gearbox with a Quick-Attach/Detach (QAD) adapter that,
once installed, stays attached to the gearbox when DC
starter/generator is removed. The DC starter/generatorconnects to the QAD with a V band clamp to secure the
assembly. The QAD has an opening that connects to an
airframe duct to exhaust cooling air outside of the engine
nacelle.
The torque force is transmitted to the DC starter/
generator through a splined shaft that connects to the
engine accessory gearbox. The DC starter/generator’s
drive shaft has a shear section that breaks to protect the
accessory gear box drive train from damage when toomuch torque is supplied.
The DC starter/generator is an electromagnetic machine.
When it rotates, it will change engine input mechanical
torque to output DC electrical power. In the starter mode,
it changes input electrical DC electrical power into a
mechanical output torque. The DC starter/generator
operation is controlled and monitored by the DC
Generator Control Unit (GCU).
The DC starter/generator is a four-pole, shunt connected,fully compensated DC generator with interpole windings,
brush commutation, and an internal cooling fan. It is
attached to the aircraft accessory gearbox.
The DC starter/generator has a permanent magnet
so that it can make output voltage without an external
excitation source. The output from the DC starter/
generator is sent to the DC GCU. The DC GCU supplies
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ATA 24 - ELECTRICAL POWER
DC Starter Generator Figure 9 –
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ATA 24 - ELECTRICAL POWER
voltage to maintain a constant voltage at the Point of
Regulation (POR) under different load conditions. The DC
starter/generator speed and voltage will increase when
the engine is operating. When the output voltage is more
than ±1 Vdc of the bus voltage, the DC GCU will cause
the bus contactor to connect the DC starter/generator to
its main feeder bus.
When the two main feeder buses are connected together
through K21 by a MAIN BUS TIE toggle switch selection
or EPCU reconguration, a signal is sent to each DC
GCU to make the DC starter/generators load share.
The DC GCU regulates the DC starter/generator’s output
and gives these protective functions:
Parameter Value
Over voltage 32.5
Differential Feeder Fault
Detection
75 A ±25 A
Bus Fault Detection 800 A
Parallel Load Sharing 10% rated current
starter/generator to control its output voltage. The
shunt eld winding current is amplied by the rotational
energy of the engine to make the rated output of 400 A
at 30 Vdc. The DC starter/generator terminal voltage is
monitored by the EPCU for CBIT functions.Inputs and outputs of the stator windings are given by
connections on the body of the DC starter/generator. Low
current connections are made with a circular connector
and high power output connections are made to studs
mounted on a terminal block. A bonding stud on the
housing of the DC starter/generator is used to connect it
to the airframe ground potential.
The DC starter/generator has a temperature switch that
closes a set of contacts when the temperature of the DCstarter/generator is excessive. It gives continuity through
two pins on the DC starter/generator connector to make
the DC GEN HOT indication come on in the caution and
warning lighting panel.
DC Generator Control Units (GCU)
The DC GCU controls the DC starter/generator start and
regulation modes. The DC GCU senses the DC starter/
generator speed and it stops the start mode when the
speed of the engine is more than 50%) NH. At the end of
the start mode, the DC GCU opens the bus contactor and
de-energizes the DC starter-generator.
When the DC GCU senses a voltage output from the
DC starter/generator, a eld control contactor in the DC
GCU switches from the eld build up circuits to a voltage
regulator circuit. The voltage regulator is a closed loop
control circuit. It adjusts the duty cycle of the exciter eld
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ATA 24 - ELECTRICAL POWER
DC Generator Control Unit (GCU)Figure 10 –
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ATA 24 - ELECTRICAL POWER
are supplied through output drives that use current
limiting for isolation.
The critical aircraft operating functions use circuits with
usually closed contactors to let the ight compartment
toggle switches control the main 28 Vdc generationsystem when the EPCU is de-energized.
The voltage and current data is supplied to the Electronic
Instruments System (EIS) through the Integrated Flight
Cabinets (IFC1, IFC2) for indication. The data supplied
for indication is not used for the control functions.
The EPCU has daughter printed circuit card assemblies
and a mother board contained in an aluminum enclosure.
The daughter printed circuit card assemblies connect to
a mother board that connects to interface connectors.The daughter circuit card assemblies connect to the
motherboard using thermal card guides for support and
thermal dissipation.
The EPCU is cooled by conduction through the mounting
feet of the unit to the airframe and by natural convection
through cooling holes near high power dissipation
devices. The high power devices are attached to heat
sinks on the circuit card assemblies that contact the
thermal card guides when it is installed.
The box enclosure has three connectors that connect
to the remaining part of the system. One connector is
used for the APU interface functions. A carrying handle
is provided for transporting the EPCU. The EPCU has
hard mounting points as an alternative to rack mounting.
The EPCU has an interface panel with two push-button
The DC Generator Control Unit (GCU) contains the
voltage regulation and protection circuits for the DC
starter/generator. Printed circuit cards, discrete power
semiconductors, and electromechanical contactors are
put in an aluminum box without the need for convectioncooling through the box.
The DC GCU is cooled by conduction through the
chassis mounting locations and by convection of air over
the surface of the box. Power dissipating components
in the DC GCU are attached to chassis heat sinks to
transfer heat to it. Electrical connections are made with
a single connector that is attached to the end of the DC
GCU enclosure.
Electrical Power Control Unit (EPCU)
The EPCU manually or automatically controls the
operation of some contactors in the main 28 Vdc
generation system. It manually controls contactors by
ight compartment toggle switch selections.
For example, a manual external DC power selection
causes the EPCU to close the external power contactor.
Automatic functions are used to reduce pilot work load
and to start protective functions that cannot tolerate thetime delay associated with a pilot selection. For example,
the EPCU automatically causes the external power
contactor to open when an over voltage condition is
sensed. Digital logic control is used as the primary control
of the main 28 Vdc generation system. Analogue to
digital circuits in the EPCU changes the analogue inputs
to digital format to control the system. The output signals
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ATA 24 - ELECTRICAL POWER
Electrical Power Control Unit (EPCU)Figure 11 –
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ATA 24 - ELECTRICAL POWER
double throw switch. The toggle lever is connected to
a mechanism located in the body of the switch that
moves in the opposite direction as the toggle lever. The
wires from the receptacle connector are connected to
the switch terminals with lugs except the bus fault reset
toggle switch that has solder wire connections. The
toggle switches are attached to the backing panel with
mounting nuts and lock washers.
switches and a digital display on its front face to access
fault codes.
The Electrical Power Control Unit (EPCU) records main
28 Vdc generation system fault conditions. It has an
interface panel on its front face to look at the fault codes
DC Control Panel
The DC control panel is attached to the overhead console
using four DZUS fasteners. The fasteners and a bonding
wire are directly connected to the chassis to make a
ground continuity connection between the backing panel
and the aircraft structure.
The DC control panel gives manual control of the main
28 Vdc generation system. It has these toggle switchesto control the systems:
Batteries●
DC starter/generators●
External DC power ●
Main feeder bus tie●
Bus fault reset.●
The DC starter/generator, standby, auxiliary, and main
battery toggle switches are positive break, two pole,
double throw, toggle switches. The battery master toggle
switch is a four pole, double throw switch.
The main bus tie and DC external power toggle switches
are positive break, single pole, double throw toggle
switches and the main bus tie toggle switch has a
locking mechanism that locks the selection to the set
position. The bus fault reset toggle switch is a four pole,
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ATA 24 - ELECTRICAL POWER
DC Control PanelFigure 12 –
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ATA 24 - ELECTRICAL POWER
a maximum of 10 seconds. The AC generator output
voltage is reduced proportionally with the frequency to
prevent damage to the TRU.
No active control to connect or disconnect the TRU from
the AC generator is necessary. A short circuit in the TRUor related aircraft wiring will open its 3 phase AC circuit
breaker.
The TRU has a thermal sensor to automatically control
the cooling fan. The cooling fan draws in ambient air and
exhausts it outside the aircraft through a ange on the
TRU attached to an airframe duct. If the TRU overheats,
the thermal sensor supplies a signal to the caution and
warning lights system.
DC Transformer Rectier Units (TRU)
The TRU are located in the nose of the aircraft. They are
accessed by opening the upper right nose compartment
door. The TRU enclosure has a rectangular shape, with
the cooling air ow driven from the front face along thelength of the enclosure to the rear face, where provision
is included to attach ducting for routing of the hot exhaust
air. The cooling air is forced through the unit using a fan
mounted in the TRU at the front face. This fan is powered
from within the TRU without the need for external
activation and is thermostatically controlled so as to
only be operational when the unit requires cooling. The
enclosure provides for mounting by incorporating four
hard mounting feet that bolt to the aircraft structure.
The Transformer Rectier Unit (TRU) is a self contained,
solid state power conversion device that changes 115
VAC variable frequency AC electrical power to 28 Vdc
electrical power.
The TRU receives three phase 115 VAC power through
a circular connector on the front face of the enclosure.
The output 28 Vdc power is available through positive
and negative studs also located on the front face of the
enclosure.The TRU supplies an indication of its status for indication
in the ight compartment through the EPCU.
The TRU stays energized during AC generator under
frequency and under voltage condition. When the
frequency and voltage are decreased because of AC
generator under speed operation during propeller
feathering, the TRU stays connected to the load for
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ATA 24 - ELECTRICAL POWER
Transformer Rectier Unit (TRU)Figure 13 –
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ATA 24 - ELECTRICAL POWER
The DCCB contains these components:
Diodes●
Current transformers●
DC circuit breakers●
Protective fuses●
DC starter/generator bus tie contactors, K1 and K2●
Main feeder bus tie contactor, K21●
Secondary feeder bus tie contactor, K22●
Secondary/main feeder bus tie contactors, K5 and K6●
DC external power contactor, Kg●
Battery bus tie contactor, K7 and K8.●
DiodesDiodes are electronic devices that are used to isolate the
direct current power sources.
Current Transformers
Two current transformers, identied CT1 and CT3,
located in the DCCB, are used to sense the current
ow in the main feeder buses. The main feeder current
transformers supply current data to the EPCU differential
current sense circuits. Each current transformer
assembly is a toroidal (ring-type) current transformers.
The cable from each phase of the system is passed
through a toroid. A multi-pin connector is used to make
electrical connections between the toroids and the other
components. The toroidal current transformers are
contained in an aluminum case and have mounting holes
that are not symmetrical to prevent improper installation.
DC Contactor Box
The DC Contactor Box (DCCB) is installed in the nose
compartment and is secured to the airframe with six
mounting bolts. It weighs 76 Ib (34.47 kg) and it is 47.5
in. (1.2 m) long, 9.2 in. (233.7 mm) high and 16 in. (406.4mm) wide.
The DCCB’ is not a Line Replaceable Unit (LRU). It
does not require replacement unless it is damaged. The
components in the DCCB are easily accessed when its
covers are removed. The components in the DCCB give
control and distribution of the DC power source from the
two DC starter/generators transformer rectier units, main
and auxiliary batteries, or DC external power source.
The DC Contactor Box (DCCB) assembly is used todistribute and control the main 28 Vdc electrical power.
The DCCB contains monitor and protection components
in a protected environment. The box has covers to allow
access to the internal Line Replaceable Units (LRU). The
electrical power connections between contactors in the
DCCB is made with bus bars. They are attached by a
mounting plate assembly that secures the bus bars in the
DCCB. Standard cable is used to make the connections
between the mounting plate assembly and the boxconnectors. The DCCB has circuit breakers to protect
the feeder cables that are routed directly to the aircraft
equipment or to the circuit breaker panels in the ight
compartment. The DCCB cover must be removed to get
access to the circuit breakers.
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ATA 24 - ELECTRICAL POWER
DC Contactor BoxFigure 14 –
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ATA 24 - ELECTRICAL POWER
DC Circuit Breakers
The circuit breakers are installed in the DCCB to protect
the feeder cables that are routed to the circuit breaker
panel in the ight compartment. Protective Fuses: Two
fuses are installed in the DCCB to protect the feedercables that go to the loads. A 100 A fuse protects the
ECS recirculation fan and a 150 A fuse protects the TRU
and the secondary feeder bus tie contactor, K22.
DC Contactors
The contactors are attached to the bus bars with
mounting screws and to a connector with two mounting
screws.
Each contactor has 400 A single pole, single throwcontact arrangement with 2 sets of 5 A auxiliary contacts.
Coil windings are used to move the contact arms.
The DC contactors in the DC contactor box connect the
DC electrical sources to the main and secondary feeder
buses.
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ATA 24 - ELECTRICAL POWER
DC ContactorsFigure 15 –
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ATA 24 - ELECTRICAL POWER
The SBCB contains these components:
Diode, CR 6, CR7, CR8●
Circuit Breakers●
DCMP power contactor, K23●
Main Bus contactor, K25●
Essential Bus contactor, K24.●
Diodes
Diodes are electronic devices that are used to isolate the
direct current power sources. A 100 A diode is located in
the SBCB. It is connected between the standby battery
and the left essential distribution bus. The diode allows
power to reach the essential bus but prevents the left
essential bus from being the source of power.
DC Circuit Breakers
The circuit breakers are installed in the DCCB to protect
the feeder cables that are routed to the hydraulic DCMP
and circuit breaker panel in the ight compartment.
DC Contactors
The contactors are attached to the bus bars with
mounting screws and to a connector with two mounting
screws. Each contactor has 400 A single pole, singlethrow contact arrangement with 2 sets of 5 A auxiliary
contacts. Coil windings are used to move the contact
arms. The DC contactors in the standby contactor box
connect the standby battery to the hydraulic DCMP and
battery buses in the ight compartment.
Standby Contactor Box
The Standby Contactor Box (SBCB) is installed in the
nose compartment or aft fuselage compartment and
is secured to the airframe with four mounting bolts. It
weighs 14 Ib (6.35 kg) and it is 12.1 in. (307.3 mm) long,5.9 in. (150 mm) high and 12.03 in. (305.56 mm) wide.
The Standby Contactor Box (SBCB) energizes the No.
3 hydraulic system DC Motor Pump (DCMP) and battery
buses in the ight compartment. The SBCB has monitor
and protection components in a protected environment.
The box has a cover to allow access to the internal Line
Replaceable Units (LRU).
The electrical power connections between contactors in
the SBCB is made with bus bars. They are attached by amounting plate assembly that secures the bus bars in the
SBCB. Standard cable is used to make the connections
between the mounting plate assembly and the box
connectors. The SBCB uses three circular connectors to
interface with the other aircraft system components in the
main 28 Vdc generation system. The SBCB has circuit
breakers to protect the feeder cables that are routed
directly to the aircraft equipment or to the circuit breaker
panels in the ight compartment. The SBCB cover mustbe removed to get access to the circuit breakers.
The SBCB is not a Line Replaceable Units (LRU). It
does not require replacement unless it is damaged. The
components in the SBCB are easily accessed when its
covers are removed. The components in the SBCB gives
control and distribution of the DC power source from the
standby battery.
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ATA 24 - ELECTRICAL POWER
STBY Contactor BoxFigure 16 –
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ATA 24 - ELECTRICAL POWER
DC Voltage Indication
The DC voltage indication shows the DC essential, main,
and secondary bus voltage in white numbers.
DC Starter/Generator and TRU Load Indication
The DC starter/generator load indication shows the DC
starter/generator output load. The numbers show the DC
starter/generator output load as a percentage of the total
rated output. A 1.00 indication is used to show 1000/0.
A + symbol before the numbers shows an overload
condition of the DC starter/generator or TRU. The +
symbol is out of view when the load is less than 100%
NOTE: When the voltage or load data malfunctions,
the digits are replaced by white dashes.
•
Controls and Indications
When the aircraft engines are operating with no external
AC and DC power connected, and the DC starter/
generator toggle switches located on the DC control
panel are set to GEN 1 and GEN 2, the DC starter/generators are connected to their related DC main feeder
buses for operation.
The DC starter/generator toggle switches are set to the
OFF position to de-energize the DC generators and reset
latched functions in the DC GCU. The bus fault toggle
switch is set to BUS FAULT RESET position to reset
latched functions in the EPCU.
The MFD1 and MFD2 reversion switches located on
the ESID Control Panel (ESCP) are used to select theelectrical page on the multi-functional displays.
The electrical page shows these DC starter/generator
indications:
Essential, main, and secondary bus voltages●
DC starter/generator loads●
TRU loads.●
The DC voltage indication has a DC BUS L and DC BUS
R title in white fonts to show the voltage of the essential,main, and secondary buses. The DC load indication has
a DC GEN1 and DC GEN2 title in white fonts to show the
load of the DC starter/generators and it has a TRU1 and
TRU2 title in white fonts to show the load of the TRU.
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ATA 24 - ELECTRICAL POWER
Controls and IndicationsFigure 17 –
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ATA 24 - ELECTRICAL POWER
DC BUS Caution Light
The DC BUS caution light comes on to show that the
EPCU has recongured the main 28 Vdc generation
system because of a source or bus fault condition. If the
bus fault is reset, the indication will go out. The cautionlight receives its input from the EPCU.
#1 DC GEN HOT, #2 DC GEN HOT Caution Light
The #1 DC GEN HOT or #2 DC GEN HOT caution light
comes on to show that the generator temperature is more
than 160 of (71.1 °C) and goes out when the temperature
is less than 155 of (68°C). The two caution lights receive
their inputs directly from the bimetal thermal sensor in
each starter/generator.
L TRU HOT, R TRU HOT Caution Light
The L TRU HOT or R TRU HOT caution light comes on
to show that the TRU has overheated. The two caution
lights receive their inputs directly from the TRU.
Caution Lights
The main 28 Vdc generation system malfunctions are
shown with these caution lights:
#1 DC GEN●
#2 DC GEN●
L TRU●
R TRU●
DC BUS●
#1 DC GEN HOT●
#2 DC GEN HOT●
L TRU HOT●
R TRU HOT.●
#1 DC GEN, #2 DC GEN Caution Light:
The #1 DC GEN or #2 DC GEN caution light comes on
to show that the generator is not connected to its bus
because of a source fault condition, the DC starter/
generator toggle switches are set to the OFF position, or
external DC power is energizing the left and right main
feeder buses. The caution lights receive their inputs from
the left GCU and right GCU.
L TRU, R TRU Caution Light
The L TRU or R TRU caution light comes on to show
that the a secondary feeder bus is not energized by its
related TRU because of a source fault condition or the
AC generator toggle switches are set to the OFF position.
The caution lights receive their inputs from the EPCU.
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ATA 24 - ELECTRICAL POWER
Caution LightsFigure 18 –
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ATA 24 - ELECTRICAL POWER
Description
The APU 28 Vdc generation system supplies 28 Vdc
electrical power from a APU DC generator to these
buses:
Left and right main feeder buses●Left and right secondary feeder buses●
Left and right main distribution buses●
Left and right secondary distribution buses●
Left and right essential buses.●
if there is an overload condition the EPCU will open the
APU contactor K26.
The APU 28 Vdc generation system is continuously
monitored by the EPCU. When a malfunction is sensed, it
disconnects the APU power from the aircraft buses.
The APU 28 Vdc generation system has passive and
active protection. The APU 28 Vdc generation system
uses circuit breakers and fuses for passive protection
so that a malfunction condition will not cause damage
to the aircraft wiring. The circuit breakers and fuses are
protection devices that open the circuit during excessive
current ow conditions.
Unlike fuses, circuit breakers can be reset without having
to be replaced. The APU 28 Vdc generation system uses
the EPCU and GCU to automatically control the APU
contactor for active protection so that a fault condition will
not cause damage to the aircraft wiring. The automatic
functions do not need a manual selection from the ight
compartment.
APU 28VDC GENERATION SYSTEM
The APU 28 Vdc generation system makes and supplies
28 Vdc electrical power to the aircraft 28 Vdc buses.
The APU supplies 28 Vdc to the main, essential and
secondary DC buses while the aircraft is on the ground.
The annunciator switches located on the APU control
panel in the ight compartment give manual control of the
APU 28 Vdc generation system.
The Engine and System Integrated Display (ESID)
system shows 28 Vdc generation system electrical
indications. After the APU is started, the APU DC
generator is available to supply power in parallel with
the batteries to assist start the aircraft engines. The APU
generator is connected to the right main feeder bus andautomatically cross ties to the left main feeder bus to
supply 28 Vdc electrical power to the DC buses.
The APU 28 Vdc electrical power system has these
components:
Generator, APU DC●
Unit, APU DC Generator Control●
Transformers, DC Current●
Shunt, Current.●
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ATA 24 - ELECTRICAL POWER
APU OperationFigure 19 –
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ATA 24 - ELECTRICAL POWER
APU DC Generator
The APU DC generator is attached to the APU gearbox
and has a fan to supply a forced air ow axially through
it to cool the rotor and starter windings, the commutator
assembly, and the bearings. Air is drawn from outside
the APU bay through NACA vents to keep the operating
temperatures satisfactory.
The APU DC generator is an electromagnetic machine.
When it rotates, it will change engine input mechanical
torque to output DC electrical power. In the starter mode,
it changes electrical input power into a mechanical output
torque. The APU DC generator operation is controlled
and monitored by the APU DC Generator Control Unit
(GCU).
The APU DC GCU supplies the APU DC generator with
its output voltage until the APU DC GCU starts regulation.
When regulating, the APU DC GCU supplies a modulated
current to the APU DC generator to maintain a 28.5 Vdc
output. The APU DC generator supplies information
to the APU DC GCU for system protection through
tapping of the interpole winding voltage. This voltage
is proportional to generator current and is continuously
monitored by the APU DC GCU to sense unusual currentlevels.
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ATA 24 - ELECTRICAL POWER
APU Starter/Generator Figure 20 –
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ATA 24 - ELECTRICAL POWER
The PWR FAIL caution light on the APU CONTROL panel
and the APU caution light on the CAWP also illuminates,
and the APU shuts down.
NOTE: The FAIL caution light on the APU CONTROL
panel and the APU caution light on the CAWPwill not illuminate if the PWR push-button is
not set, except when the re protection control
amplier has calculated an APU shutdown.
The MFD ELECTRICAL system page has a digital display
of the APU load. The digital display of the APU load in a
white box gives an accurate APU generator load value.
An APU GEN label, in white identies the APU generator
load indicator. A LOAD unit is also shown in blue with the
indication.
The numbers for the digital display of the APU generator
load change from .00 to +1.99 in 0.01 power unit
increments. A + symbol before the numbers shows an
overload condition. The numbers relate to the APU
generator load as a percentage of the maximum rate.
If there is an open or short circuit between the APU
current shunt and the EPCU, the digital display of the
APU generator load indicator will show .00.
Controls and Indications
The APU CONTROL panel has the GEN control for
the APU 28 VDC generation system. The APU starter/
generator is connected to the DC buses when the
conditions are as follows: APU is operating●
EXT PWR toggle switch power is not set●
GEN switchlight is pushed.●
The GEN switchlight is set to the off position to reset the
related APU DC GCU.
The GEN ON light on the APU CONTROL panel
illuminates when the generator is connected to the right
main feeder bus. The APU contactor K26 has connectedthe APU generator to the right main feeder bus.
The WARN caution light in the APU control panel
illuminates when contactor K26 does not connect the
generator to the right main feeder bus. The WARN
caution light will also come for conditions that are not
malfunctions as follows:
The ON/WARN switchlight is not set●
External DC power is energizing the right main feeder●
bus.
NOTE: The WARN caution light will not illuminate
if the PWR switch is not set. The GEN
OHT caution light on the APU control panel
illuminates when the APU DC starter/generator
temperature is more than 166°C (330°F).
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ATA 24 - ELECTRICAL POWER
Controls and IndicationsFigure 21 –
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ATA 24 - ELECTRICAL POWER
The DC external system has protective functions to
prevent damage to the system The DC external power is
continuously monitored for the fault conditions that follow:
Correct polarity●
Over voltage●Under voltage.●
The faults are isolated to give continued operation.
DC EXTERNAL POWER SYSTEM
The DC external power system supplies 28 VDC
electrical power to the main 28 VDC generation system.
28 VDC electrical power is supplied through a DC
external power receptacle to let an external power sourceenergize the main 28 VDC generation system when
the aircraft is on the ground and the engines are not
operating.
DC external power supplies 28 VDC electrical power
through a line contactor to the left DC electrical load
distribution systems. The EPCU controls the line
contactor. The toggle switches on the DC CONTROL
panel give manual control of the DC external power
system.Description
The DC external power operates when the conditions are
as follows:
BATTERY MASTER toggle switch is set to energize●
the EPCU
DC external power is set.●
External power contactor Kg is energized by the EPCU
and DC starter/generator contactors K1 and K2 areprevented from operating as well as APU contactor K26.
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ATA 24 - ELECTRICAL POWER
DC External - SchematicFigure 22 –
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ATA 24 - ELECTRICAL POWER
DC External Power Receptacle
The DC external power receptacle is located in the left
nose fuselage. The DC external power receptacle is a
high power electrical connector. The unit has two large
gauge pins to carry the DC current and one smaller pinfor interface control.
Controls and Indications
The DC CONTROL panel has the EXT PWR control for
the DC external power system. OVERHEAD CONSOLE
The DC external power is connected to the DC buses
when:
Good external DC power is connected to the aircraft●
The BATTERY MASTER toggle switch on the DC● CONTROL panel is set
The EXT PWR toggle switch on the DC CONTROL●
panel is set.
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ATA 24 - ELECTRICAL POWER
DC EXT. Power - Control & ReceptacleFigure 23 –
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ATA 24 - ELECTRICAL POWER
EIS DC External Power Indications
The MFD ELECTRICAL system page has a DC external
power message. The EIS ELECTRICAL systems page
shows a DC EXT PWR ON message when the:
DC external power voltage is more than 22 VDC and●less than 31 VDC
DC EXT PWR toggle switch is set●
DC external power is energizing the left and right●
buses.
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ATA 24 - ELECTRICAL POWER
EIS Indication - DC External Power Figure 24 –
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ATA 24 - ELECTRICAL POWER
DC ELECTRICAL LOAD DISTRIBUTION
28 VDC electrical power is distributed through the circuit
breaker panels in the ight compartment to services.
Some distribution is done at the standby battery and DC
contactor boxes.
Description
28 VDC electrical power is supplied through circuit
breakers in the DC contactor box to the left and right 28
VDC circuit breaker panels.
The left and right 28 VDC circuit breaker panels supply
power to the avionics circuit breaker panel. 28 VDC
electrical power is supplied through circuit breakers in
the standby contactor box to the left and right 28 VDCcircuit breaker panel and main hydraulic system NO.3.
The DC electrical load distribution system are the:●
28 VDC circuit breaker panels●
28 VDC avionics circuit breaker panel.●
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ATA 24 - ELECTRICAL POWER
DC Electrical Load DistributionFigure 25 –
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ATA 24 - ELECTRICAL POWER
28 VDC Circuit Breaker Panels
The left and right 28 VDC circuit breaker panel is installed
in the ight compartment on the left and right side of the
pilot’s and copilot’s seats.
CAUTION: DO NOT LET THE CIRCUIT BREAKERPANEL TOUCH THE ADJACENT SIDE
WINDOW. WHEN YOU INSTALL THE
CIRCUIT BREAKER PANEL, IT IS EASY
TO PUT A SCRATCH IN THE SIDE
WINDOW. THIS WILL CAUSE DAMAGE
TO THE SIDE WINDOW.
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ATA 24 - ELECTRICAL POWER
Left & Right DC Circuit Breaker PanelsFigure 26 –
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ATA 24 - ELECTRICAL POWER
28 VDC Avionics Circuit Breaker Panel
The DC avionics circuit break r panel is installed in the
ight compartment above and aft of the left 28 VDC
circuit breaker panel.
CAUTION: DO NOT LET THE CIRCUIT BREAKERPANEL TOUCH THE ADJACENT SIDE
WINDOW. WHEN YOU INSTALL THE
CIRCUIT BREAKER PANEL, IT IS EASY
TO PUT A SCRATCH IN THE SIDE
WINDOW. THIS WILL CAUSE DAMAGE
TO THE SIDE WINDOW.
The DC avionics circuit breaker panel gives distribution
for the main 28 VDC generation system.
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ATA 24 - ELECTRICAL POWER
28 VDC Avionics C/B PanelFigure 27 –
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ATA 24 - ELECTRICAL POWER
and controls the AC generator line contactors. The AC
variable frequency system also has current transformers
in the AC contactor boxes and AC generators to measure
A, B, and C phase current.
The toggle switches on the AC CONTROL panel give
manual control of the AC variable frequency system.
NOTE: 115 VAC three-phase electrical power is
also supplied through an AC external power
receptacle to let an external power source
energise the AC variable frequency system
when the aircraft is on the ground and the
engines are not operating
The AC variable frequency system units are the:
AC generators● AC generator adapter plate●
AC GCU●
AC CONTROL panel●
AC contactor boxes●
AC contactor box contactors.●
The MFD, ELECTRICAL system page and the CAWP
show AC variable frequency system indications.
AC VARIABLE FREQUENCY SYSTEM
The AC variable frequency system supplies 115 VAC
three-phase electrical power to the AC electrical load
distribution system.
The AC variable frequency system supplies electricalpower to AC systems that are not frequency sensitive,
such as:
De-icing (heaters)●
Fuel (auxiliary pumps)●
Hydraulic (standby power unit)●
Galleys.●
The AC variable frequency system also interfaces with
the Main 28 VDC generation system to supply electricalpower to the transformer rectier units (TRUs) that are
additional sources of DC electrical power.
Description
The AC variable frequency system has a left and right
system with cross-tie capabilities. Two engine driven AC
generators supply 115 VAC three-phase electrical power
through AC generator line contactors to the left and right
AC electrical load distribution systems.
It is not possible to connect the two AC generators
in parallel. The AC generator line contactors have a
centre off position to make sure that the AC generator
is disconnected before it is connected to the opposite
side (break before make). This is necessary because
variable frequency AC generators are used. The AC
GCUs regulate its related AC generator’s output voltage
C C O
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ATA 24 - ELECTRICAL POWER
AC Variable Frequency SystemFigure 28 –
ATA 24 ELECTRICAL POWER
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ATA 24 - ELECTRICAL POWER
active protection. Circuit breakers and fuses are used for
passive protection.
The AC GCUs are used to control the AC generator
line contactors for active protection. The AC variable
frequency system uses the EPCU for CBIT. The AC
variable frequency system also uses AC GCUs for
initiated built in test.
During the normal mode of operation, the AC GCU
controls the related AC generator line contactor to
connect the AC generator to the related AC bus.
NOTE: The two AC GCUs monitor the condition of
the opposite AC GCU. When an AC GCU
senses that the opposite AC generator is not
serviceable, it then controls the AC generatorline contactor to connect the bus to the
serviceable AC generator.
The galley load shed contactors are controlled by
auxiliary contacts in the AC generator line contactors.
The two galley load shed contactors are automatically
closed when the AC buses are energized by the two AC
generators or external power.
The alternating current variable frequency system modes
of operation are:
Airborne (normal)●
External AC power.●
The AC variable frequency system operates in thenormal mode when:
Two engines are operating and the propeller is not●
feathered
External AC power is not set●
GEN 1 and GEN 2 toggle switches on the AC●
CONTROL panel are set.
The AC generators are by monitored by the AC GCUs to
automatically control the operation of the AC generator
line contactors. If a fault is sensed, the AC generator
line contactors are opened or closed to automatically
recongure the system. The fault is isolated from the
remaining part of the AC variable frequency system to
give continued operation.
The AC variable frequency system has protective
functions to prevent damage to the AC generators
when there is a fault. The AC variable frequency system
continuously monitors for the following fault conditions:
Source●
Bus.●
The source and bus faults are isolated and the part of
the system that continues to operate is automatically
recongured to give continued operation. The AC
variable frequency system has passive protection and
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ATA 24 - ELECTRICAL POWER
AC Generation - Normal OperationFigure 29 –
ATA 24 ELECTRICAL POWER
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ATA 24 - ELECTRICAL POWER
the generator cooling oil. Inputs and outputs of the stator
windings are supplied through interface connections
on the body of the AC generator. The low current
connections are made with a circular connector and the
high power three-phase output connections are made
with studs attached to a terminal block.
A bonding stud is also attached to housing of the
AC generator. The generator also has a bimetallic
temperature sensor that closes a set of contacts and
gives continuity through two pins on the interface
connector when the temperature of the machine is too
much.
A three-phase current transformer is attached in the
neutral lines of the windings to give current data to the
AC GCU.
NOTE: The AC generators output rating is 45 KVA.
The rst stage of the AC generator is a Permanent
Magnet Generator (PMG). It has a magnet to make
output power without the need for an external excitation
sources. The three-phase output from the PMG is
supplied through the AC GCU to the second stage eld.
The second stage is the exciter.
NOTE The generator also has a generator positivevoltage output signal so that the terminal
voltage of each phase at the generator can be
monitored by the EPCU for CBIT functions.
AC Generators
The AC generators are attached to the propeller
reduction gearboxes of the engines with V-clamps around
the quick attach/detach (QAD) adapter plates and the
AC generator mounting anges. The QAD adapters stay
attached to the reduction gearboxes.
CAUTION: YOU MUST CORRECTLY ALIGN THE
FLANGE OF THE AC GENERATOR WITH
THE QAD ADAPTER PLATE. IF YOU DO
NOT DO THIS, YOU CANNOT INSTALL
THE V BAND CLAMP AND YOU CAN
CAUSE DAMAGE TO THE EQUIPMENT.
The generator receives a supply of pressurized oil from
the reduction gearbox to cool and lubricate the internalcomponents. Oil is transferred from the gearbox, through
the QAD adapter, to the generator using transfer tubes.
These tubes are sealed with O-rings at each end to
prevent oil leakage. Oil enters the shaft and is sprayed
outward to cool the windings and lubricate the bearings.
The used cooling oil then goes under gravity to the
bottom of the generator where it is collected in the sump.
A scavenge port is located in the sump. The engine
scavenge pump extracts the oil and returns it to theengine.
The generator is a three-stage machine, with
electromagnetic coupling between stages. (It does not
have slip rings and brushes). Each stage in the machine
has of a rotor and stator segment along its length. The
rotor is supported in the machine by ball bearings at
both ends. The bearings are lubricated and cooled by
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ATA 24 - ELECTRICAL POWER
AC Generator Figure 30 –
ATA 24 ELECTRICAL POWER
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ATA 24 - ELECTRICAL POWER
The AC GCUs give the functions that follow:
Voltage regulation●
Control of the AC generator line contactors●
Protection●
Control of the AC generator and AC bus caution lights●CBIT.●
Voltage Regulation
The AC GCU supplies power through a voltage regulator
circuit to the exciter windings of the AC generator to start
generator output when the conditions are as follows:
The related GEN 1 or GEN 2 toggle switch on the AC●
CONTROL panel is set
AC GCU senses that the AC generator is at the●correct operating speed.
The voltage regulator gives a constant voltage at the
POR under different load and speed conditions. An
average of the individual three-phase outputs are
used for regulation. The AC GCU also uses current
transformers in the system to continuously monitor the
AC generator current and limit the current if the output is
more than the permitted limits.
Control Of The AC Generator Line Contactor
When a stable output is sensed by the AC GCU, it closes
the related AC generator line contactor to connect the AC
generator to the related AC bus.
It closes the opposite AC generator line contactor to
connect the AC generator to the opposite AC bus if the
opposite AC GCU or AC generator is not operating.
AC Generator Adapter Plate
There are two quick attach/detach QAD adapter plates
installed on the aircraft, one for each AC generator. The
QAD adapter plate is not removed when an AC generator
is removed.
The QAD adapter plate has two transfer tubes to supply
engine oil to the AC generator. The QAD adapter plate
has an O-ring and the transfer tubes also have O-rings
and retaining clips. The QAD adapter plate gives an
interface between the AC generators and the reduction
gearbox of the engines.
AC Generator Control Units (GCU)
The two AC GCUs are installed under the fuselage
oor in the centre of the aircraft. The AC GCU is a fully
enclosed, non-hermetically sealed unit. A connector
is attached to one end of the AC GCU enclosure to
interface with the electrical power generation and
distribution system (EPGDS).
The AC GCU has an Initiated Built In Test (lBIT) function
to check the status of the unit. The unit is cooled by
conduction through the chassis and support bracket and
by convection air over the surface of the box. Power
dissipating components in the unit are attached tochassis heat sinks to minimize the thermal resistance
of the cooling circuit and give correct operating
temperatures.
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ATA 24 - ELECTRICAL POWER
AC GCU LocationFigure 31 –
ATA 24 ELECTRICAL POWER
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ATA 24 - ELECTRICAL POWER
NOTE: If an undervoltage and bus fault condition
is sensed at the same time, the overcurrent
protection function will occur.
Differential Current
The AC generator is de-energized and the related ACgenerator line contactor is opened by the AC GCU when
the differential current on any phase is more than 20 A.
Underfrequency
The AC generator is de-energized and the related AC
generator line contactor is opened by the AC GCU when
the AC generator frequency is less than 300 Hz. It will
automatically reconnect the AC generator when the
frequency is more than 320 Hz.
This will occur only if the system was not previously
disconnected by a different protective function. As the
AC generator frequency goes below 340 Hz, the output
voltage decreases relative to the frequency until the AC
generator is disconnected at 300 Hz
NOTE: When the condition lever is set to START &
FEATHER, the AC generator is de-energized
and disconnected from the bus because the
AC generator speed to too low for operation.The AC generator will automatically reconnect.
The operating AC GCU senses this condition through a
TRANSFER REQUEST [TR] signal. This signal is active
when there is no need for transfer and is removed when
there is a need.
If AC external power is supplied to the aircraft, a signalis supplied to the AC GCU to energize the Y coil of the
AC generator line contactor to give external AC power
system operation.
Protection
To prevent damage to the AC generators, the AC GCUs
have the protective functions that follow:
Overvoltage●
Undervoltage●Differential current●
Underfrequency●
Bus fault.●
Overvoltage
The AC generator is de-energized and the related line
contactor is opened by the AC GCU before the voltage
is more than approximately 125 VAC. This protective
function is sensed in a period of time that is inverselyproportional to the size of the sensed overvoltage
condition.
Undervoltage
The AC generator is de-energized and the related line
contactor is opened by the AC GCU before the voltage is
less than approximately 100 VAC.
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ATA 24 - ELECTRICAL POWER
AC GCUFigure 32 –
ATA 24 - ELECTRICAL POWER
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ATA 24 - ELECTRICAL POWER
AC Control Panel
The AC CONTROL panel is attached to the overhead
console with four DZUS fasteners.
The toggle switches are identied as:
GEN 1●
GEN 2●
EXT POWER.●
The three switches have an OFF label to show their off
position. The AC CONTROL panel gives manual control
of the AC variable frequency system.
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ATA 24 ELECTRICAL POWER
AC Control PanelFigure 33 –
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ATA 24 ELECTRICAL POWER
output current of the AC generators to the EPCU for AC
electrical load indications in the electronic instrument
system (EIS).
Two current transformers, identied as CT2 and
CT5, supply total generator output current of the AC
generators to the AC GCUs for differential currentsensing (protection). When an AC bus is energized by the
opposite AC generator, CT3 or CT6 is also used.
AC Contactor Boxes
K1 and K3 are installed in the left-hand side ACCB, and
K2 and K4 are installed in the right-hand side ACCB.
Also, when installed, K5 is in the left ACCB and K6 is in
the right ACCB.
CT1, CT3, and CT5 are installed in the left-hand side
ACCB, and CT2, CT4, and CT6 are installed in the
right-hand side ACCB. Connectors are attached to the
enclosure to interface with the EPGDS.
There are two ACCBs installed on the aircraft. The units
have additional components:
AC generator line contactors, K1 and K2●
External power contactors, K3 and K4●
Galley load shed contactors, K5 and K6 (optional)●
Stand by power unit (SPU) contactor, installed in the●
right hand side ACCB
Internal cable assemblies and rigid bus bars●
Circuit breakers and fuses for output protection●
Circuit breakers for heaters in the propellers anti-icing●
system
Current transformers, CT1, CT2, CT3, CT4, CT5,●
CT6.External Power Protection Unit (EPPU), installed in●
the right-hand side ACCB
The current transformers in the ACCBs monitor the
current for phase A, B, and C of the AC generator for
indication and protection. Two current transformers,
identied as CT1 and CT4, supply total generator
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ATA 24 ELECTRICAL POWER
AC Contactor Box (ACCB)Figure 34 –
ATA 24 - ELECTRICAL POWER
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They also have auxiliary contacts to control the galley
load shed contactors, K5 and K6.
Galley Load Shed Contactors
The galley load shed contactors, K5 and K6, are
50A Three-Pole Single Throw (3PST), normally opencontactors.
They have one coil winding to move the contact arms to
connect the galley buses to the AC buses.
The galley load shed contactors are only energized
when:
The two AC buses are energized by the two AC●
generators
The two AC buses are energized by the external AC● power.
AC Contactor Box Contactors
The contactors in the left-hand side and right-hand side
ACCBs attach directly to rigid bus bars.
AC Generator Line Contactors
The AC generator line contactors, K1 and K2, are 150A
Three-Pole Double Throw (3PDT), centre off contactors.
They have two coil windings, identied as X and Y, to
move the contact arms to connect the AC buses to
the related or the opposite AC generator. They also
have auxiliary contacts to control the galley load shed
contactors, K5 and K6.
NOTE: The contactor arms also move to the Y
position when the external AC power is used.
NOTE: The X position of the bus contactor is
controlled by the related AC GCU and the Y
position is controlled by the opposite AC GCU.
External Power Contactors
The external power contactor, K3 and K4, are 100A
3PDT, contactors. They have one coil winding to move
the contact arms to connect the AC buses to the AC
external power receptacle.
The external power contactors are only energized when
the conditions are as follows:
Good external AC power is supplied●
EXT PWR toggle switches on the AC CONTROL●
panel are set
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ACCB ContactorsFigure 35 –
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If there is an open or short circuit to ground potential
between the AC bus and the EPCU, the related digital
display of the AC generator voltage will show 0.
AC Load
The load part of the AC generator voltage and loadindicator shows an indication for the digital display of the
AC generators phase A, B, and C.
The numbers for the digital display of the AC generator
load change from .00 to +1.99 in 0.01 power unit
increments. A + symbol before the numbers shows
an overload condition. The numbers relate to the AC
generator load as a percentage of the maximum rate.
AC GEN Caution Lights
The #1 AC GEN or #2 AC GEN caution light illuminates
when the associated generator (GEN1 or GEN2) is not
connected to its bus.
The #1 AC GEN or #2 AC GEN caution light will also
illuminate for conditions that are not malfunctions as
follows:
The AC GEN1 or AC GEN2 toggle switch is set to the●
OFF position●
External AC power is energizing the Land R AC●buses.
If a source fault is sensed, the AC generator line
contactors K1 and K2 will not connect the AC GEN to the
115 VAC BUS.
Controls And Indication
The AC CONTROL panel has the controls for the AC
variable frequency system:
GEN 1 toggle switch●
GEN2 toggle switch.●
The AC generators are connected to the AC buses when
the:
Two engines are operating and the propeller is not●
feathered,
EXT PWR toggle switch power is not set, and●
GEN1 and GEN2 toggle switches are set.●
The GEN1 and GEN2 toggles switches are set to the
OFF position to reset the related AC GCU.
The MFD ELECTRICAL system page has an AC
generator voltage and load indicator with a digital display
for voltage and load.
AC Voltage
The voltage part of the AC generator voltage and load
indicator shows an indication for the digital display of the
AC generators phase A, B, and C.
If there is an open or short circuit between the ACgenerator current transformer and the EPCU, the digital
display of the AC generator load indicator will show .00.
The numbers for the digital display of the AC generator
voltage change from 0 to 249 in 1 volt increments.
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AC Control and IndicationsFigure 36 –
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AC BUS Caution Lights
The R AC BUS or L AC BUS caution light illuminates
when there is an overcurrent condition on the 115 VAC
BUS for more than seven seconds.
The caution light will go out if the associated toggleswitch is set to the OFF position.
AC GEN HOT Caution Lights
The #1 AC GEN HOT or #2 AC GEN HOT caution
light in the CAWP illuminates when the AC generator
temperature is more than 166°C (330°F).
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FOR TRAINING PURPOSES ONLY
Caution LightsFigure 37 –
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FOR TRAINING PURPOSES ONLY
External Power Receptacle
The AC external power receptacle is located in the right
nose fuselage. The unit has four large gauge pins to
carry the three-phase AC current and two smaller pins for
interface control.
It gives the interface connection between external AC
power source and the aircraft.
External AC Power Protection Unit
The external AC power protection unit is located in the
right AC contactor box. It is attached with screws. The
AC external power protection unit monitors the quality
of the input power to the AC external power receptacle
and prevents connection of the power to the aircraft
distribution system if the power is not satisfactory.
It monitors the parameters for:
Undervoltage, 106 VAC●
Overvoltage, 124 VAC●
Underfrequency, 370 Hz●
Overfrequency, 450 Hz●
Phase Rotation.●
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AC External Power Receptacle and EPPUFigure 38 –
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FOR TRAINING PURPOSES ONLY
Controls And Indications
The AC CONTROL panel has the EXT PWR control
for the AC external AC power system. The external AC
power is connected to the AC buses when the:
Good external AC power is connected to the aircraft,● and
BATTERY MASTER toggle switch on the DC●
CONTROL panel is set, and
EXT PWR toggle switch on the AC CONTROL panel●
is set.
The MFD ELECTRICAL system page has an AC external
power message. The EIS ELECTRICAL systems page
shows an AC EXT PWR ON message when:
AC external power voltage is more than 106 VAC●and less than 124 VAC, frequency is more than 370
Hz and less than 450 Hz, phase rotation is phase
A, phase B, then phase C, and AC EXT PWR toggle
switch is set, and
AC external power is energizing the left and right 115●
VAC buses.
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AC External Power - Controls and IndicationsFigure 39 –
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FOR TRAINING PURPOSES ONLY
CAUTION: DO NOT LET THE CIRCUIT BREAKER
PANEL TOUCH THE ADJACENT SIDE
WINDOW. WHEN YOU INSTALL THE
CIRCUIT BREAKER PANEL, IT IS EASY
TO PUT A SCRATCH IN THE SIDE
WINDOW. THIS WILL CAUSE DAMAGETO THE SIDE WINDOW.
Operation
Three-phase, 115 VAC variable frequency electrical
power is supplied through three 50 A circuit breakers on
the Land R AC contactor box to the 115 VAC variable
frequency circuit breaker panel.
Three-phase, 115 VAC variable frequency electrical
power is supplied through three 35 A circuit breakers onthe L and R AC contactor box to the propellers anti-icing
system
Three-phase, 115 VAC variable frequency electrical
power is supplied through three 60 A circuit breakers on
the R AC contactor box to the main hydraulic systems 1
NOTE: The left part of the AC electrical load
distribution is described. The right part is
similar.
ELECTRICAL LOAD DISTRIBUTION
Three-phase, 115 VAC variable frequency electrical
power is distributed through a circuit breaker panel in the
ight compartment to electrical loads. Some distribution
is done at the AC contactor boxes.
Description
Phase A, phase B, and phase C 115 VAC electrical
power is supplied through circuit breakers in the left and
right AC contactor boxes to:
115 VAC variable frequency circuit breaker panel●
Propellers anti-icing system●
Main hydraulic system No.1.●
The AC electrical load distribution has an 115 VACvariable frequency circuit breaker panel. Circuit breakers
(CB) in the 115 VAC variable frequency circuit breaker
panel and AC contactor box protect the wires in the AC
variable frequency system.
115 VAC Variable Frequency Circuit Breaker Panel
The 115 VAC variable frequency circuit breaker panel is
installed on the rear ight compartment bulkhead above
and aft of the right 28 VDC circuit breaker panel. It has
circuit breakers attached to the panel. Panel markings
identify the buses and circuit breakers.
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© J A i ti LP AC DistributionFigure 40