IEEE Electrification Magazine - December 2017 - 33

supply. Once the APU shaft speed exceeds its self-sustaining speed, the self-start mode is complete.

Main-Engine Start Mode of Operation
In main-engine start mode, the APU engine produces
mechanical power to the turbine shaft through the combustion of fuel and drives the induction machines as a generator. As illustrated in Figure 10(b), the electric actuation dc
bus is still disconnected from the system. The transformed
wye-connected induction machine, now operating as a
generator, provides electrical power through IRU 1 to the
main dc bus. This power is used by the main-engine starter/generator to spin the HP spool turbine of the main
engine. After the main engine is started, the system transitions to cooling mode.

Cooling Mode of Operation
In the cooling mode of operation, the combustor of the
APU is no longer used. The high-stage bleed air from the
main engines is first cooled down by the APU turbine and
then sent to supply cabin pressurization and air conditioning. Part of the cooled air is also circulated to provide
forced-air cooling for avionics, flight-critical electronics,
and other liquid-cooled heat loads.
During a normal flight mission, most of the electrical loads
on board are supplied by the main-engine generation system.
Hence, as shown in Figure 10(c), the main dc bus is disconnected from the APU power management system in the cooling
mode. The left side terminals of the open-end winding induction machine are shorted through IRU 1. The APU turbine shaft,
powered by the high-stage bleed air from the main engines,
is responsible for supplying/absorbing the high-peak
power demand/regeneration from the EHA/EMAs using IRU 2.

Emergency Power Mode of Operation

in-flight main-engine failure occurs, the APU is commanded
to operate at full power and produce electricity to support
the main network and potential main-engine restart. In
case of dual (all) engine failure, the APU generator is responsible for regulating the main dc bus voltage. As one of the
most critical electrical loads on board, the power of EHA/
EMAs must be secured at all times during the flight mission.
Therefore, in emergency power mode of operation, both
sides of the open-end winding induction generator are activated. IRU 2 is responsible for supplying the highly dynamic
actuator loads, while IRU 1 is used to provide disturbancefree dc power supply or voltage regulation for the main dc
bus. The closed-loop control scheme for the APU for power
generation and management system in emergency power
mode is shown in Figure 11.
Two voltage sensors are used to monitor the voltages of
two separated dc buses, while two current sensors are used
to provide stator current feedbacks. Neither the rotational
speed of the rotor nor rotor position feedback is essential for
control of the generation system. The control scheme is
developed based on FOC and instantaneous power theory.
The FOC in this scheme is implemented by an active fluxbased direct-flux observer using inverter terminal voltage
and generator stator current feedback. The inverter terminal
voltages are reconstructed using dc bus voltage feedback
and inverter gating signals. If all of the engines of the aircraft
have failed during flight mission, the main dc bus voltage is
regulated by the system through PI voltage controller 1. The
output of this controller is the main dc side power command p)dc1. In case of single engine failure, the main dc bus
*
voltage control loop is disabled, and p dc1
can be provided by
the main dc bus voltage regulator from the main-engine
system. To ensure unity power factor operation, the output
voltage vector of IRU 1 is always kept aligned with generator
current vector v )ds1, and v )qs1 can be calculated as

The emergency-mode operation of the induction machinebased APU system is shown in Figure 10(d). When an

v ds1 = v 1p)

i ds
,
is

(3)

APU Turbine Shaft

To Main
dc Bus

O

B
C

IRU 1

dc Voltage Feedback 1

ea L
s

Rs

ias

eb Ls

Rs

ibs

ec Ls

Rs

ics

OEWIS/G

Current
Feedback

A

Controller

a
o

b
c

Electric
Actuation
dc Bus

IRU 2

dc Voltage Feedback 2

Figure 9. The system configuration of the induction generator-based APU architecture for electric power generation and management.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

33



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