IEEE Electrification Magazine - June 2014 - 56

LP Shaft
Turbine System

Generator

Converter

HP Shaft
Motor/
Gen

Turbine System

28 V

dc-dc
Converter

Oil Pump
Motor

Fuel Pump
Motor

Hydraulic
Pump
Motor

Inverter

Converter/
Inverter

dc Voltage
270 V
or
Higher

+
_

Figure 8. A typical MEE system with embedded generation (K. Rajashekara, "Converging technologies for electric/hybrid vehicles and More Electric Aircraft systems," in Proc. SAE Power Systems Conf., Fort Worth, TX, Nov. 2-4, 2010, Paper No. 2010-01-1757).

required for a full authority digital electronic controller and
other flight controls are derived using a dc-dc converter
operating from the 270-Vdc supply. This 270-Vdc power is
obtained from the aircraft APU or from a battery. For
ground-starting the engine, the 270 V could also be derived
from a ground-based APU.
In embedded generation applications, the inaccessibility
of the location, harsh operating environment, and ambient
temperatures possibly exceeding 250 °C (Figure 9) require
the use of materials such as PMs and insulation materials
close to or beyond their operating limits. Hence, special
cooling and the use of alternative materials and processes
are required to enable the electric machine and the power
electronic systems to withstand these temperatures. As can
be seen, both for HEVs and for MEAs, it is necessary to
develop machines and power electronic systems capable of
operating at high temperatures.

Power Electronics
The power conversion system that is used for electric propulsion in automobiles is analogous to the power generation system in an aircraft. In EVs/HEVs, power electronics
is required for (but not limited to):
xx
conversion of the dc bus voltage to ac to control the
propulsion motor speed and torque
xx
dc-dc conversion from a high voltage to 12 V for
charging the 12-V battery
xx
a converter for charging the batteries
xx
an inverter for controlling the air conditioner motor.
In MEAs, the power conversion depends on the type of
generator being used and the nature of the required output
voltage, i.e., dc, fixed-frequency ac, or VF ac. It might also be

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2014

required to convert power from one form to another within
an aircraft's electrical system. Some typical examples of
power conversions are as follows:
xx
The VF output of the engine-driven generator must be
converted to dc (270 V or + / - 270 Vdc).
xx
DC power must be converted to ac power; inverters are
used to convert 28-Vdc to 115-Vac single-phase or threephase power.
xx
The conversion from 115-Vac to 28-Vdc power is
achieved by using TRUs.
xx
Battery charging is necessary to maintain the state of
charge of the aircraft battery by converting 115-Vac to a
28-Vdc battery charge voltage.
xx
The conversion to 115-V three-phase 400 Hz and 28 Vdc
from 270 Vdc, if the main bus voltage is 270 Vdc, is
required to power legacy equipment originally designed
to operate using these voltages.
A typical insulated-gate bipolar transistor (IGBT)-based
power converter and control system for an electric propulsion system in an EV/HEV is shown in Figure 10. The inverter converts the dc voltage to ac to control the speed and
torque of the propulsion motor. During regeneration, the
same inverter is used as an active rectifier to convert the ac
voltage of the machine to dc to recharge the batteries. A
vector control strategy is used for obtaining the desired
torque at various speeds.
The power conversion topology for the starter/generators in MEAs is similar to the EV/HEV propulsion system
shown in Figure 10. The power converter will be used as an
inverter to convert the dc voltage to VF and variable voltage
to run the same electric machine as a motor to start the jet
engine. Once the engine starts and overcomes the peak

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2014