IEEE Electrification Magazine - December 2014 - 11

Electric
Motor
Reduction
Gearbox

Ball
Screw

Figure 5. A system diagram for an EMA.

Three-Phase
Supply
Electric
Motor

Power
Converter

Fixed
Displacement
Pump
Hydraulic Ram

Figure 6. A system diagram of an EHA.

Architectural
Layer

ple
x
om

rea

ls I

tai

Functional Layer

ity

Inc
rea

se
s

switching frequency of many power converters. This layer
is representative of the actual system waveforms and can
therefore be used in the design of passive filters for harmonic and switching frequency components. This layer is
certainly the most detailed layer used for electrical power
system modeling, but the time range is usually low due to
the complexity of the models.
The functional layer is used to represent transient
behavior at frequencies typically up to a couple of hundred
hertz. The usual purpose of functional simulations is to
look at the electrical power system dynamics and stability

l
ve
Le

Behavioral Layer

The design and successful deployment of future electrical
power system architectures will involve extensive modeling
and simulation activities to ensure the stability and integrity
of the system over a very wide range of operating scenarios,
many of which will rarely be encountered during flight. It is,
therefore, necessary to have a robust and reasonably standardized approach to modeling the electrical system. For this
reason, a number of modeling levels have been defined and
are generally accepted as useful for different models and
required simulation study outcomes. Figure 7 shows these
levels; the higher the level, the more time efficient the model,
but the more the detail is sacrificed to achieve faster simulation times. Each of these levels has proved useful in the complete system design process of a More Electric Aircraft and
further refinements and optimization of modeling techniques are being made.
The device physical layer is used to represent a piece of
equipment of the device on the system. These models typically have a very high bandwidth and can represent very fast
transients within the device and its surroundings. Typically,
such models are used for equipment or device verification
and in-depth analysis of its behavior locally with the supplier
or manufacturer. This detailed level of modeling is not usually extended beyond the design of an individual component
or piece of equipment within the system and is, therefore,
rarely used for the simulation of an electrical power system.
The behavioral layer uses lumped parameter subsystem models and is capable of simulating frequencies up to
a few hundred kilohertz, a frequency range that covers the

Power
Converter

electrical Power System modeling

Three-Phase
Supply

control surface can be controlled by simply controlling the
motor. As the motor turns, it moves a ball screw, often
through a reduction gearbox. Each turn of the motor displaces the actuator by a fixed amount due to the direct
connection between the motor and the ball screw. However, there is a problem in using EMAs for primary flightcontrol applications on large aircraft as, to date, it has
been very difficult to guarantee that the ball screw will
never jam. A jam in an actuator for a flight-critical control
surface would cause problems in the current design of the
aircraft as the surface would not be controllable unless a
benign failure mode of the actuator can be guaranteed. A
jam in a ball screw is not a benign failure as another actuator on the same control surface would not be able to
move the surface if one actuator has jammed.
An alternative to the EMA is the electro-hydrostatic
actuator (EHA), which has a system driven by local
hydraulics and controlled with a fixed displacement
pump driven by an electrical motor. The actuator position
moves by a fixed displacement for each revolution of the
motor, as shown in Figure 6. There is no direct mechanical
connection between the motor and the actuator arm;
hence, the EHA has benign failure modes, giving the system a significant advantage when compared to EMAs for
primary flight-control applications.

Device Physical Layer

Figure 7. The commonly defined modeling levels for MEA electrical
power systems.

IEEE Electrific ation Magazine / d ec em be r 2 0 1 4

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