IEEE Electrification Magazine - December 2017 - 19

required for a high-speed taxi operation. The structural requirements are
quite severe for the landing shock
and operational vibration during the
taxi maneuver.

Coordination of Electromagnetic,
Thermal, and Structural
Performance

The structural
requirements are
quite severe for the
landing shock and
operational vibration
during the taxi
maneuver.

Succinct coordination is required in
the electromagnetic, thermal, and
structural designs. The motor iron
and conductor cross sections must
be large to maximize efficiency.
Larger iron cross sections reduce magnetic flux densities and iron losses (hysteresis and eddy current losses).
Larger conductor cross sections reduce the conductor
current density and resulting conduction losses. The thermal design also must have a large heat transfer surface
area, a high fin count for air cooling, and a high cooling
air flow. Finally, the structural design must have large
mechanical cross sections for mounting and structural
integrity. To minimize the volume and weight of the TM,
the interdependent needs of the electromagnetic, thermal, and structural designs must be coordinated. The
optimization of this design interdependence can be
achieved iteratively.
The TM is an energy conversion device, converting the
available electrical energy to mechanical energy. The
power electronic controller is limited by the available
power from the power source and electrical feeder circuits, typically from the APU generators. The mechanical
load requirements are limited by the load and duty cycle
requirements for the taxi service. Therefore, a significant
coordination of the power source, the mechanical load,
and the motor electromagnetic, thermal, and mechanical
performance is required.

Duty Cycle and Thermal
An acceptable motor design is quite dependent on the
required duty cycle (see Figures 10 and 11). The duty
cycle is a function of the aircraft weight and vehicle taxi
requirements. Sufficient output torque and output power

to achieve the duty cycles within the
thermal capability of the TM is needed. Therefore, a definition of the
worst-case taxi events under the
worst-case ambient conditions and
heat rejection capability is required
to ensure that the resulting motor
component temperatures are not
exceeded. The significant motor components with respect to the thermal
performance are the winding insulation and the bearings. The component life is significantly dependent
on the component temperature exposure. Typically, overtemperature causes a significant
reduction in the bearing and insulation system life.
Therefore, the coordination of the electromagnetic efficiency, the duty cycle, and the thermal heat transfer
must be achieved to limit the peak component temperatures, thereby delivering the reliability needed for a commercial electric taxi system.

Motor Type Selection
There are several types of motors that could be used for
electric taxi applications. As discussed in the "Coordination of Electromagnetic, Thermal, and Structural Performance" section, the coordination of the motor design
with the thermal and mechanical performance is significant. For these reasons, both high efficiency and high
torque density are desirable for the motor. The interior
rotor, surface-mounted PMSM is chosen. The combination
of high efficiency, high torque density, and light weight
makes this motor type attractive for electric taxi applications. Figure 12 shows two different views of the TM with
the integral cooling fan.

Motor Sizing
As stated earlier, the motor is an energy conversion
device, converting electrical energy from the power electronic controller to mechanical energy that propels the
aircraft. This energy conversion occurs in the air gap of
the machine. The air gap is the space between the stationary portion of the motor (the stator) and the rotating

200
100
0
0

1,000
Time (s)

Figure 10. The duty cycle torque versus time.

r/min Versus Time

12,000
Speed (r/min)

Torque (Nm)

Torque Versus Time

2,000

8,000
4,000
0
0

1,000
Time (s)

2,000

Figure 11. The duty cycle r/min versus time.

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

19



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