IEEE Electrification Magazine - December 2017 - 18

experienced during breakaway torque to start the aircraft moving, and the highest bus voltage, which occurs
when the motor BEMF is rectified at maximum speed.
The inverter switching frequency is selected to be fast
enough for reasonable motor current ripple and for
control loop update-rate closure, but it is kept as low as
possible to minimize switching losses.
The power stages are air cooled using 28-V powered
fans. The main inverter cooling fan is run only when the
inverter temperature gets warm and reaches a preset
point. When the temperature cools below a lower preset
point, the fan is turned off. This on-demand technique
saves energy and reduces acoustic noise. The predominant operating condition that generates the highest heat
loss requiring active cooling is the high output current
(torque) condition.

WACU EMI Filter Design
The WACU contains inverter output filtering for EMI compatibility. The inverter output passes through differentialand common-mode filters before leaving the WACU to
drive the TM. The differential-mode filter consists of three
differential-mode inductors and three line-to-line connected capacitors. The common-mode filter consists of a

common-mode inductor and three capacitors connected
from line to chassis ground. A relaxation of the conducted
and radiated emissions requirements may be possible.
However, due to other equipment installed on the aircraft,
this will be difficult.
The WACU contains a differential-mode EMI filter for
the 28-Vdc control input power.

WACU Cable interfaces
The WACU cable interfaces consist of both signal-level
and high-power connections. The signal-level connections
include the TM feedback, serial data bus communication,
contactor control, and control discretes. The high-power
connections include the ±270-Vdc input power from the
ATRU and three-phase ac output power to the TM. The
power cables are constructed with 360° overbraid shields
for EMI control.
The three-phase ac power cable that delivers the
power from the WACU to the TM has stringent requirements. Having to make the transition from the fuselage
to the landing gear, the cable is subject to harsh vibration, the possibility of runway or taxiway FOD, and
repeated flexing every time the gear is deployed and
retracted. Dividing the cable into two parallel threephase cables instead of a single larger cable allows for a
smaller cable diameter and more flexible routing to the
TM. A potential pitfall of this approach is that if one cable
were to become an open circuit, the remaining cable in
that particular phase will carry all of the current and
could overheat, without being detectable. By means of
extra current sensors and software logic, the WACU is
able to make the determination that one or more of the
six cables has become an open circuit and can report the
fault before any damage ensues.

Prototype WACU for PAS

Figure 8. The WACU prototype for the PAS.

TM Torque Speed Curves

Torque (Nm)

200

Aircraft Pushback
Aircraft Taxi

160

TM

120

TM Major Requirements

80
40
0

0

4,000
8,000
Speed (r/min)

Figure 9. The TM electromagnetic requirements.

18

A picture of the prototype WACU used in the PAS is shown
in Figure 8. Two units, left and right, were mounted in the
center fuselage location in the aircraft, one for each main
landing gear wheel. The power connectors can be seen on
the front left side, the top being the ±270-Vdc input power
connector and the two below it for the TM output power.
The control signal connectors can be seen on the righthand side. The large circular opening is the inlet for the
fan used to cool the inverter.

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017

12,000

The TM has multiple requirements. In addition to electromagnetic performance (Figure 9), mechanical and
environmental requirements are severe. The electromagnetic requirements include high torque at zero and
low speed, for pushback from the gate as well as acceleration from rest during taxi operation. Additionally, a
high-power operation up to maximum speeds is also



Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2017

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