IEEE Electrification Magazine - December 2017 - 14

system reliability. An innovative way to protect the system
with a minimal number of sensors is highly desirable.

EDS Optimization and Stability
The EDS has to be optimized for weight, volume, efficiency, reliability, and cost. The optimization should be conducted at the overall EDS level. The result will then dictate
the EDS component-level requirements. For example, the
EDS operation envelopes include a low-speed operation
point with a high-torque output and a high-speed operation point with a high-power output.
To minimize the weight of the motor controller and
overall power loss, a flux weakening technique, as
described by Jahns, is adopted. Without the optimization,
the power loss and current, as well as the weight of the
EDS, would be very high. On the other hand, if too much
flux weakening is used, the WACU will be penalized to
absorb a high back-electromotive force (BEMF) voltage

Point A:
Breakaway Current
Point B:
Point C:
Corner Point
Constant Current
Curren
nt Maximum Speed
(Constant Power)

Amps (Peak)

300

200

100

2,000 4,000 6,000 8,000 10,000
Motor Speed (r/min)

Figure 4. The TM current speed curve with flux weakening.

from the motor at high speed. Then, the ATRU will be
forced to boost the dc bus voltage to a higher level, and
both the WACU and ATRU will experience a weight penalty.
Therefore, an optimization for flux weakening is implemented. The TM with flux weakening has a current speed
curve as shown in Figure 4. The highest level of flux weakening is experienced at the maximum operating speed.
The machine BEMF at this point is 307.2 Vrms line to neutral. In case of an inverter failure, a dc voltage bus will be
charged to 752.4 Vdc, which is within the allowed maximum rating of the components used.
To perform a quantitative comparison between the performance with flux weakening and the performance without flux weakening, the WACU power losses have been
calculated over the operating profile. The chart in Figure 5(a)
shows the loss without employing flux weakening, and the
chart in Figure 5(b) shows the loss with an optimized flux
weakening. Due to the flux weakening, the WACU can be
designed to dissipate 3.2-kW peak power instead of 4.3-kW
peak power. This design strategy can improve power density up to 10%.
Many design parameters need to be optimized for
the entire system performance such as the inverter
switching frequency. A frequency of 15 kHz has been
selected based on an optimization of EMI performance
and losses. The efficiencies of the individual components must be properly allocated to achieve the overall
EDS efficiency requirement. The optimization has been
performed considering the duty cycle based on the
Schiphol airport typical taxiing duty cycle. A sensitivity
analysis of the parameters has been used predominately for the system optimization.
The EDS load can be a constant power load (CPL)
with negative impedance. The CPL is known for possibly
degrading the system performance and causing instability, e.g. see the work by Emadi et al. and Jusoh. The

Peak Power Dissipation 4.3 kW

Peak Power Dissipation 3.2 kW

X: 758
X
Y: 4,262
Y

X: 755.5
X
Y: 3,107
Y

4,000

3,000

2,000

1,000

Average

400

800
1,200
Time (s)
(a)

2,000

1,600

Average

Figure 5. The WACU power loss (a) with no flux weakening and (b) with flux weakening.

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

400

800
1,200
Time (s)
(b)

1,600

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