IEEE Electrification - June 2022 - 48

In a hydrogen-powered airplane, availability of LH2 for
machine cooling could be a game changer in future propulsion
systems. A robust cooling system for the rotor, which
would require a hydrogen-transfer coupling, separate
cooling loop, or separate cooling system such as a rotormounted
cryocooler, also needs to be implemented.
Enabling Technologies
Cryogenic Design
The main goal in the cryogenic design of machines is to
maintain SC coils below the required temperature without
the need for large auxiliary systems. The heat load
from the SC assembly needs to be minimized and be well
below the lift capability of available cryocoolers. With a
typical closed-loop cooling system, the heat load needs to
be below approximately 100 W to use compact cryocoolers
that can provide ~10 W of lift at 40-50 K, and the ΔT
must be limited to less than 10 K. Although this sounds
daunting, it is regularly achieved in LTS magnet systems
such as large magnetic resonance imaging magnets. The
difference in electric machine applications is the need to
handle centrifugal forces, torque transmission, heat leakage
from current leads, and the risk of ac losses from stator
harmonic coupling. The main components of the heat
load are
x Conduction losses in the suspensions system: These losses
need to be minimized by balancing structural and
thermal requirements over a wide temperature range.
The heat conduction occurs mainly through the current
leads and shaft (torque tube) of the rotor. On the
torque tube, advanced materials such as glass-fiberreinforced
plastic are used to minimize thermal conductivity
in addition to lengthening the conduction
path. New material systems and additive manufacturing
could lead to more optimal solutions that balance
the need for mechanical integrity while minimizing
heat leak.
x Ac losses: In fully SC machines, SC-armature windings
experience varying rotating magnetic fields, which
generate losses in the form of heat, known as ac losses.
These losses are cyclical and increase with both
applied frequency and square of the peak applied flux
density. These ac losses can be categorized into four
different losses as hysteresis (Ph), eddy current (Pe),
coupling (Pc), and transport current (Pt) losses. As electric
propulsion motors operate at high speeds and
therefore experience high electrical frequencies, losses
in armature coils are significant and determine the
feasibility of a motor's design.
x Current leads: Current leads and associated thermal
conduction are one of the major challenges in realizing
100,000
Proposed Scalable SC Motor
10 MW, 3,000 r/min
10,000
Under Development
1 MW, 8,000 r/min
1,000
PM-AIR500L
500 kW, 4,500 r/min
PM-AIR1000H
1 MW, 15,000 r/min
100
2,000
4,000
6,000
8,000
Speed (rpm)
Figure 5. A comparison of the power-torque capability of conventional and SC machines.
48
IEEE Electrification Magazine / JUNE 2022
10,000
12,000
14,000
16,000
Torque (Nm)
Large Electric Aircraft
Small Regional Jets,
Distributed, Hybrid-Electric
IEEE Electrification - June 2022

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