Superconducting Homopolar DC Motor
Homopolar motors are simple in design and have the
potential for large weight reductions when compared to
conventional motors. In SHDCM, in order to reduce size and
weight, the field coils are superconducting. As there are no
multi-pole components in the motor, it is expected to be quiet
enough acoustically to permit hard mounting directly to a
ship’s hull.
In SHDCM, brushes are the maximum limitation. Solid
carbon and metal-graphite are found to be inadequate due to
their low current density and excessive wear. Graphite fiber
brushes have had some success. Liquid metal brushes are not
suitable in many applications due to material and life
limitations. Copper fiber brushes are being considered, as they
provide a compromise between current carrying capability and
long term wear [29]. Homopolar motors are compact and,
therefore, are attractive for use in direct electric drive
applications in ships. They are considered to be a conceptually
4
superior alternative to AC motors for naval propulsion
applications. The application of SHDCM in naval ships is
presented in [29]-[30]. The design and development of these
motors is presented in [31]-[32]. General Atomics (GA) is
leading the industry in the development of SHDCM. Successful
development and testing of 3.7 MW motor led to contract to develop fullscale
36.5 MW motor from the Office of Naval Research (ONR), USA
[33]. GA will deliver a 36.5-megawatt, 120-rpm advanced
design prototype motor, motor drive, and support equipment,
which will be shipped to a US Navy facility for fullperformance
testing. The projected full load efficiency of
SHDCM is about 97.5%. The SHDCM will be used to propel
large vessels, such as destroyers, now in development.
SHDCM are significantly quieter than ac motors as they do
not have inherent time and spatially varying forces that are a
source of noise in ac electric machines. They are smaller,
lighter, and more efficient than the AC motors. Their control
is more straightforward and simpler than the AC motor
systems, and SHDCM systems are suited to simpler and less
costly ship electrical distribution architectures. The SHDCM
system, when combined with a DC link power distribution
system, might simplify the ships integrated power distribution
and motor drive systems, reduce the number of system
components, improve efficiency, and lower system acquisition
and operating costs.