Superconductor Motor Technology - Electrical Line Mag

ELECTRICAL LINE March / April 2000

Gearing Up...

Superconductor Motor Technology

American Superconductor Corporation Iron Core
(ASC), with its U.S. Department of Energy
Superconductivity Partnership Initiative HTS Winding
(SPI) partner Rockwell/Reliance, will
have the world’s first 1,000 horsepower Coolers
high temperature superconducting
(HTS) motor designed for commercial
application up and running this spring.
The coils for two SPI project 5,000hp
motors are presently being developed at
American Superconductor and the com-
pany is now under contract to begin
design on a 25,000 hp HTS ship propul-
sion motor for the U.S. Navy. Lieu-
tenant Commander Clifford A. Whit-
comb, U.S.N., Ph.D. of MIT discusses
this new technology destined to have a
major impact on industry, as well as the
military, in the years ahead.

Superconductivity is a fascinating Output Shaft
phenomenon that encompasses
much more than just simple zero Stator Winding
DC resistance at very low tempera-
tures. To completely describe the effect The AC synchronous motor design is compact and lightweight, and inherently
and its applications requires discussion quiet, but provides greater power density than that of conventional motors.
of aspects from disciplines such as
classical physics, quantum mechanics, States 25% of the electric power that is used for vehicle or ship propulsion.
thermodynamics, materials science, generated is transformed into shaft The advantages of using superconduc-
electrical engineering, to name only power by electric motors of 1,000 hp or
some of them. more. Even a small increase in motor tivity in commercial motors are attrac-
efficiency would result in substantial tive, since the size and weight are
Lossless current transport and the energy savings each year. Since there reduced to as little as 1/3 of conventional
ability to create large magnetic fields are approximately 50 million motors in motors while the losses are halved. In
based on this high current are two main operation in the industrial and commer- the past, the major disadvantages of
factors which give superconducting cial sectors alone, with more than 1 superconductivity applied to AC and DC
machines an advantage over conven- million of these greater than 5,000 hp, machines related to the thermal manage-
tional machines. These factors relate there is tremendous potential for use of ment. This included increased capital
directly to improved efficiency and superconducting motors. cost due to the necessary refrigeration
reduced size as measured by both and increased complexity of motor
weight and volume. Further, they can For the military sector, the weight design and upkeep due to incorporation
lead to initial as well as life-cycle costs. and volume reductions are equally of the cryogenic cooling system. Other
important, since most military factors included the reliability in the low
In the commercial and industrial sec- machines must be transported or are temperature thermal support systems.
tor, the efficiency translates into several
tangible results such as energy savings,
size reduction and operational cost
reductions. For instance, in the United

March / April 2000 ELECTRICAL LINE

New Materials

Although low temperature supercon-

ductivity has been known and applied

for many years, it was the discovery of

high temperature superconductivity

(HTS) materials, that began the super-

conducting transition in the tempera-

ture range of 77° - 100° K, that has

allowed for more widespread applica-

tion. The ability to operate at higher

temperatures removes a major obstacle

to the practical use of superconductiv-

ity - the need for a liquid helium refrig-

eration system. HTS compounds such

as Bi 2223 are being used in the manu-

facture of electrical conductors and a

variety of magnets. This offers the

potential to reduce operating costs due

to reduced refrigeration requirements.

The development of practical HTS High temperature superconducting (HTS) wire carries more than one hundred

materials removed the most significant times the electrical current carried by copper wire of the same dimensions.

economic roadblock, which is the cost

of cooling earlier materials to 4° K. nology on a scale such that it demon- tivity still retains the basic configura-

Large scale motor demonstrations are strates technologies applicable to larger tion and operation of conventional AC

planned, including a 1,000 hp motor. naval ship propulsion motors. motors. A superconducting magnet cre-

The initial commercial investment ates a magnetic field high enough that

impetus is in the regime of large Superconductive Motor Design iron teeth are not needed to enhance the

(>5,000 hp) industrial and commercial AC machines are the most common magnetic flux, either in the rotor or the

motors. Thus the first of the new HTS in a wide range of industrial applica- stator. This means that the current den-

motors would likely incorporate tech- tions. The application of superconduc- sities in the active regions are not lim-

ited by iron saturation. The stator only

requires the use of back iron acting pri-

marily as a shield to keep magnetic flux

inside the machine. A resulting HTS

air-core configuration, with high flux

density, is significantly smaller and

lighter than conventional AC synchro-

nous motors.

The lack of iron teeth in the rotor and

stator provides additional benefits.

First, it eliminates the need for winding

slots, thus reducing the major source of

cogging torques that lead to radiated

noise. The reduction of iron also

decreases the inherent armature (stator)

reactance, resulting in improved

machine dynamic performance. The

lack of iron leaves more room in the

stator and rotor structure for winding

conductors, thus increasing the power

density and efficiency. Dynamic char-

acteristics will be different in the super-

conducting motor. The lack of iron will

lower synchronous and subtransient

reactances, resulting in lower torque

angles during operation, thus improv-

ing transient stability. This stability

American Superconductor technicians, Rick Medeiros and Jason Montaner, improvement should be realized even

assemble HTS motor coils at company headquarters in Westborough, Massachusetts. with the reduced air core rotor inertia.

ELECTRICAL LINE March / April 2000

For naval ship propulsion, motors Amphibious Ship Comparison
must be low speed and high torque,
resulting in high weight when conven-

tional type motors are installed. The

lower weight and volume supercon- Traditional Drive
ducting motors will be able to operate

in the low speed ranges, less than 200

rpm, to directly drive the propeller,

while greatly reducing weight and vol-

ume over conventional motors. The

size reduction is important both for Propulsion / Electrical Generation Spaces

arrangement considerations in tight Intakes and Uptakes

areas of the ship, in exterior propulsion Shaft Line

implementations, and in the possible Integrated Power System
reduction of overall ship displacement.

The low weight also helps reduce

moments caused by large distances

from the longitudinal centre of gravity,

leaving greater design flexibility for the

naval architect to achieve ship balance.

Recent ASC preliminary design of a 25

MW, 120 rpm motor shows 5 to 10 fold

decrease in size and weight as com- and the Naval Research Laboratory synchronous design takes full advan-

pared with conventional motors. have recently initiated programs to tage of commercial developments in

design and build superconducting motor design, as well as the factors of

ASC Air Core ship propulsion motors that take low volume, weight, and high effi-

AC Synchronous Motor advantage of these HTS engineering ciency, making the motor attractive

The new ASC HTS conductor advancements. ASC is currently for shipboard applications. The initial

materials have made the transition under contract to design and build a implementation has a copper stator

from science to engineering and are superconducting motor directed winding and superconducting rotor

now available to begin construction specifically towards eventual use in coils. Modular construction of the

of superconducting motors today. The ship propulsion. winding assemblies simplifies the

U.S. Navy Office of Naval Research The superconducting air core AC design, production, testing, and main-

tenance of the motor. As new genera-

tions of HTS conductors are created,

they can easily be developed, manu-

factured, mass-produced, and tested

in parallel with existing coils, so that

when higher performance is ready for

insertion, the motor can be marketed

taking advantage of non-recurring

engineering costs of the design of the

balance of components. The assem-

blies can be bench tested to full volt-

age and current before installation,

improving reliability.

Heat transfer is probably the

biggest challenge in the motor design.

The most important development

pointing to a successful implementa-

tion in this area is the use of

extremely low power, low weight,

high reliability, mechanically simple

cryocoolers instead of high power,

high weight, complicated cryogenic

cooling systems used in earlier cool-

At one fifth the size and weight, ing systems. A cryocooler is a small

HTS AC synchronous motors will provide refrigeration unit that uses gas flow

ship architecture flexibility and lower installation costs. through a regenerator bed to achieve

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Head Office: 1515 Matheson Blvd. East, Unit 210, Mississauga, ON L4W 2P5
Phone (905) 238-6848, 1-800-668-6053 • Fax (905) 238-6849 or E-mail us at [email protected]

With Locations Across Canada to Serve You Better

Calgary • Mississauga • Ottawa • Montreal • Halifax

ELECTRICAL LINE March / April 2000

conductive cooling. ASC has used cry- consists of yearly change out of the motor is small and light with a high
ocoolers in many HTS magnets compressor discharge stream filter. efficiency for a propulsion motor.
including a mine clearing application. The HTS motor has no special shock Overall the motor diameter of a 25
A cryocooler suitable for a 5000 horse- considerations. MW, 120 rpm motor will be approxi-
power motor requires only about 6 mately 100 inches by 60 inches with a
kilowatts to keep the rotor at the requi- The motor system is predicted to be specific weight of 1.5 lbs/horsepower
site 30° K during operation. robust enough for at-sea operations, and an efficiency of 97.7%.
Cryocooler mainte-
nance only even in loss of cooling condi- The market for commercial supercon-
tion. A loss of cooling is ducting motors is very large, estimated
predicted to allow oper- to be at 1 billion dollars per year world-
ation at continually wide. The combination of high tech
reducing power, allow- R&D and commercial product develop-
ing time for correc- ment of modularly designed motor mag-
tive actions. Most nets should produce a viable technolog-
importantly, the
ical result that will
make sense in
both military and
commercial
aspects. Ω

The 5000 hp industrial motor is expected to be 40% THE DELTA GROUP xfo GROUP
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