Transcript Superconducting and Conventional Machines

Superconducting and
Conventional Machines
A.M.Campbell
IRC in Superconductivity
Cambridge
Machine Parameters
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Power/vol
Efficiency (including thermal)
Cooling Technology
Reliability
Lifetime Cost
Material Parameters
• Tc Jc Birr
E
J
v
We can pass a current until the Lorentz force pulls flux
lines off pinning centres.
Max force BJc/vol and E=Bv
Consequences
Maximum force/ vol is BJc.
This is what drives a motor.
When the field changes there is a loss EJc /vol
This determines AC losses.
AC Losses, Field amplitude b, or current I
Self field at Ic Loss/cycle/length moIc2/4p2
Independent of material, fairly independent of shape,
reduces with cube of current, so subdivide conductors
and work below Ic for low loss.
External field, (relevant to motors)
Loss= bJcd J/cycl/vol
d is width perpendicular to field. Since
field is also proportional to Jc, loss for
given field in given volume does not
depend on Jc or conductor material,
only on conductor width.
Maximum Possible
(Based on perfect diamagnetism and
square hysteresis loop).
Aspect ratio x applied field 2 mo
J/vol/cycle
Material Limits for
Conventional Machines
• Nearly all conventional machines can be
approximated by a hollow cylindrical iron
stator containing a solid iron rotor.
• There are current sheets fixed to each, either in
the gap or in slots.
• A sinusoidal current sheet allows simple
analytic expressions for torque and dissipation
N
S
N
S
Max B 1T, Fe saturation
Max j 800 kA/m , 0.1 T (Thermal and appoximate)
Max torque Bj /vol of rotor
Max power at 50Hz 25MW/ cubic metre of rotor.
EFFICIENCY
• Loss/Power is (d/R)2 where d is the skin
depth of copper (1 cm at 50 Hz) and R the
rotor radius.
• Large machines are very efficient, (98% for
utility generators)
• Superconductors are attractive for small
machines, but small coolers are expensive
and inefficient which cancels advantage
Now compare superconductors
• Limits depend on maximum pinning force
Force BJc
dS
Integrate the moment of the force to get torque.
Torque/vol=RBJc/3
Power x 2p frequency
Torque/vol proportional to Jc and radius.
BJc for YBCO 123
Using YBCO we get 100 times the power/vol, or 1/1000 volume
for a given power. (Weight even better)
HOW TO ACHIEVE THIS?
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Maximum BJc is at 2 tesla
Rotor can be solid or wire
Stator needs wire, and AC losses are high.
Only possible with YBCO coated
conductors
Realities
• We can come down from the ideal in many
different ways and still have major benefits
• A factor of 100 is possible, a factor of five can be
economically attractive.
Motor Types
• Conventional motor types tend to merge using
superconductors. Think in terms of force on flux
lines and E-J characteristics.
E
Induction (viscous)
Reluctance,
Permanent
Magnet
I
Hysteresis (friction)
• How do we provide a large rotating field?
• Firstly a large field
• Secondly without large losses?
B
Free space moj=0.1 T, thermal limit.
B=0.05 T (approximately)
j A/m
Iron on outside (or inside)
doubles B
B=0.1 T
(May be needed for screening)
Narrow air gap.
B=0.1Tx(radius/gap) up to saturation
2T
• A superconducting rotor is a large air gap so
only small fields can be applied with a
normal stator.
• A hybrid iron/superconducting rotor does
not increase the B at the right place so is not
helpful
• (But it is useful in reluctance machines)
Cooling
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Start with Carnot,work proportional to 1/T:T
300 : 77: 20: 4 K
Power
1: 4: 16: 80
Inefficiency, multiply by 10-50.
Large coolers are more efficient
Efficient coolers cost more
Coolants
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Liquid Nitrogen
Liquid neon
Liquid Hydrogen
He
Materials
• YBCO wires, ideal but expensive and not
yet available.
• BSCCO tape, must be shielded from field at
77K, good properties at 20-30K
• YBCO bulk. Good for rotor, especially as
permanent magnet- but then may need to be
remagnetised.
Magnesium Diboride
• Magnesium Diboride, comparable with
BSCCO at 20K, still improving. Cheaper
and easier.
• Low Tc, still the cheapest per kA m, but AC
losses in stator an expensive cooling a
barrier.
CONCLUSIONS
• Superconducting motors and generators can
increase the power per unit volume by
several orders of magnitude compared with
copper and iron.
• Efficiency gains are also significant.
YBCO
• Only YBCO wires can realise the full
benefits
• Significant improvements can be obtained
with a wide variety of different materials,
temperatures, and machine designs.