• SS304 is chosen as material of radiation shield for present study.

• Emissivity of material varies considerably in the range of temperatures

studied.

• A linear fit to the property data [4] of emissivity for chosen material is used

in the present analysis.

• Temperature of radiation shield is assumed to be constant and same as

the temperature of torque tube at anchor point.

• Temperature of torque tube area 𝐴𝑇𝐿 is assumed to be same as that ofradiation shield. Similarly temperature of 𝐴𝑇𝐻 is assumed to be same asthat of vacuum vessel (300K)

For a given location of radiation shield ′𝑥′, the temperature of anchor point𝑇𝑎𝑝 can be solved iteratively by equating the 𝜙𝑅 measured from energy

balance across torque tube and energy balance across radiation shield.

Total amount of heat-in-leak into the cold space 𝑄𝑐𝑠 is then given by

Qcs = QL + Qc ; 𝜙𝑐𝑠=𝑄𝑐𝑠𝑄𝑚𝑎𝑥

Optimum Location of Thermal Radiation Shield in Superconducting Rotating Machines

Aaditya Saikiran Pegallapati1 , V. V. Rao1

1Applied Superconductivity Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur - 721302

Poster ID : 10-P3-241

The principles of operational performance of electrical machinery have not

changed since their invention in 19th century, which makes the pursuit of

their improvement a challenging task. A.C electrical machinery with

conventional design using conventional materials has attained a high level

of performance and high output ratings over a period of time. Improvements

are possible by use of better materials and by better cooling, but these

techniques can only be applied to parts of the machine and when

availability of technology permits.

Studies reveal that the application of superconductors operating at high

current density in modest high magnetic fields will result in reduction of

space and weight along with improvement in the efficiency of large A.C

generators/ motors. This led to the development of synchronous generators

and motors with low temperature superconductors . The discovery of high

temperature superconductors raised considerable excitement, since a

higher operating temperature reduces both capital and operational cost of

the machines. A considerable effort has been made in developing

superconducting rotating machines with applications in generation (power

plant generators, hydro generators, wind turbines etc.) and transportation

(ship propulsion, air craft propulsion etc.)

Introduction Results

Thermal Issues

Heat Transfer Model

Results

Conclusions

International Cryogenic Engineering Conference - 2016. March 10, 14:00 to 15:30 IST

Variation of Total heat-in-leak (𝜙𝑐𝑠 ) into cold rotor with anchor point location for various emissivities of cold rotor

Variation of Total (𝜙𝑐𝑠 ), Conduction(𝜙𝐿 ) and Radiation (𝜙𝑐 ) heat-in-leak with anchor point location

QH =A

x T

TH

k(T)dT

QL =A

L − x TL

T

k(T)dT

QR = QL − QH

𝜙𝐻 =1 − Κ

𝜂

𝜙𝐿 =Κ

1 − 𝜂

𝜙𝑅 = (𝜙𝐿− 𝜙𝐻)

𝜙 =𝑄

𝑄𝑚𝑎𝑥

Κ =1

𝛼 𝑇𝐿

𝑇

𝑘(𝑇)𝑑𝑇

𝜂 =𝑥

𝐿

𝑄𝑚𝑎𝑥 =𝐴

𝐿 𝑇𝐿

𝑇𝐻

𝑘(𝑇)𝑑𝑇

𝛼 = 𝑇𝐿

𝑇𝐻

𝑘(𝑇)𝑑𝑇

𝑄𝑣 = 𝜎𝐴𝑟𝐹𝑟𝑣 𝑇𝐻4 − 𝑇4

𝑄𝑐 = 𝜎𝐴𝑐𝐹𝑐𝑟 𝑇4 − 𝑇𝐿

4

𝜙𝑣 =𝑄𝑣𝑄𝑚𝑎𝑥

; 𝜙𝑐 =𝑄𝑐𝑄𝑚𝑎𝑥

2𝜙𝑅 = (𝜙𝑣− 𝜙𝑐)

Superconducting rotating machines have rotors maintained at a low

temperatures, which are below the critical temperature of the

superconductor. This establishes large temperature differences between

the cold rotor (~30K for a typical HTS motor) and surroundings, resulting in

a large heat-in-leak into rotor which occurs primarily through 1) conduction

from supports (torque tubes) and connections (electrical cables), 2)

convection through surrounding medium and 3) radiation from enclosed

surfaces. Minimizing this heat leak is essential to reduce the power

expenses of cryogenic cooling system [1].

Reducing heat leak through torque tubes is challenging because of

conflicting requirements between mechanical and thermal performances.

Torque tubes are subjected to working and peak torques generated in the

machine as well as a large temperature differences across the ends.

Mechanical integrity of the torque tube calls for a larger cross sectional

area and shorter length. This is in direct conflict with thermal design

requirements, which call for lower cross sectional area and longer length.

Electrical connections like the current leads (usually vapor cooled) also

contribute to some conduction heat transferred (this component is not

relevant in present analysis, as it is unaffected by the anchoring point of

radiation shield). By ensuring a vacuum below a certain limit (<10-4 mBar),

heat transfer through residual gas conduction is neglected. Heat transfer

through radiation is reduced by providing a radiation shield anchored at a

suitable location on the torque tubes. In principle any number of radiation

shields can be provided, however, owing to other mechanical and design

complexities, superconducting rotating machines have one or sometimes

no radiation shield. Schematic arrangement of radiation shield anchored

onto a torque tube for a typical HTS motor is shown below.

This paper presents a method to determine the optimum location at which

radiation shield should be anchored, such that the total heat-in-leak into the

cold rotor is minimized for a given geometry of rotor assembly. It is

observed from the present analysis, that above a limiting value of cold rotor

emissivity (𝜀𝑐), the optimum anchor point is very close to the cold end of

torque tube. Below this limit, the optimum anchor point is closer to the hot

end of torque tube.

𝐹𝑟𝑣 =1

𝜀𝑟+

𝐴𝑟𝐴𝑣 + 𝐴𝑇𝐻

1

𝜀𝑣− 1

−1

𝐹𝑐𝑟 =1

𝜀𝑐+

𝐴𝑐𝐴𝑟 + 𝐴𝑇𝐿

1

𝜀𝑟− 1

−1

• Hot end temperature 𝑇𝐻 = 300𝐾 and cold end temperature 𝑇𝐿 = 30𝐾• SS304 is chosen as material of torque tube for present study.

• Thermal conductivity of material varies considerably in the range of

temperatures studied, hence its variation with temperature is taken into

account. [3]

• Cross sectional area of torque tube is assumed to be constant.

• Optimum location of radiation shield depends on the emissivity

of cold rotor maintained at cryogenic temperatures.

• At very high emissivity of cold rotor i.e. when the dominant heat-

in-leak is through radiation then optimum location of radiation

shield is close to cold end of the torque tube

(η = 0.8 to 1)

• At very low emissivity of the cold rotor i.e. when the dominant

heat-in-leak is through conduction from torque tube then no

radiation shield is recommended as introduction of a radiation

shield will increase the heat-in-leak.

• There exists a critical emissivity of cold rotor above which the

location of anchor point of the radiation shield for minimum heat-

in-leak is very close to the cold end of the torque tube.

• Below this critical emissivity of cold rotor the optimum location of

anchor point suddenly jumps near to hot end of torque tube. With

further decrease in emissivity this optimum point moves closer to

the hot end.

Variation of anchor point temperature (𝑇𝑎𝑝 ) with radiation shield

anchor point location

Conduction

Radiation

References

1. T.E. Laskaris, A cooling concept for improved field winding

performance in large superconducting ac generators,

Cryogenics, Volume 17, Issue 4, April 1977, Pages 201-208,

ISSN 0011-2275

2. Bejan, A, Material selection for the torque tubes of large

superconducting rotating machinery, Cryogenics, vol 14 no. 6

(1974), pp. 313-315

3. Marquardt E, Le JP, Radebaugh R. Cryogenic material

properties database. In: Proceedings of 11th international

cryocooler conference, 2000.

4. Hawks, K. H., & Cottingham, W. B. (1971). Total normal

emittances of some real surfaces at cryogenic temperatures.

In Advances in cryogenic engineering (pp. 467-474).

Springer US

5. www.mae.ncsu.edu/buckner/courses/mae535/SuperHigh.pdf

6. http://evbud.com/news/134/

5-MW motor factory test at AlstomPower Conversion in Rugby, UK. [5]

5-MW motor Artistic View [5]

EADS concept of hub-mounted superconducting motor [6]

CONTACT INFORMATION

Mr. Aaditya Saikiran Paaditya.sai.kiran@gmail.com

Prof. V. Vasudeva Raovvrao@hijli.iitkgp.ernet.in

Cold end

Hot end Cold end

Hot end

Cold end Hot end