Study of Full Wave Supercoducting Rectifier Type Flux Pumps

7/23/2019 Study of Full Wave Supercoducting Rectifier Type Flux Pumps

1/4

I EEE

TR A N SA C TI O N S O N M A G N ETI C S, V O L.

32,

NO. 4, JU LY

1996

2699

Study

of

Full-wave Superconducting Rectifier-type Flux-pum ps

Qiuliang Wang, Luguang Yan and Changlian Yi

Institute

of

Electrical Engineering, Academia Sinica, Beijing, China

Abstract-Based on a theoretical analysis, the main

components of a full-wave superconducting rectifier-type

flux-pump, including a low-loss superconducting transfor-

mer with high coupling coefficient, superconducting therm-

ally activated switches, protection system, control system

and variable frequency and amplitude power supply, are

developed. With the developed components, two flux-pump

systems have been assembled and studied experimentally

with different input current waves of various frequency and

amplitude. Quite good results have been achieved. It can

energize the superconducting magnet with different charging

rate.

The

highest rectifier frequency is

0.35

Hz and the

charging rate is 86.7 Almin with an efficiency of 96 9 . The

paper summarizes the design, development and experi-

mental results

of

the system.

I. INTRODUCTION

Fig. 1presents the basic circuit of the full-wave super-

conducting rectifier-type flux-pump for charging a load

superconducting magnet

LL.

The flux-pump consists of a

altemating current power supply PS, a superconducting

transformer with primary inductance

Lp

and secondary

inductances L s ~ ,LSZ, a mutual inductance M between

primary and secondary windings and two superconducting

switches S1 and S2 For load magnet and flux-pump

protection, there is a superconducting protection switch

Sp, and a dump resistance . There is also a control

system a t room temperature to control the primary cur-

rent wave form, the action of superconducting switches

and the protection.

In

every half cycle of the primary current, the work of

the flux-pump consists of a series of four processes, i.e.

1)

Load current pump-in. The S1 switch is closed and S2

switch is open, the primary current rises and induces a

load current through switch Si and load magnet LL;

(2)

Superconducting Switch S 2 recovery. The primary current

remains constant, the switch S 2 recovers its superconduct-

ing state;

(3)

Current commutation. By inductive or resis-

tive cominutation method the load current is transferred

from S1 circuit to S2 circuit; (4)Superconducting switch

S1 opening. The circuit is ready for the second half cycle

charging with closed

S2

and opened

S I .

During magnet

charging, the processes repeat many times until the load

current reaches its operating valuet~l.[~l.

As a power supply for a superconducting magnet, the

Superconducting rectifier-type flux-pump has some ad-

vantages, such as : 1) Only a pair of small current leads

is

used, therefore, heat leak

and

joule heat

losses

from

Manuscript received June 11, 1995 .

Q.

Wang,

L. Yan and C. Yi, fax 86-10-2560904, address: P.O.Box

2703, Beijing 100080 hina.

Flux pump

Protection

Load

%nee

Power Supply

- - I

Fig. 1 . Basic circuit of th e full-wave rectifier-type

flux-pump

for

magnet charging

current leads are small; (2) By choosing the current

commutation method and the wave form of the transfor-

mer primary current properly, a load current of arbitrary

value can be obtained; (3) The magnet current can be

easily adjusted to meet the high precision and high stabili-

ty requirements;

(4)A

high current and high stable power

supply is not needed.

For an experimental study we developed the main

components of the flux-pump, including transformer,

switches, protection and control systems. With the deve-

loped components, two flux-pumps

No 1

and

No 2

are

assembled and a quite wide experimental study with dif-

ferent primary current wave forms were performed. The

present paper summarizes the related work and results

11. COMPONENTS

A.

Superconducting

Transformer

The transformer consists of air-core solenoids, with a

primary coil wound with NbTi composite conductor and a

secondary coil wound with a superconducting braided

cable.

To

obtain a coupling coefficient as high as

pos-

sible, the secondary coil is placed between the primary

coils. To increase the axial heat conduction, 60.1

copper wires are placed between layers along the axial

direction. The transformer is impregnated with glassfibre

epoxy resin with 4040 A1203 powder.

In

total, three

transformers have been constructed and tested. Their

specifications are given in Table I.

0018-9464/96$05.00 996 IEEE


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2700

TABLE

I

TABLE

I1

SPECINCATIONS OF THE SUPERCONDUCTING

TRANSFORMER3

SPECIFICATIONSOF THE SUPERCONDUCTING SWITCHFS

No.

No.1 No.2 N0.3

Outer

diameter (mm)

126.8 62.5 88.6

Inner diameter (mm)

120 58 79.8

Primary turns

Np

1375 1466 2720

Secondary turns Ns 28 76 68

Coupling coefficient Ks 0.978 0.967 0.980

Primary conductor:

Height (mm) 56 64 160

material

CuNilNbTi

NbTilCuNilCu NbTilCuNilCulNb

diameter (mtn)

0.3 0.35

0.3

filament diameter (pm) 15 0.98

3

twist pitch (mm)

10

4 10

CulSc

1

0.91 1.2

filament number

500 4320 2640

material NbTilCu NbTilCuNilCu NbTiiCu

section (inin*) 1.137x2.26 d0.8 1.137x2.26

filament diameter (hm)

23

7.9 23

CUlSC 3.3 1.8 3.3

Secondary conductor:

twist pitch (mm) 35 15 35

During tests, with a short-circuited secondary coil, the

transforiner No. 1 showed the highest primary quench

current of 32.5 A and a charging rate of 100

MS.

No. 2

and

No.

3 have 35 A 165

IS

and 34.8 A ,

200

A / s

respectively. With an open secondary coil, the transfor-

mer No. 1 has 26 highest quench current and No. 3 has

30.24 A . The study on the influence of the primary

frequency shows that for transformer No. 3 up to 10 Hz

the degradation of quench current is quite small.

B.

Superconducting Switch

S

Four thermally-controlled superconducting switches

have bzzn constructed and tested, their specifications are

listed in Table 11. The conductor used is NbTi in CuNi

matrix, all of them are non-inductive coils and impregna-

ted with epoxy resin with 40 - 6 A 1 2 3 powder.

Different insulation material and heater structure are

used. The tests showed that the switch off-time is around

30 ms with high enough heater pulse. The minimum

heater energy

is 20

-

50

mJ at small current, the switch

recovery time is between 0.1 and

0 . 4

s .

C Prowction Switch S

For protection during quench, a protection switch

S

is

used to switch off the flux-pump, so that the load magne-

tic energy can be absorbed by the parallel dump resistor

or diode. The thermally-controlled switch is a

non-

inductive solenoid with

15

mm average diameter and 30

mm height, made of 10 m long 5x

4

0 . 3 mm NbTi/CuNi

superconducting cable. The heater is wound around the

cable uniformly with

5

cm length and 5 cm gap. test

showed that

the

switch-off time

is

10ms, the normal

No.

No. 1

Current (A) 450

Average diameter (mm) 58

Effective height (mm)

20

Heat

Insulation:

material Kapton

thickness (mm) 0.3

Conductor:

length (m) 1 o

diameter (mm) 0.5

filament diameter (pm) 24.3

twisted pitch (mm) 7.0

Resistance(Q, t 300K) 0.65

Off-time (ms) 28

Minimum heater energy(mJ) 19

Recovery-time (ms) 300

No.2

480

58

20

Kapton

0.13

1

o

0.5

24.3

7.0

0.65

30

100

30

No.3

480

25

25

Polyester

0.25

0.8

0.7

14.5

5

0.84

40

110

48

No.4

400

25

25

Kapton

0.13

0.8

0.3

15

10

0.84

25

420

23

resistance is 4 0 nd the heater energy

is

0.74 J.

D.

Load

Magnet

LL

The load magnet is a superconducting solenoid with

59

mm inner diameter,

67

mm outer diameter and 140 mm

height. The conductor used is C J 0.85 mm NbTi/Cu

conductor with 107 pm filament diameter and 15 mm

twist pitch. The total number of turns is 1200 and the

inductance is 40 mH. The magnet can be charged to 500

A without quench.

E. Control

System

As

an

important part of the flux-pump, a computer

control system has been developed. It consists of three

major units, i.e.

(1)

The unit for power supply control. It

can produce the programmable wave form of the primary

current and control the charging process in accordance

with the load current feed-back signal.

2)

The unit for

superconducting switch control. It can provide the pro-

grammable signals to close and open the superconducting

switches

in

sequence to guarantee the correct work of the

flux-pump during charging. (3) The unit for data collec-

tion and processing. It collects all the operating para-

meters of the system and provides the necessary signals

for power supply and switch control and the protection

system. Corresponding soft ware for control has

been

also developed.

111.

EXPERIMENTAL

TUDY

Specifications of the flux-pumps and main test results

are listed in Table 111.

Fig. 2 presents the three types of shapes of the trans-

former primary current A B and C used in our experi-

mental study, in accordance with the four processes

during every half cycle t, shows the current pumping


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TABLE I11

SPECIFICATIONS OF TWO SC. FLUX-PUMPS

No.

Transformer

Switch

Protection switch

Load magnet

Primary current

Maximum load current

Average charging power

Efficiency

Maximum ch rgi ng rate

No. 1 No.2

No.

1

No.3 (Tab.1)

Lp

=

25 4 mH Lp

=

184 mH

K = 0.978 K = 0 .98

No .2

+

No 3 No .2+ No .3 (Tab.2)

40

mH,

40

ms

40

mH, 40 ms

Wave

C,

A Wave B, A

Ls

=

105.6 pH

Ls

=

232 pH

% = 4 Q ,

&=In

5 = 4 n , % = l o

Ip

=

1-7A Ipz1-6A

f=0.0083-25Hz f = O . 1-0.35Hz

198A 620A

IOW 29W

Wave C:93.2 % Wave B:96.5 %

Wave A:94.8%

27A/min 87A/min

time,

t,

witch recovery time,

&

ommutation time,

th witch opening off-time. It can be seen that during

current pump-in and commutation intervals tp and t, the

primary current i is changing linearly, during switch

recovery and switch-off intervals

t .

and th the i, remains

constant.

Wave A uses constant time intervals, i.e. tp tc, t,, th

are constant for all cycles, so that the primary current

frequency is constant, the primary voltage and current

charging rate at t, and & are changing for different cycles.

Wave uses constant primary current changing rate, so

that the t, and & are changing from cycle to cycle, the

primary frequency is changing. Wave A and wave

B

use

the inductive commutation method. Wave

C

uses the

resistive coininutation method, it has constant primary

I

A-wave

I

frequency and constant current charging rate.

Experiments were performed in a 200 mm inner dia-

meter vertical dewar. The study concentrated on the

influence of the different primary current wave forms and

their parameters (amplitude, frequency, time intervals) on

the load magnet charging process and the performance of

the flux-pump. All the measured parameters during

experiments were registered with computer and X-Y

recorders.

With flux-pump

No.1

an extensive study with C-wave

of the primary current and resistive commutation was

performed. With a primary current amplitude

Ip =

7

A

and 0.0083

Hz

frequency, the load current reached 178

A

in 44 min. with an average charging rate of 3.29 Almin.

With I, = 2 A and 0.125 Hz he load current reached

121

A

in 27 min. Fig. 3a presents the experimental result

of flux-pump No.1 with C-wave and varied I,, and f

values during charging. It reached 198

A

load current in

37 min. Fig. 3b is the result of the flux-pump No.1 with

A-wave,

I,

2 .6 A , f 0.1

Hz.

The load current

reached 152 A in 15 min. For comparison there

is

shown

also the calculated charging curve, the difference reflects

the influence of the secondary losses. With improved

A-

wave the charging time is further reduced. The highest

charging rate of 26.9 Almin was obtained.

With flux-pump No.2 a mainly study with variable fre-

quency B-wave of the primary current was performed.

Fig. 4 a, presents the measured result with B-wave, I, =

5 A , f = 0.05-0.25 Hz. The load current reached 620 A

in 12.4 min with charging rate of 50 in. After select-

ing the optimal wave parameters, with the improved B-

wave the load current reached 505 A in 6 min (Fig. 4b).

The highest charging rate obtained is 86.7 Almin. With

the improved A-wave of

0.15 Hz

and variable current

amplitude from 1 A to 5 A , quite a good result has been

also obtained

(505

A in

1 1 , l

min.).

k - u l - w

Fig. 2. Wave forms of the primary current

witch recovery time,

t, ommutation time

th Switch off-time, tp urrent pumping time


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2702

200

180

160

140

120

100

80

60

40

2

10 15 20

25 3

35 40 *

Time T (min)

a) C-wave

2 4 6 8 1 0 1 2 1 4

Time

T

(min)

b)

A-wave

Fig.

3 ,

Experimental result of flux-pump NO 1

16

1 6

1

I , = 5 A

f = 0 . 0 5 - 0 . 2 5 H ~

I

L

calculated

measured

I , = 6 A

=0.1-0.351-I~

a) B-wave

b) B-wave with optimal parameters

Fig. 4. Experimental result of flux-pump No.:

Iv CONCLUSION and

an

efficiency of

more

than

96

With the developed main components,

two

full- wave

superconducting rectifier-type flux-pumps No. 1 and No.2

have been constructed and tested. Three different wave-

forms of the input primary current have been studied

experimentally. The experiments show that the variable

frequency B-wave form has the maximum charging rate

and efficiency. The best result obtained is: the primary

current frequency .35 Hz he maximum charging

rate 6.7 Nmin, the maximum load current 20 A

REFERENCES

[l] L. J. M . van d e Klundert and H .

H.

J. ten Kate, Fully supercon-

ducting rectifiers and flux

pumps

Part

1:

Realized methods for

pumping flux , Cryogenics 21 vol.

21,

pp.

195-206,

April

1981;

[2] L.

J. M. van de Klundert and H

H

J. ten Kate, On fully sugercon-

ducting rectifiers and flux pumps

Part 2:

Communication modes,

characteristicsand switches,

Cryogenics

21 vol. 21, pp. 267-2719>

May 1981.

Documents

Study of Full Wave Supercoducting Rectifier Type Flux Pumps