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AN IRON FREE ASYNCHRONOUS PULSED
GENERATOR FOR THE PRODUCTION OF PULSED HIGH MAGNETIC FIELDS
F. Sultanem, C. Bleijs, C. Postel, S. Askenazy, J. Marquez
To cite this version:
F. Sultanem, C. Bleijs, C. Postel, S. Askenazy, J. Marquez. AN IRON FREE ASYNCHRONOUS PULSED GENERATOR FOR THE PRODUCTION OF PULSED HIGH MAGNETIC FIELDS.
Journal de Physique Colloques, 1984, 45 (C1), pp.C1-67-C1-70. �10.1051/jphyscol:1984114�. �jpa-
00223611�
JOURNAL DE PHYSIQUE
Colloque C l , supplément au n° 1, Tome U-5, janvier 1984 page Cl-67
AN IRON FREE ASYNCHRONOUS PULSED GENERATOR FOR THE PRODUCTION OF PULSED HIGH MAGNETIC FIELDS
F. Sultanem, C.A. Bleijs, C. Postel, S. Askenazy* and J. Marquez*
Laboratoire d'Electroteehnique des Universites Paris VI et XI , Bdtt-ment 214, 91405 Or say, France
* Service des Champs Magnetiques Intenses**, INSA, Avenue Rangueil, S1000 Toulouse, Franee
Résumé - La fabrication de champs intenses (> 500 kgaus) puisés de lon- gue durée (T > 1 s) avec des bobines cryogéniques nécessitent de dispo- ser d'une source d'énergie de plusieurs mégajoules. Les génératrices asynchrones sans fer à régime impulsionnel sont parfaitement adaptées à ce type d'utilisation et peuvent fournir l'énergie avec un prix de reviens très compétitif.
Abstract - The production of long pulsed high fields (> 500 kgaus with T > 1 s) with cryogenic coils requires an initial energy source of se- veral megajoules. Iron free asynchronous pulsed generators are particu- lary well adapted to this application as they are able to produce this energy at a very competitive price.
A - INTRODUCTION
The iron free pulsed generator were first developed at the "Laboratoire d'Electro- technique des Universites Paris VT et XI" a few years ago in view of their use as very high pulsed power source (1). Subsequent discussions held with the "Service des champs intenses de l'INSA de Toulouse" shows that they are particulary well adapted to feed the cryogenic coils used for long pulsed field physics experiments.
Present experiments are limited by the investment necessary to atain.such high pulsed fields. Typicaly, a 50 kg copper coil cooled by liquid nitrogen and fed from a 1,5 MJ capacitor bank, can produce a field of some 40 Teslas with a crow- bar time constant of 1 sec.
We are now in position which permits us to imagine a reliable crowbarred field coil of 50 Teslas with a time constant of 1 sec, using the structure suggested by V.I. OZHOGIN (2)(reinforced copper conductor). Such a coil would require an ener- gy source of some 4 MJ. More than 10 MJ would be required for time constants of the order of 2 to 3 seconds. Capacitor banks become prohibitively expensive at these energy levels and the use of pulsed generators provide a simple low cost solution.
B - GENERATOR DESCRIPTION AND OPERATING MODE.
The generator structure is very simple. It consists of a solid aluminium alloy rotor surrounded by a multipole stator devoid of ferromagnetic material. This is an asynchronons generator requiring no mechanical contact between rotor and stator;
energy is transfered only by electromagnetic forces. For low energy levels (< 5 MJ) the rotor can store directly the initial kinetic energy. For higher levels it would be preferable to add an external iron flywheel to reduce the overall dimensions.
figure 1 indicates the two possible machine structures having a disk shaped or cylindrical rotor.
Equipe de Recherche Associee au CNRS n" 838
**Laboratoire Propre CNRS LP 120003
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984114
JOURNAL DE PHYSIQUE
windings rotor
Cylindrical mchine.
I
Disk s h a p d machine.
Fig. 1
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Iron f r e e generator is constituted with an aluminium alloy rotor surrounded by a m u l t i p l e non mqnetic s t a t o r . An iron flywheel is~ c h a n i c a l y coupled t o the rotor.
Fig. 2 shows a schematic diagram of the generator associated with i t s discharge c i r c u i t . This c i r c u j t i s composed of the reactive pawer compensation capacitors, the three phase diode bridge and the c o i l load. Fig. 3 shows a typical current discharge.
The basic operating principle of t h i s system and the experimental r e s u l t s obtained with the disk shaped protot ( 4 = 1 m) have already been f u l l y described in previous publications. ( 3 )
(v
C - CAPABILITY OF AN IRON FRGE GENERATOR - CRYOSENIC COIL SYSTEM :
Iron free generator were not i n i t i a l l y developed specifically f o r the charging of cryogenic c o i l s . This particular application does not seem t o pose any m j o r d i f f i c u l t i e s . In f a c t t h i s u t i l i s a t i o n can be defined by the following three p i n t s :
1 / The energy range.
The generator energy range must be high ( m r e than 5 W ) t o allow the produc- tion of high f i e l d s i n large volumes. For our particular application we can assme t h a t the time between successive discharges i s long (a few minutes). The mchine can therefore be constructed without a cooling system, but has t o absorb its own e l e c t r i c a l losses. The energy level i s thus defined essentialy by i t s t h e m 1 capacity. For an iron free mchine operating under these conditions it i s possible t o a t t a i n an energy density of about 15 kJ/kg a t an energy level of 10 W . For larger mchines, the admissible energy density increases hence the mchine e f f i - ciency is improved. Consequently t h e energy range needed here can be achieved easly with machines having reasonable volumes.
2/ The charging tire.
The charginq time must be relatively short t o avoid heating the cryogenic coil. With iron free mchines,the current b u i l t up time constant i s of the order of 1/10 t o 2/10 sec. As the c o i l crowbar time constant is assumed t o be 1
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2 sec,the heating during the charging sequency represents only 10 t o 20% of the t o t a l discharge c o i l heating.
Note t h a t with iron free mchine there i s no limitation due t o saturation pheno- m. The power i s always proportional t o the square of the current. However there i s a relation betwen the charging t h e and the powr factor correction capacitors energy which limits the power of a reasonably sized system.
In f a c t , i f W draw more pwer from a given machine, we w i l l need more capacitors t o produce the reactive power. This can increase significantly the system cost.
For our investigations we have always assumed that the capacitor energy m s t not exceed 1
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2% of the generator energy.3/ The f i e l d quality
The f i e l d produced i n the c o i l have t o be m o t h without any mdulation o r interference t o avoid pertubations of the measunrents. This question i s now under study. with the existing system the current c o r n t a t i o n s i n the diodes brid- ge (see f i g . 3) produce a modulation on the load current which can a t t a i n up t o 5% of the t o t a l value. The suppression of t h i s modulation during charging phase i s practicaly impossible a s they are specific of the system operation mode.
Nevertheless, a t the end of the charging phase, it i s possible t o isolate the mchine electrically and t o c r o w the coil.
Such an o ~ r a t i o n mode can be achieved by replacing the diodes Dl, D2 and D3 (see f i g . 2) by thyristors and by using the supplercentary freewheel diode D7.
The thyristor triggers can then be c o m d e d t o open the c i r c u i t when the current reach i t s m i m a l value.
Fig. 2
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The qenerator and h i s discharge c i r c u i t .Some preliminary experiments have been done recently with an ordinary (hot) c o i l t o t e s t t h i s new operation mode a t low energy level ( E SO kJ) : f i g . 4 shows one such experimental discharge. W e can compare f i g . 3 and 4 and note that the mchine current has stopped and the c o i l current continues t o flow through the free-wheel diode.
These r e s u l t s are encouraging, but there still remain some problems which are due t o the over voltage produced on the thyristors.
JOURNAL DE PHYSIQUE
Fiy. 3
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A typical discharge Fig. 4-
Thyristors are used t o isolate the mchine with an ordinary c o i l . Note from the load c o i l a t the end of the charning sequen- that the machine alternatinu ce, The machine current stops flowing and the load current continue t o flow current diverts through the freewheel d i d e . The cur- durinq the c o i l crowbar. rent modulation i s thus eliminated.The broken l i n e indicates the wave form obtainable by cooling the load c o i l with liquid nitrogen.
D
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CONCLUSION :Achieving long pulsed f i e l d of over 50 T in a cryogenic c o i l having a crowbar tim constant of over 1 sec seems t o be possible with iron free generators a t very rea- sonable cost. The cooperation between the "Laboratoire dlElectrotechnique des UniversitCs Paris V I e t X I " and the "Secvice des champs intenses de 1'mSA de Toulouse" w i l l allow t h i s goal t o be attained. A cryogenic c o i l adapted t o our machine ( $ = 1 m) has been constructed recently. This c o i l has a crowbar tinle constant of 1.2 sec. I t should be possible t o charge it t o about 300 kJ with a t i n e constant of about.2 sec. The succesfull completion of these preliminary experiments w i l l pemcit us t o design a f u l l scale high f i e l d system.
1
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C. RIOUX, F. SULTANEM-
prcc.
of the 9th sym. on fusion technology Gannish Partenkirchen ( 1976).
2
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V . I . OZHCGIN and a l l .-
proc. o f - t h e int. sym. on high f i e l d magnetism-
Ozaka (sept. 1982) p 267.
3 - F. SULTANEM, C.A. BLEIJS, C. RIOUX and F. RIOUX
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M d a u , J. Appl.phys. 52 (5)
,
my 1981.4
