United States Patent [19] [ii] 4,045,716 Bartram et al. [45] Aug. 30, 1977

[54] MAGNETIC FIELD SYSTEMS EMPLOYING [56] References Cited A SUPERCONDUCTING D.C. FIELD COIL U.S. PATENT DOCUMENTS 2,604,616 7/1952 Edwards 318/154 [76] Inventors: Trevor Carlisle Bartram, Fossway, 2,929,975 3/1960 Caldwell et al 318/154 Newcastle upon Tyne NE6 2YD; 3,263,142 7/1966 Adoutte et al ...318/154 . Paul Alexander Hazell, Ministry of 3,450,952 6/1969 Nelson 317/123 Defence (Navy), D.G. Ships, 3,864,965 2/1975 Anderson 318/154 Foxhill, Bath, Somerset, both of Primary Examiner—William H. Beha, Jr. England Attorney, Agent, or Firm—Lawrence Rosen; E. Janet Berry [21] Appl. No.: 577,506 [57] ABSTRACT Method and equipment for transferring energy to or [22] Filed: May 14,1975 from a direct-current superconducting field coil to change the magnetic field generated by the coil in which a second direct-current superconducting coil is Related U.S. Application Data used as a storage coil, and energy transfer between the [63] Continuation-in-part of Ser. No. 398,359, Sept. 18, field coil and the storage coil is effected automatically 1973, abandoned. in dependence upon a control programe. Preferably, the control programe acts upon a variable transformer [51] Int. CI.* H02P 7/34; H02J 15/00 which is coupled by respective rectifier/inverters to the [52] U.S. Q 318/154; 361/141; field and storage coils and also serves for initial supply 363/27 of energy to the coils. [58] Held of Search 322/16; 318/153, 154; 323/44 F; 321/8 CD, 2; 317/123, DIG. 6 12 Claims, 11 Drawing Figures U.S. Patent Aug. 30, 1977 Sheet 1 of 5 4,045,716

F/G.2 U.S. Patent Aug. 30,1977 sheet 2 of 5 4,045,716

FIG.3

POWER ' MONITOR

23 -L-. M m 19 20 21 22 — DRIVE MOTOR 24 — 7- CONTROLLER

* * * Sr St MR MT VF

26^ 2&*x>

w U.S. Patent Aug. 30,1977 sheet 3 of 5 4,045,716

TIME — TIME

FioAb FIG.Sb

CURRENT CURRENT COIL S COIL M IS, 0

J TIME „ TIME TF

FIG Ac FIG.-5 c VOLTAGE VOLTAGE COIL S COIL M Vs Vf U.S. Patent Aug. 30, 1977 Sheet 4 of 5 4,045,716

Fie.6 U.S. Patent Aug. 30,1977 sheet 5 of 5 4,045,716

Fig. 7

MULTIPLYING ADDING CIRCU/T CIRCUIT Vf •102 Vsd 103

if DIVIDING CIRCUIT -i/sm

23 1 -Gvmnr-

1 -IT- - J 11 11 tr22 Vs - ML mw^wnn is , 21 4,045 716 2 the required conditions for initiating energy transfer MAGNETIC FIELD SYSTEMS EMPLOYING A and maintaining balance of electrical power transfer SUPERCONDUCTING D.C. FIELD COIL rate between each coil and the energy transfer means, This application is a continuation-in-part of our appli- while transfer of energy is initiated and controlled by cation Ser. No 398,359, filed Sept. 18, 1973, now aban- 5 the variable ratio transformer coupling arrangement doned. between the two rectifier/inverters. BACKGROUND OF THE INVENTION The variable coupling arrangement may be formed by secondary windings on a transformer, the primary The present invention relates to the transfer of energy winding of which is connected to the electrical power to or from a direct-current superconducting field coil to 10 source. The variable ratio coupling may be adjusted by change the magnetic field produced by the coil. an induction regulator of the rolling contact type and Recent developments in the practical application of the transformer may be arranged as an auto-trans- superconductivity have led to the use of d.c. supercon- former. ducting coils for such purposes as dynamo-electric ma- In alternative systems, a fixed ratio coupling between chine field producing coils and plasma containment 15 rectifier/inverters of the energy transfer means could be magnet coils. In most applications of this type the stored adopted if their firing angles were controlled to draw energy in the magnetic field produced by a supercon- reactive power from the power source and such reac- ducting coil is of considerable magnitude, giving rise to tive power flow was acceptable in the system. problems of energy transfer and disposal when it is required to vary, remove, or reverse the magnetic field. 20 The invention may be applied in a system basically requiring only a single superconducting d.c. coil, for BRIEF SUMMARY OF THE INVENTION example, in a plasma confining magnetic field coil ar- rangement for power generation, in which case the In accordance with the present invention there is provided a method of transferring energy to or from a storage coil is essentially an auxiliary item. direct-current superconducting field coil to change the 25 In the preferred embodiment the invention provides a magnetic field produced by the coil comprising, provid- system for controlling an electric motor drive in which ing a storage coil which is a direct-current supercon- the motor is of the homopolar type having a supercon- ducting coil generating a magnetic field when energized ducting field winding and is part of a motor-generator and transferring energy between the field coil and the set in which the generator is also of the homopolar type storage coil by way of an energy transfer device in 30 having a superconducting field winding. Speed control accordance with a control program based on electrical of the motor is effected by field control of the generator parameters of the two coils. in this system and the superconducting field coil of the There is also provided equipment for transferring generator either receives energy from or transmits en- energy to or from a direct-current superconducting ergy to the motor field coil according to the required field coil to change the magnetic field produced by the 35 drive operation, the motor field coil thus also serving as coil comprising a storage coil which is a direct-current a storage coil for the generator coil. The generator may superconducting coil generating a magnetic field when be driven by a prime mover which also drives an a.c. energized, controllable electrical energy transfer means exciter acting as a power source for the generator and coupled between said field coil and said storage coil, motor field windings. Alternatively, a separately avail- and control means connected to said transfer means to 40 able a.c. supply may be used. This type of system is control the rate of energy transfer between said field particularly suitable for marine propulsion and in such coil and said storage coil in accordance with a predeter- an application the vessel's auxiliary a.c. generating sup- mined program. ply may satisfy the a.c. supply requirement of the sys- The transfer means may also serve for the supply of tem. power to the coils from a power source, especially for 45 initial energization of the coils, and in this case the trans- BRIEF DESCRIPTION OF THE DRAWINGS fer means are adjusted during an energy transfer opera- The scope and nature of the invention will now fur- tion to minimize power flow from the source. The rate ther be made clear by the following description with of electrical energy transfer from one coil to the energy reference to the accompanying drawings, of which: transfer means must be substantially equal to the rate of 50 FIG. 1 shows a basic circuit diagram for an energy energy transfer from the transfer means to the other coil transfer system between two superconducting d.c. coils at all times. That is to say the total energy stored in the according to one aspect of the invention; two coils remains constant throughout the energy trans- FIG. 2 shows a schematic diagram of a motor drive fer operation. system in accordance with the preferred embodiment of The energy transfer means may comprise alternating 55 the invention; current devices requiring that both active and reactive FIG. 3 shows a simplified circuit diagram of appara- power from an alternating current power source are tus comprising energy transfer means for the machine minimized. In alternative arrangements, however, the field coils of the system shown in FIG. 2; energy transfer means may comprise direct current FIGS. 4a - 4c show the generator field coil electrical devices, for example direct-coupled rotary d.c. ma- 60 parameters for the system shown in FIG. 2 during a chines, in which case control to minimize direct power discharge of the generator field into the motor field; only to the energy transfer means is required. FIGS. 5a - Sc show corresponding required motor The energy transfer means preferably comprises a field parameters for the system shown in FIG. 2 to the rectifier/inverter for each coil, there being a controlled generator parameters shown in FIGS. 4a-4c for the variable ratio transformer coupling between the two 65 discharge of the generator field into the motor field;an d rectifier/inverters. Adjustment of the firing angles of FIG. 6 shows a basic circuit diagram for an alterna- rectifying devices in the rectifier/inverters prior to and, tive form of energy transfer system between two super- if required, during an energy transfer period governs conducting d.c. coils to that shown in FIG. 1. 4,045,716

FIG. 7 shows a typical circuit diagram for the con- rectifier devices in the rectifier/inverters using the con- troller of FIG. 3 in order to satisfy a program requiring trol device to effect such control. The current may a linear decrease of the generator field coil current. provide an additional input to the control device but current is allowed to flow in the connections between DETAILED DESCRIPTION OF THE 5 the source P and the transformer V to permit the trans- INVENTION fer of reactive power only between the two. In the system shown in FIG. 1, M is a main supercon- Referring to FIG. 2, the drive system shown is one ducting direct-current field coil and S is a storage coil intended for marine propulsion and consists principally which is also a superconducting direct-current field of a motor-generator set comprising a motor 14 and a coil. The coils M and S are coupled by an energy trans- 10 generator 15 providing a continuously variable speed fer system T which also serves to connect a power drive for a propeller 16. The drive system is basically source P to the coils. The source P is in this case a three powered by a prime mover 17 which may have any phase alternating current supply. power/speed characteristic. The operation of the energy transfer system T to Motor 14 and generator 15 are both homopolar ma- transfer energy between the coils M and S is controlled 15 chines of the type having a superconducting d.c. field by a control device 10 which operates in accordance coil, coil M being the field coil for generator 15 while with a predetermined program. As will be shown math- coil S is the field coil for motor 14. Speed control of ematically below, this program is dependent on the motor 14 is effected by varying the ratio of the field initial states of the two coils. Current sensors 11 and 12 current of generator 15 to the field current of motor 14, associated with the coils M and S, respectively, feed the 20 for any generator speed, such that the total magnetic required electrical parameters to the control device 10. stored energy of the motor and generator field coils Thus, if it is required for example to reduce the mag- remains substantially constant. netic field of the coil M from a maximum value to zero, The resultant stored magnetic energy variations in the the initial voltage, current and stored energy conditions generator field coil M are accommodated by energy for coil M will be known as will the desired final values 25 transfer between the coil M and the motor field coil S, of these parameters. The corresponding values of the in accordance with the invention, the transfer being initial and final parameters of coil S are also known. effected via the energy transfer means T. Hence it is possible to calculate the intermediate values In a preferred system of the type shown in FIG. 2, the of these parameters at any time during the period of nominal stored energy capacity of the motor field coil S energy transfer between coils M and S for a desired 30 may exceed that of the generator field coil M by a factor transfer mode (for example, linear rate of decrease of of three, or greater. It is thus possible to charge the current in coil M) for the condition that, at all times motor field coil S initially, prior to operation of the during the period, the rate of energy transfer from coil system, from an exciter power source P of compara- M is substantially equal to the rate of energy transfer to tively low power as compared with the system power, coil S via the energy transfer means T, and to apply a 35 this initial charging being permitted to take place over a control program within the means T based on the calcu- substantial time period as compared to the short time lated results. periods required for energy transfer between coils S and As indicated in FIG. 1, the energy transfer means T M which the drive is in operation. Hence it is possible to preferably comprises a rectifier/inverter RIS associated dispense with the provision of a large, highly rated, with coil S and a similar rectifier inverter RI^ asso- 40 generator for field coil M, resulting in substantial space ciated with coil M, together with a variable ratio trans- and weight saving in a marine vessel. In many cases the former V interconnecting the a.c. sides of the two recti- power source P may be derived from conventionally fier inverters. The control device 10 is arranged to specified auxiliary generators for a vessel, dispensing control the transformer V to vary the coupling between with the requirement for a separate exciter for the mo- the a.c. sides of the rectifier/inverters RIsand Rl^at the 45 tor-generator drive system. transformer. The mode of operation of each rectifier/in- The energy transfer means T is preferably arranged verter, that is to say rectifying or inverting mode, may on the general lines of the circuit diagram shown in be initially set by the control program to ensure the FIG. 3. Each field coil has a high voltage silicon con- required direction of energy transfer. trolled rectifier (SCR) bridge for use during energy The a.c. power source P is connected to a primary 50 transfer and a low voltage SCR bridge for use during winding of the transformer V and in addition to provid- steady running. The generator high voltage bridge is ing initial energization of either coil S or coil M it acts designated Mrand the low voltage bridge in FIG. 3, as the frequency control for the inverters of rectifier/in- while the corresponding motor bridges are designated verters RIsand RI^. Since the control effected by the S^ and Sjj. control device 10 may not be sufficiently accurate to 55 For simplicity, the alternating current section of the maintain the energy transfer rates between the coils M circuit is shown as single-phase in FIG. 3, but in prac- and S and the transfer system T in balance, the firing tice it would preferably be three-phase. This section angles of the rectifier/inverters RIS and RI^ are con- comprises a transformer V with a primary coil 18 fed trolled by a current sensor 13 responsive to current flow from the a.c. exciter source P, secondary windings 19 between the source P and the transfer system T to mini- 60 and 20 respectively supplying bridges SR and M^and an mize such current flow and thereby minimize supply of auto-transformer secondary winding 21 connected to power by the source P during an energy transfer opera- the high voltage bridges S^and Mras shown. The con- tion. nection to the motor field coil high voltage bridge Sris In an alternative embodiment of the invention which preferably made by way of a rolling contact 22 of the is not shown in the drawings, the connection of the 65 Brentford Regulator type on the auto-transformer transformer V to the two rectifier/inverters is by a fixed winding 21. ratio coupling. Control of the energy transfer system T The contact 22 is moved by a drive motor 23 under is effected solely by varying the firing angles of the the control of a controller 24 which receives a program 4,045,716 5 6 input and also an initial condition input from one of two sensors 25 and 26 associated with the coils M and S (l) Total respectively. As in the embodiment of FIG. 1, a power Energy = Motor Field coil energy + Generator field coil enersv monitor 27 acting to control the firing angles of the 5 rectifiers S^and M7serves to maintain minimum power b = E„ + Efo transfer between the source P and the transformer V. , ! The operation of the drive system shown in FIGS. 2 B = T + T and 3 will now be described for a speed change from maximum speed to zero requiring a reduction of motor During generator field coil discharge armature current from maximum to zero, by way of 10 explanation of the system control. dis v = L The initial conditions of the generator field coil for / ~ '~dT = v"since "/maintained the operation to be described are that its field current is constant of maximum value at maximum value and the stored magnetic energy of dif vf the field coil is also at maximum value. The voltage '' dt ~ across the generator field coil is required to be constant throughout the operation. At the end of the operation Integrating, the current in the generator field coil is required to be zero, when the stored energy in the generator field coil 2q (2) must also be zero, and thus the maximum stored energy , _ _ f> ; c in the generator field coil must be transferred to the Lf 1 motor field coil during the operation. The initial conditions of the motor field coil for the Since, at t = o, i/= If, then C\ = If operation under consideration are that its field current is 25 Substituting for Cj in (2), initially at a base maximum value for motor field re- quirements but that its stored magnetic energy at this vf (3) current value is at considerably less than the maximum >/='/- — a linear expression value, since the stored energy capacity of the motor field coil must be great enough to accept also the energy 30 At end of operation t = Tfi and if = O, to be transferred to it from the generator field coil. Referring to FIGS. 4a-4c and 5a-Sc, the behavior of T from 3 the system for the operation may be considered mathe- •''• /~ vf ^ < ) matically using the following symbols for the system parameters: 35 At any instant total energy if — instantaneous generator field coil current If = maximum generator field coil current , j Vf = instantaneous generator field coil voltage Vr= maximum generator field coil voltage , ,,, • , » 0 .-. from (l). e.f = instantaneous stored energy of generator field coil 40 E/0 = initial stored energy of generator field coil L

Thus, dis is Vj = is ij dt Thus, the final energy of the motor field coil 30 (7)

Es, = Ls Is} = -i- ]}+ LJZ (10)

35 Vf From (3) If= if+ —jr— t i.e. the sum of the initial generator field coil energy and the initial motor field coil energy. ••• w- vntj-' The energy transfer means need not necessarily com- 40 prise an a.c. power transfer arrangement coupled with Subst. for VFIF in (10) rectifier inverters. In an alternative form of the inven- tion, schematically indicated in FIG. 6, two direct shaft- vf vfvf coupled d.c. electrical machines, each capable of mo- tor/generator operation, are used for energy transfer in ••• '>, = excitation of the machine field windings 32 and 33 re- spectively, according to the required program, by winding 21, where vs = Vs. The firing angles of the means of an excitation controller 34 which receives rectifiers in bridge Mr are arranged to set the bridge at 55 full invert throughout the operation, while those in inputs from a program generator 35 and a power sensor 36. Initial excitation of the coil or coils in the system bridge Sr are arranged to set the bridge at full rectify throughout the operation. shown is by way of a motor 37 coupled to the motor/- Contact 22 would next be moved according to the generator shafts, motor 37 being, for example, an a.c. required control function to effect the required energy 60 machine supplied from a power source P. Should the transfer. Movement of contact 22 may be made suffi- program generating means for the system shown in ciently accurately with unsophisticated control func- FIG. 6 give an insufficiently accurate control of coil tion generating means, in which case the rectifier firing parameter variation during energy transfer, a further angles are not adjusted during the operation. Should control may be imposed on the motor/generator field such means not adequately balance the power transfer 65 excitation by the sensor 36 in accordance with the con- rates at all times, however, small variations of the firing dition that minimal direct power flow from source P to the energy transfer means T should occur during an angles of the rectifiers in bridges Mr and S7- may be made under the control of the detector 27 monitoring energy transfer operation. 4,045,716 10 FIG. 7 illustrates a simple program and control cir- perconducting coil generating a magnetic field when cuit for controller 24. Thus, for the energy transfer case energized, energy transfer means interconnecting said described previously in which the decrease of current in field coil and said storage coil, said energy transfer the generator field coil i/is linear, the expression: means being electrically controllable to initiate energy 5 V transfer between said field coil and said storage coil in '>!= 'f f either direction and to control the rate of such energy transfer, and electrical control means connected to said represents a program in accordance with which v„ the transfer means for the electrical control thereof in ac- instantaneous motor field coil voltage must be con- cordance with a predetermined program whereby to trolled by means of moving contact 22 in FIG. 3. Note 10 effect a desired transfer of energy between said coils. that: 2. Equipment as claimed in claim 1 including a power source coupled to the coils by way of the transfer means J!L_ = V to provide initial energization of the coils. dt f 3. Equipment as claimed in claim 2 including means 15 for sensing direct power flow between the power Thus: source and the transfer means during transfer of energy between the coils, and means for adjusting the energy ."•^Vr transfer means under the control of said sensing means to minimize said direct power flow. 20 4. Equipment as claimed in claim 3 wherein said sens- While the program is governed by linear decrease in ing means is responsive to current flow between the iy; the parameter initiating the program is V/which is power source and the transfer means and said adjusting constant for linear rate of decrease of iy. Prior to initiat- means is operative to minimize said current flow. ing the program, V/is zero, although the generator field 5. Equipment as claimed in claim 1 in which the cur- coil is at maximum stored energy, because of the super- 25 rent transfer means comprises a transformer coupled by conductive nature of the coils (enabling current flow a first rectifier/inverter to the field coil and by a second without voltage drop). On initiating the the program, rectifier/inverter to the storage coil. V/i the maximum value of the generator field coil volt- 6. Equipment as claimed in claim 5 wherein said con- age is demanded and this causes the change in mode of trol means is connected to control the firing angles of operation of the rectifier/inverter sets to that required 30 the first and second rectifier/inverters. for commencement of energy transfer. 7. Equipment as claimed in claim 2, in which the FIG. 7 shows the circuitry required (basically a servo transfer means comprises a variable transformer cou- mechanism) for the control of contact 22 in accordance pled by first and second rectifier/inverters to the field with the program. In FIG. 7, the controller contains and storage coils respectively and the said control dividing circuit 101, multiplying circuit 102, and adding 35 means is connected to control the transformer. circuit 103. Circuits 101,102 and 103 are conventional. 8. Equipment as claimed in claim 7 in which the The dividing circuit has two inputs which are con- power source is connected to a primary winding of the nected to signal sources related to i/and respectively said transformer. and the output of divider 101 is the quotient i/is. Multi- 9. Equipment as claimed in claim 8 in which first and plier circuit 102 has two inputs connected to the pro- 40 second secondary windings of said transformer are cou- gram value V/and the quotient i/isand multiplies these pled respectively to the field and storage coils by way two values. The output of multiplier 102 is the de- of rectifiers separate from the rectifier/inverters to pro- manded value Vjjof the motor field coil voltage. Thus, vide power for energization of the coils. value v^is then compared with the measured value vsm 10. Equipment as claimed in claim 8 in which the of the motor field coil voltage, and the difference be- 45 rectifier/inverters are connected to a single auto-trans- tween v^and vsm is the output of adding circuit 103, and former winding of the said variable transformer, one of is used to drive motor 23 in a direction to reduce the the rectifier/inverters being connected to the said wind- difference to zero. ing by a moving contact upon which the said control Although this invention has been described with re- means acts. spect to preferred embodiments, it should be under- 50 11. Equipment as claimed in claim 3 in which the said stood that many variations and modifications will now adjusting means is connected to control the firing angles be obvious to those skilled in the art, and it is preferred, of two rectifier/inverters coupling the field and storage therefore, that the scope of the invention be limited, not coils respectively to a variable transformer controlled by the specific disclosure herein, but only by the ap- by the said control means. pended claims. 55 12. Equipment as claimed in claim 1 in which the field We claim: coil is the field winding of a motor and the storage coil 1. Equipment for transferring energy to or from a is the field winding of a generator which is drivable by direct-current superconducting field coil to change the a prime mover and is connected to supply current to the magnetic field produced by the coil, said equipment said motor. comprising a storage coil which is a direct-current su- 60 *****

65