Title of Invention	A CHARGING CIRCUIT FOR SUPER CONDUCTING (SC) COIL FOR SUPER CONDUCTING MAGNETIC BASD ENERGY STORING AND UNITERUPTED POWE SUPPLY SYSTEMS.
Abstract	There is provided an improved charging circuit for a Super Conducting coil (SC) for SMES/UPS systems comprising. An input unit having a circuit breaker (2) for power supply from the three phase mains (1), a set for two three phase auto- transformer (3). (4). connected to said circuit breaker (2). which auto-transformers (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer there being provided pair of step-down transformers (5), (6) connected to the said second auto-transformer for deriving current there from. Each said step down transformer being connected to a three phase diode rectifier bridge circuit (7). (8) to get a DC voltage. Said to diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge And The said twelve pulses rectifier bridge being connected through a CB2 contactor (9) to the SC coil 12 to charge the same, the SC coil being connected to a LEM make current Sensor 10) for sensing the SC coil.

Title of Invention

A CHARGING CIRCUIT FOR SUPER CONDUCTING (SC) COIL FOR SUPER CONDUCTING MAGNETIC BASD ENERGY STORING AND UNITERUPTED POWE SUPPLY SYSTEMS.

Abstract

There is provided an improved charging circuit for a Super Conducting coil (SC) for SMES/UPS systems comprising. An input unit having a circuit breaker (2) for power supply from the three phase mains (1), a set for two three phase auto- transformer (3). (4). connected to said circuit breaker (2). which auto-transformers (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer there being provided pair of step-down transformers (5), (6) connected to the said second auto-transformer for deriving current there from. Each said step down transformer being connected to a three phase diode rectifier bridge circuit (7). (8) to get a DC voltage. Said to diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge And The said twelve pulses rectifier bridge being connected through a CB2 contactor (9) to the SC coil 12 to charge the same, the SC coil being connected to a LEM make current Sensor 10) for sensing the SC coil.

Full Text	Introduction to the Filed of the Invention: This invention relates to the design, development and fabrication of a new cost effective charging circuit for a super-conducting coil used in a UPS (Uninterrupted Power Supply) and other similar Applications based on storage of magnetic energy. Prior Art & Drawback: Charging circuits for super conducting (SC) coils are already known and different types are in use. a. The charging circuits available commercially are mainly transistor-regulator type. Here the mains AC supply is rectified, filtered and passed to the SC(Super Conducting) coil through a variable dynamic resistor (a transistor whose base current is controlled depending on the output current). The output current is compared to a reference-current signal and the base current of the series transistor is controlled accordingly. These circuits give a very stabfe, ripple free output current but are very expensive for large currents. Their overall efficiency is also poor, which is a definite disadvantage when current is large as in SMES (Super Conducting Magnetic based Energy Storing Systems) coils. b The other types of charging circuits use thyristor converters. While these can handle large currents, the ripple of the output voltage of the converter and the waveform depend heavily on the average output voltage. For low output voltage, the ripple content is very high and waveform (of voltage) is quite distorted. The losses in the SC coil could be quite high unless very thin filament SC (Super Conducting) wires are used. The cost of the charging circuit, for the same current output, may not be as high as that of a transistorized power supply unit, but it is still high. The control of the circuit is also sufficiently complex. Thyristors may be replaced by IGBTs (Insulated Gate Bipolar Transistors) or GTOs (Gate Turn-off Thyristors) but suffer from similar disadvantages. c. Yet another charging circuit consists of a rectifier and chopper circuit. The chopper is generally a high frequency, multi-phase chopper. The disadvantages are once again due to high cost, complexity of the control circuits. It is known that in a charging circuit, if the charging rate is too fast the coil develops excessive loss and may quench. On the other hand too slow a charging may not suit the user's requirement. 2 Objects of the invention: It is therefore an object of this invention to propose an improved type of charging circuit which is cost effective, rugged and relatively simple for use in super-conducting coil which is used in a UPS (uninterrupted Power Supply) and other similar applications based on storage of magnetic energy in a SC (Super Conducting) coil It is a further object of this invention to propose such an improved charging arcuit in which the mains AC supply charges the SC (Super Conducting) coil when the supply is healthy and the stored energy can be recovered back and transformed into useful form for example as (3 phase, 400 V, 50 Hz) to feed critical loads during power failure / power glitches etc It is yet another object of this invention to propose such a charging circuit which can charge the SC (Super Conducting) coil at a controlled rate, such as 1 amp per sec or 5 amps per sec etc. It is still further object of this invention to propose such a charging arcuit, which can charge the SC (Super Conducting) coil to a preset value such as 200A, 300A etc. and maintain the current at that value. Yet another object is to propose such a charging circuit which will disconnect itself from the SC (Super Conducting) coil so as to protect the latter in the event of quench and other malfunctions. A further object is to propose such a charging arcuit which preferably has a visual indicating display such as . LED to indicate the preset value of the current and ramp rate etc. These and other objects of the invention will be apparent from the following paragraphs: Basis of the Development: In our efforts to develop the charging circuit to satisfy all the objectives in mind, we conducted practical trial and experiments with the Designs developed Dy us keeping in mind the speafic requirements of the UPS-SMES (Uninterrupted Power Supply Systems and Super Conducting magnetic based Energy Storage) system. It is known knowledge that in the SMES-UPS (Uninterrupted Power Supply Systems and Super Conducting magnetic based Energy Storage) system, the charging circuit need not be reversible. In our design the SC coil is charged with large DC current and the energy stored is fed to load rather than to supply. Hence we can use a non-reversible converter like diode rectifier. 3 We have employed a bridge rectifier. In a bridge rectifier, the ripple voltage is quite low. Particularly with two 3 phase bridges in series with their outputs shifted by 30 degree, we get a reasonably smooth DC current compared to thyristor converter and at much smaller cost. Due to low ripple current in the voltage the losses in the SC coil (hysteresis and eddy current) are less and the chances of quench during charging are low. We have preferred the diode rectifier for the simple reason that the diodes are most nigged and simple devices, they are cheaper and are available in large current ratings. They do not have to be controlled by gate drive circuits etc. Moreover due to lower ON state voltage drops of diodes, the losses in the charging circuit are reduced. We have designed charging circuit in such a way that the control of ramp rate of current is achieved by manual setting of one auto-transformer and the final value of the set current is achieved and maintained through a servo controlled (second) auto-transformer. The first auto-transformer can also be eliminated by incorporating some additional control features in the servo unit. Another feature in our charging circuit is that we use a hall current sensor to see that the output current is maintained constant at the preset value. If needed, we have provisions in our charging circuits to include LED display and protection features. Brief Statement of the Invention: According to this invention there is provided an improved charging circuit for a superconducting coil (SC) comprising i. an input unit having a circuit breaker for power supply from the mains. ii. at (east one auto-transformer, preferably a set of auto-transformers, a first transformer and a second transformer connected to said circuit breaker, iii. said first auto-transformer adapted to feed current to. the said second auto-transformer, 4 iv. there being provided a pair of step down transformers (one delta-delta another delta-star) connected to the said second auto-transformer for deriving current therefrom, v each said step down transformer being connected to a 3 phase diode rectifier bhdge circuit to get a DC voltage,. vi- said two diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge and vii the said 12 pulses rectifier bridge being connected to the SC coil to charge the same through a controlled switch. In a preferred embodiment, there are provided two auto-transformers connected in series in addition to two fixed-ratio step down transformers. The first auto-transformer is manually controlled and is set to control the ramp rate of the charging current- The output of the first auto-transformer goes as input to the second auto-transformer, which is servo controlled. The purpose of the second auto-transformer is to see that the SC coil is charged to the desired level of current and to see that this current is maintained. By incorporating an adjustable upper voltage limit on the output of the second auto-transformer, it is possible to eliminate the first auto-transformer- Thus, both the controls, namely, ramp rate control as well as set current control can be achieved using a single servo controlled auto-transformer. The output of the servo controlled auto-transformer goes to two three-phase step down transformers. In a preferred embodiment, the first of the two 3 phase step down transformer is connected in delta on the primary as well as on the secondary and the second of the 3 phase step down transformer is connected in delta in the primary but has a star on the secondary. Instead of using two separate step down transformers, it is possible to use a single three winding (with two sets of secondaries), three phase transformer to achieve the same purpose at reduced cost. Each of the secondaries is connected to a 3 phase diode bridge, each having 6 diodes and the diode bridges are connected in series to obtain the required DC voltage. In this embodiment, each of the two diode bridge circuits is provided wrth a common anode tap point and a common cathode tap point and these two tap points respectively-constitute the negative and positive output terminals of the rectifier bridge. The common cathode tap point of the diode bridge which is connected to the step down transformer, whose primary and secondary have delta connections, is connected to one end (inlet) of the SC coil through a circuit breaker, 5 while the common anode tap point of other diode bridge, which is connected to the primary in delta fashion and the secondary in star fashion of the second step down transformer, is connected directly to the other end (outlet) of the SC coil. The primary to secondary turns for each of the step down transformer are so selected as to have a ratio which will step down 400 volts across the primary lines to 20 volts across the secondary lines. In the experimental embodiment, the two 3 phase rectifier bridges are connected to the secondary of the respective step down transformers and are connected in series and the output (maximum of about 50 volts) is fed to the SC coil. The controlled switch between the rectifier bridge output and the SC coil is either an electro magnetic or solid state contactor switch with an ON-OFF mechanism for connecting to the SC coil and there is also provided a current sensor to sense the SC coil current. The ON-OFF mechanism is either a manually operated ON-OFF switch or automatic controlled. Brief Description of the Accompanying Drawings: The invention is illustrated with the help of the accompanying schematic diagrams in which, Figure 1 schematically represents the charging circuit. Figure 2 represents a stepper motor controller and a LED based display unit used in the circuit. Figure 3 represents a power supply circuit used for supplying power to the circuit of Figure 2. Figure 4 illustrates the stepper motor connection used for the servo controlled auto-transformer. Detailed Description of the Drawings: Referring to the schematic diagram of Fig 1, it will be noticed that the charging circuit developed for the present purpose takes its input from a 3 phase 400 volts supply (1) from the mains circuit-breaker (2). The input supply is first stepped down through a manually controlled auto-transformer (3). The output of the manually controlled auto-transformer is fed to another servo-controlled auto-transformer (4). Output of the second auto-transformer is then, fed to two 6 3- phase step down transformers (5, 6) one of which (5) is connected in delta on the primary as well as on the secondary while the second transformer (6) has delta on primary and star on the secondary The outputs of these two transformers go to two 3-phase diode bridges (7, 8) connected in cascade to get a DC voltage with 12 ripple since the two secondaries give output voltages which have 30 degree phase shift between them. The manually controlled auto-transformer can be eliminated altogether by incorporating in adjustable upper-limit output voltage control of the servo-controlled auto-transformer. This will further reduce the cost of the charging unit- This modification is now under the process of implementation. The primary to secondary turns ratio is so chosen for each transformer to give 400/20 volts across lines. Each secondary is followed by a three phase diode rectifier bridge and the two bridge outputs (7. 8) are then connected in series, thus constituting a twelve pulse rectifier bridge as already explained Output from the twelve -pulse rectifier is fed to the SC coil (12) through the normally open contacts of a contactor CB2 (9). The contactor CB2 is operated through ON -OFF push buttons but in case of quench fault or coil over voltage fault the contractor opens up automatically SC(Super Conducting) coil current is sensed through a Hall effect based current sensor (10) and the coil current information is fed to the servo motor controller (13) for the second auto-transformer. The controller for the servo driven auto-transformer comes into action after switching on the input supply (3 phase. 400V). LED Display unit: The LED display unit and the steeper motor controller is shown in Fig. 2. The reference current for the coil is set through a ten -turn pot (R51) as shown in Fig 2. Fig. 2 also shows rest of the servo motor control circuit. An array of 9 LEDS (D1 through D9) is used to indicate the value of the set coil current. A 4.7 volts zener (Z1) reference voltage is applied across the fixed ends of the 10 turn, 10 K potentiometer (R51). This same zener reference voltage is applied across a series of 10 equal resistors (R2, R6, R8, R12, R15, R18, R21, R24, R27 and R52) of 1 K each. Thus the 4 7 volts zener reference voltage is subdivided in 10 equal parts to give reference voltages of magnitudes -0.47 V, -0.94 V, -1.41 V etc. The pot voltage is compared with each of these smaller reference voltages using the comparators U1/1, U1/4, U1/3, U2/4, U2/3, U3/1, U3/4, U3/3 and U3/2 A low comparator output voltage would light up the LED connected to that particular comparator output. At the minimum pot (R51) voltage (set voltage =0) none of the LEDs glow. When pot voltage decreases below -0.47 volts, LED D1 glows. If the pot voltage setting decreases by another -0.47 volt one more LED (D2) glows. In this way the nine LEDs (D1 through D9) indicate the range of the set pot voltage. This is how the LED display unit for the set current works. 7 The LED indicators are used as a visual guard against any unintentional excess current setting for the coil. The Feed back current Controller: The pot (R51) voltage (Fig 2) is passed through two buffer stages using op-amps U1/2 and U5. Output of op-amp U5 that essentially indicates the set coil current is compared with the actual SC coil current signal. The coil current is measured using LEM 100 hall effect current sensor. The LEM sensor voltage is output across the 50 ohm resistor (MR). Voltage across 'MR' is linearly proportional to the coil current 1. 0 volt across MR represents 400 amperes through the single turn primary side of the LEM (in our experimental set-up , however, this meant 80.0 amp through the SC coil as the coil current flows through two parallel conductors with identical physical dimensions and the LEM sensor was used to measure current through one of the conductors ) Op-amp U1/2 is used to scale the pot (R51) reference voltage such that the set current for the coil does not exceed 210 amperes. Op-amp U5 works as a unity follower when the switch SWI is in position 1. However, when SWI is in position 2 the reference current command set by the pot is overruled and the servo motor controller brings down the auto-transformer output voltage to zero. A normally closed auxiliary contact of contractor CB2 is also placed across the pole 2 of SWI Thus, when CB2 is off, the pot current command is once again overruled and the applied voltage to the coil remains zero CB2 is turned off when it is desired to retrieve the coil energy. CB2 is also turned off during quench fault circuit activation LEM output voltage is passed through an Op-amp (U4) based unity follower buffer stage and is compared with the validated set reference current voltage (output of Op-amp U5). The comparison between the set and the actual currents is done in parallel at three comparator stages consisting of Op-amps U6, U7 and U8. Comparators using Op-amps U6 and U7 are hysteresis type comparators, whereas the comparator using Op-amp U8 has negligible hysteresis R35, R33 and R34. Similarly R38, R40 and R41 decide the hysteresis band for the comparator using Op-amp U7. U7 output goes high with increase in magnitude of the actual coil current whereas U6 output goes high when the coil current magnitude decreases. The hysteresis bands of the two comparators are so chosen that they are not overlapping, i.e. simultaneously both U6 and U7 outputs will not go low (or high). U6 output is NANDed (signals fed to a NAND logic gate), with U8 output using the NAND gate U9/4 and the U7 output is NANDed (signals fed to a NAND logic gate.) with the inverted U8 output using NAND gate U9/2. These NAND gate outputs are amplified using p-n-p transistors Q1 and Q2 and are used to drive. 8 the 12 volt coils of two electromagnetic control relays. When a particular NAND gate output is low the corresponding relay turns on. Anti-Hunt Feature: Reference may be made to the block marked as "current controller with anti-hunt feature" in Fig. 2. Turning on of relay #1 is used to decrease the applied voltage to the coil whereas turning on of relay #2 is used to increase the applied voltage. The effect of the three comparators and the associated logic is to make relay #2 on when the actual current is less than the set current and make relay #1 on when the reverse happens, however they have a dead zone in-between when the two current magnitudes are close enough and in that case none of the relays are on As will be discussed later, introduction of this dead zone eliminates chattering of the relays. Fig. 4 illustrates a slow speed (60 rpm) synchronous servo-motor connection used for controlling the servo controlled auto-transformer. The motor (M) has two windings with capacitor split phase connections. The common point of the two windings is connected directly to one pole of the 50 Hz, single phase supply. Depending on which of the two relays (relay #1 or relay #2) is closed the motor will run either in clock-wise or counter clock-wise direction. In both the directions, the motor rotates at the same speed (60 rpm). When the auto-transformer spindle shaft (carrying the carbon brushes) hits either the maximum voltage position or the minimum voltage position, the motor movement is stopped by the action of limit switches and the motor is de-energized. The anti-hunt feature of the current controller can also de-energize the motor when the coil current is within the set limits. The complete charging control circuit including its power supply is housed in an enclosure. The ten turn pot (R51) and the switch SWI are mounted on the cover of the enclosure. The control power supply (+12 V, -12 V and ground) circuit for the charger is shown in Fig. 3. The function and the sequence of circuit operation during SC coil charging is as follows: - As mentioned in the beginning, the 400 Volts, 3-phase input supply is fed to two auto-transformers in tandem. The first auto-transformer, which is manually controlled decides the peak charging rate for the given coil. For slower charging the auto-transformer output should be set at lower voltages (the set voltage being proportional to the peak charging rate). In our experimental set-up we started by very low voltages and then increased It gradually. 9 Once the peak charging rate is fixed, the ten-turn pot (R51) is set to fix the reference current for the coif. The LED arrays give an approximate indication of the set current. As the current setting is increased the LEDs start lighting up one by one. The first LED lights up when the set current is about 21 amps. At 42 amps setting both first and second LEDs light up. An increase of 21 amps in the set current lights up one more LED. Thus when say. the first six LEDs light up the set current will approximately be 21x6=126 amperes. After setting the reference current for the coii, as mentioned, the ON push button for the contactor CB2 is activated. Turning on of contactor CB2 validates the coil current setting and the servo motor controller for the auto-transformer comes into action. The SC coil inductance being very large and the allowed charging voltage being limited the coil current starts building slowly, Positive error between the set and the actual coil currents cause the servomotor to move in a direction in which the auto-transformer output increases. The given servomotor takes around 8 seconds to move the auto-transformer shaft from zero voltage position to its maximum output voltage position. After reaching the maximum voltage position the motor movement is stopped by limit switch actton and the coil charging takes place at this maximum rate. The set and the actual coil currents are continuously compared using the set of three comparators discussed above. Due to the built in hysteresis in the comparators the actual current slightly overshoots the set current before relay #1 turns on to bring the auto-transformer voltage back to zero. With zero applied voltage across the SC coil the coii current will remain constant, however due to the conduction voltage drop of diodes in the 12 pulse rectifier feeding the SC coil the actual voltage applied to the coil becomes slightly negative and the coil current instead of remaining constant starts decreasing at a slow rate. When the slowly decreasing coil current crosses the lower hysteresis band of the current comparators relay #2 turns on to once again increment the applied dc voltage to the coii and the process repeats. By proper choice of the hysteresis band of the current comparators the coil current be kept close to the set value and at the same time the frequent turning on and off of the relays (#1 & #2) can be avoided. The charging circuit thus described keeps the coii in a charged state. For retrieving back the energy stored in the coil the charging circuit is disconnected by opening the contacts of contractor CB2. in the present set-up contactor CB2 is opened by pressing the stop button. As C82 turns off the chopper circuit automatically comes into operation to retrieve the coil energy. As described before, turning off of CB2 also results in resetting of servo driven auto-transformers output voltage to zero. Advantages of the Invention: 1. The Charging Circuit can charge the SC Coil at a steady controlled rate to avoid too fast or too slow charging. 2. It can work at a preset value and maintain the current at that value. 3 It can disconnect itself from the SC coil so as to protect the latter in the event of quench and other malfunctions. 4 It has visual display indicators to indicate the preset value of the current and ramp rate. 5. The use of non-reversible converters enables us to have the circuit to function at very low ripple current. 6. We are able to get smooth DC current. 7. We are able to save on cost compared to known charging circuits. 8. Since we use diode rectifier the charging circuit is rugged and more reliable in operation. Moreover, due to lower ON state voltage drops of diodes, the losses in the charging circuit are reduced. 9 Because of the availability of large current rated diodes and transformers this type of charging circuit can be used for large SMES units. 10. The cost of two auto-transformers and diode rectifier bridges works out cheaper than a converter using controlled devices and has the advantage of lower ripple content in the output Moreover the cost of the servo motor unit remains relatively constant (even at large output power) compared to the cost of other equipments and devices. 11. It is highly dependable, easily to operate cost of manufacture is lesser than known charging circuits and moreover, the maintenance costs are very low. 12 Of the two auto-transformers, one can be eliminated by incorporating some additional control features in the servo unit. 11 We claim 1. An improved charging circuit for a super conducting coil (SC) for SMES -UPS systems comprising: An input unit having a circuit breaker (2) for power supply from the three phase mains (1), a set of two three phase auto- transformer (3), (4), connected to said circuit breaker (2) with auto - transformer (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer, there being provided pair of step; down transformers (5), (6) connected to the said second auto-transformer for deriving current therefrom, each said step down transformer being connected to a three phase diode rectifier bridge circuit ( 7), (8) to get a DC voltage, said two diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge and the said twelve pulses rectifier bridge being connected through the CB2 contactor (9) to the SC coil(12)to charge the same, the SC coil being connected to a LEM make current sensor (10) for sensing the SC coil. 2. An improved charging circuit as claimed in Claim 1 wherein, the first auto- transformer is used to achieve the desired rate of charging current (e.g. 1 Amp/Sc). 3. An improved charging circuit as claimed in claims 1& 2, wherein, the second auto-transformer is a servo-controlled transformer which is used to maintain the current in SC coil at the set reference value. 4 An improved charging circuit as claimed in Claim 3, wherein, the first of the two 3-phase step down transformer (connected to the output of second auto-transformer) is connected in delta on the primary as well as on the secondary. 12 5. An improved charging circuit as claimed in claims 4 and 6, wherein, the second of the 3-phase step down transformer is connected in delta on the primary but has a star on the secondary. 6. An improved charging circuit as claimed in claims 1 to 5 wherein the delta of the secondary of the first transformer and the slar of the secondary of the second transformer are each connected to 9 3 phase diode bridge, each having 6 diodes, 7. An improved charging circuit as claimed in claim 6, wherein each phase of the two diode bridge circuit is provided with a common anode tap point and a common cathode tap point. The common cathode tap point of the diode bridge which is connected to the step down transformer whose primary and secondary have delta connections, is connected to one end (inlet) of the SC coil through a circuit breaker, while the common anode tap point of other diode bridge which is connected to the primary in delta fashion and the secondary in star fashion of the second step down transformer is connected directly to the other end (outlet) of the SC coil. 8. An improved charging circuit as claimed in claims 1 to 7, wherein there is provided only one auto-transformer namely a servo controlled, auto-transformer thereby eliminating the manually controlled auto-transformer by incorporating an adjustable upper limit output voltage control of the servo controlled auto transformer. 9. An improved charging circuit as claimed in claims 1 to 8, wherein the primary to secondary turns for each of the step down transformer are so selected as to have a ratio which will give 400/20 volts across lines. 10. An improved charging circuit as claimed in claims 7, wherein the two 3 phase rectifier bridges connected to the secondary of the respective step down transformers are connected in series and the output is fed to the SC coil. 11. An improved charging circurt as claimed in claim 10, wherein a contactor is provided with a ON-OFF push buttons for connecting to the SC coil. 12. An improved charging circuit as claimed in claims 1 to 10, wherein there is provide a LEM make current sensor based circurt to sense the SC coil current. 13 An improved charging circuit as claimed in claims 1 to 12 wherein, the SC coil is associated with a servo motor controller for the second auto-transformer. 14. An improved charging circuit as claimed in claims 1 to 13 wherein, the circuit breaker could either be electro-magnetic type or solid state type. 15. An improved charging circuit as claimed in claims 1 to 14, wherein, an LED based display is provided to indicate the set coil current and the ramp rate. 16. An improved charging circuit as claimed in claims 1 to 15 wherein a mechanism is provided to automatically disconnect the charging circuit in case a fault signal is received. 18 . An improved charging circuit substantially as herein described with reference to the accompanying drawings. There is provided an improved charging circuit for a Super Conducting coil (SC) for SMES/UPS systems comprising. An input unit having a circuit breaker (2) for power supply from the three phase mains (1), a set for two three phase auto- transformer (3). (4). connected to said circuit breaker (2). which auto-transformers (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer there being provided pair of step-down transformers (5), (6) connected to the said second auto-transformer for deriving current there from. Each said step down transformer being connected to a three phase diode rectifier bridge circuit (7). (8) to get a DC voltage. Said to diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge And The said twelve pulses rectifier bridge being connected through a CB2 contactor (9) to the SC coil 12 to charge the same, the SC coil being connected to a LEM make current Sensor 10) for sensing the SC coil.

Full Text

Introduction to the Filed of the Invention:
This invention relates to the design, development and fabrication of a new cost effective charging circuit for a super-conducting coil used in a UPS (Uninterrupted Power Supply) and other similar Applications based on storage of magnetic energy.
Prior Art & Drawback:
Charging circuits for super conducting (SC) coils are already known and different types are in use.
a. The charging circuits available commercially are mainly transistor-regulator type. Here the mains AC supply is rectified, filtered and passed to the SC(Super Conducting) coil through a variable dynamic resistor (a transistor whose base current is controlled depending on the output current). The output current is compared to a reference-current signal and the base current of the series transistor is controlled accordingly. These circuits give a very stabfe, ripple free output current but are very expensive for large currents. Their overall efficiency is also poor, which is a definite disadvantage when current is large as in SMES (Super Conducting Magnetic based Energy Storing Systems) coils.
b The other types of charging circuits use thyristor converters. While these can handle large currents, the ripple of the output voltage of the converter and the waveform depend heavily on the average output voltage. For low output voltage, the ripple content is very high and waveform (of voltage) is quite distorted. The losses in the SC coil could be quite high unless very thin filament SC (Super Conducting) wires are used. The cost of the charging circuit, for the same current output, may not be as high as that of a transistorized power supply unit, but it is still high. The control of the circuit is also sufficiently complex. Thyristors may be replaced by IGBTs (Insulated Gate Bipolar Transistors) or GTOs (Gate Turn-off Thyristors) but suffer from similar disadvantages.
c. Yet another charging circuit consists of a rectifier and chopper circuit. The chopper is generally a high frequency, multi-phase chopper. The disadvantages are once again due to high cost, complexity of the control circuits.
It is known that in a charging circuit, if the charging rate is too fast the coil develops excessive loss and may quench. On the other hand too slow a charging may not suit the user's requirement.
2

Objects of the invention:
It is therefore an object of this invention to propose an improved type of charging circuit which is cost effective, rugged and relatively simple for use in super-conducting coil which is used in a UPS (uninterrupted Power Supply) and other similar applications based on storage of magnetic energy in a SC (Super Conducting) coil
It is a further object of this invention to propose such an improved charging arcuit in which the mains AC supply charges the SC (Super Conducting) coil when the supply is healthy and the stored energy can be recovered back and transformed into useful form for example as (3 phase, 400 V, 50 Hz) to feed critical loads during power failure / power glitches etc
It is yet another object of this invention to propose such a charging circuit which can charge the SC (Super Conducting) coil at a controlled rate, such as 1 amp per sec or 5 amps per sec etc.
It is still further object of this invention to propose such a charging arcuit, which can charge the SC (Super Conducting) coil to a preset value such as 200A, 300A etc. and maintain the current at that
value.
Yet another object is to propose such a charging circuit which will disconnect itself from the SC (Super Conducting) coil so as to protect the latter in the event of quench and other malfunctions.
A further object is to propose such a charging arcuit which preferably has a visual indicating display such as . LED to indicate the preset value of the current and ramp rate etc.
These and other objects of the invention will be apparent from the following paragraphs: Basis of the Development:
In our efforts to develop the charging circuit to satisfy all the objectives in mind, we conducted practical trial and experiments with the Designs developed Dy us keeping in mind the speafic requirements of the UPS-SMES (Uninterrupted Power Supply Systems and Super Conducting magnetic based Energy Storage) system.
It is known knowledge that in the SMES-UPS (Uninterrupted Power Supply Systems and Super Conducting magnetic based Energy Storage) system, the charging circuit need not be reversible.
In our design the SC coil is charged with large DC current and the energy stored is fed to load rather than to supply. Hence we can use a non-reversible converter like diode rectifier.

3

We have employed a bridge rectifier. In a bridge rectifier, the ripple voltage is quite low. Particularly with two 3 phase bridges in series with their outputs shifted by 30 degree, we get a reasonably smooth DC current compared to thyristor converter and at much smaller cost. Due to low ripple current in the voltage the losses in the SC coil (hysteresis and eddy current) are less and the chances of quench during charging are low.
We have preferred the diode rectifier for the simple reason that the diodes are most nigged and simple devices, they are cheaper and are available in large current ratings. They do not have to be controlled by gate drive circuits etc. Moreover due to lower ON state voltage drops of diodes, the losses in the charging circuit are reduced.
We have designed charging circuit in such a way that the control of ramp rate of current is achieved by manual setting of one auto-transformer and the final value of the set current is achieved and maintained through a servo controlled (second) auto-transformer. The first auto-transformer can also be eliminated by incorporating some additional control features in the servo unit.
Another feature in our charging circuit is that we use a hall current sensor to see that the output current is maintained constant at the preset value.
If needed, we have provisions in our charging circuits to include LED display and protection features.
Brief Statement of the Invention:
According to this invention there is provided an improved charging circuit for a superconducting coil (SC) comprising
i. an input unit having a circuit breaker for power supply from the mains.
ii. at (east one auto-transformer, preferably a set of auto-transformers, a first
transformer and a second transformer connected to said circuit breaker,
iii. said first auto-transformer adapted to feed current to. the said second auto-transformer,

4

iv. there being provided a pair of step down transformers (one delta-delta another delta-star) connected to the said second auto-transformer for deriving current therefrom,
v each said step down transformer being connected to a 3 phase diode rectifier bhdge
circuit to get a DC voltage,.
vi- said two diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge and
vii the said 12 pulses rectifier bridge being connected to the SC coil to charge the same through a controlled switch.
In a preferred embodiment, there are provided two auto-transformers connected in series in addition to two fixed-ratio step down transformers. The first auto-transformer is manually controlled and is set to control the ramp rate of the charging current- The output of the first auto-transformer goes as input to the second auto-transformer, which is servo controlled. The purpose of the second auto-transformer is to see that the SC coil is charged to the desired level of current and to see that this current is maintained. By incorporating an adjustable upper voltage limit on the output of the second auto-transformer, it is possible to eliminate the first auto-transformer- Thus, both the controls, namely, ramp rate control as well as set current control can be achieved using a single servo controlled auto-transformer. The output of the servo controlled auto-transformer goes to two three-phase step down transformers.
In a preferred embodiment, the first of the two 3 phase step down transformer is connected in delta on the primary as well as on the secondary and the second of the 3 phase step down transformer is connected in delta in the primary but has a star on the secondary. Instead of using two separate step down transformers, it is possible to use a single three winding (with two sets of secondaries), three phase transformer to achieve the same purpose at reduced cost. Each of the secondaries is connected to a 3 phase diode bridge, each having 6 diodes and the diode bridges are connected in series to obtain the required DC voltage.
In this embodiment, each of the two diode bridge circuits is provided wrth a common anode tap point and a common cathode tap point and these two tap points respectively-constitute the negative and positive output terminals of the rectifier bridge. The common cathode tap point of the diode bridge which is connected to the step down transformer, whose primary and secondary have delta connections, is connected to one end (inlet) of the SC coil through a circuit breaker,

5

while the common anode tap point of other diode bridge, which is connected to the primary in delta fashion and the secondary in star fashion of the second step down transformer, is connected directly to the other end (outlet) of the SC coil.
The primary to secondary turns for each of the step down transformer are so selected as to have a ratio which will step down 400 volts across the primary lines to 20 volts across the secondary lines.
In the experimental embodiment, the two 3 phase rectifier bridges are connected to the secondary of the respective step down transformers and are connected in series and the output (maximum of about 50 volts) is fed to the SC coil.
The controlled switch between the rectifier bridge output and the SC coil is either an electro magnetic or solid state contactor switch with an ON-OFF mechanism for connecting to the SC coil and there is also provided a current sensor to sense the SC coil current.
The ON-OFF mechanism is either a manually operated ON-OFF switch or automatic controlled.
Brief Description of the Accompanying Drawings:
The invention is illustrated with the help of the accompanying schematic diagrams in which,
Figure 1 schematically represents the charging circuit.
Figure 2 represents a stepper motor controller and a LED based display unit used in the circuit.
Figure 3 represents a power supply circuit used for supplying power to the circuit of Figure 2.
Figure 4 illustrates the stepper motor connection used for the servo controlled auto-transformer.
Detailed Description of the Drawings:
Referring to the schematic diagram of Fig 1, it will be noticed that the charging circuit developed for the present purpose takes its input from a 3 phase 400 volts supply (1) from the mains circuit-breaker (2). The input supply is first stepped down through a manually controlled auto-transformer (3). The output of the manually controlled auto-transformer is fed to another servo-controlled auto-transformer (4). Output of the second auto-transformer is then, fed to two

6

3- phase step down transformers (5, 6) one of which (5) is connected in delta on the primary as well as on the secondary while the second transformer (6) has delta on primary and star on the secondary The outputs of these two transformers go to two 3-phase diode bridges (7, 8) connected in cascade to get a DC voltage with 12 ripple since the two secondaries give output voltages which have 30 degree phase shift between them. The manually controlled auto-transformer can be eliminated altogether by incorporating in adjustable upper-limit output voltage control of the servo-controlled auto-transformer. This will further reduce the cost of the charging unit- This modification is now under the process of implementation.
The primary to secondary turns ratio is so chosen for each transformer to give 400/20 volts across lines. Each secondary is followed by a three phase diode rectifier bridge and the two bridge outputs (7. 8) are then connected in series, thus constituting a twelve pulse rectifier bridge as already explained Output from the twelve -pulse rectifier is fed to the SC coil (12) through the normally open contacts of a contactor CB2 (9). The contactor CB2 is operated through ON -OFF push buttons but in case of quench fault or coil over voltage fault the contractor opens up automatically SC(Super Conducting) coil current is sensed through a Hall effect based current sensor (10) and the coil current information is fed to the servo motor controller (13) for the second auto-transformer. The controller for the servo driven auto-transformer comes into action after switching on the input supply (3 phase. 400V).
LED Display unit:
The LED display unit and the steeper motor controller is shown in Fig. 2.
The reference current for the coil is set through a ten -turn pot (R51) as shown in Fig 2. Fig. 2 also shows rest of the servo motor control circuit. An array of 9 LEDS (D1 through D9) is used to indicate the value of the set coil current. A 4.7 volts zener (Z1) reference voltage is applied across the fixed ends of the 10 turn, 10 K potentiometer (R51). This same zener reference voltage is applied across a series of 10 equal resistors (R2, R6, R8, R12, R15, R18, R21, R24, R27 and R52) of 1 K each. Thus the 4 7 volts zener reference voltage is subdivided in 10 equal parts to give reference voltages of magnitudes -0.47 V, -0.94 V, -1.41 V etc. The pot voltage is compared with each of these smaller reference voltages using the comparators U1/1, U1/4, U1/3, U2/4, U2/3, U3/1, U3/4, U3/3 and U3/2 A low comparator output voltage would light up the LED connected to that particular comparator output. At the minimum pot (R51) voltage (set voltage =0) none of the LEDs glow. When pot voltage decreases below -0.47 volts, LED D1 glows. If the pot voltage setting decreases by another -0.47 volt one more LED (D2) glows. In this way the nine LEDs (D1 through D9) indicate the range of the set pot voltage. This is how the LED display unit for the set current works.
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The LED indicators are used as a visual guard against any unintentional excess current setting for the coil.
The Feed back current Controller:
The pot (R51) voltage (Fig 2) is passed through two buffer stages using op-amps U1/2 and U5. Output of op-amp U5 that essentially indicates the set coil current is compared with the actual SC coil current signal. The coil current is measured using LEM 100 hall effect current sensor. The LEM sensor voltage is output across the 50 ohm resistor (MR). Voltage across 'MR' is linearly proportional to the coil current 1. 0 volt across MR represents 400 amperes through the single turn primary side of the LEM (in our experimental set-up , however, this meant 80.0 amp through the SC coil as the coil current flows through two parallel conductors with identical physical dimensions and the LEM sensor was used to measure current through one of the conductors )
Op-amp U1/2 is used to scale the pot (R51) reference voltage such that the set current for the coil does not exceed 210 amperes. Op-amp U5 works as a unity follower when the switch SWI is in position 1. However, when SWI is in position 2 the reference current command set by the pot is overruled and the servo motor controller brings down the auto-transformer output voltage to zero. A normally closed auxiliary contact of contractor CB2 is also placed across the pole 2 of SWI Thus, when CB2 is off, the pot current command is once again overruled and the applied voltage to the coil remains zero CB2 is turned off when it is desired to retrieve the coil energy. CB2 is also turned off during quench fault circuit activation LEM output voltage is passed through an Op-amp (U4) based unity follower buffer stage and is compared with the validated set reference current voltage (output of Op-amp U5).
The comparison between the set and the actual currents is done in parallel at three comparator stages consisting of Op-amps U6, U7 and U8. Comparators using Op-amps U6 and U7 are hysteresis type comparators, whereas the comparator using Op-amp U8 has negligible hysteresis R35, R33 and R34. Similarly R38, R40 and R41 decide the hysteresis band for the comparator using Op-amp U7. U7 output goes high with increase in magnitude of the actual coil current whereas U6 output goes high when the coil current magnitude decreases. The hysteresis bands of the two comparators are so chosen that they are not overlapping, i.e. simultaneously both U6 and U7 outputs will not go low (or high). U6 output is NANDed (signals fed to a NAND logic gate), with U8 output using the NAND gate U9/4 and the U7 output is NANDed (signals fed to a NAND logic gate.) with the inverted U8 output using NAND gate U9/2. These NAND gate outputs are amplified using p-n-p transistors Q1 and Q2 and are used to drive.
8

the 12 volt coils of two electromagnetic control relays. When a particular NAND gate output is low the corresponding relay turns on.
Anti-Hunt Feature:
Reference may be made to the block marked as "current controller with anti-hunt feature" in Fig. 2. Turning on of relay #1 is used to decrease the applied voltage to the coil whereas turning on of relay #2 is used to increase the applied voltage. The effect of the three comparators and the associated logic is to make relay #2 on when the actual current is less than the set current and make relay #1 on when the reverse happens, however they have a dead zone in-between when the two current magnitudes are close enough and in that case none of the relays are on As will be discussed later, introduction of this dead zone eliminates chattering of the relays.
Fig. 4 illustrates a slow speed (60 rpm) synchronous servo-motor connection used for controlling the servo controlled auto-transformer. The motor (M) has two windings with capacitor split phase connections. The common point of the two windings is connected directly to one pole of the 50 Hz, single phase supply. Depending on which of the two relays (relay #1 or relay #2) is closed the motor will run either in clock-wise or counter clock-wise direction. In both the directions, the motor rotates at the same speed (60 rpm). When the auto-transformer spindle shaft (carrying the carbon brushes) hits either the maximum voltage position or the minimum voltage position, the motor movement is stopped by the action of limit switches and the motor is de-energized. The anti-hunt feature of the current controller can also de-energize the motor when the coil current is within the set limits.
The complete charging control circuit including its power supply is housed in an enclosure. The ten turn pot (R51) and the switch SWI are mounted on the cover of the enclosure. The control power supply (+12 V, -12 V and ground) circuit for the charger is shown in Fig. 3.
The function and the sequence of circuit operation during SC coil charging is as follows: -
As mentioned in the beginning, the 400 Volts, 3-phase input supply is fed to two auto-transformers in tandem.
The first auto-transformer, which is manually controlled decides the peak charging rate for the given coil. For slower charging the auto-transformer output should be set at lower voltages (the set voltage being proportional to the peak charging rate). In our experimental set-up we started by very low voltages and then increased It gradually.

9

Once the peak charging rate is fixed, the ten-turn pot (R51) is set to fix the reference current for the coif. The LED arrays give an approximate indication of the set current. As the current setting is increased the LEDs start lighting up one by one. The first LED lights up when the set current is about 21 amps. At 42 amps setting both first and second LEDs light up. An increase of 21 amps in the set current lights up one more LED. Thus when say. the first six LEDs light up the set current will approximately be 21x6=126 amperes. After setting the reference current for the coii, as mentioned, the ON push button for the contactor CB2 is activated.
Turning on of contactor CB2 validates the coil current setting and the servo motor controller for the auto-transformer comes into action. The SC coil inductance being very large and the allowed charging voltage being limited the coil current starts building slowly, Positive error between the set and the actual coil currents cause the servomotor to move in a direction in which the auto-transformer output increases. The given servomotor takes around 8 seconds to move the auto-transformer shaft from zero voltage position to its maximum output voltage position. After reaching the maximum voltage position the motor movement is stopped by limit switch actton and the coil charging takes place at this maximum rate.
The set and the actual coil currents are continuously compared using the set of three comparators discussed above. Due to the built in hysteresis in the comparators the actual current slightly overshoots the set current before relay #1 turns on to bring the auto-transformer voltage back to zero. With zero applied voltage across the SC coil the coii current will remain constant, however due to the conduction voltage drop of diodes in the 12 pulse rectifier feeding the SC coil the actual voltage applied to the coil becomes slightly negative and the coil current instead of remaining constant starts decreasing at a slow rate. When the slowly decreasing coil current crosses the lower hysteresis band of the current comparators relay #2 turns on to once again increment the applied dc voltage to the coii and the process repeats.
By proper choice of the hysteresis band of the current comparators the coil current be kept close to the set value and at the same time the frequent turning on and off of the relays (#1 & #2) can be avoided. The charging circuit thus described keeps the coii in a charged state. For retrieving back the energy stored in the coil the charging circuit is disconnected by opening the contacts of contractor CB2. in the present set-up contactor CB2 is opened by pressing the stop button. As C82 turns off the chopper circuit automatically comes into operation to retrieve the coil energy. As described before, turning off of CB2 also results in resetting of servo driven auto-transformers output voltage to zero.

Advantages of the Invention:
1. The Charging Circuit can charge the SC Coil at a steady controlled rate to avoid too fast
or too slow charging.
2. It can work at a preset value and maintain the current at that value.

3 It can disconnect itself from the SC coil so as to protect the latter in the event of quench
and other malfunctions.
4 It has visual display indicators to indicate the preset value of the current and ramp rate.

5. The use of non-reversible converters enables us to have the circuit to function at very low
ripple current.
6. We are able to get smooth DC current.
7. We are able to save on cost compared to known charging circuits.
8. Since we use diode rectifier the charging circuit is rugged and more reliable in operation.
Moreover, due to lower ON state voltage drops of diodes, the losses in the charging
circuit are reduced.
9 Because of the availability of large current rated diodes and transformers this type of
charging circuit can be used for large SMES units.
10. The cost of two auto-transformers and diode rectifier bridges works out cheaper than a
converter using controlled devices and has the advantage of lower ripple content in the
output Moreover the cost of the servo motor unit remains relatively constant (even at
large output power) compared to the cost of other equipments and devices.
11. It is highly dependable, easily to operate cost of manufacture is lesser than known
charging circuits and moreover, the maintenance costs are very low.
12 Of the two auto-transformers, one can be eliminated by incorporating some additional control features in the servo unit.

11

We claim
1. An improved charging circuit for a super conducting coil (SC) for SMES -UPS systems comprising:
An input unit having a circuit breaker (2) for power supply from the three phase mains (1),
a set of two three phase auto- transformer (3), (4), connected to said circuit breaker (2) with auto - transformer (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer,
there being provided pair of step; down transformers (5), (6) connected to the said second auto-transformer for deriving current therefrom,
each said step down transformer being connected to a three phase diode rectifier bridge circuit ( 7), (8) to get a DC voltage,
said two diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge and
the said twelve pulses rectifier bridge being connected through the CB2 contactor (9) to the SC coil(12)to charge the same, the SC coil being connected to a LEM make current sensor (10) for sensing the SC coil.
2. An improved charging circuit as claimed in Claim 1 wherein, the first auto-
transformer is used to achieve the desired rate of charging current (e.g. 1 Amp/Sc).
3. An improved charging circuit as claimed in claims 1& 2, wherein, the second auto-transformer is
a servo-controlled transformer which is used to maintain the current in SC coil at the set
reference value.
4 An improved charging circuit as claimed in Claim 3, wherein, the first of the two 3-phase step down transformer (connected to the output of second auto-transformer) is connected in delta on the primary as well as on the secondary.
12

5. An improved charging circuit as claimed in claims 4 and 6, wherein, the second of the 3-phase step down transformer is connected in delta on the primary but has a star on the secondary.
6. An improved charging circuit as claimed in claims 1 to 5 wherein the delta of the secondary of the first transformer and the slar of the secondary of the second transformer are each connected to 9 3 phase diode bridge, each having 6 diodes,
7. An improved charging circuit as claimed in claim 6, wherein each phase of the two diode bridge circuit is provided with a common anode tap point and a common cathode tap point. The common cathode tap point of the diode bridge which is connected to the step down transformer whose primary and secondary have delta connections, is connected to one end (inlet) of the SC coil through a circuit breaker, while the common anode tap point of other diode bridge which is connected to the primary in delta fashion and the secondary in star fashion of the second step down transformer is connected directly to the other end (outlet) of the SC coil.
8. An improved charging circuit as claimed in claims 1 to 7, wherein there is provided only one auto-transformer namely a servo controlled, auto-transformer thereby eliminating the manually controlled auto-transformer by incorporating an adjustable upper limit output voltage control of the servo controlled auto transformer.

9. An improved charging circuit as claimed in claims 1 to 8, wherein the primary to secondary turns for each of the step down transformer are so selected as to have a ratio which will give 400/20 volts across lines.
10. An improved charging circuit as claimed in claims 7, wherein the two 3 phase rectifier bridges connected to the secondary of the respective step down transformers are connected in series and the output is fed to the SC coil.
11. An improved charging circurt as claimed in claim 10, wherein a contactor is provided with a ON-OFF push buttons for connecting to the SC coil.
12. An improved charging circuit as claimed in claims 1 to 10, wherein there is provide a LEM
make current sensor based circurt to sense the SC coil current.
13

An improved charging circuit as claimed in claims 1 to 12 wherein, the SC coil is associated
with a servo motor controller for the second auto-transformer.
14. An improved charging circuit as claimed in claims 1 to 13 wherein, the circuit breaker could either be electro-magnetic type or solid state type.
15. An improved charging circuit as claimed in claims 1 to 14, wherein, an LED based display is provided to indicate the set coil current and the ramp rate.

16. An improved charging circuit as claimed in claims 1 to 15 wherein a mechanism is provided
to automatically disconnect the charging circuit in case a fault signal is received.
18 . An improved charging circuit substantially as herein described with reference to the accompanying drawings.
There
is provided an improved charging circuit for a Super Conducting coil (SC) for SMES/UPS systems comprising.
An input unit having a circuit breaker (2) for power supply from the three phase mains (1),
a set for two three phase auto- transformer (3). (4). connected to said circuit breaker (2). which auto-transformers (3), (4) are operatively connected to a stepper motor control 13, said first transformer being adapted to feed current to the said second transformer
there being provided pair of step-down transformers (5), (6) connected to the said second auto-transformer for deriving current there from.
Each said step down transformer being connected to a three phase diode rectifier bridge circuit (7). (8) to get a DC voltage.
Said to diode rectifier bridges being connected in series to give a twelve pulse rectifier bridge And

The said twelve pulses rectifier bridge being connected through a CB2 contactor (9) to the SC coil 12 to charge the same, the SC coil being connected to a LEM make current Sensor 10) for sensing the SC coil.

Documents:

00557-cal-2001-abstract.pdf

00557-cal-2001-claims.pdf

00557-cal-2001-correspondence.pdf

00557-cal-2001-description(complete).pdf

00557-cal-2001-drawings.pdf

00557-cal-2001-form-1.pdf

00557-cal-2001-form-18.pdf

00557-cal-2001-form-2.pdf

00557-cal-2001-form-3.pdf

00557-cal-2001-form-5.pdf

00557-cal-2001-letters patent.pdf

00557-cal-2001-p.a.pdf

557-CAL-2001-CORRESPONDENCE 1.1.pdf

557-CAL-2001-CORRESPONDENCE 1.2.pdf

557-CAL-2001-FORM 27.pdf

557-CAL-2001-OTHERS 1.1.pdf

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Patent Number

206867

Indian Patent Application Number

557/CAL/2001

PG Journal Number

20/2007

Publication Date

18-May-2007

Grant Date

15-May-2007

Date of Filing

01-Oct-2001

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY,

Applicant Address

KHARAGPUR 721 302, WEST BENGAL, INDIA, AN INDIAN EDUCATIONAL INSTITUTION AND DEPARTMETN OF SCIENCE & TECHNOLOGY, TECHNOLOGY BHAVAN, NEW MEHRAULI ROAD, NEW DELHI - 110 016, INDIA, AN INDIAN GOVERNMENT BODY.

Inventors:

#	Inventor's Name	Inventor's Address
1	KONDEPUDY VENKATARATNAM	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302
2	SAILENDRA NATH BHADRA	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302
3	VUTUKURU VASUDEVA RAO	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302
4	SABYASACHI SENGUPTA	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302
5	TAPAS KUMAR BHATTACHARJEE	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302
6	DINKAR PRASAD	INSTITUTE OF TECHNOLOGY KHARAGPUR-721302

PCT International Classification Number

H 01 F 7/22

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA