FORM 2
THE PATENT ACT 1970
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION (See section 10 and rule 13)

1. TITLE OF THE INVENTION:
"Solid State Device Assembly for Arc less Switching of Electrical Circuit"
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(c) ADDRESS: Larsen & Toubro Limited
Electrical & Automation North Wing, Gate 7, Level 0, Powai Campus, Saki Vihar Road, Mumbai 400 072, INDIA
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

Solid State Device Assembly for Arc Jess Switching of Electrical Circuit
Field of invention
The present invention relates to arc less switching devices involving versatile loads, and more particularly to a solid state device adapted for arc less switching of the electrical circuit.
Background of the invention
Automatic or manually operated electromechanical switching devices are well known for their use in breaking AC or DC electrical circuits. However, the electromechanical switching devices are incompetent to prevent arcing and additionally have a tendency to erode away the contact material between the cathodes and anodes thereby creating carbonaceous tracking path which is responsible for creating a flashover condition with premature failure of these switching devices. Also, the electrical and mechanical life of these electromechanical switching devices is very poor.
The use of solid state contactors, such as relays or contactors is an alternative option to electromechanical switching devices. The solid state contacting devices use back to back SCR or TRIAC for switching AC power and MOSFETS or High power transistors (1GBT) for switching DC power. These devices are capable of preventing the arc during making and breaking of the electrical circuit. However, when these devices switch ON, the inductive load on components, such as for example squirrel cage induction motor, transformer, incandescent lamp, luminaries, and capacitor banks, is subjected to very high inrush current of the order 10 to 100 that lasts for at least 100 ms to seconds. In such situation, the power electronics devices associated with the solid state contactors get heated up.

Moreover, these devices continuously carry rated current thereby consuming high power. For example, the device having 100 A rated current is likely to consume a continuous power of 200 watt that is high enough to cause a very high temperature and thermal runaway of the solid state switching device. Due to higher power consumption, these prior art solid state devices essentially need to have bulkier heat sinks for heat dissipation. Also, the performance of these solid state devices deteriorates in heated condition due to variation in the ambient temperature and other environmental condition. In addition, the cooling of these devices needs to be done by forced cooling means which unnecessarily increases overall size and cost of the solid state device.
Therefore, there is a need of a solid state device which facilitates arc-less switching of the electrical circuit under high load conditions. Also, there is a need of a solid state device which facilitates faster rate of heat dissipation with least power consumption. Further, there is a need of a solid state device that overcomes all the drawbacks of the prior art.
Objects of the invention
An object of the present invention is to provide a solid state device for arc less switching under various types of loads such as for example a resistive load, a slightly inductive load and a capacitive load.
Another object of the present invention is to provide a solid state device facilitating low temperature rise across the contacts of the solid state device under high rated current without the need of using bulkier heat sink for controlling temperature.
Further object of the present invention is to provide a solid state device whose overall thermal resistance is very low for conducting heat at a faster rate during making and breaking of the electrical circuit.

Summary of the invention
Accordingly, the present invention provides a solid state device assembly including an electromechanical switch connected in parallel connection with a solid state device. The solid state device includes a printed circuit board assembly with an alternating current power supply having a positive half cycle and a negative half cycle. The printed circuit board assembly is configured with a control circuit and a power circuit. The solid state device is includes a constant voltage direct current source having a bridge rectifier unit, a zener diode and a filter circuit adapted for supplying an uninterrupted constant direct current voltage to the control circuit. The solid state device includes a silicon controlled rectifier module adapted for facilitating a low thermal resistance during making and breaking of the power circuit with reduced heating effect. The solid state device includes a microcontroller having an input connecting to the constant voltage direct current source. The microcontroller is having an output connecting to a pair of optocouplers via a respective pair of transistors. The microcontroller is adapted for delaying operation of the silicon controlled rectifier module and the electromechanical contactor by a predefined amount of time. The solid state device includes at least one resistor adapted for controlling the point on wave or firing angle of the incoming voltage. The solid state device includes a snubber circuit along with a varistor connecting in parallel connection with the silicon controlled rectifier module for suppressing the surge voltage during switching of the power circuit. The snubber circuit controls a rate of change of incoming voltage. The solid state device includes at least one diode along with a parallel resistor for the operation of the silicon controlled rectifier module during the positive half cycle and the negative half cycle.
Brief description of the drawings
FIG. 1 illustrates a solid state device assembly including a solid sate device and an electromechanical device constructed in accordance with the present invention;

FIG. 2 is a perspective view of the solid state device of FIG. 1;
FIG. 3 is a circuit diagram illustrating connection between the solid state device and the electromechanical device of the solid state device assembly of FIG. 1; and
FIG.4 illustrates a PCB assembly adapted for the solid state device of FIG. 1.
Detailed description of the invention
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.
Referring to FIG. I, a solid state device assembly having an electromechanical device I coupled with a solid state device 2 is illustrated. The solid state device 2 is connected in parallel connection with the electromechanical device 1 wherein anode and cathode terminals of the solid state device 2 are connected in parallel to main contacts of the electro-mechanical switch 1.
Referring to FIG. 2, the solid state device 2 is having a housing 3 and a cover 4 as illustrated. The housing 3 and the cover 4 are made of insulating material in this one particular embodiment. The housing 3 and the cover 4 are adapted to securely enclose a printed circuit board (PCB, hereinafter) assembly 5 which is configured with a power circuit and a control circuit.
Referring now to FIGS. 3 and 4, the PCB assembly 5 includes a constant voltage source unit 6, a micro-controller 7, a first optocoupler 8, a second optocoupler 9 and a back to back anti-parallel silicon controlled rectifier (SCR, hereinafter) module 10. The constant voltage source 6 is a direct current (DC, hereinafter) source that supplies voltage to the control circuit. The micro-controller 7 is

provided with an integrated circuit (IC, hereinafter) socket in this one particular embodiment. The first optocoupler 8 is adapted for isolation of the control circuit and power circuit. The second optocoupler 9 is adapted for isolation between individual control circuits. The back to back anti parallel SCR module 10 is adapted for making and breaking of the electrical circuit.
The constant voltage source 6 in this one particular embodiment is facilitated by a rectifier unit 11; a zener diode 12 and a filter circuit 13 such that a constant shunt voltage regulation from the AC control supply is enabled. The zener diode 12 is configured to remain open for very low voltage until a saturation voltage Vz is reached. The zener diode 12 is adapted to be shorted beyond voltage Vz for sharing increase in current. The zener diode 12 ensures that the voltage available at its output is always constant. The PCB assembly 5 includes a LED 14 that is configured to receive an output voltage through a current limiting resistor for indicating power up status of the solid state device 2. The constant voltage regulator 6 is connected to the Vcc and ground of the micro-controller 7.
The micro-controller 7 is configured and programmed such that its respective I/O pins P2 and P8 are fed as an input to the first optocoupler 8 and the second optocoupler 9 via a first transistor 15 and a second transistor 16 respectively. The first transistor 15 and the second transistor 16 are NPN type transistors in this one particular embodiment. The first transistor 15 and the second transistor 16 are adapted to switch ON low branch current to high current circuit for input supply to the first optocoupler 8 and the second optocoupler 9 respectively. The micro controller 7 is programmed to develop a positive delay of at least 20 ms between the operations the SCR 10 and the electromechanical contactor 1. The first optocoupler 8 and the second optocoupler 9 respectively have an output configured to drive a gate circuit of back to back SCR 10 and electromechanical contactor 1 respectively. The optocoupler 8 in this one particular embodiment acts as a positive isolation between the control circuit and the power circuit of the solid state device 1,

The PCB assembly 5 includes a fixed resistor 17, a snubber circuit 18, a varistor 19, a first resistor 20, a second resistor 21, a first diode 22, a second diode 23 a third resistor 24 and a fourth resistor 25, The fixed resistor 17 is adapted to control the point on wave or firing angle of the incoming voltage of the power circuit in the range of about 0° to 90°. The snubber circuit 18 is positioned across the terminals T2 and T4 of the antiparallel SCR device 10 to which AC supply is fed. The snubber circuit 18 is adapted for controlling the dv/dt. The varistor 19 is connected in parallel to the snubber circuit 18 for suppressing any surge produced during switching of the power circuit. The first resistor 20 is adapted to limit the current in the input circuit of the optocoupler 9. The second resistor 21 is adapted to limit the current in the input circuit of the optocoupler 8. The first optocoupler 8 positively isolate the control circuit and the coil circuit in this one preferred embodiment. In the optocoupler 9, the output is fed to a contactor coil through AC supply wherein the power to the coil is internally derived from the control supply without requiring any separate power supply to the coil.
In this one particular embodiment, the supply is having a positive half cycle and a negative half cycle. The positive half cycle consists of path L - (22) - 6(8) - 4(8)
- (17) -1(10) - 2(10) -N. The negative half cycle consists ofpathN- (23) -(17)
- 4(8) - 6(8) - 5(10) - 4(10) - L. The first diode 22 and the third resistor 24 are adapted to block the operation of the SCR 10 during the positive half cycle. The second diode 23 and the fourth resistor 25 are adapted to block the operation of the SCR 10 during negative half cycle. Also, during switching OFF operation, the back to back SCR 10 breaks the electrical circuit by natural commutation thereby developing a reverse recovery voltage that lasts for few micro seconds due to high inductive load. In such a situation, the snubber circuit 18 absorbs the energy and dampens the surge so as to protect the power device.
In Operation:

Referring to FIGS. 1-4, the solid state device 2, in operation, is advantageously usable with a single or multi pole electromechanical device I to facilitate arc less switching. The solid state device 2 essentially facilitates two circuits namely the control circuit and the power circuit that operate with AC power supply of 10 to 50 A, 240V, 50Hz. An electromagnetic coil needs to be connected to the solid state device 2 via AC supply. A band of control voltage at which it will pick up is 195 - 270 V. A drop out voltage of the solid state device 2 is less than 90V. When AC supply is fed to the control circuit of the device 2, the bridge rectifier unit 11 generates a DC voltage that is fed to the zener diode 12 to derive constant DC output voltage. This constant DC output voltage is fed to two parallel branch circuits consisting of the capacitor 13 and the LED 14 for status indication. Further, the same voltage is supplied as Vcc for the microcontroller 7 to a pin PI of the microcontroller 7. Ground is connected to pin P14 of the microcontroller 7. An output of the microcontroller 7 from the pin P2 & P8 is connected to the base of the first transistor 15 and the second transistor 16 respectively. The microcontroller 7 is programmed in such a way that it develops positive delay between signal coming from pins P2 & P8 respectively. When the base current of first transistor 15 becomes high it turns ON and the collector circuit consisting of the first optocoupler 8 input. In this condition, the first resistor 21 draws current due to the Vcc available. This collector current is sufficient to build voltage across input of the photo diode 22 which is more than its forward voltage required. The photo diode 22 conducts and generates infrared, which optically couples with the silicon bilateral device to turn on the output terminal. This generates the firing pulses to the back to back SCR 10, which turns ON in both the positive and negative half cycle of the AC supply.
The load current is conducted through the terminal T2 & T4 of the back to back SCR 10 such that resistive or inductive load are connected in series with it. After a delay of approximately 20 ms, the analogue output of pin P8 of the microcontroller 7 is fed to the second transistor 16 which turns ON if it gets sufficient base current. The collector circuit of the second transistor 16 is

connected in series with input of the second optocoupler 9 and the resistor 20 to Vcc. Upon activation the diode generate infrared which optically couples with phototriac to turn ON the second optocoupler 9, Output of this is connected in series with the electromagnetic coil. The electromechanical contactor 1 picks up after positive delay of approximately summation of pick up time and one cycle. This ensures that contact of the mechanical contactor 1 never create an arc due to contact bouncing as the voltage across it is < 2 V. When its contact gets fully closed with sufficient contact pressure due to very low contact resistance in mohm, the voltage available across the anode & cathode terminal of the SCR module 10 will be in few mV which is very low as compared to the minimum voltage required for gate triggering which is less than 12 V. Due to which the SCR 10 is turned OFF. In this condition, the total current gets transferred to the main contact of the electromechanical contactor 1. Since its power consumption is very low hence overall temperature of the device I is within the specified limit. When the control supply is switched OFF, the electromechanical contactor 1 drops down within < 20 ms., when the voltage across mechanical contact goes beyond 12 V, the back to back SCR is turned ON during breaking of the circuit. Eventually, with a delay of few ms, the back to back SCR 10 will turn OFF due to natural commutation.
Examples:
Example- 1:
The solid state device assembly having the electromechanical device 1 (MNX 25 / 3 poles type) coupled to the solid state device 2 is tested under operative voltage of 240 V AC with rated current of 50A for resistive and slightly inductive load at a supply frequency of 50Hz. The making and breaking test done upon the device at the various currents is as follows-

Voltage M/B Load Power Time (ms) operation

(V)
Current(A) factor cleared
240 10 Resistive 500 10
240 19.8 Resistive 419 10
240 31.4 Resistive 415 10
240 51.7 Resistive 416 10
240 101 Resistive 430 10
240 280 Resistive 337 5
Table -1 Observations:
Initially few make and break operations were done only upon one pole of the contactor 1 without connecting its load terminal in parallel to the Anode -Cathode terminal of the solid state device 2. An arcing was observed across the contacts with slight erosion of the contact tip material.
Subsequently, the same test was then conducted with one adjacent clean pole of the same contactor 1 connected in parallel across the SCR power terminal. No arcing was observed during making and breaking operation. No erosion or pit mark observed upon the contact tip. As shown in table -1, it was observed that the overload withstanding capacity of the device 1 is suitable up to a resistive load current of 280 A for 337 ms. Conclusion:
The test results reveal that the arc less switching device is capable for switching a load of 32A. 240V, 50Hz, AC 53 or 200A, AC51 utilization category.
Advantages of the present invention are:
1. The solid state device assembly including the electromechanical connector 1 and the solid state device 2 is suitable for versatile load, such as inductive and resistive load (up to 240V, 100A, 50Hz) preferably with AC 51 & AC53 utilization category, which advantageously avoids arcing across the contacts during making & breaking of the electrical circuit.

2. The solid state device assembly including the electromechanical connector 1 and the solid state device 2 is suitable for wide band of AC control supply.
3. The electromechanical contactor 1 and the solid state device 2 operate such that the operation between the electromechanical connector 1 and the solid state device 2 is positively delayed during the making and breaking of the circuits.
4. The control circuit facilitated by the electromechanical connector 1 and solid state device 2 consume low voltage and power
5. The electromechanical contactor 1 and the solid state device 2 provides positive isolation between the coil circuit and control circuit with ease of termination
6. The electromechanical contactor 1 and the solid state device 2 positive isolation between the control circuit and the power circuit
7. The solid state device 2 provides dv/dt protection via the snubber circuit 18
8. The solid state device 2 advantageously suppresses any surge produced during switching of the power circuit through the varistor 19.
9. The solid state device 2 advantageously provides peak load control and short circuit protection in the control circuit and the power circuit respectively
10. The solid state device 2 advantageously provides low temperature rise of during the making and breaking of the circuit without the need of special cooling means such as heat sink
11. The solid state device 2 is easily mountable with the mechanical switching device
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed,

and obviously many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We Claim:
1. A solid state device connected in parallel connection with an
electromechanical switch for arc-less switching of an electrical circuit, the solid
state device comprising:
a printed circuit board assembly with an alternating current power supply facilitating a positive half cycle and a negative half cycle, the printed circuit board assembly having a control circuit and a power circuit;
a constant voltage direct current source having a bridge rectifier unit, a zener diode and a filter circuit adapted for supplying an uninterrupted direct current voltage to the control circuit;
a silicon controlled rectifier module having a low thermal resistance adapted for making and breaking the power circuit with reduced heating effect;
a microcontroller having an input connecting to the constant voltage direct current source, the microcontroller having an output connecting to a pair of optocouplers via a respective pair of transistors for delaying operation of the silicon controlled rectifier module and the electromechanical contactor by a predefined amount of time;
at least one resistor for controlling the point on wave or firing angle of the incoming voltage;
a snubber circuit including a varistor connecting in parallel connection with the silicon controlled rectifier module for suppressing the surge voltage during switching of the power circuit to control a rate of change of incoming voltage; and
at least one diode connecting in parallel connection with a resistor for operating the suitable silicon controlled rectifier module during the positive half cycle and the negative half cycle.
2. The solid state device assembly as claimed in claim 1, wherein the opto
couplers optionally drive a gate circuit of the silicon controlled rectifier module

and a gate circuit of the silicon controlled rectifier module to facilitate isolation between the control circuit and the power circuit.
3. The solid state device assembly as claimed in claim 1, wherein the firing angle is in the range of about 0° to 90°.