High pressure rapid gas quenching vacuum furnace utilizing an isolation transformer in the blower motor power system to eliminate ground faults from electrical gas ionization

Abstract

An integral high pressure rapid quenching vacuum furnace utilizing an electrical isolation transformer in the blower motor power control system in order to isolate the motor windings, reduce the possibility of gas ionization and eliminate ground faults, particularly when quenching in argon gas, is described. In order to achieve the desired mechanical properties of certain metal alloys being quenched using argon gas as a quenching medium in the high pressure gas vacuum furnace chamber, a 600 HP-460 Volt motor is required. A 460 Volt primary-460 Volt secondary [delta-delta] isolation transformer, having input and output windings separated by an electrostatic shield connected to ground is placed between the power source and the gas blower motor in the quenching chamber filled with argon gas. The 460 Volt power source is connected to a variable frequency drive (VFD) and the VFD is connected to the primary transformer winding. The secondary transformer winding connects 460 Volts to the blower motor windings. The full electrical isolation of the transformer secondary winding results in zero ground fault voltage.

Claims

1. A high pressure vacuum furnace for heat treating and rapid gas quenching in argon atmosphere in the same furnace comprising a single chamber having blower means therein, the vacuum furnace comprising: power supply means, and isolation transformer means operatively connected to said power supply means, and wherein the blower means being operatively connected to said isolation transformer means, said isolation transformer means having primary winding means, secondary winding means and electrostatic shield means therebetween, said primary winding means receiving electric power from said power supply means, and said blower means receiving electric power from said secondary winding means.

2. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace further includes variable speed drive means and metal oxide varistor means both operatively connected to said power supply means, and wherein all of said power supply means, said variable speed drive means and said metal oxide varistor means are operatively connected to ground.

3. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace further includes motor terminator means operatively connected to the blower means, and wherein all of the blower means, said motor terminator means and said isolation transformer means are operatively connected to ground.

4. A vacuum furnace in accordance with claim 1 wherein the power from said power supply means to said primary winding means is 460 Volts, 3-phase, 60 cycles.

5. A vacuum furnace in accordance with claim 1 wherein the blower means includes motor means, and wherein the power to said motor means from said secondary winding means is 460 Volts, 3-phase, 60 cycles.

6. A vacuum furnace in accordance with claim 1 wherein the pressure in said vacuum furnace is up to 10 Bar.

7. A vacuum furnace in accordance with claim 1 wherein the pressure in said vacuum furnace is in excess of 10 Bar.

8. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace includes baffle means, and wherein said baffle means is in the form of a chevron configuration.

9. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace includes variable speed drive means, and wherein said variable speed drive means is operatively connected on its input side to said power supply means, and is operatively connected on its output side to said isolation transformer means.

10. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace includes variable speed drive means, and wherein said variable speed drive means is operatively connected on its input side to said power supply means, and is operatively connected on its output side to said isolation transformer means.

11. A vacuum furnace in accordance with claim 1 wherein the vacuum furnace includes 3-phase metal oxide varistor means, and wherein said 3-phase metal oxide varistor means is operatively connected in parallel with said power supply means to the input side of said variable speed drive means.

12. A vacuum furnace in accordance with claim 5 wherein the vacuum furnace includes motor terminator means, and wherein said motor terminator means is operatively connected in parallel with said secondary winding means to said blower motor means. A vacuum furnace in accordance with claim 11 wherein said motor terminator means comprises varistor means.

13. A high pressure vacuum furnace for heat treating and rapid gas quenching in argon atmosphere in the same furnace comprising a single chamber and access means, the chamber being segregated into an outer portion and an inner portion, the inner portion of the chamber being a hot zone and being adapted to receive the work piece to be heat treated through the access means, the furnace further including movable door means in the chamber outer portion in the form of movable doors formed to be closed during the heat treating cycle and opened during the quenching cycle, the furnace chamber outer portion further including heat exchanger means, blower means and baffle means formed to deflect the radiant energy of the hot zone passing into the outer portion of the chamber through an opening created by the movable doors being in the open position back through the opening into the inner portion hot zone of the chamber, and wherein the baffle means is further formed to diffuse the convective heat energy of the hot gases passing through the opening and to distribute the convective heat energy evenly over the full surface area of the heat exchanger means during the quenching cycle, the baffle means being located in the outer portion of the chamber juxtaposed from the movable doors, and wherein the heat exchanger means being located in proximity to the baffle means and the blower means, and the blower means being located in proximity to the heat exchanger means for circulating argon gas into the inner portion hot zone of the chamber to quench the work piece, the improvement comprising: power supply means, and isolation transformer means operatively connected to said power supply means, and wherein said blower means being operatively connected to said isolation transformer means, said isolation transformer means having primary winding means, secondary winding means and electrostatic shield means therebetween, said primary winding means receiving electric power from said power supply means, and said blower means receiving electric power from said secondary winding means.

14. A vacuum furnace in accordance with claim 13 wherein the vacuum furnace further includes variable speed drive means and metal oxide varistor means both operatively connected to said power supply means, and wherein all of said power supply means, said variable speed drive means and said metal oxide varistor means are operatively connected to ground.

15. A vacuum furnace in accordance with claim 13 wherein the vacuum furnace further includes motor terminator means operatively connected to the blower means, and wherein all of the blower means, said motor terminator means and said isolation transformer means are operatively connected to ground.

16. A vacuum furnace in accordance with claim 13 wherein the power from said power supply means to said primary winding means is 460 Volts, 3-phase, 60 cycles.

17. A vacuum furnace in accordance with claim 13 wherein the blower means includes motor means, and wherein the power to said motor means from said secondary winding means is 460 Volts, 3-phase, 60 cycles

18. A vacuum furnace in accordance with claim 13 wherein the pressure in said vacuum furnace is up to 10 Bar.

19. A vacuum furnace in accordance with claim 13 wherein the pressure in said vacuum furnace is in excess of 10 Bar.

20. A vacuum furnace in accordance with claim 13 wherein the vacuum furnace includes baffle means, and wherein said baffle means is in the form of a chevron configuration.

21. A vacuum furnace in accordance with claim 13 wherein the vacuum furnace includes variable speed drive means, and wherein said variable speed drive means is operatively connected on its input side to said power supply means, and is operatively connected on its output side to said isolation transformer means.

22. A vacuum furnace in accordance with claim 13 wherein the vacuum furnace includes 3-phase metal oxide varistor means, and wherein said 3-phase metal oxide varistor means is operatively connected in parallel with said power supply means to the input side of said variable speed drive means.

23. A vacuum furnace in accordance with claim 17 wherein the vacuum furnace includes motor terminator means, and wherein said motor terminator means is operatively connected in parallel with said secondary winding means to said blower motor means.

24. A vacuum furnace in accordance with claim 23 wherein said motor terminator means comprises varistor means.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 depicts in perspective a side horizontal closed door cross-section view of a high temperature vacuumhigh pressure quench heat treating furnace 100.

[0028] FIG. 2 depicts a prior art step-down transformer circuit used in a high temperature vacuumhigh pressure quench heat treating furnace 100.

[0029] FIG. 3 depicts an isolation transformer circuit used in a high temperature vacuumhigh pressure quench heat treating furnace 100 in accordance with the present invention.

[0030] FIG. 4 depicts the complete blower motor circuit in accordance with the present invention.

[0031] FIG. 5 depicts a comparison of quench rates for two identical runs using 100% argon versus 20% helium/80% argon.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Referring to the drawings wherein like reference numerals refer to the same or similar elements across the multiple views, FIG. 1 depicts in perspective a side horizontal, closed door cross-section view of a high temperaturehigh pressure integral quench heat treating furnace 100. As fully described in Wilson et al., the disclosure of which is fully incorporated herein by reference, the term integral includes the movable doors, baffles, heat exchanger and blower assembly all interconnected within a single chamber, all of which will be fully described below. The circuitry for the blower assembly, using an increased horsepower motor greater than the 300-400 HP motor described in Wilson et al. in a 100% argon quench gas, is the key improvement of the present invention.

[0033] FIG. 1 shows furnace 100 that includes a hinged door 150 which is opened to allow the insertion of a work piece to be heat treated, and then closed during the heat treating cycle. Outer wall 101 and inner wall 102 of furnace 100 form the radial boundaries of a furnace water jacket 103 used for cooling outer furnace wall 101. The outer chamber of furnace 100 thus is a cylindrical double-walled, water-cooled vessel. Inner wall 102 also forms the outer wall of a spacious gas plenum chamber 105, which is a large annular cavity that is important to high velocity, rapid quenching.

[0034] The inner wall 102 of gas chamber 105 forms a hot zone 106 of vacuum furnace 100. Hot zone 106 includes a work zone 107 for heat treating a work piece placed in the furnace. It should be understood that the term work piece can refer to a single piece or multiple pieces to be heat treated and rapidly quenched. It should also be understood that the dimensions of the hot zone could be advantageously varied to accommodate larger sized work pieces. Reference is made to Wilson et al., the disclosure of which is fully incorporated herein by reference for a complete description of the arrangement of furnace 100.

[0035] Still referring to FIG. 1, at the rear end of hot zone 106 is a circular wall (not shown) which comprises an opening 115 containing movable radiation baffle doors 116 and 117. When doors 116 and 117 are opened, baffles 118 are exposed to direct gasses from hot work zone 110 outward into a water-cooled copper-finned heat exchanger 119, and thereafter to a recirculating fan wheel 120. Recirculating fan wheel 120 receives its power from a 600 HP-460 Volt blower motor 121, which is attached to fan wheel 120. Baffles 118 serve two purposes. The first purpose is to act as a radiation barrier between the hot work zone 110 and the heat exchanger 119. Upon opening the radiation baffle doors 116 and 117, all radiant energy from the hot work zone 110 would otherwise transfer immediately into and overwhelm heat exchanger 119, leading to its rapid failure. Baffles 118 serve to deflect radiation energy back into the hot work zone 110 in a similar fashion as a metal heat shield in a typical all metal hot zone, which reflects radiant heat back towards the work piece during a heating cycle, and also serves to avoid heat losses during the heating cycle. This leaves only convective heat energy via the hot gases as the source of heat that must be removed by heat exchanger 119. Reducing the effects of any source of radiant heat energy decreases the heat load placed on heat exchanger 119 during the quenching cycle, thus minimizing various maintenance issues typically required for heat exchangers that deal with both radiation and convection heat loads.

[0036] FIGS. 2 and 3 are simplified electrical schematic drawings of the ground to current voltage that is always present when a 460 Volt motor is connected to the power supply. The use of a prior art autotransformer 650 is shown in FIG. 2, and the present invention isolation transformer 660 showing power source and voltage to ground is shown in FIG. 3. As depicted in FIG. 2, prior art blower motors used in high pressure gas quenching vacuum furnaces rely on an autotransformer 650 to drop the 460 Volts entering the building down to 230 Volts. Autotransformer 650 consists of a single winding 651 that is connected to the initial power supply 550 on its input side, and is connected to blower motor 121 on its output side with the voltage reduced to 230 Volts. With this arrangement a measure of the voltage to ground using a voltmeter 400 connected in parallel to the output of winding 651, the reading could be as high as 277 Volts to ground. This high voltage to ground can result in extraneous electrical current inside blower motor 121 and cause ionization of the argon gas, resulting in an electrical short circuit. Conversely, the use of an isolation transformer 660 shown in FIG. 3 eliminates such a possibility. A simplified electrical schematic in FIG. 3 consists of a power supply 550 connected to the primary winding 661 of isolation transformer 660. Primary winding 661 is separated from the secondary winding 662 by an electrostatic shield 663, that acts as a voltage or current isolator between the primary and secondary windings that could possibly ionize the argon gas. Both electrostatic shield 663 and secondary winding 662 are connected to ground. The output of secondary winding 662 is also at 460 Volts and is connected to the input of blower motor 121, which is connected to the same ground as isolation transformer 660. With this arrangement a measure of the voltage to ground using a voltmeter 400 connected in parallel to the output of secondary winding 662, the reading would be essentially zero, thus eliminating any ground voltage that could ionize the argon gas and cause an electrical short circuit.

[0037] As stated previously in the background of the invention, there is a recognition that submerging a motor with greater than 230 Volts into an ionizing gas significantly increases the probability of creating an arc which would damage not only the motor, but also the furnace and any material being heat treated. The National Fire Protection Association standards and other recognized electrical codes for these type of vacuum furnaces include recommendations that a motor cannot exceed 230 Volts in the presence of an ionizing gas such as argon. Since the applicable standards and the established prior art have included the use of an autotransformer, the present invention represents an improvement when using integral high pressure argon gas quench systems. The inclusion of a 460 Volt motor submerged in an ionizing gas such as argon in order to create gas quenching speeds required to meet certain strict cooling rates has not previously been utilized.

[0038] FIG. 4 depicts in its entirety the actual power connections between the utility power supply, the various components including the connection of the variable speed drive 225 used to regulate the speed of the blower fan, the isolation transformer 660, and the blower motor 121. Also shown are two different types of varistors that serve as insurance against random voltage spikes that occur during the transfer of power from the power supply to the blower motor. A varistor is a variable resistor, sometimes referred to as a voltage dependent resistor. Each component of the design plays a role in significantly reducing the probability of an ionizing occurrence in the presence of argon gas.

[0039] In FIG. 4 the utility power supply 550 is connected to the input side of a variable speed drive 225. A 3-phase metal oxide varistor (MOV) is connected in parallel to the input side 224 of variable speed drive 225. The output side 226 of variable speed drive 225 is connected to primary winding 661 of isolation transformer 660. Secondary winding 662 is connected to the input of blower motor 121. As an added layer of protection, an electrostatic shield 663 is located between the primary 661 and secondary 662 windings of isolation transformer 660 to shield the windings from any electrical voltage spikes that may occur between the two windings. Also attached to the blower motor in parallel with secondary winding 662 is a motor terminator 664, which is a varistor that protects the motor from unwanted distorted waveforms from the variable speed drive 225 to the blower motor 121. When the variable speed drive 225 is used, the typical non-distorted or pure sinusoidal voltage waveform is squared off. Over time the squared-off waves can result in transients or voltage spikes that can enter the blower motor 121 and cause damage to the motor winding insulation. Damage to the wire insulation would result in a short circuit current within the motor windings. Motor terminator 664 will work as a sacrificial current absorber and keep such transient voltage peaks from entering the motor.

[0040] Although the use of isolation transformers in the electrical technology is not new, for this particular application the use of a 460 Volt rated motor in the presence of specifically argon gas, but also other quench gases such as nitrogen and helium, is new and inventive The present invention goes beyond the current teachings regarding use of a motor in an ionizing gas and provides the opportunity for quenching in argon gas at pressures that have not previously been achieved because of prior art industry electrical limitations.

[0041] While there has been described what is believed to be a preferred embodiment of the present invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit and scope of the invention. It is therefore intended to claim all such embodiments that fall within the true scope of the invention.