Patent Grant
10925121
This invention relates to air cooled, self contained, handheld induction heaters used to heat rusted nuts and bolts for removal thereof such as for repair of machinery, automobiles, trucks, rails, chemical plants, petrochemical, etc. Such handheld induction heaters are totally self-contained; that is, they are air cooled and include a high frequency inverter. One end of the tool has a power connecter for connection to utility power, i.e. 120 or 240 VAC, and the other end has attachment terminals for a variety of work coils. This variety of work coils allows efficient heating of different sized nuts, bolts and the like needed in industrial, automotive or other equipment disassembly and/or repair. Other uses for such handheld induction heating equipment include expanding bearing races or housings for bearing removal/replacement, and even annealing metals such as brass shells in firearms ammunition reloading procedures.
Historically, such hand held induction heaters have been limited to a power of about 1000 watts. The reason for this is the limited volume and physical constraints in a handheld tool. A generally cylindrical shape for such tools is preferred allowing ergonomic compatibility with the human hand. As such, the gripping section of the tool is limited to no more than a 3″ diameter, and preferably less. Length is limited for operator ease of use, as are balance and weight. These physical constraints result in the power limitation of about 1000 watts for all handheld, self contained induction heaters on the market. It would be very desirable to increase this power limitation so that small nuts and bolts and the like can be heated more quickly, and larger nuts and bolts, etc. can be heated sufficiently for expansion and thus easy removal.
According to the invention there is provided a handheld, self contained, air cooled induction heater including an input connector for engaging a source of A.C. power, a rectifier to convert A.C. to D.C., an inverter which converts the D.C. to A.C. operating at substantially higher frequency than the A.C. power line frequency, one or more high frequency step-down transformers having magnetic cores which are of various embodiments and being connected to heat dissipating terminals capable of functional engagement to at least one work coil.
As will become clearer after a thorough perusal of this application, three main principles of electronics are applicable to the invention as listed below.
Resistance of Each Winding
Rp or Rs=MLT*Rcu*N where: Rp=Primary Winding Resistance Rs=Secondary Winding
Resistance Rcu=Copper Resistance (μΩ/cm) N=Turn Count
Temperature Rise Estimate
ΔT=(PΣ/At)0.833 where: ΔT=Temperature Rise in ° C. P Σ=Total Transformer Losses in mW/cc
(Power dissipated in the form of heat) At=Surface Area of Transformer in cm2
Creation of the Exponent in the Temperature Rise Formula
x=ln(PΣ@1st ΔT/PΣ@2nd ΔT)/ln(1st ΔT/2ndΔT)
The need for the transformer embodiments of the present invention is two-fold. First, the high frequency inverter within these devices operates at power line voltages, typically 100-240 volts, necessitated by their direct connection to standard power line voltage supplies. The work coils in contrast, operate at several 10's of volts and high current to allow for a small, practical number of work coil turns, typically 1 to 5 turns, of heavy gauge wire. The one or more transformers “match” the high voltage, low I inverter to the low voltage, high current work coil. Second, the high frequency one or more transformers galvanically isolate the dangerous high voltage from the power line-connected inverter from the user-accessible work coil.
Due to the dimensional constraints of the handheld, self contained induction heater, especially the maximum diameter comfortable for the human hand, the transformer design choices are limited and a set of 2 windings is needed to complete a transformer.
For example, the temperature rise of the conventional transformer was compared to that of the new transformer embodiments. The same work coil consisting of two turns of #8 AWG wire, loaded by a 1.5″ diameter mild steel pipe was used to test temperature rise of both transformers. The A.C. input voltage was 120.0 VAC in both cases. Starting temperature was 30° C. in all cases. Temperature was measured by type K thermocouples and yielded results as shown in the following table.
It was discovered that the temperature rise for the transformer embodiments of the present invention was considerably lower, as expected. An unexpected result was the 250-300 watt increase in measured power. The number of turns on the prior art and present transformer embodiments was the same, 30 turns primary and 4 turns secondary. It was determined that the reason for the increased power was due to the vastly reduced primary to secondary leakage inductance in the instant
transformer embodiments. Leakage inductance may be approximately measured by shorting the secondary and measuring the primary inductance. A perfect transformer, having zero leakage, would measure zero micro-Henries with its secondary shorted. Real transformers have finite leakage inductance. Leakage inductance acts as a series impedance with the load, reducing current, voltage and therefore power to the load. Prior art transformer showed 13.2 μH leakage inductance, while the instant transformer embodiments only showed 5.3 μH, explaining the increased power.
The lower leakage inductance and lower temperature rise are very desirable results. The new transformer embodiments deliver ⅓ more power for twice the time before overheat. The improved induction heater using the disclosed transformer embodiments is therefore capable of heating larger nuts, bolts and metallic objects to a given temperature in shorter time, or heating large objects to a higher temperature before transformer thermal limitations are reached.
Inasmuch as the core and legs thereof of the transformer 12 are clearly shown and described in the pending parent application, for the sake of simplicity which one in the art would grasp instantly, they are left out of the figures herein and the variety of combinations of primary and secondary windings are shown independent of the legs of the core about which they are wound.
As illustrated in
Next shown are the transformer 12 embodiments of the present invention wherein winding half of the turns of the primary and secondary winding 24 and 26, respectively, on a first leg (not shown) and half on a second leg (not shown) leads to dramatic reductions in peak winding temperature, for a given power transferred, and thus much higher power output capability at greater duty cycle. The legs upon which the windings are wound are not shown for the sake of simplicity, as they are already defined in the parent case, the teachings of which are incorporated herein, The herein below descriptions of the transformer 12 embodiments only describe modifications to the right of reference numeral 12 in the figures.
Beginning with
The descriptions of the modifications made to the windings of the at least one transformer 12 of the present invention are not very complex but are extremely beneficial for use in their ambient environment and do not require much in the way of description as those skilled in the art would understand immediately the workings of the much improved high frequency step down transformers 12 of the present invention. It will also be understood by those skilled in the art that the windings 13 and 17 may be of more than one part into which they may be split, as necessary.
Next the secondary windings 26 are each connected to corresponding heat dissipating terminals 32 which are also capable of functional engagement to a work coil 34 which allow various work coils 34 to be installed, depending on the application of the device. Finally the work coil 34 converts high frequency high current alternating current into a high frequency alternating magnetic field which heats a metallic or conductive object (not shown) which is placed within the magnetic flux lines emanating from the work coil 34. Here it will be understood by those skilled in the art how the above equations were applied to create any one of the plurality of disclosed embodiments of the transformer 12 of the present invention.
It will be appreciated by those skilled in the art that the single dual legged transformers described above are electrically similar to two transformers electrically interconnected also as described above. Such a prototype set was built and tested. Two sets of ETD-44 cores, available as part number 44-00-00 from Ferrite International were used to construct two transformers. Such a transformer has a maximum dimension of 2.25 inches and fits easily within a case with an inside diameter of 2.4 inches. The primary winding of each consisted of 15 turns of 150×#36 AWG Litz wire, single layer wound. These two 15 turn primary windings were interconnected in series, effectively providing 30 turns total. The secondary windings were each wound over the primary and consisted of 4 turns each of 420×#36 AWG Litz wire, and were interconnected in parallel. The cores in both interconnected transformers were gapped to 0.030 inch.
Testing at 120.0 VAC input to the induction heating inverter using the same workcoil/load as above yielded the following results:
At 5 Minutes: Power=1210 watts, temperature=110 C
At 10 Minutes: Power=1225 watts, temperature=145 C
The starting temperature was again 30 C.
The somewhat lower power of this embodiment was attributed to the slightly higher leakage inductance of the two transformer combination which measured 6.8 microhenrys as opposed to 5.3 uH in the single transformer dual winding embodiment. Leakage inductance was measured by shorting the parallel connected four turn windings and measuring the leakage inductance across the series connected primary turns, totaling thirty turns.
The lower temperature rise of another exemplary two transformer embodiment is attributed to the two transformers having more surface area for heat dissipation. The use of more than two transformers would increase this area farther at the expense of electrical and mechanical complexity
As described above, the induction heater 10 of the present invention provides a number of advantages, some of which have been described above and others of which are inherent in the invention. Further, modifications may be proposed without departing from the teachings herein. Accordingly the scope of the invention should only be limited as necessitated by the accompanying claims.
This application is a continuation-in-part of U.S. application Ser. No. 15/044,754 filed on Feb. 16, 2016, and entitled “Hand Held Induction Heater and Transformer Therefor”, the teachings of which are incorporated by reference in their entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4017701 | Mittelmann | Apr 1977 | A |
| 5122947 | Hishiki | Jun 1992 | A |
| 5764500 | Matos | Jun 1998 | A |
| 20030121909 | Riess | Jul 2003 | A1 |
| 20120248093 | Ulrich | Oct 2012 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15044754 | Feb 2016 | US |
| Child | 15936304 | US |
