US2042233A - Electric range - Google Patents

Description

y 1936. E. H. LOCKWOOD 2,042,233

ELECTRI C RANGE Filed Feb. 21, 1954 W|TNES$ESI INVENTOR FaW/n H lock wood I KTTORNE til Patented May 26, 1936 UNITED STATES PATENT OFFICE ELECTRIC RANGE Pennsylvania Application February 21, 1934, Serial No. 712,342

2 Claims.

My invention relates to electric ranges and has particular relation to the construction and control of range heating units designed for energization by low-voltage currents supplied through transformers from the usual residence-potential power circuits.

Low-Voltage surface or exposed type heating units for domestic and other electric ranges possessa number of inherent advantages, a proper appreciation of which it now appears may cause such units to supplement or even replace the conventional line-voltage devices which at the present time are so universally employed in cooking-device applications. For example, by designing the unit for low-voltage energization, little or no insulation is required on the heatproducing conductor and a correspondingly higher efiiciency of thermal conductivity to receptacles and materials may be realized. Likewise, the energizing voltage may be reduced to a completely non-dangerous value with an attending elimination of all possibility of electrical shock to the housewife or other user.

In addition, the body or cross-section of the heavy-current heat-producing conductor may be made sufficiently large to afford mechanical ruggedness of an unbreakable order and to prevent appreciable diminution due to a flaking off of the outer material at the high operating temperatures. The life of such a unit is, therefore, practically unlimited. Furthermore, the presence of a transformer, the ratio of which may readily be changed by Winding tap control, in the power supply circuit makes practical a variation of energizing voltage in a large number of steps and thereby facilitates heat-intensity adjustments.

The present invention contemplates improvements which will make low-voltage heating systems more practical.

One object of the invention is to provide improved designs for low-voltage heavy-current range heating units of the surface or exposed heat-producing-conductor type.

Another object of the invention is to provide an improved heat-adjustment control scheme for units or devices which are energized through a trasformer from a multi-voltage supply circuit.

My invention itself, together with additional objects and advantages thereof, will best be understoodthrough the following description of specific embodiments when taken in conjunction with the accompanying drawing, in which:

Figure 1 is a top or plan view of one preferredform of parallel-path type of heating unit con- 55; structed in accordance with my invention paratus and circuits arranged to effect heat-in-' tensity adjustment in accordance with my invention;

Fig. 4 is a top or plan view of a second form of parallel-path type of heating unit constructed in accordance with my invention; and

Fig. 5 is a section view taken on line V-V of the device in Fig. 4.

Referring to the drawing, and particularly to Figures 1 and 2 thereof, the improved heating unit there represented comprises a plurality of parallel-arranged heat-producing conductor sections .lll connected between two terminal members l2 and 14 which are positioned on opposite sides of a heater-plate area. While as represented this area is of the usual circular contour, it will be understood that the improved heating unit of my invention may also be constructed in square or rectangular form.

Energizing current is supplied from the usual 110 or 220 volt residence power circuit through a suitable step-down transformer 20 conveniently mounted beneath the range cooking-platform 30 through the top of which the heating unit protrudes. The voltage applied to the heating element may be as low as one or two volts. As shown, this transformer comprises a low-voltage heavy-current secondary winding I8 connected by means of bus bars I6 to the heating unit terminal members l2 and I4, and a primary winding 22 arrange-dtoreceive energization from the power source, which in the drawing is represented in the form of circuit conductors 25 to 27, inclusive. To control and adjust the intensity of heater energization, the'primary winding supply circuit of the transformer includes a tap-selecting or ratio changing switch 24 the actuating knob 28 of which protrudes through the front of the range structure in the manner shown.

Insofar as the heating unit of myinvention is concerned, it will be understood that the specific details of the supply transformer 20 and its mounting underneath the cooking platform 30 are relatively immaterial, except that a device having separate and electrically insulated primary and secondary windings is preferable to a single winding or auto-transformer since the former permits the low voltage heater circuit to be completely isolated from-the higher voltage supply circuit and thereby effectively protects the user from all possibility of electrical shock.

In addition, it is important that the resistance, and hence the length, of the high-current secondary leads or bus bars I6 be kept as low as possible in order that the major portion of the heat produced in the secondary circuit of the transformer may be localized in the conductors ll of the surface heating unit. Hence, the bus bars should be of generous cross-section to accommodate the high heating-unit current, which at the low voltages named may be of the order of 1000 amperes or more. They may be joined with the terminal portions I2 and [4 of the heating unit in any suitable satisfactory manner as by brazing,

One preferred manner of forming the heating unit structure, which as illustrated in Figs. 1 and 2 is mounted to slightly protrude above the top surface of the cooking platform 30 in order that receptacles or materials to be heated may be set directly thereupon, is to cut from a plate of suitable metal, such as nickel-chromium alloy or other conducting material, having a high coefficient of resistance in addition to being mechanically strong, parallel slots so spaced as to leave the desired parallel conducting strips 10 through which the heating current may pass. Because of the exceedingly low voltage, no insulation whatever is required on the unit assemblage which further is so mechanically strong as to be completely break-proof. High thermal efiiciency and unlimited life are the respective advantageous results.

Distribution of current among the several parallel heat-producing conductors l0 proper to produce uniform or other predetermined heat production among them may be effected by varying their relative cross-sections in a manner to be more fully described by a later portion of the specification.

Instead of forming the element from a fiat sheet of metal as just described, an equivalent structure may be produced in the form of a casting of the desired conducting material. Such a casting is in practice, found to be equally satisfactory.

The described heating unit may be supported in the illustrated position in any suitable manner as through the utilization of angle piece or clip members 23 attache-d to the underside of the platform member 30 and disposed to engage the edges of the terminal portions l2 and I4 as shown. Preferably, sections of insulating material 29 are interposed between these supporting members and the heater structure to prevent a by-pass circuit from being established through the platform or surface member material 30.

Another form of low voltage surface unit structure of the parallel path type is illustrated in Figs. 4 and 5. In this structure, I utilize a plurality of conducting material bars 34, the ends of which are secured in cooperating holes made in the solid and common terminal members 36 and 38, the necessary low resistance electrical connection being achieved by brazing the ends 40 to the body portions of the terminal members or by attaching in other suitable manner. These terminal members 36 and 38 may be directly connected with the heavy current low voltage output or secondary winding of the supply transformer (not shown) or connected therewith through the medium of bus bars of the general form indicated at 42. As in the first explained constructions, any heat distribution among the bars desired may be obtained by varying their cross-sections to change the distribution of current therethrough.

Further relative to the problem of regulating heat distribution, it is important to note that the heat produced by each of the parallel conductors varies as the first power of the resistance of that conductor and as the square of the current traversing it. Consequently, increasing the resistance, which proportionately reduces the current, causes the total heat dissipated by the conductor to .decrease. Assuming that the voltage impressed upon all of the several parallel conductors in a heating unit of my invention is equal, the heat distribution thereamong may be made uniform by making the conductor resistances equal, in which case the currents traversing them will be equal.

The resistance of each conductor varies directly with its length and inversely as its cross sectional area. Consequently, the several conductors in a heating unit may be made to present equal resistances by so adjusting their relative areas that the quotient of the length of each conductor divided by its area has the same value I as does that of all the other conductors.

In the case of the plate type of heating unit construction described in connection with Figures 1 and 2, assuming that the plate material is of uniform thickness, this means that to give the uniformity of heat distribution mentioned, the quotient of the effective length of each of the con ducting paths I0 divided by the width of that path should be made equal to that of each of the other conducting paths. For other constructions, typified by that shown in Figs. 4 and 5, the computation must take account of the coefficient of resistance of the bar materials, in case all are not alike, as well as their cross-sectional areas.

Considering now the improved heat-adjusting or control system of my invention which is applicable to situations involving a transformer and a multi-voltage power supply circuit, reference may be had to the diagrammatic representation of Figure 3, in which the several elements of the complete range heating unit system previously described in connection with Figs. 1 and 2 are identified by corresponding reference numerals. The particular arrangement illustrated is capable, in combination with the -220 volt power supply circuit represented by circuit conductors 25, 26 and 21, of effecting six different adjustments in heat intensity through the utilization of but three tap connections, indicated at a, b and c, on the primary winding 22 of the stepdown transformer 20.

The tap selecting or control switch 24 requires in effect, two blades, the first of which, indicated at 46, is joined with the central conductor 26 of the power supply circuit and the second of which, indicated at 48, is similarly joined with one of the outer conductors as 25 of the power circuit. To the remaining outer conductor 2'! of the power circuit, the right-hand end of the transformer winding 22 is joined by means of a conductor 50.

When the blades of switch 24 occupy the extreme upper or off position in which they are illustrated, the transformer energizing circuit is interrupted. As the switch blades are moved downwardly to active position No. 1, there is completed an energizing circuit for winding 22 which extends from power source conductor 26 through conductor 52, the switch blade 46, conductor 54, winding tap a, the winding 22, and conductor 50 back to power source conductor 21. This applies 75 the potential appearing between conductors 26 and 2'! of the supply circuit, designated in the drawing as 110 volts, to the complete winding and produces a minimum value of voltage in the heater supply secondary winding [8.

Actuation of the switch blades to position No. 2 transfers the energizing connection just described from winding tap a to winding tap b, thereby causing the same supply potential to act upon a smaller number of primary winding turns. As a result, the heater energizing voltage induced in secondary winding I8 is correspondingly raised. Further downward movement of the switch blades to active position No. 3 transfers the described energizing connection to tap c of winding 22 and still further raises the voltage induced in the Winding I8.

As the blades of switch 24 are advanced downwardly to active position No. 4, the transformer energizing circuit is transferred from blade 46, connected with the 110-volt supply, to blade 48 which is connected with the 220-volt supply conductor 25. In this new position, the transformer energizing circuit may be traced from power source conductor 25 through conductor 58, switch blade 48, conductor 60, tap connection a, the winding 22, and conductor 5!! back to power source conductor 21. Under these conditions, the 220- volts are impressed across the complete winding 22. The number of turns and tap connecting points of this winding are so proportioned that there is now produced in the secondary winding l8 a voltage somewhat higher than resulted when the 110 volts were applied to winding tap 0.

Further downward movement of the switch blades to active position No. 5 transfers the energizing connection just described from tap a to tap b and thereby further increases the voltage induced in secondary winding l8. Additional advancement of the blades to active position No. 6 further transfers the 220-volt energizing connection to tap c of winding 22 to produce the maximum value of heater voltage in winding l8. In the illustrated connection, active position No. 6 constitutes the extreme limit of travel of the blades of switch 24 in the downwardly or voltage increasing direction.

To summarize the manner of operation of the improved control system of my invention, I have consolidated the descriptive information just given in the form of the following table:

The heater voltage values presented by this table apply to the 1% volt maximum parallel path type of heating unit described in connection with Figs. 1 and 2. Since these specific values of voltage induced in the secondary winding [8 of the step-down transformer are for illustrative purposes only, it will be understood that in case heating units of other maximum voltage ratings are to be controlled, the values stated will all be modified accordingly. A similar comment applies to the values listed in the supply voltage column. Instead of the 110-220 volt combination, it will be apparent that other potentials differing not only in absolute magnitude but also in relative values lend themselves to utilization by my improved control system.

It is, in this connection, further significant to note that supply voltage circuits providing more than two different values of voltage may also be adapted to the control system of my invention. Nor is the system restricted to the six difierent heat adjustments shown and described, since, practically any number desired may be incorporated by increasing the number of transformer winding taps and active control switch position. The important feature resulting from its use is the reduction of the required number of transformer taps for a given number of heat adjustmerits. Thus, in the illustrated two voltage supply circuit combination, six different values of heater voltage are obtained through the use of only three winding taps. If a three voltage supply circuit were available, the same three taps could be caused to produce nine heat adjustments, and

so on.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the scope of the appended claims.

I claim as my invention:

1. An electric range low-voltage heating unit comprising a pair of spaced terminal members, and a plurality of conductors of dissimilar lengths, parallel-positioned in a common horizontal plane, interconnecting said members, the cross-section of each of said conductors being so related to its length that the resistance of that conductor is the same as is that of each of the others.

2. An electric range low-voltage heating unit comprising a flat metal plate having parallel slots which provide within a substantially circular area a plurality of parallel-positioned heat-producing conductor strips of dissimilar lengths through which current may be caused to flow in parallel from the plate material on one side of said area to the material on the opposite side, the width of each of said strips being so related to its length that the resistance of that strip is the same as is that of each of the others.

EDWIN H, LOCKWOOD.

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