US2849589A - Limit switches for electric heating elements - Google Patents

Description

Aug- 26, 1958 R. w. LANCASTER I 2,849,589

LIMIT SWITCHES FOR ELECTRIC HEATING ELEMENTS Filed July 5, 1957 2 Sheets-Sheet 1 INVENTOR. RODNEY W. LANCASTER A 7' TOR/V5 Y 1958 R. w. LANCASTER 2,849,589

LIMIT SWITCHES FOR ELECTRIC HEATING ELEMENTS Filed July 5, 1957 2 Sheets-Sheet 2 P040?! Sup 04y INVENTOR.

ROD/V6) IV. Z4/VCA57'EA? LIMIT SWITQHES FUR ELECTRIC IEATING ELEMENTE) Rodney W. Lancaster, Red Bank, N. 3.

Application duty 5, 1957, Serial No. 670,156

11 Claims. (Cl. film-39) This invention relates to a limit switch for electric heating devices and especially for electric room air heaters.

To date, limit switches of solder type links and thermostats have been used as protecting devices in electric heaters but such devices are unable to accurately control external radiant energy. The commonly used solder fuse is usually located close to the heating element in order to sense the temperature thereof. Since much of the damaging force of an electric heater is in the form of radiant heat energy, the solder type links cannot respond accurately because of the variable mass of the solder from fuse to fuse, and the discoloration of the surface of the solder by aging, radiant energy being reflected by some surfaces and absorbed by others. Another disadvantage of the solder type ling when applied to an open coil type heating element is found in that the momentum of the blade of the link when opening the circuit will cause it to swing far enough to contact the heating element itself and short circuit the unit. Sometimes the solder holding the blade will only soften and allow the blade to move slowly toward the open position but the circuit is never really opened since the solder drawn like tatfy will sustain the circuit.

It is an object of the instant invention to provide a limit switch which can be located other than in close proximity to the heating element.

A further object is to create a limit switch that can be located so as to be more accessible to the user than heretofore possible.

Another object is to provide a limit switch which will give ample protection to plastic material when used in some parts of electric heaters.

Other objects of the instant invention will become apparent in the course of the following specification.

In the attainment of the aforesaid objectives, subject limit switch is made in live embodiments, the first three of which are concerned with protection for an electric heating element that has been incorporated in a room air cooling unit while the remainnig two are concerned with a room air electric heat exclusively. In the first embodiment, the limit switch is constituted of a removable fusible link in series in the circuit of an open helical coil heating element added to a room air cooling unit, the fan of the cooling unit circulating the room air therethrough. The second embodiment is like the first except that some of the heated room air is concentrated against the removable fusible link. In the third embodiment, an open coil heating element is constricted at one point in lieu of the fusible link of the first two embodi: ments. The fourth embodiment covers the use of a fusible link in series in the circuit of an enclosed helical coil heating element in an electric room air heater rather than the combined cooling and heating units of the first three embodiments. The fifth embodiment is similar to the fourth but with the coil of the heating element constricted at one point as a substitute for the removable fusible link of the fourth embodiment.

The invention will appear more clearly from the following detailed description when taken in conjunction with the accompanying drawings showing by way of example the preferred embodiments of the inventive concept.

In the drawings:

Figures 1, 2, and 2a show the first embodiment of the limit switch constructed in accordance with the principles of this invention, where like reference numerals indicate like parts, and in which:

Figure 1 is a perspective view in part section of a room air cooling unit in which the limit switch in the form of a removable fusible link is connected in series in the circuit of an open helical coil heating element added to the cooling unit;

Figure 2 is an enlarged perspective view of the limit switch shown in Figure 1 as viewed from the back; and

Figure 2a is a wiring diagram for the removable fusible link.

Figures 34 show the second embodiment of the limit switch, and in which:

Figure 3 is an enlarged fragmentary side view in part section of the removable housing for the limit switch shown in Figure 1; and

Figure 4 is a View along the line 4-4 of Figure 3.

Figure 5 shows the third embodiment of the limit switch and is a fragmentary perspective view of an open helical coil heating element a segment of which is constricted to replace the limit switch shown in the first two embodiments.

Figures 67 show the fourth embodiment of the limit switch and in which:

Figure 6 is a cross-sectional view of an electric fan heater in which the limit switch shown in Figure 2 is inserted in series in the circuit of an enclosed coil heating element; and

Figure 7 is a sectional plan view of the electric fan heater shown in Figure 6.

Figure 8 shows the fifth embodiment of the limit switch and is an enlarged detailed view in cross section of an enclosed coil heating element with a segment of the helical coil constricted in lieu of the limit switch shown in Figure 2.

Referring now in greater detail to the first embodiment of the limit switch shown in Figures 1, 2, and 2a, reference numeral 20 indicates a room air cooling unit or air conditioner, and 21 the limit switch for the open helical coil heating element 27 which has been added thereto.

The room air cooling unit or air conditioner 20 is similar to the unit illustrated and described in applicants copending application, Serial No. 568,007, filed February 27, 1956. However, in the present application an open helical coil heating element 27 has been added to the cooling unit of the co-pending application. Also, the housing 22 of the present application, in contrast with that illustrated and described in the co-pending application, is made removable by thes crews 23, 24, 25, and the like which are threaded into the casing 26 of the cooling unit 20 to facilitate the renewal of the fusible link of the limit switch when blown.

The open helical coil heating element 27 is placed in front of the squirrel cage fan 28 which circulates the room air through the casing of the air conditioner by drawing it first through the removable filter 29 and the protective screen 30 located over the room air entrance. Any known means may be used to discontinue the operation of the cooling components of the air conditioner when the unit is being used as a heater.

ported in the air stream by any known electrical and heat resistant devices such as the ceramic supports 32 which are held by the spaced uprights 33 and 34.

One terminal 41 of the heating element 27 (Figs. 1 and 2a) is connected by the conductor 55 (Figs. 1, 2, and 2a) to the terminal 46 (Figs. 2 and 2a) of the limit switch 21, thence by the removable fusible link 52 (Figs. 2 and 2a) to the terminal 4-3 thereof, thence by the conductor 55a (Figs. 2 and 2a) to contact 38d (Fig.

2a) of the three-way switch 38, and thence by the conductor 56a (Fig. 2a) to one side of the source ofelectrical energy. Another conductor 56 connects the'terminal 35 (Figs. 1 and 2a) of the element 27 to the opposite side of the source of electrical energy while the same side of the electrical energy source is connected to one terminal of the motor 23a (Fig. 2a) which actuates the fan 28 when the three-way switch 38 is thrown to either of the two on positions. The switch 38 in the on position 38:: connects the free terminal of the motor 280 with the other side of the source of electrical energy through the conductor 56a. In the on position 38g, the switch 38 conects both the free terminal of the motor 280 and the terminal 48 of the limit switch 21 with the oposite side of the source of electrical energy. Such known circuits prevent the operation of the heating element unless the fan is turned on also.

The limit switch 21 (Figs. 1 and 2) is constituted of a ceramic or other electrical and heat resistant base 41 which is secured, for example, to the inner surface of the front of the removable housing 22 (Fig. 1) by screws which are first passed through the integrally formed and constricted ends (Fig. 2) of the base 41 and then threaded into the front of the housing 22 or by other known means. Unlike the known solder type links, subject limit switch can be located in proximity to or remote from the heating element 27. The removable fusible link 52 (Fig. 2) may be a very short length of the same type of wire as that forming the open helical coil of the heating element 27 but of less diameter and removably connected by thumb nuts, as illustrated, between the terminals 46 and 48 of the base 41. The shorter the length of the link 52, the less the mass, and the less likelihood of causing a short circuit when fused.

Referring now to the second embodiment of the limit switch shown in Figures 3 and 4, reference numeral 60 indicates the limit switch which is secured to the inner surface of the housing 61, the. housing being removably secured to a combined room air cooling and heating unit or air conditioner as in the first embodiment.

The limit switch 60 is similar to that of the prevoiusly described first embodiment except that in the second embodiment there is the addition of the hollow conical frustum 63, open at both ends, the end 64 of maximum diameter being directed toward the room air exit 65 of the unit 62 where the exit is protected by the louvers 72. The open end 66 of minimum diameter of the frustum is substantially centered over the removable fusible link 67 (Fig. 4) of the limit switch 60. Ears which are fastened to or integrally formed with the periphery of the conical frustum 63 at the end 64 of maximum diameter are turned, as most clearly shown in Figure 4, and then fastened to opposite sides of the removable housing 61 by the screws 7 and 71 or by any other known means.

The frustum or funnel 63 is actually a magnifying means to establish more air density over the link 67 and is especially practical for general application since a small change in air density over the heating element, which in some cases might allow the element to operate at too high a temperature, yields a more sizable air change at the location of the link 67. This magnifying feature allows the designer more latitude in selecting the link for any given set of conditions, either excessive amperage or insufiicient air flow.

In the third embodiment shown in Figure 5, reference numeral 80 indicates an open helical coil heating element. As in the previously described embodiments, the heating element is supported in a room air cooling unit or air conditioner by ceramic supports, one of which is indicated by the reference numeral 81, the ceramic supports being attached to spaced uprights such as the one indicated by the reference numeral 82.

The difference between the heating element St) and the previously described heating elements of the first two embodiments is that a segment of the element 811 is constricted, for example, at 83 which obviates the need for the removable fusion link of the first two embodiments.

The terminals, not shown, of the heating element 311 and the motor of the electric fan, not shown, are connected by a switch, not shown, to a source of electrical energy and preferably in such a manner that the fan is turned on simultaneously with or prior to the turning on of the heating element as illustrated and described in connection with the first embodiment.

When the heating element 80 becomes overheated as, for example when the circulation of the air through the casing is interrupted by the clogging of the filter, loose fan blade, locked rotor, or broken belt, the coil of the heating element 80 will break at the constricted segment 83 and open the circuit through the heating element. To prevent the thus parted ends 84 and 85, on opposite sides of the constricted segment 83, from falling and perhaps shorting the unit, a washer 94 of mica or the like with a central opening may be threaded on the helical coil until it rests on top of the ceramic support 81 to hold the upper strand of the coil 30 as illustrated. In addition, two spaced and like mica supports 86 and 87 are provided. The mica support 36 with the oppositely directed longitudinal slits 3i; and 89 is fitted, as illustrated, between the upper and lower strands of the coil 80 on one side of the constricted segment 83. The other mica support 87 with like oppositely directed longitudinal slits is fitted, as further illustrated, between the upper and lower strands but on the opposite side of the constricted segment 83. Of course, the washer 94 is added protection and may be eliminated.

In the fourth embodiment, illustrated in Figures 6 and 7, reference numeral indicates a fan heater, and 111 the fan heater limit switch.

The fan heater 110 is constituted of the casing 112 with room air entrances 113 (Fig. 6), and the louvers 113 which form a protection for the room air exits, the :air being circulated through the entrances and out of the exits by the electric fan 115.

Above the fan 115 is an enclosed coil heating element 116 constituted of the usual outer sheath 151, as shown in Figure 8, around a known helical coil 152 held in position therein by an insulating material, such as magnesium oxide, in a known manner. in each end of the enclosed coil heating element 116 is an electrode 118 or 119 (Fig. 7), the inner end of each of which is electrically connected to an end of the helical coil and then insulated electrically from the outer sheath 151 (Fig. 8) by any suitable means. The enclosed coil heating element 116 is supported in the casing 112 of the fan heater by the spaced supports 120, 121, 122, 123, and 124 (Fig. 7) and is protected against accidental contact by the upwardly spaced screen 126.

The limit switch 111 is substantially the same as the similar component illustrated and described in the first embodiment, that is it has an insulated base 41 (Fig. 2), and terminals 46 and 48 between which is the removable fusible link 52. One electrode 118 (Fig. 7) of the heating element 116 is connected in series with the removable fusible link and with the switch 134 and the conductor 135 to one side of the source of electrical energy. The other electrode 119 (Fig. 7) is connected by a conductor 136 to the opposite side of the source of electrical energy. Preferably, the switch 134 has three positions so that it first turns on in a known manner the motor of the fan 115 and then both the motor of the fan 115 and the heating element 116.

In Figure 7, the limit switchlllisshown on'the-side of the casing while in Figure 6 it is shown under the baflle 128, the baffle being rcmovably inserted over the top of the casing 112 by the screws 129, 130, and the like. However, since subject limit switch is responsive to its own radiant energy which is designed to be proportional to that of the element 116, the location in order to function is immaterial.

Referring now to the fifth embodiment of the. fan heater limit switch shown in Figure 8, referencenumeral 150 indicates the enclosed coil'heating element.

The enclosed coil heating element 150 is constituted, as in the fourth embodiment, of an outer sheath 151 substantially in the center of which is a helical coil 152 electrically connected at one endto an electrode 153 which is sealed therein by any suitable insulation 154. The helical coil 152 is supported in the outer sheath 151 by any known insulating material.

Unlike the helical coil of the fourth embodiment, however, a segment 156 of the helical coil 152 is constricted, and with such construction the enclosed coil heatingelement may be connected to a source of electrical energy without the use of the removable fusible link of the first, second, and fourth embodiments. Obviously, well known circuits may be used whereby the turning on of the heating element is simultaneous with or follows the turning on of the fan. Of course, an open coil heating element could be used in both the fourth and fifth embodiments.

In operation:

A typical table for a copper alloy limit switch would be:

Elapsed time in seconds after fan failure for circuit to break Amperes Applying the above table to a 1,500 watt heater rated at 115 volts, and developming 13 amperes at 115 volts, if the air velocity fell to 0, subject limit switch would terminate the circuit in about 18 seconds. If, however, the voltage was low, for example, around 90 volts, 10.3 amperes, the circuit would sustain itself for 26 seconds which would be permissible since the heater is putting out only 900 watts at 90 volts or 60% of the rated power of 1,500 watts.

Based upon the above, the following would be the protection afitorded by subject limit switch for a heater using a heating element rated at 1,500 Watts at 115 volts, which is a common rating because of the 15 ampere fuse limitation now prevalent:

On fan failure Volts Amperes trip time of circuit in seconds Extreme voltage expected". 125 14. 2 7 Rated voltage 116 13.0 18 Minimum voltage expected 105 12.0 23

many modifications and changes can be made in the details of construction within the scope of the appended claims.

What is claimed is: p

1. A thermal limit switch in combination with the circuit of an electrically operated radiant heater comprising a fusible link in the circuit of the heater, the link measuring the intensity of the heat energy given off by the heater by sensing its own self-energy as well as the amperage therethrough.

2. In a heating unit for a fluid, the unit having an electrical heating element and means for passing the fluid in heat transfer relationship with the element; a limit switch for the element, a source of electrical energy for the element and switch, and a circuit connecting the switch with the element and the source of electrical energy, the switch having'self-energy proportional to that of the element.

3. In a heating unit for a fluid, the unit having an electrical heating element and means for passing the fluid in heat transfer relationship with the element; a fusible limit switch for the element, a source of electrical energy for the element and switch, and a circuit connecting the switch with the element and the source of electrical energy, the switch having a breaking point proportional to the temperature of the element and. the current in the circuit.

4. In a room air heating unit having a casing, the casing having an electric fan and heating element, and the fan circulating the air through'the casing in heat exchange relationship with the element; a fusible limit switch for the element, the switch comprising a removable fusible link, a source of electrical energy for the element and link, and a circuit connecting the link with the elements and the source of electrical energy, the link being in series with the element and further being responsive only to the same air flow or a portion thereof that cools the element as well as the flow of electrical energy in the circuit.

5. In a room air heating unit having a casing, the casing having an electric fan and heating coil, and the fan circulating the air through the casing in heat exchange relationship with the coil; a fusible limit switch for the coil, the switch comprising a constricted segment of the coil, a source of electrical energy for the fan and coil, and a circuit connecting the fan and coil to the source of electrical energy, the constricted segment being responsive only to the same air flow or a portion thereof that cools the element and the flow of electrical energy in the circuit and resisting the flow of electrical energy more than any other segment of the coil.

6. In a room air cooling unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an electric heating element disposed in the casing in the path of the circulated air, a removable housing disposed over at least a portion of the exit,

-the housing having a heated room air inlet and outlet, a

fusible limit switch for the element disposed in the housing, the switch comprising a removable fusible link, the link having a breaking point proportional to the maximum safe operating point of the element and located in at least a portion of the air flow, a source of electrical energy for the fan and element, and means for connecting the fan and element to the source of electrical energy with the link in series with the element.

7. In a room air cooling unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an electric heating element disposed in the casing in the path of the circulated air, a removable housing disposed over at least a portion of the exit, the housing having a heated room air inlet and outlet, a fusible limit switch for the element disposed in the rousing, the switch comprising a removable fusible link, the link having a breaking point proportional to the heat energy given oil? by the element and substantially unresponsive to the heat energy given oil by the element, a source of electrical energy for the element and link, a circuit connecting the fan and element to the source of electrical energy with the link in series with the element, and means for concentrating at least some of the heated room air against the link.

8. In a room air cooling unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an electric heating element disposed in the casing in the path of the circulated air, a removable housing disposed over at least a portion of the exit, the housing having a heated room air inlet and outlet, a fusible limit switch for the element disposed in the housing, the switch comprising a removable fusible link in series with the element, the link measuring the intensity of the heat energy given oil. by the element as well as the amperage therethrough, a hollow conical frusturn with open ends disposed in the housing, the open end of maximum diameter being directed toward the exit and the opposite end toward the link, a source of electrical energy for the link and element and for the fan, and means for connecting the fan and element to the source of electrical energy whereby the element operates only when the fan is in operation.

9. In a room air heating unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an open coil heating element disposed in the casing in the path of the circulated air, a segment of the coil being constricted and measuring the intensity of the heat energy given off by the element as well as the amperage therethrough, support means for the constricted segment on at least one side thereof, and means for connecting the fan and coil to the source of electrical energy whereby the coil operates only when the fan is in operation.

10. In a room air heating unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an enclosed coil heating element disposed in the casing in the path of the circulated air, a fusible limit switch for the coil comprising a removable fusible link in series with the element, a source of electrical energy for the link and element and the fan, the link yielding self-energy proportional to that of the element and further being of a size to fracture when a predetermined current is attained in the circuit, and means connecting the fan and element to the source of electrical energy whereby the element operates only when the fan is in operation.

11. In a room air heating unit having a casing with room air entrance and exit and an electric fan circulating the air therethrough; an enclosed coil heating element disposed in the casing in the path of the circulated air, a fusible limit switch for the coil comprising a constricted segment of the coil, the segment measuring the intensity of the heat energy given on by the element as well as the amperage therethrough, a source of electrical energy for the fan and coil, and means connecting the fan and coil to the source of electrical energy whereby the coil operates only when the fan is in operation.

References Cited in the file of this patent UNITEDSTATES PATENTS 1,991,280 Hynes Feb. 12, 1935 2,006,810 Kunz July 2, 1935 2,015,251 Browning Sept. 24, 1935

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