SE541696C2 - Selfregulating heater - Google Patents

Abstract

The invention relates to a self-regulating heating circuit comprising at least one resistive element (43) having a substantially temperature independent resistance (R) and at least one PTC element (44) having a temperature dependent resistance(RPTC) . These elements (43, 44) are arranged in heat transfer relationship with a flowing medium of a heat circuit. The characteristics of the PTC element(s) (44) are selected such that when the temperature of the flowing medium reaches a predetermined value, the resistance of the PTC element(s) (RPTC) has reached a level such that the voltage across the resistive element(s) drops. The resistive and PTC elements (43, 44) are coupled to each other in a configuration such that the combined power delivered by the resistive element(s) and PTC element(s) (43, 44) is reduced when the resistance of the PTC element(s) (RPTC) has reached said level.

Description

SELFREGULATING HEATER The present invention relates to electrical heaters in general, and in particular to heaters for vehicles, automotive as well as maritime. More in particular it relates to a self-regulating heater circuit using a combination of ordinary resistive heating elements and resistive elements with a Positive Temperature Coefficient (PTC elements).

Background of the Invention Resistive heaters are well known in the prior art and have been used extensively for many decades. Originally resistive heating elements having essentially temperature independent characteristics have been used. However, a problem with prior art heaters is that when ordinary resistive heating elements are used these require thermostat based feed-back systems in order to regulate the heat. Such systems can be fairly complex and also potentially unreliable at least over time.

In order to avoid the drawbacks of purely resistive heaters PTC elements have been used instead. However, the drawback of these systems is that they are fairly expensive.

A few examples of prior art PTC heaters are listed below.

In US-4,721,848 (HEAT TRACE LTD) there is disclosed a self-limiting electrical heater comprising an elongate resistance heating element and a PTC resistor. The resistor is elongate and extends side-by-side with and along the length of the heating element so as to be responsive to the temperature of the entire length of the heating element. The heating element and resistor are connected in series, the resistor having a positive temperature coefficient such that its electrical resistance is substantially less than that of the heating element when at a normal operating temperature but increases rapidly when exposed to temperatures above the normal operating temperature.

A self-regulating electric heater is shown in US-3,940,591 (TEXAS INSTRUMENTS INC). The heating element in this apparatus is a self-heating, positive temperature coefficient (PTC) resistor having low initial resistance which increases abruptly as its temperature rises above a given level.

In US-4,730,103 (GTE PROD CORP) a compact resistor heater device is disclosed comprising a resistor body of ceramic material having a positive temperature coefficient of resistance and having electrically conductive elements disposed upon the resistor body for conducting current from one side of a power supply to one side of the resistor body.

Summary of the Invention In order to overcome the drawbacks of the prior art systems the inventors have devised a novel system employing an inventive combination of ordinary resistive heating elements and PTC elements.

The novel system is defined in claim 1.

The inventiveness of the novel system lies in the combination of PTC with resistive heating elements in a heater for automotive /marine applications, wherein selfregulation is provided by one or more PTC heating elements, while the bulk of the power output is provided by one or more cheap resistive heating elements.

Thus, the invention provides a self-regulating heating circuit comprising at least one resistive element having a substantially temperature independent resistance and at least one PTC element having a temperature dependent resistance, said elements being arranged in heat transfer relationship with a flowing medium of a heat circuit. The characteristics of the PTC element(s) are selected such that when the temperature of the flowing medium reaches a predetermined value, the resistance of the PTC element(s) has reached a level such that the voltage across the resistive element(s) drops, and the resistive and PTC elements are coupled to each other in a configuration such that the combined power delivered by the resistive element(s) and PTC element(s) is reduced.

Preferably the resistive elements and the PTC elements are provided in separate compartments in a housing.

The resistive elements are suitably provided in a first compartment having an inlet for heating medium wherein the PTC elements are provided in a second compartment having an outlet for heated heating medium.

In particular the PTC elements, if there are more than one PTC element, are coupled in series or in parallel with each other.

The resistive elements if there is more than one the resistive element are coupled in series or in parallel with each other.

Furthermore, the resistive elements as a unit are coupled in series with the PTC elements as a unit. It does not work if the PTCs are in parallel with the resistive elements. If the resistance of the PTCs goes up, all current will flow through the resistive elements instead and there is no self-regulation.

The circuit can be implemented in any vehicle, such as cars, trucks, boats.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus not to be considered limiting on the present invention, and wherein Fig. 1 schematically illustrates the self-regulating heating system; Fig. 2 is a graph showing resistance vs temperature for resistive and PTC elements; Fig. 3 is a graph showing power vs temperature for the self-regulating heating system; Fig. 4 illustrates an embodiment of a self-regulating heating system; Fig. 5a-b show exemplary set-ups of resistive and PTC elements; and Fig. 6a-b illustrate further embodiments comprising switching in and out of elements.

Detailed Description of Preferred Embodiments The present invention relates to an electrical heater unit for the transport sector (automotive and/or maritime). It is designed for heating of liquid heat transport media (such as water, glycol (or mixtures of water and glycol), ...). It comprises a combination of resistive heating elements and PTC heating elements that are electrically connected together. The specific properties of these elements (such as cold resistance) is chosen in such a way that they provide a means of selfregulation of the output power. Control electronics may be part of the heater unit in order to switch the heater on or off, to regulate the maximum power output, to control a fluid pump (when such a pump is part of the heater unit), and/or provide feedback to the host regarding the heater condition. It can also provide redundant safety features, for example against overheating and/or overvoltage.

The invention is based on the realization that “self-regulation” of heating circuits involving a fluid (e.g. water or other heating or cooling media such as glycol) can be achieved by using so called PTC elements (PTC = Positive Temperature Coefficient) for heating in combination with resistive heating elements, where the PTC and resistive elements are coupled in series, as shown in Fig. 1. By this combination one can refrain from a feed-back loop, and still use inexpensive resistive element.

In this invention, both PTC and resistive elements take part in the heating. The resistive elements can either be put in parallel or in series. Likewise, the PTC part can consist of several elements put in series or in parallel to obtain the wanted characteristics .

At least one of the PTC elements is preferably placed at the flow exit, so that a good control of the temperature of the water flowing out is obtained.

The high voltage (HV) power supply is provided externally. A pump for circulating the liquid heat transport media may be provided externally, or may be part of the heater unit.

Resistive heating elements are characterized by a resistance that is more or less constant over the complete temperature range. PTC heating elements are characterized by a resistance that is more or less constant for low temperatures, and a sharp increase in resistance at a specific ‘transition’ temperature. See Fig. 2 for a graphical representation of the resistive characteristics as function of temperature. Fig. 2 illustrates the principle of self-regulation. Shown characteristics are idealized since there will always be some influence of temperature on resistance.

Fig. 3 illustrates the power output of a self-regulated system.

The basic function of the novel system is as follows. Reference is made to Fig. 1 which is a schematic illustration of a self-regulating heating system, generally designated with reference numeral 10.

Thus, there is provided a flowing and circulating liquid medium for heat transport and a channel system (not explicitly shown) for conducting the liquid.

An arrangement of resistive heating elements 12 are provided as well as an arrangement of PTC heating elements 14. The resistive and PTC elements are suitably coupled in series. Power is supplied from an external high voltage supply 16.

The system is set-up for a predetermined target temperature for the flowing liquid, although this target temperature may of course be selectable to suit different situations of use.

When the flowing liquid, i.e. the heat transport medium, such as water, glycol (or mixtures of water and glycol), illustrated by a horizontal arrow Fnqin Fig. 1, is below its target temperature, most of the heat is transferred to the liquid heat transport media by the resistive heating element(s) 12. When the temperature of the liquid increases, the resistance of the PTC heating element(s) 14 will also increase and consequently, the total power output will be reduced. Finally, when the liquid has reached its target temperature, the heater output power is reduced to a level where no further increase of the average temperature of the liquid takes place.

Referring to Fig. 1 , the power produced by the resistive and PTC heating elements is given by the following equation (see also Figs. 2 and 3 which illustrate resistance and power as a function of temperature): PPTC<=>Ih<2>RPTC Pres= Ih<2>Rres and the total power distributed by resistive and PTC elements is P total = PPTC+ Pres- PPTCmeans the power output from the PTC element(s) Ihmeans the supply current RPTCmeans the resistance in the PTC element(s) Presmeans the power output from the resistive element(s) Rresmeans the resistance in the resistive element(s) Ptotalmeans the total power output The current is dependent on both Rresand RPTC. When VHV= Vres+ VPTC, the current is given by: Ih = VHV / (Rres+ RPTC)? VHVmeans the voltage delivered by the high voltage supply Vresmeans the voltage across the resistive element(s) VPTCmeans the voltage across the PTC element(s) Ih, Rresand RPTChave the same meaning as above.

It is clear from the equations that an equal amount of power is dissipated by the resistive element and the PTC element when RPTC= Rres. For the PTC, this is also the point of maximum power dissipation.

The choice of resistive and PTC elements is now governed by the following rules (RPTCin below equations is taken when the system is cold, i.e. well below the transition temperature): Rres+ RPTC= VHV2 / Ptotal PPTC,max<=>0,25 VHV2 / Rres Pres, max<=>VHV2 / ( Rres(1 RPTC/Rres)2) PPTC.maxmeans the maximum power that the arrangement of PTC element(s) will dissipate.

Pres, maxmeans the maximum power that the arrangement of resistive element(s) will dissipate.

In addition, in order to have the majority of the heating done by the resistive element: RPTC< Rres.

Example 1: Consider the following example: High voltage supply: VHV = 450 VDC Max. requested electrical power: Ptotal= 5000 W With these values it follows that: RPTC+ Rres= 450<2>/ 5000 = 40,5 Ohm Choose Rres= 30 Ohm, RPTC= 10 Ohm, then it follows that: PPTC.max= 0,25 . 450<2>/ 30 = 1687,5 Watt Pres, max= 3796,9 Watt The PTC element should thus be able to dissipate approximately 1700 Watt and (well below the transition temperature) it should have a resistance of 10 Ohm. The resistive element should be able to dissipate approximately 3800 Watt, and have a resistance of 30 Ohm.

Notice that resistance and power dissipation can be achieved by placing components in parallel or in series. For the above example, this means that a single resistive element of 3800 W/30 Ohm could be realized by an equivalent combination of four separate resistive heater elements, each with a resistance of 120 Ohm and a maximum power dissipation of 950 W, connected in parallel (see Fig. 5 a).

The total resistance Rresis given by Image available on "Original document" Alternatively, a single resistive element of 3800 W/30 Ohm could also be realized by an equivalent combination of two separate resistive heater elements, each with a resistance of 15 Ohm and a maximum power dissipation of 1900 W, connected in series (see Fig. 5 b).

The total resistance Rresis given by Image available on "Original document" The same principle applies to the PTC element, however it must be realized that due to tolerances in transition temperature and resistance, the maximum power dissipation of each separate PTC element must be chosen carefully.

The possibility to place heating elements (either resistive or PTC) in series or in parallel thus serves two purposes.

Firstly, when the characteristics of a single element does not fulfill the requirements for resistance or maximum power dissipation, those requirements can be fulfilled by connecting two or more elements in a series and/or parallel configuration.

Secondly, by having the heating elements in a series and/or parallel configuration, there is a simple possibility to implement a varying maximum power output, by switching elements in or out. An example of this is given in Fig. 6a and b.

Example 2: As an example, consider the configuration in Fig. 6a. When we choose: Rres, l = 60 Ohm, Pmax, resl = 2500 W, Rres,2 = 60 Ohm, Pmax, res2 = 1900 W, RPTC = 10 Ohm, Pmax, PTC = 1700 W, VHV = 450 VDC, and the switch is closed, we have a heater with similar characteristics as the heater from example 1, with a total maximum power output is approximately 5000 W. Notice though that in this case the maximum power dissipation of resistive element number 1 is overdimensioned with respect to the total power that the heater can produce. The reason for this will become clear when we consider opening of the switch.

When we open the switch, it is apparent from the previous equations that the total maximum power output of the heater drops to approximately 2900 W, since the total resistance of the heater increases. At a temperature well below the transition temperature, the resistive element number 1, which is participating in the heating process, will deliver approximately 2500 W while the PTC element delivers the remaining 400 W.

In the example in Fig. 6 only two resistive elements in parallel or in series are considered. When more resistive elements are used, more switching options are possible and the maximum output power can be controlled in more steps.

The transition temperature is remaining to be chosen and will depend on what maximum temperature the liquid heat transport fluid is supposed to reach, as well as mechanical details that affect the heat transfer from the PTC element to the liquid, and heat losses from the heater to the environment.

Turning now to Fig. 4 an embodiment of a self-regulating heating system will be described.

The entire heating system, here generally designated 40 (components are given designations beginning with<*>4<*>), is enclosed in a compact housing H, suitably made of aluminium, although other materials may be usable. Inside the housing H there are two compartments 48, 49 in fluid communication (illustrated by a vertical arrow) and partially separated by a partition wall P.

There are also further spaces S within the housing H accommodating electrical components for power supply.

There is provided an inlet 4 1 to the first compartment 48 and an outlet 42 from the second compartment 49. The inlet 41 and outlet 42 are connected to a system (not shown) for circulating heating/ cooling medium. Flow direction is indicated by arrows In and OUT, respectively.

In the first compartment 48 there is provided at least one, preferably a plurality of resistive heating elements 43, in the shown embodiment there are three elements. In the embodiment shown in Fig. 4 these resistive elements 43 are coupled in parallel with each other. As indicated above they could also be coupled in series. In the second compartment at least one PTC element 44 is provided, in Fig. 4 only one PTC element is provided. If a plurality of PTC elements would be provided they could be connected in parallel as well as in series. At least one PTC element is suitably placed at or near the outlet (42). The resistive element(s) 43 and the PTC element(s) 44 are connected to a first circuit board CB1 inside the space S, via leak tight through connections, comprising sealing means 412 for leakage protection, i.e. to prevent liquid form entering into the space S where the electrical components are provided. The circuit board CB1 is coupled to a high voltage input 45 in the wall of the housing H by means of suitable cables. It is also coupled to a low voltage input 46, also in the wall of the housing H, via a further circuit board CB2, coupled to the first circuit board CB1 via suitable electrical cables. Sealing means 411 for preventing ingress of liquid or other contaminants into the space S are provided at these voltage inputs 45, 46.

The pattern on the first circuit board CB 1 is designed so as provide the required coupling between resistive and PTC element 12, 14, i.e. serial or parallel or possibly a combination of both.

A self-regulating heating circuit according the claims and the disclosure herein is suitable for use in electrical heaters for vehicles.

The self-regulating heating apparatus can be implemented in heaters for any kind of vehicle, such as cars, trucks, boats to mention a few.

Claims (11)

CLAIMS:

1. Self-regulating heating circuit comprising at least one resistive element (43) having a substantially temperature independent resistance (R) and at least one PTC element (44) having a temperature dependent resistance (RPTC), said elements (43, 44) being arranged in heat transfer relationship with a flowing medium of a heat circuit, wherein i) the characteristics of the PTC element(s) (44) are selected such that when the temperature of the flowing medium reaches a predetermined value, the resistance of the PTC element(s) (RPTC) has reached a level such that the voltage across the resistive element(s) drops and ii) the resistive and PTC elements (43, 44) are coupled to each other in a configuration such that the combined power delivered by the resistive element(s) and PTC element(s) (43, 44) is reduced when the resistance of the PTC element(s) (RPTC) has reached said level, wherein the resistive elements (43) and the PTC elements (44) are provided in separate compartments (48, 49) in a housing (H) , wherein the resistive elements (43) are provided in a first compartment (48) having an inlet (41) for heating medium wherein the PTC elements (44) are provided in a second compartment (49) having an outlet (42) for heated heating medium

2. Circuit as claimed in any preceding claim, wherein the PTC elements (44) if there are more than one PTC element are coupled in series or in parallel with each other.

3. Circuit as claimed in any preceding claim, wherein the resistive elements (43) if there are more than one the resistive element are coupled in series or in parallel with each other.

4. Circuit as claimed in any preceding claim, wherein the resistive elements (43) as a unit is coupled in series with the PTC elements (44) as a unit.

5. Circuit as claimed in any preceding claim, wherein at least one PTC element is placed at or near the outlet (42).

6. Circuit as claimed in any preceding claim, wherein the power produced by the resistive and PTC heating elements is given by the following equations: PPTC- Ih<2>RPTC Pres = Ih<2>Rres and the total power distributed by resistive and PTC elements is Ptotal- PPTC+ Pres· wherein PPTCmeans the power output from the resistive element(s) Ih means the supply current RPTCmeans the resistance in the PTC element(s) (taken when the system means cold, i.e. well below the transition temperature) PPTCmeans the power output from the PTC element(s) Rresmeans the resistance in the resistive element(s) Ptotalmeans the total power output

7. Circuit as claimed in any preceding claim, wherein the current is dependent on both Rresand RPTC, according to the equations: VHV= Vres+ VPTC, the current is given by: Ih= VHV/ (Rres+ RPTC) wherein VHV means the voltage delivered by the high voltage supply Vresmeans the voltage across the resistive element(s) VPTCmeans the voltage across the PTC element(s) Ih,Rresand RPTChave the same meaning as above.

8. Circuit as claimed in any preceding claim, wherein properties of the resistive and PTC elements are governed by the following rules: Rres+ RPTC= VHV2 / Ptotal PPTC.max<=>0,25 VHV2 / Rres Pres, max= VHV2 / ( Rres( 1 RPTC/Rres)2 ) wherein PPTC.maxmeans the maximum power that the arrangement of PTC element(s) will dissipate. Pres, max means the maximum power that the arrangement of resistive element(s) will dissipate. and VHV, Vres, VPTC, Ih, Rresand RPTChave the same meaning as above.

9. Circuit as claimed in any preceding claim, wherein, RPTC< Rres whereby the majority of the heating is done by the resistive element.

10. Electrical heater for vehicles, comprising a self- regulating heating circuit according to any of claims 1 to 9.

11. Vehicle comprising an electrical heater according to claim 10.