WO2018041787A1

WO2018041787A1 - Electric heater and method for detecting overheating in such an electric heater

Info

Publication number: WO2018041787A1
Application number: PCT/EP2017/071567
Authority: WO
Inventors: Alexander Krämer; Rainer Heck
Original assignee: Dbk David + Baader Gmbh
Priority date: 2016-08-30
Filing date: 2017-08-28
Publication date: 2018-03-08
Also published as: EP3508029B1; DE102016216295A1; CN109644525A; CN109644525B; EP3508029A1

Abstract

The invention relates to an electric heater (1) for heating fluid streams with a heating element (2) and a control device (3) for controlling a heat output produced by the heating element (2), wherein the heating element (2) has an inductance (L_RHK) that changes on the basis of the temperature, and wherein an overheating detection device (4) is provided that is configured to sense a change in the current (Iheat) flowing through the heating element (2) caused by a change in the inductance (L_RHK) and to compare said sensed change with a previously defined limit value and to determine overheating in the electric heater (1) in the event of the limit value being exceeded. The invention further relates to a method for detecting overheating in such an electric heater (1).

Description

Electric heater and method for detecting overheating

The present invention relates to an electric heater and a

Method for detecting overheating of such an electric heater.

Such heaters can be used, for example, in motor vehicles for heating (heating) the room air in the passenger compartment and for heating the battery, for preheating the cooling water of water-cooled engines, for preheating the spark plugs in auto-ignition internal combustion engines, for heating fuel, for thawing operating fluids such as or

Headlight cleaning fluid and the urea solution of an SCR catalyst, etc. are used. Furthermore, such heaters can be used in so-called white goods such as a tumble dryer or a washing machine.

Especially in modern vehicles, such as electrically powered vehicles, hybrid vehicles or fuel cell vehicles, which use high electrical voltages in their electrical system, heating circuits must be heated due to lack of or only temporarily available heat sources, such as an internal combustion engine. Typically, the heating circuits are water circuits or a

Water circulation system.

Since the engine heat in such modern vehicles is not or only to a limited extent available as a heat supplier, electrical heaters are typically used which a PTC resistance wire as

Have heating element (see, for example, DE 10 2014 108 074 A1). However, the startup behavior of PTC resistance wires at low temperatures is often characterized by undesirably high current peaks, which may be a factor of 3.5 above the value corresponding to the temperature according to the characteristic curve. Furthermore, the maximum allowable operating point is usually at 600 ° C so that the average operating point should not exceed about 400 ° C so as not to excessively shorten the life of the PTC resistance wire. As a rule, only heating wires made of special material (such as Nifethal®, a nickel-iron alloy) can be used as PTC resistance wires.

It is an object of the present invention to provide an electric heater with a long service life, which has a high functional

Security provides and in which in particular undesirable tarnishing behavior is avoided or reduced. It is a further object of the invention to provide a method for detecting overheating of such an electric heater.

This object is achieved by an electric heater for heating fluid streams and a method for detecting overheating of such an electric heater having the features of the independent claims.

The heater according to the invention for heating fluid streams, in particular of liquid such as water or water-glycol mixtures, has a heating element, a control device for controlling a heat output generated by the heating element and a

Overheat detection device on. The heating element has an inductance which varies as a function of the temperature. Preferably, the inductance increases up to a limit temperature. The

Overheat detection means is configured to detect a change in the current flowing through the heating element caused by a change in the inductance of the heating element, with one in advance

the defined / specified limit value is exceeded and if the limit value is exceeded

Limit can detect overheating of the electric heater or its heating element by changing the current flowing through the heating element current. Overheating can occur in particular when the electric heater is running dry, ie, for example, in the event of a fault, the fluid does not flow around it. By recognizing overheating can thus also a Dry run can be detected.

Under the detection of a change of the inductance of the

Heating element caused change of the current flowing through the heating element is also understood as the detection of a physical quantity from which this current or its change can be derived, in particular a physical quantity which is proportional to the current or its change. For example, when the current flowing through the heating element is measured by means of a shunt resistor connected in series with the heating element, the physical quantity is the voltage across the shunt resistor which is proportional to the current flowing through the heating element.

It can be provided more than one heating element, and the

Control device may be designed to control the heat output generated by more than one heating element. Accordingly, the

Overheat detection means configured to detect a change in the respective inductances of a plurality of heating elements caused changes in the currents flowing through the plurality of heating elements and to evaluate them, as explained above for a heating element by limiting value comparison with respect to overheating.

In addition to the temperature varying inductance, the

Heating element have a resistance that can also change with temperature.

The heating element preferably comprises a material whose relative permeability increases up to a limit temperature. The increase of the relative permeability of the heating element with increasing temperature results in an increase of the inductance of the heating element with increasing temperature. Preferably, the heating element consists of such a material.

Particularly preferably, the heating element comprises a ferromagnetic material. Ferromagnetic material is characterized by a particularly high relative permeability of μτ »1. Preferably, the heating element is made such a ferromagnetic material. As ferromagnetic material are, for example, iron, nickel and / or cobalt in question. Likewise, alloys comprising iron, nickel and / or cobalt are suitable with a relative permeability of pr »1. In particular, an alloy of iron, chromium and aluminum can be used.

The electric heater according to the invention is preferably as

Pipe radiator configured, which comprises the heating element. The heating element is preferably designed as a heating wire. Heating wires made of the above-mentioned materials, in particular alloys of iron, chromium and aluminum, are advantageously standard heating wires, which are freely available on the market.

Heating elements of the above materials have

advantageously substantially no undesired tarnishing behavior, i. No unwanted current peaks during start-up / heating from the cold state. If the heater according to the invention, for example, supplied with a voltage from a vehicle electrical system of a vehicle, therefore, a Bordnetzwelligkeit when starting the electric heater in comparison to PTC resistance heating wires using heaters can be reduced and thus improved. Furthermore, higher operating points can be achieved than with the use of PTC resistance heating wires. Thus, for example, when using a heating wire made of an alloy of iron, chromium and aluminum as a heating element, a maximum operating point of 1350 ° C can be achieved, resulting in a higher power density and a longer life. Furthermore, heating elements made of the materials mentioned by a minimum thermal capacity and inertia, which allows rapid detection of overheating.

The overheating detection device of the electric heater according to the invention, the functional safety of the electric heater can be ensured. By comparison with a predefined / established limit value, the overheat detection device determines whether the values determined by the Temperature-induced change in the inductance caused change in the current of the heating element exceeds a permissible value, so that overheating of the electric heater is detected and corresponding

Measures such as switching off the electric heater can be initiated.

Advantageously, in the case of the electrical heater according to the invention, the sensor element corresponds to the heating element. That The heating element is not only used for heating, but also for detecting overheating. A component, namely the heating element, can thus be used for two different purposes.

The overheating detecting means of the electric heater according to the invention preferably comprises a current measuring means for measuring the current flowing through the heating element, a differentiator for detecting a change in the current flowing through the heating element, a peak detector for detecting a maximum change in the current through the heating element

Heating element flowing current and a comparing means for comparing the maximum change of the current flowing through the heating element with the pre-defined limit. The overheating detection device is furthermore preferably designed such that it switches off the electric heater when initiating overheating or initiates a shutdown. For this purpose, the

Comparing means send a signal to the controller of the electric heater, which causes the control device to turn off the electric heater. As described above, the current flowing through the heating element is also understood to mean a physical quantity, from which this current can be derived, in particular one which is proportional to this current

physical quantity such as a voltage drop across a shunt resistor connected in series with the heating element. The

Overheating detection means may further comprise an amplifier.

In the inventive method for detecting overheating of an electric heater according to the invention is one of a change of Inductance of the heating element caused change in the current flowing through the heating element determined, compared the determined change in the current flowing through the heating element with a pre-defined limit and found when exceeding the predefined limit overheating. Preferably, upon detection of overheating of the electric heater is turned off via the control device.

In the method according to the invention, preferably, the current flowing through the heating element is measured, for determining the change of the current flowing through the heating element, the derivative thereof is formed, wherein the

Derivative represents the change in the current, the peak value of the derivative of the current flowing through the heating element determined and the determined

Peak value compared to the pre-defined limit. As explained above, the current flowing through the heating element is also understood to be a physical quantity from which this current can be derived.

Further advantageous embodiments of the invention will become apparent from the

Subclaims and from the embodiments illustrated below with reference to the drawings. Show it:

1 shows an exemplary course of an inductance of a

electric heater according to the invention or of the heating element in

Depending on the temperature,

Fig. 2 is a schematic diagram of an inventive

electric heater,

Fig. 3 is a schematic representation of a

Overheating detection device of an electric heater according to the invention (Fig. 3a)), schematic curves of the in the

Overheat detection device occurring physical quantities over time in case of overheating (Figure 3b)) and schematic curves of the same occurring in the superheat detection device physical quantities as in Fig. 3b) in the normal state (Fig. 3c)) and

4 shows a flowchart of a method according to the invention for Detecting overheating of an electric heater according to the invention. Dimensions given in the figures are purely exemplary in nature.

The electric heater 1 according to the invention, as shown in FIG. 2 described below, is preferably designed as a water heater, as e.g. is provided in the circulation of an operating fluid, such as cooling water, in a vehicle. For this purpose, the electric heater 1 is preferably designed as a tubular heating element (RHK) or a cylindrical heating element with a wire-shaped heating element 2 (cf., FIG. 2). A corresponding water heater is known, for example, from DE 10 2010 060 446 A1 of the Applicant, so that with regard to the specific construction of the water heater, reference may be made to the statements in this document. For the understanding of the present invention, however, the concrete structure plays only one

subordinate role, since different types of heaters can be designed and operated according to the invention.

The electrical heater 1 is preferably supplied with voltage from a vehicle electrical system of a vehicle, the vehicle electrical system voltage in particular being a high-voltage voltage used in a vehicle, which is typically between approximately 120 V and approximately 450 V. The heating element 2 (and thus the electric heater 1) has an inductance L (also called inductance L_RHK), which increases with increasing temperature.

Figure 1 shows an example of a course of a temperature-dependent inductance of the electric heater 1 designed as a tubular heater over the temperature. As shown in Figure 1, the inductance increases up to one

Limit temperature too. The working range of the electric heater 1 according to the invention is below this limit temperature of the heating element. 2

Figure 2 shows an embodiment of the electric heater 1 according to the invention. The electric heater 1 has a heating element 2 which has an inductance L RHK and a resistance R RHK connected in series. The inductance L_RHK is temperature dependent and increases with increasing temperature on. For this purpose, the heating element 2 is preferably made of ferromagnetic material having a relative permeability μτ ».

The heating element 2 is connected to a supply voltage U_HV, e.g. A voltage of a vehicle electrical system or a battery of a vehicle, and to the other terminal via a switch SW1 to ground (in Figure 2: reference potential "0") .The series circuit of inductance L_RHK and resistor R_RHK is preferably a diode D1 connected in parallel The cathode of the diode D1 is connected to the

Supply voltage U_HV connected.

The switch SW1 is formed, for example, as a transistor circuit, and its state is controlled by the control device 3. The switch SW1 may be part of the control device 3. Is the switch SW1 in

closed state, so current (heating current Ι ΗΘΙΖ) flows through the heating element 2 and the heating element 2 generates heat. If the switch SW1 is in the open state, then no current flows through the heating element 2. The ratio between the time duration of the closed state of the switch SW1 and the duration of the open state of the switch SW1 thus the heat generated by the heating element 2 by the control device 3 are controlled. Accordingly, the control device 3 is preferably designed as power electronics in the form of a pulse width modulation (PWM) circuit, which at preferably constant frequency or period duration, the duty cycle of a rectangular pulse, i. controls the width of the square pulse forming pulses, the square pulse forms the input and control signal for the switch SW1.

The electric heater 1 according to the invention further comprises a

Overheat detection device 4 for detecting overheating of the electric heater 1 or its heating element 2 (see Figures 2 and 3a)). The overheat detection device 4 preferably comprises a

Current measuring device 5, a differentiator 6, a peak detector 8 and a comparator 9. Further, an amplifier 7 may be provided, the is preferably connected between the differentiator 6 and the peak detector 8.

The current measuring device 5 measures a current flowing through the heating element 2. The current measuring device 5 is preferably formed by a shunt resistor R_Shunt (Rshunt in Figure 3a)), which is connected in series with the heating element 2 and is connected between the switch SW1 and the ground 0. The current flowing through the shunt resistor R_Shunt causes at this a voltage drop (voltage Ushunt in FIG. 3), which is proportional to the current flowing through the heating element 2. That the voltage Ushunt dropped across the shunt resistor R_Shunt is proportional to the current Ι ΗΘΙΖ flowing through the heating element 2. Changes

Due to the temperature, the inductance L_RHK of the heating element 2, so changes according to the current flowing through the heating element 2 i heating and thus the voltage across the shunt resistor R_Shunt voltage Ushunt.

In the case of a pulse-width-modulated current flow through the heating element 2, the edge steepness of the current pulses of the current IHeiz is dependent on the size of the inductance L_RHK of the heating element 2, which in turn depends on the temperature. Accordingly, the slope is the of the

Current measuring device 5 detected pulses of the voltage Ushunt on the size of the inductance L RHK of the heating element 2, since the voltage Ushunt is proportional to the current flowing through the heating element 2 ΙΗΘΙΖ. If the heater 1 according to the invention is supplied with an alternating voltage, such as

For example, when used in so-called white goods, so can be provided as a current measuring device 5 a measuring bridge circuit, which the phase difference or the zero crossing of the current through the

Heating element measures.

In the event of an error in the event of overheating, the inductance L_RHK of the

Heating element 2 is lower than normally without overheating, is

Accordingly, the edge steepness of the current I Heiz or the voltage Ushunt in the event of an error of overheating higher than normally, as in Figures 3b) and 3c) shown.

Downstream of the current measuring device 5 has the

Overheat detection device 4 on a differentiator 6. The

Differentiator 6 determines the change in the current iHeiz or the voltage Ushunt, which follows in particular from a temperature-dependent change in the inductance L_RHK of the heating element 2. Preferably, the differentiator 6 forms the derivative of the voltage Ushunt, which is proportional to the derivative of the current IHeiz and which is referred to in Figure 3 as UDitr. The differentiator 6 may be formed, for example, as a high-pass circuit with a capacitor C1 and a resistor R1, wherein a terminal of the capacitor C1 to an unspecified node between the switch SW1 and the shunt resistor R_Shunt and the other terminal of the capacitor C1 to the resistor R1 is connected, which in turn is connected to the other terminal to ground. The output voltage UDiff of

Differentiator 6 is higher in magnitude in the event of a fault overheating than in the normal case (see Figures 3b) and 3c)) because of the higher edge steepness of the voltage Ushunt.

In particular, in order to achieve a better signal resolution, the differentiator an amplifier 7 is connected downstream, which by a

Operational amplifier 10 can be realized, which is operated with a supply voltage VCC. The operational amplifier 10 is preferably as

noninverting amplifier, at whose noninverting input the output of the differentiator 6, i. the tension Uratf, is applied. To the output of the operational amplifier 10, a voltage divider is connected to the series-connected resistors R2 and R3, wherein the inverting input of the operational amplifier 10 is connected between the resistors R2 and R3. The output voltage Uveret (U1A in FIG. 2) of the

Operational amplifier 10 (and the amplifier 7) corresponds to the amplified output voltage UDiff of the differentiator 6, wherein the output voltage Uverst of the amplifier 7 in the event of an error of overheating in terms of magnitude higher in the normal case (see Figures 3b) and 3c)). The amplifier 7 can be so

be configured such that it outputs as output voltage Uverst only the positive pulse of a period of the output signal UDIIT of the differentiator 6.

The amplifier 7 is followed by a peak detector 8, which determines the maximum change, in particular the maximum derivative, of the current iHeiz flowing through the heating element 2 during operation.

In particular, the peak detector 8 determines the maximum value of

Output voltage Uverst of the amplifier 7, i. the peak value of the voltage Uverst. The peak detector 8 has the output voltage Uverst as

Input signal. The peak detector 8 has a diode D2 for rectifying the output voltage Uverst of the amplifier 7, wherein the anode of the diode D2 is connected to the output of the amplifier 7 and the cathode of the diode D1 to a terminal of a resistor R4 of the peak detector 8. The other terminal of the resistor R4 is connected to a terminal of a

Condenser C2 of the peak detector 8 is connected, whose other terminal is grounded. The voltage U peak falling across the capacitor C2 corresponds to the peak value of the voltage Uverst after one period of the voltage Uverst.

In the event of an overheating fault, the output voltage Uverst of the amplifier 7 (ie the amplified output voltage UDOT of the differentiator 6), which corresponds to the increased edge steepness of the current iHeiz flowing through the heating element 2, is greater than the output voltage Uverst of the amplifier 7 in the normal case. Accordingly, the output voltage is US _P it _ze of

Peak detector 8 at an overheating greater than normal (see Figures 3b) and 3c)).

The peak detector 8 is a comparator 9

downstream, which is preferably formed by a microcontroller 11, which comprises a working and program memory (not shown, hereinafter referred to as: memory). The microcontroller 11 is connected between the resistor R4 and the capacitor C2 of the peak detector 8. In the store a predefined limit value for overheating detection is preferably stored, with which the output voltage U peak of the peak value detector 8 is compared.

The comparison device 9 or its microcontroller 11 leads

to a signal evaluation of the output signal U peak of

Peak detector 8 by that it compares the peak voltage U peak with the pre-defined limit (in Figure 3: error threshold). The comparison can be carried out by a stored in the memory of the microcontroller 11 program. The limit value is defined in advance such that a

Exceeding the limit value means overheating of the electric heater 1 or of its heating element 2. If the peak value Uspitze exceeds the predefined limit value, then the comparison device 9 (and thus the overheating detection device 4) determines overheating of the electrical heater 1. If the peak value U peak is below the limit value or if the peak value U peak is equal to the limit value, then the comparison device 9 determines that the electrical heater 1 is in the normal state and there is no overheating. In the case of overheating controls the comparator 9 and their

Microcontroller 11, the control device 3 of the electric heater 1 preferably in such a way that it opens the switch SW1 and thus the power supply of the heating element 2 interrupts to ensure the functional safety.

FIG. 4 shows a flow chart of a preferred embodiment of the inventive method for detecting overheating of an electric heater 1 according to the invention, as shown by way of example in FIG. 2

is shown. In step 20, the current flowing through the heating element 2 I Heiz or a physical quantity from which this current is derivable, such as the voltage Ushunt on the shunt resistor R_Shunt (also: Rs unt) of

Current measuring device 5, by means of the current measuring device 5 of

Overheat detection device 4 measured.

In the subsequent to step 20 step 21 is for determining a change, in particular a temperature-induced change, by the heating element 2 flowing current i heating the derivative of the current iHeiz determined by means of a differentiator 6 of the overheat detection device 4. In the subsequent step 22, the determined change in the current IHeiz is amplified by an amplifier 7 of the overheating detection device 4.

In the following step 23, the peak value of the derivative of the current IHeiz determined in step 21 and amplified in step 22 is determined by means of a

Peak value detector 8 of the overheating detection device 4 determined. In step 24, the determined peak value is compared with the predefined limit value with the aid of the comparison device 9 of the overheating detection device 4. If the peak exceeds the limit, then the

Comparative device 9 and the overheat detection device 4 in step 24 determines that there is an overheating of the electric heater 1.

In the subsequent step 25 transmits the

Overheat detection device 4 of the control device 3 of the electric heater 1, that there is an overheating, whereupon the control device 3 opens the switch SW1 of the electric heater 1, thus interrupting the power supply of the heating element 2.

LIST OF REFERENCES

1 electric heater

2 heating element

3 control device

4 overheating detection device

5 measuring device

6 differentiators

7 amplifiers

8 peak detector

9 comparison device

10 operational amplifiers

11 microcontroller

20, 21, 22, 23, 24, 25 method steps

C1, C2 capacitors

D1, D2 diodes

iHeiz current flowing through the heating element

L_RHK inductance of the heating element

R1, R2, R3, R4 resistors

R_RHK resistance of the heating element

R_Shunt, Rshunt shunt resistor

SW1 switch

t time

U_HV, VCC supply voltages

Ushunt Output voltage of the measuring device UDiff Output voltage of the differentiator Uverst Output voltage of the amplifier

U peak output voltage of the peak detector

Claims

claims

1 . Electric heater for heating fluid flows with a heating element (2) and a control device (3) for controlling a heat output generated by the heating element (2), characterized in that the heating element (2) has an inductance (L_RHK) which varies depending on changes from the temperature, and that an overheat detection device (4) is provided which is configured to detect a change of the inductance (L_RHK) caused by the change of the heating element (2) flowing current (Ι) and with a pre-defined limit to compare and to determine if the limit is exceeded, overheating of the electric heater (1).

2. An electric heater according to claim 1, wherein the inductance (L_RHK) of the heating element (2) increases up to a limit temperature.

3. An electric heater according to claim 1 or 2, wherein the heating element (2) comprises a material whose relative permeability increases up to a limit temperature.

4. An electric heater according to claim 3, wherein the heating element (2) comprises a ferromagnetic material.

5. Electric heater according to claim 4, wherein the heating element (2) comprises as material iron, nickel or cobalt or an iron, nickel and / or cobalt-containing alloy.

The electric heater according to any one of the preceding claims, wherein the overheat detection means (4) comprises a current measuring means (5), a differentiator (6) for detecting a change in the current (IHeiz) flowing through the heating element (2), a peak detector (8) for Determining a maximum change of the current flowing through the heating element (2) and a comparison device (9) for comparing the maximum change of the current (IHBIZ) flowing through the heating element (2) with the predefined limit value.

7. A method for detecting overheating of an electric heater according to one of the preceding claims, characterized by the following steps:

Determining a change in the inductance (L_RHK) of the heating element (2) of the electric heater (1) caused by the heating element (2) current flowing (IHeiz),

- Comparing the determined change of the heating element (2) flowing current (ΙΗΘΙΖ) with a pre-defined limit and

- Detecting overheating of the electric heater (1) when the predefined limit value is exceeded.

8. The method of claim 7, wherein

upon detection of overheating of the electric heater (1), the electric heater (1) is turned off.

9. The method of claim 7 or 8, wherein

the current flowing through the heating element (2) (IHBIZ) is measured,

- To determine the change of the heating element (2) flowing Current (ΙΗΘΙΖ) a derivative of the current flowing through the heating element (2) current (IHeiz) is formed,

- The peak value of the derivative of the current flowing through the heating element (2) current (IHeiz) is determined and

- the determined peak value is compared with the predefined limit value.