DE2837934A1 - Electric water flow heater control system - has heating coil divided into three sections, with two sections temp. controlled - Google Patents

Abstract

The system for controlling the output temperature of a continuous electrically heated water supply system with a loading up to 22 kilowatts, has the eating coil divided into three sections. The main section has the heaviest load, the next section is smaller, and the last section is smaller still. The main section is not continuously switched off and on, in response to the water discharge temperature. This is switched in response to the flow of water. The other two sections are switched in response to the water discharge temperature, operated by separate temperature sensors. The switching frequency of the controller for the final section is higher than that of the controller for the intermediate section. The controllers operate through triacs with a zero voltage switch.

Description

Device for regulating the outlet temperature at

electric water heaters The invention relates to a device for regulating the outlet temperature in electric instantaneous water heaters, in which the heating power from a temperature sensor arranged in the outlet by means of a Controller is controllable.

The regulation of the outlet temperature for electric water heaters offers particular difficulties. Electric water heaters have a very high installed heating capacity of, for example, 22 kilowatts. The outlet temperature must be adhered to very precisely and may only be slightly adjusted to the desired temperature vary. It is difficult to regulate the heating power accordingly sensitively.

The electrical heating power can be adjusted using a phase control regulate continuously. A phase control of the high power here however, is because of the harmonics generated and thereby evoked Interferences not allowed. It is only permissible to switch the alternating current fully on or off, whereby a zero voltage switch ensures that the switch-on and the switch-off takes place at times when the alternating voltage goes through zero anyway, so that there is no phase control.

A regulation of the heating power by switching on and off requires but a relatively high switching frequency if the requirement is to be met, that a desired outlet temperature is maintained with high accuracy. if the heating power would be switched on and off with a low switching frequency, then a water heater would alternately run out of cold or too hot water, even if the average heating power corresponds to the desired temperature.

A high switching frequency for switching a high heating output on and off however leads to other difficulties: Affected by the line impedance the high heating output of the electric water heater the mains voltage. One high switching frequency would therefore cause an unpleasant flicker of electric light bulbs entail. The switched heating power approved by the utility company is therefore the smaller, the higher the switching frequency. These demands stand against the effort to reduce the outlet temperature of a high-performance instantaneous water heater to regulate with high accuracy.

Accordingly, the object of the invention is to provide a device to regulate the outlet temperature in electric water heaters, which also applies to water heaters with a high output of, for example, 22 kilowatts allows precise regulation of the outlet temperature.

According to the invention this object is achieved in that the installed Heating power of the water heater is divided into at least two stages and only one of the levels is switched for temperature control.

According to the invention is thus a solid on the flow heater Heating power switched, which is not influenced by the control cycle of the temperature controller is. The temperature control takes place through a stage of the heating power, which is essential can be smaller than the total heating power of the water heater. The minor one The heating output of this level can then be set with a correspondingly high level in the control cycle Frequency can be switched so that a desired outlet temperature can be precisely maintained is guaranteed.

The invention can be practiced in various ways.

Further refinements of the invention are the subject of the subclaims.

The invention is based on a few exemplary embodiments below With reference to the accompanying drawings explained in more detail: Fig. 1 shows a first Embodiment of the invention with a permanently switchable level of heating power and two stages, each switched by a controller.

Fig. 2 shows a second embodiment with a water flow Permanently switchable level of heating power, one from the controller for temperature control switched stage and two further stages that depend on the controller output signal are permanently switchable.

Fig. 3 shows in detail the actuator in the embodiment according to Fig. 2.

Fig. 4 shows a third embodiment of the invention in which one level that can be permanently switched on when the water flows through, and one from the controller for temperature control switched stage is realized by a heating coil arrangement each while one third, middle heating coil arrangement depending on the output signal of the controller one of two heating powers is applied.

FIG. 5 shows the actuator in the embodiment according to FIG. 4.

Fig. 6 shows the logic circuit in the embodiment of Fig. 5.

Fig. 7 shows a fourth embodiment of the invention in which a heating coil arrangement, both the "control stage" and one or more "additional stages" represents.

FIG. 8 shows the actuator in the embodiment according to FIG. 7.

Fig. 9 shows wave trains of the alternating voltage as they are in the embodiment 7 and 8 are placed on the heating coil.

For the sake of simplicity of illustration, it is shown in the figures as alternating voltage each shows a phase that is given to heating coil arrangements, which are also each represented by a single heating coil. In practice the electric water heater is operated with three-phase current, with each heating coil arrangement of at least three heating coils acted upon by the different phases of the three-phase current consists.

In Fig. 1, 10 is the flow channel of an electrical water heater which leads from a cold water connection 12 to an outlet 14. At the outlet a nozzle 16 is arranged. In the flow channel 10 are connected in series in terms of flow a heating coil assembly 18, a Heating coil assembly 26 and a Heating coil arrangement 22 is provided. The heating coils can be those indicated in FIG Have heating capacities and 10 kilowatts, 7.8 kilowatts and 4.2 kilowatts, so the total heating output of the water heater is 22 kilowatts. The heating coil arrangement 18 represents the first stage of the heating output, which is applied by one to the water flow responsive switch 24 with the switch contact 26 is turned on as soon as the Water flow exceeds a specified value.

Downstream of the representing a second stage of the heating power A first temperature sensor 28 is arranged in the heating coil arrangement 20 in the flow channel 10. The temperature sensor 28 supplies an actual temperature value for a PID controller 30. A temperature setpoint is provided by a potentiometer 32.

The PID controller 30 switches via a triac with zero voltage switch 34 the second stage of heating power.

A second temperature sensor 36 is arranged in the outlet 14.

It supplies an actual temperature value for a switching PID controller 38. The PID controller 38 likewise receives a setpoint value from the potentiometer 32.

The difference between the mean temperatures is still on the PID controller 30 switched on, which are measured by the temperature sensors 36 and 28.

The PID controller 30 is designed to operate with a switching frequency of about 10 operations per minute works. The PID controller 38, on the other hand, is designed in such a way that that it delivers about 100 cycles per minute.

The heating coil arrangement 20 creates a second, relatively large step the heating power switched with a relatively low switching frequency. The switching of the heating output of 7.8 kilowatts with 10 switching operations per minute is permissible because of the low switching frequency. With this second stage one takes place "Pre-regulation" of the temperature. The minor one Output of 4.2 kilowatts is switched with a high switching frequency. You just need the temperature fluctuations out to regulate, which is after the pre-regulation by the controller 30 and the second Level still surrendered. Because of the low heating output of the third stage, this be switched with a high switching frequency, so that in the outlet 14 a substantially uniform temperature, which with high accuracy a desired, am Potentiometer 32 corresponds to the setpoint set.

In Fig. 2, 42 is the flow channel of an electrical flow heater denotes, which again runs from a cold water connection 44 to an outlet 46, on which a nozzle 48 is provided. The heating power is in this embodiment divided into four levels. A first stage, represented by a heating coil arrangement 50 with a heating power of 10 kilowatts is about one on the water flow through the flow channel 42 responsive switch 52 with switching contact 54 fixed can be switched on as soon as a predetermined water flow through the flow channel 42 after opening the valve 48 is reached. A second and a third stage in the outlet 46 of 3.9 kilowatts each are represented by heating coil arrangements 56 and 58, via an actuator 60 of a continuous PID controller 62 in yet to be described Way are permanently switchable. A fourth stage of the heating output is through an outlet side arranged heating coil arrangement 64 represents the temperature control by the actuator 60 can be switched on and off.

A temperature sensor 66 is arranged in the outlet 46, which provides an actual temperature value for the PID controller 62 delivers. A temperature setpoint is provided by a potentiometer 68 specified.

The actuator 60 is shown in detail in FIG. On one Input 70 of actuator 60 is the output signal of the PID controller. This output signal is connected in parallel to the inputs of three comparators 72,74,76. At the comparator 72 is a first reference signal 78. A second Reference signal is at an input 80 of the second comparator 74. From the output of the comparator 72 is a switching stage 82 for switching on the second stage of the heating power (heating coil 56) controllable. From the output of the comparator 74 is a switching stage 84 for control the third stage of the heating power (heating coil 58) controllable. As a reference voltage the signal of a triangle generator 86 is used at the third comparator 76. The reference voltage The output signals of the comparators 72 are also output via lines 88 and 90 and 74 superimposed.

The output of the comparator 76 controls one zero voltage switch each 92,94,96 and one triac 98,100,102 each, the three phases R, S, T of the three-phase current for the heating coil arrangement 64.

The arrangement described works as follows: The triangle generator 86 in conjunction with the comparator 76 forms a pulse width moulator. The output signal of the continuous PID controller 62 is converted into a continuously variable pulse width. Ever the ratio between the switch-on time and the switch-off time is determined by the level of the signal 70 the heating output of the fourth stage changes and thus the average heating output, which is emitted by the heating coil arrangement 64. The switching frequency is through the triangle generator 86 is fixed and can be relatively high, for example 100 switchings per minute. When the output of the PID controller 62 a limit value predetermined by the reference voltage at the input 78 of the comparator 72 exceeds, which indicates that the heating power supplied by the heating coil arrangement 64 is not sufficient to produce the desired outlet temperature, then the Switching stage 82 the second stage of the heating power with the heating coil 56 switched on.

This switched-on heating power is taken into account at the same time, that via line 88 the output signal of the comparator 72 corresponds to the reference signal of the Triangle generator 86 is superimposed. The fourth level of heating power with Heating coil arrangement 64 then only needs to raise the remaining amount of heating power required to achieve it of the temperature setpoint in outlet 46 is required. The connection is also sufficient the heating coil assembly 56 to achieve the temperature setpoint is not off, then If the output signal of the PID controller 62 also exceeds the reference signal at the input 80 of the comparator 84, whereby the third stage of the Heating power is switched on via the heating coil arrangement 58. This connection too is generated via line 90 by a corresponding increase in the reference signal at the comparator 76 taken into account.

In the embodiment of FIG. 4 are in a flow channel 104 of the water heater, which again from a cold water connection 106 to a Runs outlet 108 and in the outlet of which a nozzle 110 is arranged, three Heating coil arrangements 112, 114, 116 arranged one behind the other in terms of flow. The heating coil arrangement 112 again represents a basic level of heating power, that of a switch responsive to the flow through the passageway 104 118 with switching contact 120 is firmly switched on as soon as the flow through the Passage channel 104 exceeds a predetermined value. The heating coil arrangement 114 represents, in a manner to be explained, a second and third stage of the Heating output that can be switched on when the fourth stage with the heating coil arrangement 116 taking place temperature control is not sufficient to at the outlet 108 the desired To achieve outlet temperature.

In the outlet 108, a temperature sensor 122 is arranged, the one Provides actual temperature value for a continuous PID controller 124. The temperature setpoint of the PID controller 124 is again specified by a potentiometer 126. The output signal of the PID controller 124 is applied to an input 126 of an actuator 128, which switches on the heating power to the heating coil arrangement 114 and 116.

The actuator 128 is shown in FIG.

The actuator 128 again has three comparators 130, 132 and 134 to which the output signal appearing at the output 126 of the PID controller 124 in parallel is fed. Comparators 130 and 132 receive inputs 136 and 138, respectively graded reference signals. This serves as the reference signal for the comparator 134 Output of a triangle generator 140. As in the embodiment of FIG the outputs of comparators 130 and 132 on lines 142 and 144, respectively Output signal of triangle generator 144 as reference voltages for the comparator 134 superimposed. The output of the comparator 134 controls similarly to the embodiment 3 via zero voltage switches 146,148 and 150 and triac 152,154, 156 the three phases R, S, T, which are fed to the heating coil arrangement 116.

The output signals of the comparators 130 and 132 are each at one Input B or C of a logic circuit 158. At an input A of the logic circuit 158 is a square wave voltage, which is obtained by the fact that the line voltage converted into a square wave voltage by a switching stage 160 and by a binary divider 162 is converted into a square wave voltage of half the line frequency. This latter Square-wave voltage is applied to input A of logic circuit 158.

The output of logic circuit 158 controls zero voltage detectors 160,162 and 164 and triacs 166,168, 170 supply power to the heating coil assembly 114.

The logic circuit is shown in detail in FIG. The two Inputs A and B 'are common to an AND element 172. The output of the AND element 172 is at one input of an OR gate 174. The second input of the OR gate 174 is formed by the input C of the logic circuit 158. The output of the OR gate 174 forms the output of the logic circuit, which is sent to zero voltage switch 160, 162 and 164 is present.

The arrangement described works as follows: Via the heating coil arrangement 116 becomes a heating power in the manner already described in connection with FIG. 3 of 4.2 kilowatts with a pulse width ratio on and off that directly is proportional to the output signal of the controller 124. The switching frequency is again 100 switchings per minute. If by the heating coil arrangement 116 supplied heating power, the temperature setpoint in outlet 108 cannot be reached can, then output signals occur at the controller 124, which either only the comparator 130 or both the comparator 130 and the comparator 132 to respond. If neither of the comparators 130 and 132 responds, then the logic circuit switches No heating power to the heating coil arrangement 114. Only the comparator 130 speaks on, then every second wave train of the alternating voltage is sent to the heating coil arrangement 114 let through. When both comparators 130 and 132 respond, the full AC voltage is applied to the heating coil arrangement 114. The heating power of the heating coil arrangement 114 is therefore between the maximum heating output of 7.8 kilowatts, half the heating output of 3.9 kilowatts and the heating output 0 switchable, depending on the response of the Comparators 130 and 132.

At the input A is a square wave voltage with a frequency that corresponds to half the network frequency. Are the signals at inputs B and C both 0, then the output of the AND element 172 is also in the 0 state, as is the OR element 174 supplies the output signal 0. No triac 166,168,170 is then passed through. If only the output signal of the comparator 130 is "L", that is to say logical 1, then appears at the output of the AND element 172 a square-wave voltage of half the mains frequency. One period of the alternating voltage is thus allowed to pass alternately in each case and the next period blocked if the comparator 132 responds as well. then lies to the Input C constantly the signal "L", so that the OR gate 174 constantly supplies the output signal "L". Then all the periods of alternating voltage switched through.

In the embodiment according to FIG. 7, a flow channel 176 connects a cold water connection 178 and an outlet 280 in which a nozzle 182 is arranged. A heating coil arrangement is arranged one behind the other in terms of flow in the flow channel 184 and a heating coil arrangement 186. The heating coil arrangement 184, which is a basic stage the heating power of 10 kilowatts is represented in the usual way when it is reached of a predetermined flow rate can be permanently switched on via a contact 188.

In the outlet 180, a temperature sensor 190 is arranged, the one Supplies the actual temperature value for a continuous PID controller 192. The temperature setpoint of the PID controller 192 is again supplied by a potentiometer 194. The output signal An actuator 198 is present at the output 196 of the PID controller.

The actuator 198 is shown in more detail in FIG.

The actuator 198 contains m comparators 200.1 ... 200.m-1 and 200.m. The output voltage at output 196 of PID controller 192 is parallel across all of them Comparators 200.1 to 200.m.

Stepped reference voltages are available at inputs 202.1 ... 202.m-1 of the Comparators 200.1 to 200.m-1. Is at the input 202.m of the comparator 200.m a triangle generator 204.

The reference voltage from the triangle generator 204 are via leads 206.1 to 206.m-1 are superimposed on the output signals of the comparators 200.1 to 200.m-1.

The outputs of the comparators 200.1 to 200.m are connected to a logic circuit 208 at. The logic circuit 208 also receives the m outputs of a binary counter 210 with m flip-flops. the Mains voltage is supplied in a rectifier and switching stage 212 rectified and converted to a square-wave voltage of twice the line frequency implemented. This square wave voltage is switched to the dual counter.

The output of logic circuit 208 controls zero voltage switches 214,216,218 and triacs 220,222 and 224 the three phases R, S, T of the three-phase current, the is connected to the heating coil arrangement 186.

The zero voltage switches 214,216,218 and triacs 220,222, 224 are controlled by the logic circuit 208 so that for temperature control as a function of from the output of the pulse width modulator, that of the comparator 200.m in connection is formed with the triangle generator 204, a switchover between two successive, by a sequence of wave trains of the alternating voltage determined stages of the heating power he follows.

In FIG. 9, for m = 3, those in stages O, I, II and III of the heating power are shown Shown on the heating coil assembly 86 given wave trains of the alternating voltage. In stage 0, no alternating voltage is applied to the heating coil arrangement 186.

In stage I, every third half-wave is applied to the heating coil arrangement switched through. As you can see, one half-wave is positive and the next half-wave negative, so that the DC mean value disappears as required.

In stage II, one period of the alternating voltage is applied the heating coil arrangement 186 is switched through. Then a half-wave is blocked and then switched through again for a full period. Here, too, is the mean DC value 0. Finally, in the III stage, the full heating power is applied to the heating coil arrangement 186 switched. The pulse width modulator 200.m, 204 now effects with the pulse width ratio, that is determined by the output signal of the PID controller 192, the heating power is switched between two adjacent stages, so for example between level I and level II. Between which levels this changeover takes place will determines which of the comparators 200.1 to 200.m-1 have responded.

If the heating coil assembly 186 for an output of 12.6 kilowatts is designed, then there are stages of heating power of 0 kilowatts, 4.2 kilowatts, 8.4 kilowatts and 12.6 kilowatts. Overall is the maximum heating output of the instantaneous water heater again about 22 kilowatts. With the high switching frequency of 100 operations per minute however, only a small output of 4.2 kilowatts is switched in each case.

In a further embodiment of the invention, the heating coil arrangement 184 can be omitted. Instead, the heating output can be that of the heating coil arrangement 186 at m = 5 e.g.

between the levels O kilowatt, 4.4 kilowatt, 8.8 kilowatt, 13.2 kilowatt, 17.6 kilowatts and 22 kilowatts can be switched.

The switching of the power with a small subharmonic of the mains frequency, as it happens in the embodiments according to FIGS. 4 and 7, is so fast that it does not appear as a flicker on a lamp. The harmonic content, which occurs due to the omission of half-waves, remains with the circuit in each case in the zero crossing within acceptable limits.

Claims (10)

Claims device for regulating the outlet temperature at electric water heaters, in which the heating power of one in the outlet arranged temperature sensor can be controlled by means of a controller, characterized in that that the installed heating capacity of the water heater in at least two stages is divided and only one of the levels is switched for temperature control will. 2. Apparatus according to claim 1, characterized in that one Stage (18; 50; 112) of the heating power depending on the water flow but independent can be switched on permanently from the outlet temperature. 3. Apparatus according to claim 2, characterized in that the water heater at least three, each represented by a heating coil arrangement (18, 20, 22) in terms of flow having stages of heating power connected in series, of which the inlet-side Stage (18) can be permanently switched on that between the middle stage (20) and the outlet-side stage (22) and downstream of the outlet-side stage (22), the one compared to the middle level (20) has a reduced heating capacity, each has a temperature sensor (28,36) is arranged so that the temperature sensors (28,36) each have a regulator (30,38) switch the stage (20, 22) arranged upstream thereof and that the switching frequency of the regulator (38) switching the outlet-side stage (22) is higher than that of the Controller (30) that switches the middle stage (20). 4. Apparatus according to claim 3, characterized in that on the the middle stage (20) switching controller (30) the difference between the mean values of the Temperatures is connected, which is measured by the two temperature sensors (28,36) will. 5. Apparatus according to claim 1 or 2, - characterized in that by an actuator (60,128) acted upon by the output signal of the controller (62,124) depending on the controller output signal, at least one stage (56, 58; 104) of the heating power is permanently switchable. 6. Device according to one of claims 1 to 5, characterized in that that the controller (62,124) is a continuous PI or PID controller and that the actuator (60,128) for controlling the heating power a pulse width modulator (86,76; 140,134) with a fixed clock frequency, which is determined by the output signal of the controller (60,124) is controllable, and that of the pulse width modulator via zero voltage switch (92,90,96; 146,148,150) Triacs (98,100,102; 152,154,156) for switching the one Level (64,116) of the heating power is controlled. 7. Device according to claims 5 and 6, characterized in that that the actuator (60,128) contains at least one comparator (72,74; 130,132), to which the output signal of the controller (60, 124) and a reference signal are switched and the one stage (56,58; 114) of the heating power is permanently switched on. 8. Apparatus according to claim 7, that for the step to be switched on Separate heating coil arrangement (56,58) is provided upstream of a heating coil arrangement (64) is arranged, which is the level of heating power switched for temperature control represents. 9. Apparatus according to claim 7, characterized in that for several permanently switchable levels of heating power a common heating coil arrangement (114) it is provided that the output signals of several comparators (130,132) to which the output signal of the controller (124) and one of a series of graded reference signals are switched on, together with a signal that is generated by frequency division by means of a frequency divider (162) is generated from the mains frequency, a logic circuit (158) are supplied, and that the logic circuit (158) output signal sequences for Control of a zero voltage switch and triac arrangement (160,162,164,166,168,170) supplies for the heating power of the common heating coil arrangement (114) in such a way that depending on the output signals of the comparators (130, 132) different Consequences of wave trains of the alternating voltage on the common heating coil arrangement (114) can be given. 10. Apparatus according to claim 9, characterized in that the common Heating coil arrangement (186) also provided for the level that can be switched for temperature control is, that on the logic circuit (208) and the output of the pulse width modulator (200.m, 204) is switched on and that the zero voltage switch and triac arrangement (214, 216,218,220,222,224) are controlled by the logic circuit (208) so that for temperature control depending on the output of the pulse width modulator a Switchover between two consecutive, by a sequence of Wave trains of the alternating voltage take place in certain stages of the heating power.