Resistive liquid heater

Abstract

A liquid heater is described comprising a chamber for receiving a liquid, a pair of electrodes within the chamber for applying an electric current to the liquid, input terminals for connection to a power supply, a plurality of bridge arms connected in parallel to the input terminals, and a control unit for controlling switches of the bridge arms. The plurality of bridge arms comprises a respective bridge arm for each pair of electrodes and a common bridge arm, and each bridge arm comprises a pair of switches and a node between the switches. A first electrode of each pair of electrodes is connected to the node of the respective bridge arm and a second electrode is connected to the node of the common bridge arm. The switches have a plurality of different states for selectively connecting the pair of electrodes to the input terminals in one of a plurality of electrode configurations, in each electrode configuration the electrodes have a different total resistance.

Description

Technical Field

[0001] This invention relates to a liquid heater that uses resistance heating to heat liquids. Background Technology

[0002] Liquid heaters can employ resistance heating, also known as Joule heating or ohmic heating, to provide instantaneous or on-demand heating of liquids. As liquid passes through the heater, an electric current is applied to the liquid via electrodes, thus heating the liquid. Summary of the Invention

[0003] The present invention provides a liquid heater, comprising: a chamber for containing a liquid; an electrode pair located within the chamber for applying an electric current to the liquid; an input terminal for connection to a power source; a plurality of bridge arms connected in parallel to the input terminal, the plurality of bridge arms including a respective bridge arm for each electrode pair and a common bridge arm, each bridge arm including a pair of switches and a node located between the switches; and a control unit for controlling the switches, wherein: a first electrode of each electrode pair is connected to the node of the respective bridge arm; a second electrode of each electrode pair is connected to the node of the common bridge arm; and the switches have a plurality of different states for selectively connecting the electrode pair to the input terminal in one of a plurality of electrode configurations, the electrodes having a different total resistance in each electrode configuration.

[0004] By providing a common bridge arm in this way, the number of possible electrode configurations is increased. This allows for improved control of liquid heating. For example, higher thermal fidelity can be achieved by having additional electrode configurations, each with a different total resistance. Furthermore, a relatively wide range of total resistances can be achieved while ensuring that the difference in total resistance between two ordered electrode configurations is not excessive.

[0005] The two sorted electrode configurations should be understood as two consecutive electrode configurations when sorting the total resistance.

[0006] Heaters can be used to heat liquids with a wide range of electrical conductivity. By having many different electrode configurations, the heating of such liquids can be better controlled. For example, the electrode configuration can be selected based on the liquid's conductivity, allowing the same or similar heating levels to be achieved regardless of the conductivity.

[0007] A liquid heater may include at least three electrode pairs. Therefore, the heater has at least thirteen possible electrode configurations. Thus, the liquid heater is capable of achieving a given range of total resistance using the small average and / or maximum difference in total resistance between two ordered electrode configurations. Therefore, relatively good thermal fidelity can be achieved at liquid temperatures due to the small difference in total resistance between each pair of ordered electrode configurations.

[0008] Each electrode pair has a different resistance. Therefore, it is possible to have a greater number of electrode configurations with varying total resistances, and thus more precise thermal control can be achieved.

[0009] The resistance of the electrode pair can have a maximum value Rmax and a minimum value Rmin, where Rmax / Rmin is at least 10. Therefore, a relatively wide dynamic range of total resistance can be achieved for various electrode configurations.

[0010] The total resistance of an electrode configuration can have a minimum value RTmin and a maximum value RTmax. Furthermore, the difference in total resistance between any two sorted electrode configurations can have the maximum value of Rmaxdiff. Therefore, RTmax / RTmin can be at least 20, and Rmaxdiff / (RTmax-RTmin) can be no greater than 35%. This provides a relatively good balance between the dynamic range (RTmax / RTmin) and resolution (Rmaxdiff) of the total resistance. Specifically, the heater has a dynamic range of at least 20, while ensuring that the difference in total resistance between any two sorted configurations does not exceed 35% of the total range.

[0011] The control unit can control the switch so that an AC voltage is applied to the electrodes in each configuration. Therefore, electrolysis of the electrodes can be avoided.

[0012] The switch can have a first state and a second state, in which the electrode is excited by a positive voltage and in which the electrode is excited by a negative voltage. The control unit can switch the switch between the first and second states at a switching frequency of at least 300 kHz. Therefore, the electrode is excited by an AC voltage at a frequency of at least 150 kHz. By exciting the electrode at such a high frequency, a smaller electrode can be used to deliver the same electrical energy without electrolysis. Therefore, the heater can achieve a higher power density.

[0013] The power supply can provide AC voltage, and the control unit can control the switch such that, in at least one setting, the electrode is excited only during every Nth half-cycle of the AC voltage, where N is at least 2. As a result, higher thermal fidelity can be achieved. For example, the control unit can include a first setting where the electrode is excited during every second half-cycle (N=2), instead of every half-cycle of the AC voltage. As a result, the electrical input power of this electrode configuration will be halved. Similarly, the control unit can include a second setting where the electrode is excited during every third half-cycle (N=3). As a result, the electrical input power of this electrode configuration will be one-third. Therefore, a wider range of electrical input power and a wider range of heating rates are possible.

[0014] The power supply can provide AC voltage, and the control unit can control the switch such that, in at least one setting, the electrodes are excited only during one or more portions of each half-cycle of the AC voltage. As a result, higher thermal fidelity can be achieved. In particular, lower electrical input power can be achieved by exciting the electrodes only during portions of each half-cycle. Furthermore, the electrical input power can be adjusted by changing the size or length of these portions.

[0015] The liquid heater may include a temperature sensor for sensing the liquid temperature, and a control unit may control a switch to select an electrode configuration based on the liquid temperature and a temperature setpoint. Specifically, the control unit may select an electrode configuration with lower total resistance in response to a larger difference between the liquid temperature and the temperature setpoint. This allows for good thermal control. For example, when the difference between the liquid temperature and the setpoint is large, the control unit can select an electrode configuration with lower total resistance. Conversely, when the difference between the liquid temperature and the setpoint is small, the control unit can select an electrode configuration with higher total resistance. Therefore, rapid and precise heating of the liquid can be achieved.

[0016] The liquid heater may include a temperature sensor for sensing the liquid temperature, and the control unit may control a switch such that the electrodes are excited by a voltage having a duty cycle defined by the liquid temperature and a temperature set point. By exciting the electrodes with a voltage having a variable duty cycle, finer control of the liquid temperature can be achieved. Specifically, changing the duty cycle can be used to achieve electrical input power between two electrode configurations. Therefore, higher thermal fidelity can be achieved.

[0017] The control unit can control the switch so that the electrodes are excited by a voltage with a variable duty cycle of not less than 70%. As mentioned in the previous paragraph, the duty cycle can be varied to obtain higher thermal fidelity. Additionally or alternatively, when switching between electrode configurations, the electrode in the configuration with lower total resistance can be excited with a voltage with a lower duty cycle. Therefore, when switching between electrode configurations, harmonics introduced into the current drawn from the power supply can be reduced, thus allowing the use of filters with lower impedance. Exciting the electrodes with a voltage with a duty cycle less than 100% introduces a period during which no voltage is applied to the electrodes, and therefore the electrodes do not draw current from the power supply. However, by ensuring that the duty cycle is not less than 70%, relatively good control of heating can be achieved using filters with relatively low impedance.

[0018] The power supply can provide AC voltage, and the switch can be a bidirectional switch. The advantage of this is that the electrodes can be excited with AC voltage regardless of the polarity of the power supply. Furthermore, the electrodes can be excited with AC voltage at a frequency higher than the power supply voltage frequency, without requiring an AC-DC stage or PFC circuitry.

[0019] The power supply can provide an AC voltage with a first frequency, and the control unit can control the switch so that the electrodes are excited by an AC voltage with a second, higher frequency. Therefore, electrolysis can be avoided even though the power supply voltage frequency is low. The first frequency can be no greater than 60 Hz, and the second frequency can be no less than 150 kHz. Thus, a heater with smaller electrodes can be powered by mains electricity (which typically has a frequency of 50 Hz or 60 Hz), however, electrolysis can be avoided by exciting it at a frequency exceeding 150 kHz. Attached Figure Description

[0020] Embodiments will now be described by way of example with reference to the accompanying drawings, in which:

[0021] Figure 1 This is a block diagram of a liquid heater;

[0022] Figure 2 This is the circuit diagram for the heater;

[0023] Figure 3 The possible states of each switch of the heater are shown;

[0024] Figure 4 It is a table detailing the various excitation states, in which the electrodes of the heater are excited in different configurations;

[0025] Figure 5 It is a table detailing the total resistance of each electrode configuration of the heater;

[0026] Figure 6 It shows when in Figure 4 The state transition sequence of the heater switch when switching between excitation states 1 and 2;

[0027] Figure 7 It is shown Figure 5 A graph showing the total resistance behavior of the electrode configuration;

[0028] Figure 8 The various power settings for the heater are explained in detail;

[0029] Figure 9 The diagram illustrates how variations in the base resistance of the heater electrodes affect the dynamic range (top figure) and the maximum difference in total resistance between the two electrode configurations (bottom figure); and

[0030] Figure 10 It is a table that details the total resistance, dynamic range, and maximum and average difference in total resistance between two electrode configurations for electrodes with different base resistances. Detailed Implementation

[0031] Figure 1 and 2The liquid heater 10 includes a chamber 20, an electrode 30, and a control system 40.

[0032] The chamber 20 receives the liquid to be heated and includes an inlet 21 and an outlet 22 through which the liquid enters and exits the chamber 20.

[0033] Electrode 30 includes three pairs of electrodes E1-E3 located within chamber 20. Each pair of electrodes 30 defines a channel through which liquid flows from inlet 21 to outlet 22 of chamber 20. The first pair of electrodes E1 is upstream of the second pair of electrodes E2, which in turn is upstream of the third pair of electrodes E3. Therefore, the liquid first passes between the first pair of electrodes E1, then between the second pair of electrodes E2, and finally between the third pair of electrodes E3.

[0034] Each pair of electrodes 30 has a different resistance; that is, when the chamber 20 is filled with liquid, the resistance of one pair of electrodes (e.g., E1) will be different from the resistance of the other two pairs of electrodes (e.g., E2 and E3). Different resistances can be achieved by electrodes with different cross-sectional areas and / or separation distances.

[0035] The control system 40 includes an input terminal 41, a filter 42, a converter 43, a temperature sensor 44, a current sensor 45, a zero-crossing detector 46, and a control unit 47.

[0036] Input terminal 41 can be connected to a power supply 50 that provides AC voltage, such as mains power.

[0037] The filter 42 includes an inductor L1 and a capacitor C1, which attenuate high-frequency harmonics in the current drawn from the power supply 50.

[0038] The converter 43 includes multiple bridge arms 60 connected in parallel across the two ends of the input terminal 41. Thus, each bridge arm 60 can be said to include a first end 61 connected to one input terminal 41 and a second end 62 connected to another input terminal 41. Each bridge arm 60 includes a pair of switches Sn (e.g., S1 and S2) and a node 63 located between the two switches.

[0039] The switch Sn in each bridge arm 48 is bidirectional. For example... Figure 3As shown, each switch Sn has four possible states: (1) open, where the switch is not conducting in either direction; (2) closed, where the switch is conducting in both directions; (3) diode mode #1, where the switch is conducting in only one direction (e.g., B->A); and (4) diode mode #2, where the switch is conducting in only the other direction (e.g., A->B). Therefore, each switch Sn can be controlled in both directions, meaning that each switch can be made to conduct and not conduct in one or both directions. Thus, switch Sn differs from, for example, a MOSFET with a body diode or an IGBT with an anti-parallel diode, which, while capable of conducting in both directions, can only be de-conducted in one direction. Switch Sn is a gallium nitride switch, which has a relatively high breakdown voltage, making it well-suited for operation at mains voltage. Furthermore, gallium nitride switches are capable of relatively high switching frequencies, the advantages of which will be detailed below. However, other types of bidirectional switches capable of being controlled in both directions can be used alternatively.

[0040] The converter 43 includes separate bridge arms (e.g., S1 and S2) for each pair of electrodes (e.g., E1), and a common bridge arm (e.g., S7 and S8) shared by all electrode pairs 30. In this particular embodiment, the heater 10 includes three pairs of electrodes 30, so the converter 43 includes a total of four bridge arms 60.

[0041] For each pair of electrodes (e.g., E1), the first electrode (e.g., E1a) is connected to node 63 of its respective bridge arm (e.g., S1 and S2), and the second electrode (e.g., E1b) is connected to node 63 of the common bridge arm (e.g., S7 and S8). Thus, the converter 43 and the electrode 30 resemble a three-phase four-wire Y-connection system.

[0042] The switch Sn has multiple different states for selectively energizing one or more pairs of electrodes E1-E3 (i.e., applying voltage).

[0043] Figure 4 The various states of switch Sn used to excite different electrode configurations are described in detail. Figure 4 In this context, " / / " indicates a parallel connection, while "+" indicates a series connection. Therefore, for example, the electrode configuration "(E1 / / E2)+E3" should be understood as meaning that the first pair of electrodes E1 and the second pair of electrodes E2 are connected in parallel, and then this parallel pair is connected in series with the third pair of electrodes E3.

[0044] from Figure 4 As can be seen, there are two states for exciting the electrodes of each electrode configuration: one state is when a positive voltage is applied to the electrode, and the other state is when a negative voltage is applied to the electrode. Figure 4The polarity of the applied voltage is based on the positive power supply voltage on the upper line of converter 43; of course, if the power supply voltage on the upper line is negative, the polarity will be reversed.

[0045] If the voltage applied to the first electrode (e.g., E1a) is positive, then it can be said that a positive voltage is applied to a pair of electrodes (e.g., E1). Therefore, in Figure 4 In this context, the polarity of the applied voltage refers to the polarity applied to the first electrode pair in the list, as well as any electrode pair connected in parallel with the first listed electrode pair. However, the voltage applied to an electrode pair connected in series with the first listed electrode pair will have the opposite polarity. Therefore, when referring to the excitation of selected electrode pairs, it should be understood that the electrode pairs can be excited with voltages of the same or opposite polarities.

[0046] Figure 4 The table lists thirteen different electrode configurations. Each electrode configuration has a different total resistance. Figure 5 The total resistance for each electrode configuration is described in detail. Figure 5 The total resistance is based on the base resistances of the first pair of electrodes E1 (65Ω), the second pair of electrodes E2 (500Ω), and the third pair of electrodes E3 (1000Ω). Although this document mentions electrode configurations with specific base resistances or specific total resistances, it should be understood that the resistance is generated by the liquid between the electrodes, and the electrodes themselves have relatively low (ideally zero) resistance.

[0047] When the selected electrode 30 is energized, the electrical input power dissipated as heat in the liquid depends on the total resistance of the electrode configuration. More specifically, for a given power supply voltage (e.g., RMS voltage), the electrical input power is inversely proportional to the total resistance of the electrode configuration. Therefore, by selecting an electrode configuration with lower resistance, higher electrical input power can be drawn from the power supply 50, thereby achieving a higher level of heating.

[0048] Temperature sensor 44 senses the liquid temperature at outlet 22 of chamber 20 and outputs a signal TEMP to control unit 47. In this particular example, temperature sensor 44 includes a thermistor RT1.

[0049] Current sensor 45 senses the current drawn from power supply 50 and outputs a signal I_AC to control unit 47. In this particular example, current sensor 45 includes a current transducer CT1, such as a current transformer or a Hall effect sensor.

[0050] Zero-crossing detector 46 senses the voltage V of power supply 50. AC The zero-crossing detector 46 outputs a signal Z_CROSS to the control unit 47. In this particular example, the zero-crossing detector 46 includes a pair of clamping diodes D1 and D2.

[0051] Control unit 47 is responsible for controlling the operation of heater 10. Control unit 47 receives the setpoint temperature T_SET, as well as signals output by temperature sensor 44, current sensor 45, and zero-crossing detector 46. In response, control unit 47 outputs a control signal to converter 43 to control the state of switch Sn.

[0052] Control unit 47 selects an electrode configuration based on the liquid temperature and a temperature setpoint. Control unit 47 then outputs a control signal to converter 43 to excite the electrodes according to the selected electrode configuration. Control unit 47 can employ various control algorithms to select the electrode configuration. In one example, control unit 47 can initially select the electrode configuration based solely on the setpoint temperature T_SET. If the liquid temperature TEMP subsequently exceeds the setpoint temperature or stabilizes below the setpoint temperature, control unit 47 can select a different electrode configuration based on the temperature difference. In another example, control unit 47 can select the electrode configuration based on the liquid temperature (or temperature setpoint) and the temperature difference between the liquid temperature and the temperature setpoint. Therefore, control unit 47 selects an electrode configuration with a total resistance that depends not only on the temperature difference between the liquid and the setpoint but also on the liquid's initial (or final) temperature. In yet another example, control unit 47 can use PID control or other feedback mechanisms to select the electrode configuration based on the liquid temperature and the temperature setpoint.

[0053] As mentioned above, each electrode is configured with two excitation states: one is a positive voltage +V AC Another type is a negative voltage -V applied to electrode 30. AC An AC voltage is applied to electrode 30. When stimulating electrode 30 in each electrode configuration, control unit 47 switches between two excitation states, causing electrode 30 to be excited by an AC voltage. Furthermore, control unit 47 switches between states at a switching frequency of at least 300 kHz. Therefore, electrode 30 is excited by an AC voltage of at least 150 kHz; this is much higher than the frequency of power supply 50, which is typically 50 Hz or 60 Hz for mains power. By exciting electrode 30 with such a high-frequency AC voltage, a smaller electrode can be used to heat the liquid without electrolysis, as will be explained below.

[0054] For each electrode (e.g., E1a), a double-layer capacitance is generated at the interface between the electrode and the liquid. This double-layer capacitance varies depending on the electrode material and surface area. For a given electrode material, the capacitance decreases as the surface area decreases due to the smaller contact area with the liquid. The voltage across the double-layer capacitance is a function of the double-layer capacitance and the frequency of the applied voltage. Therefore, as the electrode size decreases, the capacitance decreases, and the voltage across the electrode increases. Electrolysis occurs when the voltage across the electrode exceeds the decomposition potential of the liquid. It is generally believed that electrolysis will not occur when the electrode is excited at a frequency of 50 Hz or 60 Hz, i.e., at the frequency of the mains power supply. In fact, this is true with appropriate electrode size. However, by exciting the electrode at a much higher frequency (e.g., at least 300 kHz), the same heating power can be delivered to the liquid using a much smaller electrode. Therefore, a heater 10 with a higher power density can be realized.

[0055] Switch Sn is a bidirectional switch. Therefore, an AC voltage can be applied to electrode 30, while the power supply voltage V... AC The polarity is independent. These switches are gallium nitride switches, which are not only able to operate at these relatively high switching frequencies (i.e., at least 300 kHz), but also have relatively low switching losses at these frequencies.

[0056] When switching between excitation states within each electrode configuration, or when switching between two electrode configurations, the control unit 47 controls the state of switch Sn to prevent shoot-through while also providing a path for any induced current. Figure 6 It shows when in Figure 4 The state transition sequence when switching between excitation states 1 and 2. This sequence is... Figure 6 Starting in (a), switches S1 and S8 are closed, causing a positive voltage to be applied to the first electrode pair E1. The sequence then moves to... Figure 6 (b) In this configuration, switches S1 and S8 remain closed, allowing a positive voltage to continue being applied to electrode pair E1. However, switches S2 and S7 are now in diode mode. More specifically, G1 is turned on and G2 is turned off, causing switches S2 and S7 to be in diode mode. Figure 6 The sequence is activated in the direction shown in (b). The sequence moves to... Figure 6 (c) where switches S1 and S8 are open. At this time, no voltage is applied to electrode pair E1 (i.e., the electrodes are no longer energized). S2 and S7 remain in diode mode, providing a path for the induced current to flow, such as... Figure 6 As indicated by the arrow in (c). This sequence is... Figure 6 (d) ends where switches S2 and S7 are closed, so that a negative voltage is applied to electrode pair E1.

[0057] exist Figure 6In the example shown, there is a period of time, often referred to as the standstill time, during which no current flows from power source 50. Figure 6 In the example, this occurs when switch Sn is in Figure 6 In the state shown in (c), this relatively short stagnation introduces a relatively high-frequency ripple into the current drawn from power supply 50. Filter 42 then attenuates this high-frequency ripple. Because the stagnation is relatively short, filter 42 can use relatively low-impedance components (e.g., L1 and C1) to attenuate the high-frequency ripple, thereby reducing the size and cost of control system 40.

[0058] The total resistance can change significantly when switching between two electrode configurations. This may even be true when switching between two adjacent electrode configurations with different total resistance rankings. For example, in Figure 5 In the table, the maximum difference in total resistance between any two adjacent electrode configurations is 435Ω (this occurs when switching between configurations 9 and 10 and between configurations 12 and 13). Therefore, switching between two electrode configurations can introduce significant harmonics into the current drawn from power supply 50. Consequently, control unit 47 uses the power supply voltage V sensed by zero-crossing detector 46... AC The zero-crossing point switches between two electrode configurations. When the voltage of power supply 50 is zero or close to zero, the harmonic content caused by the sudden change in the total resistance of electrode 30 can be significantly reduced by changing the electrode configuration. Therefore, the electrode configuration can be changed without significantly increasing the impedance of filter 42. It is conceivable that control unit 47 can change the electrode configuration at any time, and the resulting harmonics can be attenuated by filter 42. However, this would require a significant increase in the impedance of filter 42. As a further alternative, when changing between the two electrode configurations, control unit 47 can excite electrode 30 with a voltage with a duty cycle of less than 100%; this will be described in more detail below.

[0059] The heater 10 has thirteen different electrode configurations, each with a different total resistance. By having a relatively large number of electrode configurations, each providing a different electrical input power, a relatively high thermal fidelity can be achieved. Furthermore, by having a large number of electrode configurations, a relatively wide dynamic range of total resistance (and thus electrical input power) can be achieved, while ensuring that the average and / or maximum difference in total resistance between two ordered electrode configurations does not become excessive. For example, for Figure 5 The total resistance of electrode 30 ranges from 54Ω to 1500Ω, corresponding to a dynamic range of 28:1. However, the average and maximum differences in total resistance are 121Ω and 435Ω, respectively, representing 8% and 30% of the total range.

[0060] By providing common arms (e.g., S7 and S8), a large number of electrode configurations become possible. Without common arms, heater 10 would only have six different configurations; these are... Figure 5 * indicates the number of electrodes. Besides requiring fewer electrode configurations, the dynamic range is significantly reduced without a common bridge arm. Specifically, using... Figure 5 The dynamic range will decrease from 28:1 (54Ω to 1500Ω) to 4:1 (398Ω to 1500Ω) due to the reduced resistance. Furthermore, the average and maximum differences in total resistance between any two adjacent electrode configurations will increase from 121Ω to 220Ω and from 435Ω to 493Ω, respectively. By providing only two additional switches, the total number of electrode configurations is more than doubled, the dynamic range is significantly increased, and the average and maximum differences in total resistance between any two adjacent configurations are reduced.

[0061] Figure 7 It shows the use of Figure 5 The behavior of the total resistance across various electrode configurations is observed. Significant variations in total resistance can be seen between configurations 4 and 5 (268Ω), 9 and 10 (435Ω), and 12 and 13 (435Ω). Considering only configurations 4 and 5, the total resistance jumps from 65Ω to 333Ω. This represents a significant change in electrical input power. For example, if the RMS value of the supply voltage is 230V, the electrical input power will change from 814W in configuration 4 to 159W in configuration 5. It might be desirable to heat a liquid at an electrical input power between these two values. This would provide better control over the liquid's temperature (i.e., better resolution / higher fidelity).

[0062] One way to achieve AC input power is to adjust the power supply voltage V. AC Electrode 30 is excited during each Nth half-cycle. For example, by supplying power to electrode 30 during each second half-cycle of the power supply voltage instead of every half-cycle, the electrical input power of that particular electrode configuration will be halved. Therefore, in order to obtain an electrical input power value between electrode configurations 4 (814 W) and 5 (159 W), control unit 47 can excite electrode configuration number 1 to: (i) obtain an electrical input power of 490 W during each second half-cycle (N=2); (ii) obtain an electrical input power of 327 W during each third half-cycle (N=3); (iii) obtain an electrical input power of 245 W during each fourth half-cycle (N=4); and (iv) obtain an electrical input power of 196 W during each fifth half-cycle (N=5).

[0063] Figure 8 The various power settings of heater 10 are described in detail. For each power setting, control unit 47 employs a specific electrode configuration and operates at a power supply voltage V. ACElectrode 30 is excited during each Nth half-cycle. The listed electrical input power values ​​are based on the RMS value of the supply voltage, 230V. It can be seen that a wide range of different electrical input powers are possible by selecting different electrode configurations and by changing the length of the excitation (i.e., by changing the value of N). In particular, instead of jumping from 814W (power setting 4) to 159W (power setting 9) as in the previous case, heater 10 is now able to input power of 490W, 327W, 245W, and 196W (power settings 5 ​​to 8).

[0064] Another method to achieve AC input power is to only apply it to the power supply voltage V. AC The control unit 47 excites electrode 30 for a portion of each half-cycle. For example, after the power supply voltage crosses zero, the control unit 47 may wait for a period of time before exciting electrode 30. The control unit 47 continues to excite electrode 30 until the next zero-crossing, after which the control unit 47 waits again for a period of time before exciting electrode 30. By adjusting the time period between the zero-crossing and the start of excitation, the control unit 47 is able to adjust the electrical input power. Controlling the excitation in this way may increase the harmonic content in the current waveform. However, due to the clipping shape of the current waveform, the largest increase is likely in the lower-order harmonics, for which the regulations are generally more lenient. Therefore, with a moderate increase in the impedance of filter 42, harmonics can be attenuated to the specified level. In addition to delaying the start of excitation, the control unit 47 may stop excitation before the next zero-crossing. In particular, the control unit 47 may stop excitation before the next zero-crossing for the same duration as the time used to delay the start of excitation. Therefore, the shape of the current waveform is more symmetrical, thereby reducing the amplitude of harmonics. In another example, the control unit 47 may excite the electrode at the beginning and end of each half-cycle and pause excitation during the middle portion of the half-cycle, where the amplitude of the power supply voltage is at its maximum. This allows for a significant reduction in electrical input power during a shorter pause in the excitation. By pausing the excitation for a shorter time, the harmonic content in the current waveform can be reduced. The control unit 47 can employ different excitation modes to achieve a given reduction in electrical input power while minimizing the amplitude of current harmonics.

[0065] Another method to achieve AC input power is to excite electrode 30 with a voltage having a variable duty cycle. That is, the time period of the excitation electrode can be less than 100% of the cycle time. For example, by exciting electrode 30 with a voltage of 70% duty cycle, the electrical input power of this particular electrode configuration is approximately halved. Exciting electrode 30 with a voltage of less than 100% inevitably introduces a period during which no voltage is applied to electrode 30, and therefore no current flows out of power supply 50. As a result, harmonics are introduced into the current waveform and must then be filtered by filter 42. As the duty cycle of the applied voltage decreases, the amplitude of the harmonic content increases, and therefore the required impedance of filter 42 increases. Therefore, control unit 47 can excite electrode 30 with a voltage of not less than 70% duty cycle. Thus, a relatively low impedance filter 42 can achieve relatively good thermal control.

[0066] The control unit 47 can employ two or more of the methods described above to obtain different electrical input power. For example, in Figure 8 In the example, power setting 4 has an electrical input power of 814 W, and power setting 5 has an electrical input power of 490 W. Similarly, it may be desirable to heat the liquid at an electrical input power between these two values. Therefore, control unit 47 can select power setting 4 and excite electrode 30 with a duty cycle of less than 100% to achieve an electrical input power between these two values. For example, by exciting electrode 30 with a duty cycle of 90% or 80%, electrical input powers of 659 W and 521 W can be achieved. In another example, control unit 47 can initially increase the duty cycle to reduce the electrical input power within a specific electrode configuration. However, when the duty cycle reaches 70%, control unit 47 can employ a different excitation mode (e.g., excitation every Nth half-cycle or excitation only during a portion of each half-cycle) to further reduce the electrical input power. By combining different methods, higher thermal fidelity can be achieved.

[0067] It is conceivable that heater 10 could employ a single-electrode configuration with the lowest possible total resistance (e.g., 54 Ω), and control unit 47 could control the duty cycle of the applied voltage to achieve all other values ​​of the electrical input power. However, for the same range of electrical input power, control unit 47 would need to employ a relatively large range of duty cycles. For example, to achieve the same... Figure 8For the same electrical input power range shown, the duty cycle needs to vary between 100% (980W) and 19% (35W). However, a 19% duty cycle requires a filter with a considerably high impedance. Conversely, by switching between many different electrode configurations, changing the excitation mode (e.g., excitation every Nth half-cycle or only during a portion of each half-cycle), and exciting the electrodes with a voltage that has a variable duty cycle of not less than 70%, a similar level of thermal fidelity can be achieved with a filter 42 of lower impedance.

[0068] As mentioned above, the total resistance can change significantly when the two electrode configurations are changed. Therefore, the control unit 47 responds only to the power supply voltage V. AC The control unit 47 switches between electrode configurations based on the zero-crossing point. Therefore, switching between electrode configurations can be achieved without significantly increasing the impedance of filter 42. Additionally or alternatively, the control unit 47 can change the duty cycle of the applied voltage to reduce the difference in electrical input power when switching between different electrode configurations. More specifically, when switching between a first electrode configuration with higher total resistance and a second electrode configuration with lower total resistance, the control unit 47 can excite the electrode 30 of the second configuration with a voltage having a lower duty cycle. Therefore, the difference in electrical input power between the two electrode configurations is reduced. Thus, harmonics introduced into the current when switching between configurations can be reduced, allowing the use of a filter 42 with lower impedance. That is, the control unit 47 responds only to the supply voltage V. AC Compared to schemes that switch between different configurations based on the zero-crossing point, a higher impedance filter is still required. However, the increase in impedance can be relatively modest, the zero-crossing detector 46 can be omitted, and the control unit 47 can switch between electrode configurations at any time.

[0069] Figure 9 It shows the Figure 5 The figure below shows how adjusting the base resistance of the electrode pairs affects the dynamic range (top figure) and the maximum difference in total resistance between two adjacent electrode configurations. It can be seen that at these values, the resistances of the first electrode pair E1 and the third electrode pair E3 have the greatest impact on the dynamic range. It can also be seen that the resistance of the first electrode pair E1 has almost no effect on the maximum difference. Furthermore, any change in the resistance of the second electrode pair E2, whether increasing or decreasing, only increases the peak difference. It has been found that a good balance between dynamic range and peak difference can be achieved by ensuring that the resistance of the second electrode pair E2 is approximately half the resistance of the third electrode pair E3, i.e., 0.45 ≤ R2 / R3 ≤ 0.55.

[0070] Figure 10The total resistance of electrodes 30 with different base resistances is shown. It can be seen that a relatively wide dynamic range (i.e., about 20:1 or greater) can be achieved by ensuring that the resistance of the third pair of electrodes E3 is at least ten times that of the first pair of electrodes E1, i.e., R3 / R1 is at least 10.

[0071] Using the heater described above, it is possible to achieve a relatively wide dynamic range while ensuring that the total resistance difference between any two sorting configurations is not excessive. In particular, if the total resistance of the electrode configurations has a minimum value RTmin and a maximum value RTmax, and if the total resistance difference between any two sorting configurations has a maximum value Rmaxdiff, it is possible to achieve an arrangement where RTmax / RTmin is at least 20 (i.e., the dynamic range is at least 20:1) and Rmaxdiff / (RTmax-RTmin) is not greater than 35% (i.e., the maximum difference between the two sorting configurations is not greater than 35% of the dynamic range).

[0072] For the control system described above, not all possible electrode configuration sequences are possible. Specifically, the following configurations cannot be configured: (E1+E2) / / E3, (E1+E3) / / E2, (E2+E3) / / E1, and E1+E2+E3. While additional configurations are required, some of these non-existent configurations may have a similar total resistance to existing configurations. For example, (E1+E2) / / E3 is likely to have a similar total resistance value to E1 / / E3, and (E1+E3) / / E2 is likely to have a similar total resistance value to E1 / / E2. It is conceivable that one or more missing configurations could be obtained by adding two or more additional switches to the converter. However, for the same number of switches, a greater number of electrode configurations can be achieved by having four pairs of electrodes and five arms. Using this particular arrangement, the switches can be configured to selectively excite the electrodes in one of 36 possible electrode configurations.

[0073] Heater 10 may need to heat liquids with varying electrical conductivities. For example, the conductivity of tap water varies significantly between different countries and even between different regions within the same country. The base resistance of each pair of electrodes E1-E3, as well as the total resistance of each electrode configuration, will depend on the conductivity of the liquid. In particular, for liquids with lower conductivity, the total resistance of each electrode configuration will be higher, and therefore the electrical input power will be lower. Conversely, for liquids with higher conductivity, the total resistance of each electrode configuration will be lower, and therefore the electrical input power will be higher. Therefore, when heater 10 needs to heat liquids with varying conductivity, the significant variation in conductivity can make it difficult to achieve rapid and precise heating of the liquid. Therefore, control unit 47 can select an additional power setting or electrode configuration based on the liquid conductivity to achieve better thermal control. There are various methods to achieve this. For example, after heater 10 is installed, control unit 47 can select a power setting (i.e., electrode configuration, excitation mode, and / or voltage duty cycle) based on the setpoint temperature T_SET. For liquids with nominal conductivity, the selected power setting should heat the liquid to the setpoint temperature. However, if the liquid temperature TEMP exceeds the setpoint temperature or stabilizes below the setpoint temperature, the control unit 47 can adjust the power setting (e.g., different electrode configurations, excitation modes, and / or voltage duty cycles) until the setpoint temperature is reached. This adjustment to the power setting can then be stored by the control unit 47. When a different setpoint temperature is subsequently received, the control unit 47 can again select the power setting (based on the nominal conductivity of the liquid) and then apply the stored adjustment to the selected power setting. This particular type of control is relatively simple and well-suited for applications where the liquid conductivity is constant but unknown (e.g., tap water supply). In another example, the control unit 47 can utilize a current sensor 45, which is primarily used by the control unit 47 to monitor and avoid excessive current. For a given supply voltage, the current drawn by the heater 10 is proportional to the total resistance of the electrode configuration. Therefore, the control unit 47 can use current measurement to indirectly measure the liquid conductivity. For example, the control unit 47 can select a power setting based on the setpoint temperature and then adjust the power setting based on the magnitude of the current drawn from the power supply 50.

[0074] In the example above, heater 10 includes three pairs of electrodes E1-E3. However, heater 10 may include any number of electrode pairs. Converter 43 includes a corresponding bridge arm for each pair of electrodes, as well as a common bridge arm shared by all electrode pairs.

[0075] As already mentioned, the advantage of providing a common bridge is that it significantly increases the number of electrode configurations and the dynamic range of the total resistance. However, despite these advantages, some applications may not require such a large number of electrode configurations and / or a wide dynamic range. In such cases, the common bridge can be omitted.

[0076] In each electrode configuration, the control unit 47 controls the switch Sn of the converter 43, causing the electrode 30 to be excited by an AC voltage with a frequency of at least 150 kHz. As described above, by exciting the electrode 30 with such a high-frequency AC voltage, a smaller electrode can be used to heat the liquid without electrolysis. Depending on the electrode material and size, and the magnitude of the applied voltage, electrolysis can be avoided at lower frequencies. However, by exciting the electrode with an AC voltage with a frequency of at least 150 kHz, a significant reduction in electrode size can be achieved at mains voltage.

[0077] The converter 43 includes a bidirectional switch. Furthermore, the control unit controls the switch Sn such that the electrode 30 is excited by discontinuous or unregulated electrical power. More specifically, the electrical input power drawn from the power supply 50 has a sinusoidal square waveform. Therefore, the control system 40 operates as a direct AC / AC converter and is capable of exciting the electrode 30 with a high-frequency AC voltage without requiring a rectified power supply voltage, or providing an AC-to-DC stage, active power factor correction, or energy storage.

[0078] The heater 10 described above is intended for use with a power supply 50 that provides AC voltage. However, the heater 10 can also be used with a power supply 50 that provides DC voltage. The control unit 47 continues to control the switch Sn of the converter 43 such that the electrodes 30 of each configuration are excited by AC voltage. Therefore, the converter 43 continues to include a corresponding bridge arm for each pair of electrodes. However, since the power supply voltage is no longer alternating but remains constant in polarity, the switch Sn does not necessarily have to be bidirectional. Therefore, the switch of the converter 43 can be a conventional MOSFET or IGBT.

[0079] In the example above, the control system 40 includes a temperature sensor 44 for sensing the output temperature of the liquid. The control unit 47 then uses this temperature measurement to select or adjust the power setting or electrode configuration. As mentioned above, the control unit 47 may also use the output of the current sensor 45 to select or adjust the power setting or electrode configuration. The control system 40 may include additional sensors that the control unit 47 can use to select or adjust the power setting or electrode configuration. For example, the control system 40 may include additional temperature sensors for measuring the liquid temperature at various points within the chamber, or include a flow sensor for measuring the flow rate of the liquid flowing through the chamber 20. Furthermore, the control system 40 may include a flow valve or other device for controlling the flow rate of the liquid moving through the chamber 20.

[0080] Although specific embodiments have been described so far, it should be understood that various modifications may be made without departing from the scope of the invention as defined by the claims.

Claims

1. A liquid heater, comprising: A chamber used to receive liquid; An electrode pair, located within the chamber, is used to apply an electric current to the liquid; Input terminals are used to connect to a power source; Multiple bridge arms are connected in parallel to the input terminal, the multiple bridge arms including a respective bridge arm for each electrode pair and a common bridge arm, each bridge arm including a pair of switches and a node located between the switches; and Control unit, used to control the switch. in: The first electrode of each electrode pair is connected to the node of the corresponding bridge arm; The second electrode of each electrode pair is connected to a node in the common bridge arm; and The switch has multiple different states for selectively connecting the electrode pair to the input terminal in one of multiple electrode configurations, in each electrode configuration having a different total resistance.

2. The liquid heater according to claim 1, wherein, The liquid heater includes at least three electrode pairs.

3. The liquid heater according to claim 1 or 2, wherein, Each electrode pair has a different resistance.

4. The liquid heater according to claim 1 or 2, wherein, The resistance of the electrode pair has a maximum value Rmax and a minimum value Rmin, wherein Rmax / Rmin is at least 10.

5. The liquid heater according to claim 1 or 2, wherein, The total resistance of the electrode configuration has a minimum value RTmin and a maximum value RTmax, and the difference between the total resistances of any two sorted electrode configurations has a maximum value Rmaxdiff, where RTmax / RTmin is at least 20 and Rmaxdiff / (RTmax-RTmin) is not greater than 35%.

6. The liquid heater according to claim 1 or 2, wherein, The control unit controls the switch such that, in each configuration, the electrode is excited by an AC voltage.

7. The liquid heater according to claim 6, wherein, The switch has a first state and a second state, in which the electrode is excited by a positive voltage and in which the electrode is excited by a negative voltage in the second state, and the control unit switches the switch between the first state and the second state at a switching frequency of at least 300 kHz.

8. The liquid heater according to claim 1 or 2, wherein, The power supply provides an AC voltage, and the control unit controls the switch such that, in at least one setting, the electrode is excited only during every Nth half-cycle of the AC voltage, where N is at least 2.

9. The liquid heater according to claim 1 or 2, wherein, The power supply provides an AC voltage, and the control unit controls the switch such that, in at least one setting, the electrode is excited only during one or more portions of each half-cycle of the AC voltage.

10. The liquid heater according to claim 1 or 2, wherein, The liquid heater includes a temperature sensor for sensing the liquid temperature, and the control unit controls the switch to select the electrode configuration based on the liquid temperature and a temperature setpoint.

11. The liquid heater according to claim 1 or 2, wherein, The liquid heater includes a temperature sensor for sensing the liquid temperature, and the control unit controls the switch such that the electrode is excited by a voltage having a duty cycle defined by the liquid temperature and a temperature setpoint.

12. The liquid heater according to claim 1 or 2, wherein, The control unit controls the switch so that the electrode is excited by a voltage with a variable duty cycle of not less than 70%.

13. The liquid heater according to claim 1 or 2, wherein, The power source supplies AC voltage, and the switch is a bidirectional switch.

14. The liquid heater according to claim 1 or 2, wherein, The power supply provides an AC voltage with a first frequency, and the control unit controls the switch so that the electrode is excited by an AC voltage with a second, higher frequency.

15. The liquid heater according to claim 14, wherein, The first frequency is not greater than 60 Hz, and the second frequency is not less than 150 kHz.

Images