JP3946175B2 - Power saving motor start system - Google Patents

Description

  The present invention relates to a motor starting system for driving a motor such as a single-phase induction motor, and more particularly to a power saving motor starting system using a positive temperature coefficient thermistor (PTC).

  FIG. 4 shows a motor starting circuit used for a motor such as a refrigerator or an air conditioner. In the figure, a positive temperature coefficient thermistor (PTC) 110 is connected in series to a start winding S of a motor 100 having a start winding S and a main winding M, and a common terminal C between the start winding S and the main winding M is shown. Is connected to the overload protection device 120. Since the internal resistance of the PTC 110 is small at a normal temperature when starting the motor, a sufficient starting current flows through the starting winding S. After the motor is started, the PTC 110 self-heats due to the current flowing through it, whereby the internal resistance rapidly rises to a high resistance state and maintains a steady state with a holding current of several tens of milliamps. When the motor 100 is overloaded or restrained, the overload protection device 120 opens the circuit in response to the overcurrent and / or the temperature of the winding.

  However, since such a motor starting circuit needs to keep the PTC 110 at a high temperature and a high resistance in order to suppress the current to the starting winding S even during the steady operation of the motor, a power of several watts due to the holding current is required. Was wasted. In order to solve this problem, for example, Patent Document 1 discloses that a starting positive temperature coefficient thermistor 230 and a triac 240 are connected in series to a starting winding 210 of a motor 200 having a main winding 210 and a starting winding 220 as shown in FIG. Are connected to each other, and the triac control positive temperature coefficient thermistor 250 is connected in parallel with the starting positive temperature coefficient thermistor 230, and the triac control thermistor 250 is connected to the gate terminal of the triac 240. Thereby, the power consumption of the starting positive characteristic thermistor 230 is reduced by turning off the triac 230 by the PTC 250 for triac control during the steady operation after the motor is started.

  Further, Patent Document 2 includes a second switch circuit composed of a bimetal element and a resistance heating element connected in series between a motor start winding and a power supply line. When the motor is started, the bimetal element closes the circuit and generates a large starting current. After the start, the bimetal element opens the circuit to zero the current flowing through the start winding. The bimetal element receives heat from the resistance heating element of the second switch circuit and the resistance heating element of the overload protection device and opens the circuit after starting.

JP-A-9-285168 JP-A-11-31445

  However, the starting device shown in Patent Document 1 has the following problems. When the motor is started, a trigger signal is applied to the triac 240 through the triac control thermistor 250 and the triac 240 is turned on, whereby a current flows through the starting positive characteristic thermistor 230. After a certain time from the start-up, the starting positive temperature coefficient thermistor 230 increases its resistance value due to self-heating, and the triac control thermistor 250 also increases its resistance value and is applied to the gate terminal of the triac 240. Becomes smaller and the triac 240 is turned off. As a problem of such a circuit, there is a case where the thermistor for triac control 250 is easily affected by an external temperature and cannot always perform a stable operation. That is, when the temperature of the operating environment is high, the temperature of the triac control thermistor 250 also rises, and the triac 240 may be turned off at an earlier time than expected. For example, when restarting after overload operation or restraint operation, the surroundings of the motor are not sufficiently cooled down, and this is particularly noticeable in summer. As a result, the motor could not be started successfully and had to be started many times.

  In the case of Patent Document 2, in order to allow a large starting current to flow at the start of the motor, the bimetal element of the second switch circuit must have a low resistance, which increases the size of the bimetal element and makes it difficult to reduce the size. There are drawbacks.

  Further, in the motor starting circuit shown in FIG. 4, since the PTC element is kept at several hundred degrees even after the motor is started, the casing for housing the PTC element is made of an expensive heat-resistant and fire-resistant member in order to improve safety. It didn't happen if you didn't compose.

  An object of the present invention is to solve the above-described conventional problems and provide a motor starting system with low power consumption. Another object of the present invention is to provide a motor start system, a motor start circuit, and a motor drive device that are highly safe, reliable, low cost, and can be made compact.

A motor starting system according to the present invention is a system for driving a motor having a main winding and a starting winding, and has the following configuration. A protective device including a heater and a switch electrically connected between a common terminal of the main winding and the starting winding and a first power supply terminal; and first and second electrodes, and the first electrode A first positive temperature coefficient thermistor connected to the start winding, first and second electrodes, and a gate, and the second electrode is electrically connected to the second electrode of the first positive temperature coefficient thermistor A triac that is connected and electrically connected to the second power supply terminal, and a second positive temperature coefficient thermistor that is connected in parallel to the triac and has the first and second electrodes. The first electrode is connected in series to the second electrode of the first positive thermistor, and the second electrode is electrically connected to the gate of the triac. The second positive temperature coefficient thermistor is thermally coupled to the heater of the protective device.

  Preferably, the second positive temperature coefficient thermistor is in a high resistance state by being heated by a heater (for example, a resistance heating element) after the motor is started, the holding current supplied to the gate terminal is reduced, and the triac is disabled ( Off). Preferably, the second positive temperature coefficient thermistor consumes about a tenth of the holding current as compared with the first positive temperature coefficient thermistor. Furthermore, the Curie temperature of the second positive characteristic thermistor is set lower than the temperature of the heater during motor operation, and the temperature is stably raised to a constant temperature by the heat generated by the heater.

  Still another motor starting system of the present invention includes a protection device including a heater and a first switch electrically connected between a common terminal of the main winding and the starting winding and a first power supply terminal, A positive temperature coefficient thermistor having a second electrode, the first electrode being connected to the starting winding, a first electrode, a second electrode, and a gate, wherein the second electrode is a first positive electrode A triac that is electrically connected to the second electrode of the characteristic thermistor, the first electrode being electrically connected to the second power supply terminal, and a first terminal and a second terminal. Is electrically connected to the second electrode of the positive temperature coefficient thermistor, the second terminal is electrically connected to the gate of the triac, and the first and second terminals can be opened and closed by a thermally responsive member. And the second switch is thermally coupled to the heater of the protective device. .

  Preferably, the second switch opens between the first and second terminals by the heat of the heater and the positive temperature coefficient thermistor after the motor is started, and turns off the triac. Preferably, the protection device is an overload protection device that includes a bimetal switch and turns off the bimetal switch when an overcurrent flows in the motor. The protective device can have a function of shutting off the bimetal switch in response to an overcurrent or shutting off the bimetal switch in response to heat of a motor winding or the like during overload operation or restraint operation of the motor. .

  The motor start system of the present invention may be configured by electrically connecting a plurality of devices, or may be configured by a single device including a plurality of functions. For example, the system may be configured by connecting an overload protection device, a motor starting device, a triac, or the like, or may be configured as a system that includes these functions in a single device.

  According to the present invention, the second positive temperature coefficient thermistor for conducting the triac is brought into a high resistance state by the heat of the heater of the protective device after starting the motor, and the triac is made non-conductive (off). The holding current of the first positive temperature coefficient thermistor is cut after the motor is started or when the motor is in steady operation, and the power consumption can be greatly reduced compared to the conventional case. Furthermore, since the second positive temperature coefficient thermistor is thermally coupled to the heater of the protective device and heated or heated by the heat from there, it is relatively less susceptible to temperature changes due to the external environment and is stable. The action can be performed. Furthermore, according to the present invention, a second positive temperature coefficient thermistor or a second switch is provided to control the ON / OFF operation of the triac so that the starting current at the time of starting the motor flows through the positive temperature coefficient thermistor and the triac. Therefore, the overall configuration can be made compact.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a motor start system or a start circuit according to the present embodiment. In the figure, the motor starting system 1 includes a triac 20, first and second positive temperature coefficient thermistors 22, 24, and an overload protection device OLP.
A single-phase induction motor (motor) 10 includes a main winding M and a starting winding S, and a common terminal C thereof is connected to a power supply terminal L1 via an overload protection circuit OLP. A triac 20 and a first positive characteristic thermistor 22 are connected in series between the power supply terminal L2 and the starting winding S. The triac 20 is a bidirectional control rectifier (bidirectional thyristor), and includes electrodes T1 and T2 and a gate G. The triac 20 is turned on / off between the electrodes T1 and T2 according to a trigger current applied to the gate G. Control.

  The electrode T1 of the triac 20 is connected to the power supply terminal L2, and the electrode T2 is connected to one electrode of the first positive characteristic thermistor 22. The other electrode of the first positive characteristic thermistor 22 is connected to the starting winding S of the motor 10. A connection node N1 between the triac 20 and the first positive characteristic thermistor 22 is connected to one electrode of a second positive characteristic thermistor 24 for triac control. The resistors R1 and R2 are connected in series between the other electrode of the second positive temperature coefficient thermistor 24 and the power supply terminal L2, and the connection node N2 of the resistors R1 and R2 is connected to the gate G of the triac 20.

  The overload protection device OLP includes a heater 32 and a bimetal switch 34 connected in series between the common terminal C and the power supply terminal L1. The main point of the present invention is that the second positive temperature coefficient thermistor 24 for triac control is arranged so as to be thermally coupled to the heater 32 of the overload protection device OLP. It is a point that the triac 20 is turned off by changing from a low resistance state to a high resistance state in response to heat from the heater 32.

  The characteristic of the first positive temperature coefficient thermistor 22 is selected according to the conditions for starting the motor. For example, the characteristic of the first positive temperature coefficient thermistor 22 is about 5.6 ohms when having a Curie temperature of about 120 degrees and below the Curie temperature. If the value is exceeded, the resistance value rises rapidly, resulting in a high resistance of about 5 kilohms. The characteristics of the second positive characteristic thermistor 24 are selected according to the conditions for controlling the triac 20. The Curie temperature of the second positive temperature coefficient thermistor 24 is set, for example, about 20 to 30% lower than the Curie temperature of the first positive temperature coefficient thermistor, and is about 80 degrees here. Is about 150 ohms, and when the Curie temperature is exceeded, it becomes a high resistance of about 30 kilohms. Further, the holding current when the first positive temperature coefficient thermistor 22 is in the high resistance state is about several tens of milliamperes, whereas the holding current of the second positive temperature coefficient thermistor 24 is about 1/10 of that. The one with a few milliamps is selected.

  The first and second positive temperature coefficient thermistors 22 and 24 are preferably formed of circular pills, and electrodes are formed on opposite surfaces thereof. The second positive characteristic thermistor 24 suffices to supply a trigger current for controlling the triac 20, and an inner volume smaller than that of the first positive characteristic thermistor 22 through which the starting current flows can be used. For example, the diameter of the pill electrode of the second positive temperature coefficient thermistor 24 may be about 1/5 smaller than the diameter of the first positive temperature coefficient thermistor 22. Therefore, the heat capacity of the second positive temperature coefficient thermistor 24 is considerably smaller than that of the first positive temperature coefficient thermistor 22.

  The overload protection device OLP opens the circuit by the bimetal switch 34 at the time of motor overload operation or motor restraint operation. During overload operation or motor restraint operation, a larger current flows than during normal operation, so the heater 32 generates heat and the bimetal switch 34 is inverted in response. At the same time, during such operation, the winding temperature of the motor rises and the shell temperature of the motor also rises, so that the bimetal switch 34 may reverse in response to the raised temperature. For the heater 32, a heating resistor such as a nichrome wire can be preferably used, and the temperature is about 90 ° C. during normal operation of the motor. The bimetal switch 34 is inverted at a temperature of 80 ° C., for example.

  The resistance values of the resistors R1 and R2 are determined by the resistance value when the second positive characteristic thermistor 24 is in the high resistance state and the turn-on sensitivity of the triac 20. That is, a resistance value is selected such that the triac 20 is not turned on by the current supplied to the gate G in such a state. In this example, resistor R1 is 220 ohms and R2 is 10 ohms. The resistor R2 is determined according to the characteristics of the second positive temperature coefficient thermistor 24, but the resistor R2 is not necessarily used.

  When an AC voltage of 100 volts is applied between the power supply terminals L1 and L2 at the start of the motor, the overload protection device OLP, the start winding S, the first positive characteristic thermistor 22 and the second positive characteristic thermistor 24 are passed through. Thus, the trigger current is applied to the gate G of the triac 20. A trigger voltage of about 1.5 volts is generated between the electrode T1 and the gate G, and the electrodes T1 and T2 of the triac 20 are electrically connected (ON).

  Due to the conduction of the triac 20, a current of about 20 amperes flows through the starting winding S through the first positive characteristic thermistor 22. The first positive temperature coefficient thermistor 22 reaches the Curie temperature after approximately 2 seconds, has a resistance value of 5 kilohms, and enters a high resistance state with a holding current of about 20 milliamperes.

  At the same time, a current also flows through the heater 32 of the overload protection device OLP, and the heater 32 is heated. Since the second positive temperature coefficient thermistor 24 is thermally coupled to the heater 32, the second positive temperature coefficient thermistor 24 is instantaneously heated by the heat from the heater 32 and transitions to a high resistance state of about 30 kilohms when the Curie temperature is exceeded. . As a result, a trigger current sufficient to maintain conduction is not supplied to the gate G of the triac 20, and the electrodes T1 and T2 of the triac 20 become non-conductive.

  When the triac 20 is turned off and the start of the motor is completed, the electric power supplied between the power supply terminals L1 and L2 flows through the main winding M of the motor 10. During operation of the motor 10, the heater 32 of the overload protection device OLP continues to generate heat, so the second positive temperature coefficient thermistor 24 maintains a high resistance state. Since the holding current does not flow through the first positive temperature coefficient thermistor 22, itself is cooled by heat dissipation and returns to the initial low resistance state. A minute current flows through the second positive characteristic thermistor 24 via the first positive characteristic thermistor 22 having a low resistance, but this value is 5 milliamperes or less. This current is consumed by the self-heating of the second positive temperature coefficient thermistor 24 and contributes somewhat to maintaining the high resistance state. In this way, the second positive temperature coefficient thermistor 24 receives heat from the heater 32 and can maintain a high resistance state with a current value smaller than the holding current during the single operation. Further, during normal operation of the motor, the holding current flowing through the second positive temperature coefficient thermistor 24 is not sufficient to turn on the triac 20, so that the triac 20 remains off.

  The conventional motor starting circuit (FIG. 4) consumes several watts of power wastefully because a holding current of several tens of milliamperes flows to hold the positive temperature coefficient thermistor 110 after starting the motor. In the embodiment, the first positive temperature coefficient thermistor 22 is completely turned off, and the holding current of several milliamperes only flows through the second positive temperature coefficient thermistor 24. Can be reduced to 1.

  Further, when considering the restartability of the motor, the first positive temperature coefficient thermistor 22 has a short heat generation time and is in a cooled state after the motor is started. Therefore, when starting the motor after a short pause of several minutes However, the starting current can flow properly. At the same time, shortening the heat generation time of the positive temperature coefficient thermistor can suppress cracks, ignition and the like resulting from the deterioration of the PTC element, and improve the reliability.

  Furthermore, in the conventional motor starting circuit (FIG. 4), the positive temperature coefficient thermistor is maintained at a high temperature of several hundred degrees, so there is a possibility of ignition in a poor environment, and an expensive material is used to increase safety. In the present embodiment, the temperature around the element can be lowered as a result, so that the housing (housing case) can be formed from an inexpensive material.

  Next, a second embodiment of the present invention will be described with reference to FIG. In addition, the same number is attached | subjected to the same thing as the structure shown in FIG. The motor starting system 1 according to the second embodiment is configured by using a second bimetal switch 40 instead of the second positive temperature coefficient thermistor 24 of the first embodiment. The second bimetal switch 40 includes a thermally responsive bimetal element that opens and closes between contacts, and the bimetal element is thermally coupled to the heater 32 of the overload protection device OLP. However, it can also be thermally coupled to the first positive temperature coefficient thermistor 22 when the motor is started.

  At the time of starting the motor, the second bimetal switch 40 is closed between the contacts. When an AC voltage is applied between the power supply lines L1 and L2, a sufficient trigger current is applied to the gate G of the triac 20 via the resistor R2. Is applied, the triac 20 is turned on, and a large starting current flows through the first positive temperature coefficient thermistor 22 connected to the starting winding S. When the starting current flows for several seconds, the first positive temperature coefficient thermistor 22 becomes several hundred degrees due to self-heating, and shifts to a high resistance state, and the motor starts. At this time, the second bimetal switch 40 is heated by receiving heat from the heater 32 and the first positive temperature coefficient thermistor 22. By setting the operating temperature of the second bimetal switch 40 to be several tens of degrees lower than the heat generation temperature of the heater 32 and the temperature of the first positive characteristic thermistor 22 during self-heating, the second bimetal switch 40 The bimetal element is inverted and the contact is opened with a delay from the saturation point of the first positive characteristic thermistor 22. As a result, a trigger current sufficient to turn on the triac 20 is not supplied to the gate G of the triac 20, the triac 20 is turned off, and the current flowing through the first positive characteristic thermistor 22 becomes zero. After the motor is started, the second bimetal switch 40 receives the heat from the heater 32 that generates heat due to the motor operating current, thereby maintaining the inverted state (off state) of the bimetal element.

  According to the second embodiment, since the current flowing through the contact point of the second bimetal switch 40 is as small as several tens of milliamperes after starting the motor, and the bimetal element is less damaged, the bimetal element can be snap-action. Not limited to this, a long-life slow-motion bimetal element can be used, and multiple contacts can be used in order to avoid a conduction failure due to the entry of a foreign substance such as an insulator between the contacts. Furthermore, by accommodating the second bimetal switch 40 in a metal case or the like and eliminating the influence of dust, an opening / closing operation of 500,000 times or more can be achieved with high reliability.

  Next, FIG. 3 shows a state where the motor starting system according to the first embodiment is mounted. FIG. 4A is a plan view for explaining the internal arrangement when the upper part of the housing is cut away, and FIG. 4B is for explaining the internal arrangement when the side part of the housing is cut off. FIG.

  A motor starting device 70 shown in FIG. 1 includes a housing 50 formed of a thermoplastic or heat-resistant resin, and the starting system shown in FIG. 1 is disposed in the housing. The housing 50 is mounted on a cover 60 for a fusite pin formed on the top of the shell of the motor. At the bottom of the housing 50, three openings 52 are formed corresponding to the positions of the fusing pins, and through these openings 52, a start winding pin 54S, a main winding pin 54M, and a common terminal No. pin 54C is inserted. Further, openings for the power supply lines L1 and L2 are formed at the bottom of the housing 50.

  The pin 54 </ b> S is supported in a circular hole 56 a of a spring metal terminal 56 in the housing 50, and the metal contact terminal 56 b contacts with the electrode surface 22 a of the first positive temperature coefficient thermistor 22. It has. The pin 54M is connected to the terminal T1 of the triac 20 and the power supply line L1, and the pin 54C is connected to the bimetal switch 34 of the overload protection device OLP, but these wirings are omitted.

  The first positive temperature coefficient thermistor 22 is disposed substantially at the center between the pin 54S and the pins 54M and 54C. The other electrode surface 22b is electrically connected to the first electrode surface 24a of the second positive temperature coefficient thermistor 24 via the spring-like metal terminal 58a, and the second electrode surface 24b of the second positive temperature coefficient thermistor. Is connected to the gate G of the triac 20 via the spring-like metal terminal 58b. By positioning the first positive temperature coefficient thermistor 22 between the pin 54S and the pins 54M and 54C, the motor starter 70 can be reduced in size.

  The second positive temperature coefficient thermistor 24 is disposed close to the overload protection device OLP, and is thermally coupled to the heater 32 (shown by a broken line in the drawing). In order to easily receive the heat from the heater 32, the metal terminal 58b preferably has a contact structure that increases the contact area with the electrode surface 24b of the second positive temperature coefficient thermistor 24. In the figure, the metal terminal 58b has a corrugated shape, but a contact structure having a flatter surface can be used. Furthermore, in order to make it easy to transfer the heat from the heater 32 to the second positive temperature coefficient thermistor 24, a heat transfer wall 58c (dashed line) may be formed in the housing.

  Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to the specific embodiment, and various modifications are possible within the scope of the gist of the present invention described in the claims. Can be changed. The motor starting system, the motor starting circuit, and the motor starting device used in the above-described embodiments should not be interpreted in a limited sense, but are interpreted as including at least a function of starting the motor.

  The present invention can be used for AC motors for compressors such as refrigerators, freezers, and air conditioners.

It is a figure which shows the circuit of the motor starting system which concerns on 1st Example of this invention. It is a figure which shows the circuit of the motor starting system which concerns on the 2nd Example of this invention. It is a figure when the starting system of FIG. 1 is mounted in the housing. It is a figure which shows the structural example of the conventional motor starting circuit. It is a figure which shows the structural example of the other conventional motor starting circuit.

Explanation of symbols

1: motor starting system,
10: Motor 20: Triac 22: First positive temperature coefficient thermistor (first PTC)
24: Second positive temperature coefficient thermistor (second PTC)
32: Heater 34: Bimetal switch 40: Second bimetal switch OLP: Overload protection device 50: Housing 70: Motor starter

Claims (9)

A motor starting system having a function of starting a motor having a main winding and a starting winding,
A protective device including a heater and a switch electrically connected between a common terminal of the main winding and the starting winding and a first power supply terminal;
A first thermistor having first and second electrodes, the first electrode being connected to the starting winding;
Having a first electrode, a second electrode, and a gate, the second electrode being electrically connected to the second electrode of the first positive temperature coefficient thermistor, and the first electrode being connected to the second power supply terminal; An electrically connected TRIAC;
A second positive temperature coefficient thermistor connected in parallel to the triac, having first and second electrodes, wherein the first electrode is connected in series to the second electrode of the first positive temperature coefficient thermistor. , and a second positive temperature coefficient thermistor is the second electrode is electrically connected to the gate of the triac,
The motor starting system, wherein the second positive temperature coefficient thermistor is thermally coupled to a heater of the protection device. 2. The motor start system according to claim 1, wherein the second positive characteristic thermistor is heated by the heater after the motor is started to be in a high resistance state, and the triac is disabled. 3. The motor start system according to claim 2, wherein a Curie temperature of the second positive temperature coefficient thermistor is lower than a temperature of the heater after the motor is started. The motor starting system according to claim 1 , wherein the protection device includes a bimetal switch and is an overload protection device that turns off the bimetal switch when an overcurrent flows in the motor. A motor starting circuit having a function of starting a motor having a main winding and a starting winding,
A protection circuit including a heater and a switch electrically connected between the common terminal of the main winding and the starting winding and the first power supply terminal;
A first thermistor having first and second electrodes, the first electrode being connected to the starting winding;
Having a first electrode, a second electrode, and a gate, the second electrode being electrically connected to the second electrode of the first positive temperature coefficient thermistor, and the first electrode being connected to the second power supply terminal; An electrically connected TRIAC;
A second positive temperature coefficient thermistor connected in parallel to the triac, having first and second electrodes, wherein the first electrode is connected in series to the second electrode of the first positive temperature coefficient thermistor. , and a second positive temperature coefficient thermistor is the second electrode is electrically connected to the gate of the triac,
The motor driving circuit, wherein the second positive temperature coefficient thermistor is thermally coupled to a heater of the protection device. A motor driving device for driving a motor having a main winding and a starting winding,
A protective device including a heater and a switch electrically connected between a common terminal of the main winding and the starting winding and a first power supply terminal;
A first thermistor having first and second electrodes, the first electrode being connected to the starting winding;
Having a first electrode, a second electrode, and a gate, the second electrode being electrically connected to the second electrode of the first positive temperature coefficient thermistor, and the first electrode being connected to the second power supply terminal; An electrically connected TRIAC;
A second positive temperature coefficient thermistor connected in parallel to the triac, having first and second electrodes, wherein the first electrode is connected in series to the second electrode of the first positive temperature coefficient thermistor. , and a second positive temperature coefficient thermistor is the second electrode is electrically connected to the gate of the triac,
A housing case for housing the protective device, the first and second positive temperature coefficient thermistors, and the triac;
The motor driving device, wherein the second positive temperature coefficient thermistor is disposed in proximity to the protection device so as to be thermally coupled to the heater of the protection device within the housing case. The housing case has an opening for inserting a plurality of fusite pins electrically connected to a winding of a motor, and when the plurality of fusite pins are inserted into the housing case, The motor driving apparatus according to claim 8, wherein a positive temperature coefficient thermistor is disposed at substantially the center of the plurality of fusite pins. The first and second positive temperature coefficient thermistors have a disk shape with electrode surfaces formed on opposing surfaces, and the second positive temperature coefficient thermistor is between the first positive temperature coefficient thermistor and the protection device. The motor drive apparatus of Claim 6 or 7 arrange | positioned. The motor starter according to claim 7 or 8, wherein the second positive temperature coefficient thermistor is thermally coupled to the heater of the protection device via a metal terminal connected to one of the electrode surfaces.

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