A valve leak-proof material control circuit based on two-way relay and travel switch
Patent Information
- Application Number
- CN202522630667.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-11
- Publication Date
- 2026-10-27
- Estimated Expiration
- 2035-12-11
AI Technical Summary
[0003]然而，现有阀门防漏料控制电路的设计中，通常将两台推杆电机的控制回路与同一个行程控制回路串联，即行程开关触发的继电器触点接入总控制回路，默认两台推杆能完全同步动作，但实际运行中，两台推杆，尤其是电液动推杆会因电机转速差异、电液推杆的液压油量波动、液压压力不稳定等设备自身特性，无法保证动作速度完全一致；
本技术方案通过双路独立行程控制模块，为两台推杆电机分别配置专属的行程开关与关联继电器，且每个继电器的触点仅串联在自身对应的电机接触器控制回路中，当两台推杆因电机转速、液压油量等因素出现速度差异时，先到达行程位置的推杆会触发行程开关，仅切断自身电机的控制回路使其停止运行，另一台速度较慢的推杆电机控制回路保持导通，可继续运行至行程到位后再自行停机，避免了先到位推杆触发停机导致另一台推杆未关严的漏料问题，确保阀门始终能完全闭合，防漏料可靠性大幅提升。
Smart Images
Figure CN224904174U_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of electrical control technology for fluid control equipment, specifically a valve anti-leakage control circuit based on dual-channel relays and limit switches. Background Technology
[0002] Existing valve leakage prevention control circuits are mainly used to control the dual-push rod actuators that drive the valve, such as the start, stop, forward and reverse rotation of two push rod motors, to realize the opening and closing of the valve. At the same time, they are equipped with basic electrical protection components such as overload protection and emergency stop to ensure the safe operation of the motor and equipment. Its core is to try to avoid the leakage problem of the valve not being closed tightly by synchronously controlling the movement of the two push rods.
[0003] However, in the design of existing valve anti-leakage control circuits, the control circuits of two push rod motors are usually connected in series with the same stroke control circuit. That is, the relay contacts triggered by the limit switch are connected to the main control circuit. It is assumed that the two push rods can move completely synchronously. However, in actual operation, the two push rods, especially the electro-hydraulic push rods, cannot guarantee that their movement speeds are completely consistent due to the characteristics of the equipment itself, such as the difference in motor speed, the fluctuation of hydraulic oil volume of the electro-hydraulic push rod, and the instability of hydraulic pressure. When the faster push rod reaches the travel position first and triggers the limit switch, the corresponding relay contact will disconnect the main control circuit and directly cut off the power supply to the two motors. At this time, the slower push rod has not yet reached the target position, causing the valve to fail to close completely and material to leak out from the valve gap. This means that the existing valve anti-leakage control circuit cannot reliably solve the risk of leakage, and will also bring additional economic and production losses due to remedial measures. Utility Model Content
[0004] The purpose of this invention is to provide a valve anti-leakage control circuit based on dual-channel relays and limit switches to solve the problems mentioned in the background art.
[0005] The technical solution adopted by this application to solve its technical problem is: A valve anti-leakage control circuit based on dual-channel relays and limit switches includes a main circuit unit and a control circuit unit. The main circuit unit includes a main power isolation switch QS1, a first motor control branch, and a second motor control branch. The input terminal of the main power isolation switch QS1 is connected to a three-phase power supply, and the output terminals of the main power isolation switch QS1 supply power to the first motor control branch and the second motor control branch, respectively. The control circuit unit includes a control power module, a start control module, and a dual-path travel control module. The control power module is connected to a single-phase power supply and supplies power to the start control module and the dual-path travel control module. The start control module is used to control the on / off state of the first motor control branch and the second motor control branch. The dual-path stroke control module includes a first push rod stroke control unit and a second push rod stroke control unit. The first push rod stroke control unit is used for stroke control of the first motor control branch, and the second push rod stroke control unit is used for stroke control of the second motor control unit.
[0006] Preferably, the first motor control branch includes a motor M1 for controlling the first push rod of the valve, a contactor KM1 for driving the motor M1 to rotate forward, a contactor KM2 for driving the motor M1 to rotate in reverse, and a thermal relay FR1 for overload protection of the motor M1. The contactors KM1 and KM2 are connected in parallel, and the main contacts of the contactors KM1 and KM2 are connected in series with the thermal relay FR1 and then connected to the power supply circuit of the motor M1.
[0007] Preferably, the second motor control branch includes a motor M2 for controlling the second push rod of the valve, a contactor KM3 for driving the motor M2 to rotate forward, a contactor KM4 for driving the motor M2 to rotate in reverse, and a thermal relay FR2 for overload protection of the motor M2. The contactors KM3 and KM4 are connected in parallel, and the main contacts of the contactors KM3 and KM4 are connected in series with the thermal relay FR2 and then connected to the power supply circuit of the motor M2.
[0008] Preferably, the control power module includes a fuse FU1 and an emergency stop button STOP, which are connected in series to a single-phase power supply.
[0009] Preferably, the start control module includes a selection switch SA2, a start button SB1, a start button SB2 corresponding to motor M1, a start button SB3 corresponding to motor M2, and relays KA1 and KA2 for self-locking operation. The start button SB2 is connected in series with the coil of relay KA1 and the normally closed contact of relay KA2, and the start button SB3 is connected in series with the coil of relay KA2 and the normally closed contact of relay KA1. The normally closed contact of the selector switch SA2 is connected in series with the start button SB1. The start button SB2 is connected in parallel with the normally open contact of the relay KA1, and then connected in series with the normally closed contact of the relay KA3. The normally closed contacts of the relay KA3 and the relay KA5 are connected in parallel, and then connected in series with the normally closed contact of the relay KA2. The start button SB3 is connected in parallel with the normally open contact of relay KA2, the start button SB3 is connected in series with the normally closed contact of relay KA4, the normally closed contacts of relay KA4 and relay KA6 are connected in parallel, and the normally closed contacts of relay KA4 and relay KA1 are connected in series.
[0010] Preferably, the first push rod stroke control unit includes limit switches SQ11 and SQ12 corresponding to the extension stroke of motor M1, relays KA3 and KA4, wherein the coil of limit switch SQ11 is connected in series with the coil of relay KA3, and the coil of limit switch SQ12 is connected in series with the coil of relay KA4.
[0011] Preferably, the second push rod stroke control unit includes limit switches SQ21 and SQ22 corresponding to the extension stroke of motor M2, relays KA5 and KA6, wherein the coil of limit switch SQ21 is connected in series with the coil of relay KA5, and the coil of limit switch SQ22 is connected in series with the coil of relay KA6.
[0012] Preferably, in the first motor control branch, the coil of contactor KM1 is connected in series with the normally closed contact of contactor KM2, the coil of contactor KM2 is connected in series with the normally closed contact of contactor KM1, the normally open relay KA1 and the normally closed relay KA3 are connected in series, and then connected in series with the normally closed contact of contactor KM2, and the normally open relay KA2 and the normally closed relay KA4 are connected in series, and then connected in series with the normally closed contact of contactor KM1. In the second motor control branch, the coil of contactor KM3 is connected in series with the normally closed contact of contactor KM4, the coil of contactor KM4 is connected in series with the normally closed contact of contactor KM3, the normally open relay KA1 and the normally closed relay KA5 are connected in series, and then connected in series with the normally closed contact of contactor KM4, and the normally open relay KA2 and the normally closed relay KA6 are connected in series, and then connected in series with the normally closed contact of contactor KM4.
[0013] Preferably, the normally closed contact of the thermal relay FR1 is connected in series to the end of the coil control circuit of contactor KM1 and contactor KM2, and the normally closed contact of the thermal relay FR2 is connected in series to the end of the coil control circuit of contactor KM3 and contactor KM4.
[0014] The beneficial effects of this application are: This technical solution uses a dual-path independent stroke control module to equip each of the two push rod motors with a dedicated limit switch and associated relay. The contacts of each relay are connected in series only in the control circuit of its corresponding motor contactor. When the two push rods have speed differences due to factors such as motor speed and hydraulic oil volume, the push rod that reaches the stroke position first will trigger the limit switch, cutting off only its own motor's control circuit to stop it from running. The control circuit of the other push rod motor, which is slower, remains conductive and can continue to run until it reaches the stroke position before automatically stopping. This avoids the problem of material leakage caused by the push rod that reaches the stroke position first triggering the stop, resulting in the other push rod not closing completely. This ensures that the valve can always be fully closed, greatly improving the reliability of preventing material leakage.
[0015] In addition to the purposes, features, and advantages described above, this application has other purposes, features, and advantages. A further detailed description of this application will be provided below with reference to the figures. Attached Figure Description
[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a circuit diagram of the main circuit unit of this utility model; Figure 2 This is the overall circuit diagram of the control circuit unit of this utility model; Figure 3 This is a circuit diagram of the first motor control branch and the second motor control branch of this utility model; Figure 4 This is a circuit diagram of the start-up control module of this utility model; Figure 5 This is a circuit diagram of the dual-path stroke control module of this utility model. Detailed Implementation
[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.
[0018] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present application.
[0019] Please see Figure 1-5 The embodiments provided by this utility model are as follows: like Figure 1As shown, a valve anti-leakage control circuit based on dual-channel relays and limit switches includes a main circuit unit and a control circuit unit. The main circuit unit includes a main power disconnect switch QS1, a first motor control branch, and a second motor control branch. The input terminal of the main power disconnect switch QS1 is connected to a three-phase power supply, and the output terminals of the main power disconnect switch QS1 supply power to the first motor control branch and the second motor control branch, respectively. The main power disconnect switch QS1 is the main power switch of the main circuit. When QS1 is closed, the three-phase power supply (L1 / L2 / L3) is divided into completely independent power supplies for the first motor control branch and the second motor control branch through the three main contacts of the main power disconnect switch QS1. When the main power disconnect switch QS1 is open, the power supply of the entire circuit can be cut off, so that the power supply of the two push rod motors controlling the valve, namely motor M1 and motor M2, does not interfere with each other, and avoids the failure of one motor affecting the operation of the other. Specifically, the first motor control branch includes a motor M1 for controlling the first push rod of the valve, a contactor KM1 for driving the motor M1 to rotate forward, a contactor KM2 for driving the motor M1 to rotate in reverse, and a thermal relay FR1 for overload protection of the motor M1. Contactors KM1 and KM2 are connected in parallel, and the main contacts of contactors KM1 and KM2 are connected in series with the thermal relay FR1 and then connected to the power supply circuit of the motor M1. The second motor control branch includes a motor M2 for controlling the second push rod of the valve, a contactor KM3 for driving the motor M2 to rotate forward, a contactor KM4 for driving the motor M2 to rotate in reverse, and a thermal relay FR2 for overload protection of the motor M2. Contactors KM3 and KM4 are connected in parallel, and the main contacts of contactors KM3 and KM4 are connected in series with the thermal relay FR2 and then connected to the power supply circuit of the motor M2.
[0020] Contactors KM1 and KM2 are connected in parallel because the forward and reverse rotation of the motor requires switching the power phase sequence. After being connected in parallel, the direction of rotation can be switched by selecting one of the contactors. For example, when contactor KM1 is energized, motor M1 rotates forward, and when contactor KM2 is energized, motor M1 rotates in reverse. The main contacts of the contactors are connected in series with thermal relay FR1 and then connected to motor M1. When motor M1 is running, the current will flow through the thermal element of thermal relay FR1. If the current is overloaded, the thermal element will deform and trigger the normally closed contact of thermal relay FR1 to open.
[0021] Among them, such as Figure 3 As shown, in the first motor control branch, the coil of contactor KM1 is connected in series with the normally closed contact of contactor KM2, the coil of contactor KM2 is connected in series with the normally closed contact of contactor KM1, the normally open relay KA1 and the normally closed relay KA3 are connected in series, and then connected in series with the normally closed contact of contactor KM2, and the normally open relay KA2 and the normally closed relay KA4 are connected in series, and then connected in series with the normally closed contact of contactor KM1. In the second motor control branch, the coil of contactor KM3 is connected in series with the normally closed contact of contactor KM4, the coil of contactor KM4 is connected in series with the normally closed contact of contactor KM3, the normally open relay KA1 and the normally closed relay KA5 are connected in series, and then connected in series with the normally closed contact of contactor KM4; the normally open relay KA2 and the normally closed relay KA6 are connected in series, and then connected in series with the normally closed contact of contactor KM4. The normally closed contact of thermal relay FR1 is connected in series to the end of the coil control circuit of contactor KM1 and contactor KM2, and the normally closed contact of thermal relay FR2 is connected in series to the end of the coil control circuit of contactor KM3 and contactor KM4.
[0022] When contactor KM1 coil is energized and engaged, its normally closed contact opens, cutting off the control circuit of contactor KM2 coil and preventing contactor KM2 from engaging. Conversely, when contactor KM2 is engaged, its normally closed contact opens, preventing contactor KM1 from engaging. Relay KA1 is the starting self-locking relay for motor M1, and relay KA3 is the trigger relay for motor M1's extension stroke. Relays KA1 (normally open, closes after startup) and KA3 (normally closed, closes before the stroke is complete) are connected in series, and then connected in series with the normally closed contactor KM2, forming the control circuit for forward rotation of motor M1. Contactor KM1 can only engage when motor M1 is not started, has not reached its extension stroke, or is not reversing. Electrical relay KA2 is the self-locking relay for starting motor M2, and relay KA4 is the trigger relay for shortening the stroke of motor M1. Similarly, they form the control circuit for reversing motor M1. The principle of the second motor control branch is the same as that of the first motor control branch, so it will not be elaborated on here. Thermal relay FR1 is normally closed and connected in series at the coil ends of contactors KM1 and KM2. When motor M1 is overloaded, thermal relay FR1 normally closed opens, causing the coils of contactors KM1 and KM2 to lose power, and motor M1 stops immediately. Thermal relay FR2 has the same overload protection logic as motor M2, thereby preventing the forward and reverse contactors of the same motor from engaging simultaneously and avoiding a three-phase power short circuit.
[0023] like Figure 2 As shown, the control circuit unit includes a control power module, a start control module, and a dual-path travel control module. The control power module is connected to a single-phase power supply and includes a fuse FU1 and an emergency stop button STOP. The fuse FU1 and the emergency stop button STOP are connected in series to the single-phase power supply. The control power module supplies power to the start control module and the dual-path travel control module. The control circuit uses a single-phase power supply, usually 220V, which is safer than a three-phase power supply. The fuse FU1 is a short-circuit protection element. When a short circuit occurs in the control circuit, the fuse of the fuse FU1 will blow, cutting off the control power supply. The emergency stop button STOP can directly cut off the control power supply. In case of abnormalities such as valve jamming or material leakage, it can quickly cut off all control logic to ensure equipment safety. The start control module is used to control the on / off state of the first motor control branch and the second motor control branch. The dual-path stroke control module includes a first push rod stroke control unit and a second push rod stroke control unit. The first push rod stroke control unit is used for stroke control of the first motor control branch, and the second push rod stroke control unit is used for stroke control of the second motor control unit.
[0024] Specifically, such as Figure 4 As shown, the start control module includes a selection switch SA2, a start button SB1, a start button SB2 corresponding to motor M1, a start button SB3 corresponding to motor M2, and relays KA1 and KA2 for self-locking operation. The start button SB2 is connected in series with the coil of relay KA1 and the normally closed contact of relay KA2. The start button SB3 is connected in series with the coil of relay KA2 and the normally closed contact of relay KA1. The normally closed contact of selection switch SA2 is connected in series with start button SB1. Start button SB2 is connected in parallel with the normally open contact of relay KA1 and then in series with the normally closed contact of relay KA3. The normally closed contacts of relay KA3 and KA5 are connected in parallel and then in series with the normally closed contact of relay KA2. Start button SB3 is connected in parallel with the normally open contact of relay KA2 and then in series with the normally closed contact of relay KA4. The normally closed contacts of relay KA4 and KA6 are connected in parallel and then in series with the normally closed contact of relay KA1.
[0025] The control circuit will only be activated when relay SA2 is pressed first, followed by the start button SB1, to prevent accidental motor starting. When start button SB2 is pressed to start motor M1, the normally closed relay KA2 connected in series with start button SB2 is closed. If motor M2 has already started, relay KA2 will be energized, and its normally closed state will be opened, preventing start button SB2 from triggering relay KA1. Similarly, when start button SB3 is pressed to start motor M2, the normally closed relay KA1 connected in series will be closed, achieving interlocking to prevent the two motors from starting simultaneously. After start button SB2 is pressed, the coil of relay KA1 is energized. This causes the normally open relay KA1 to close. When the start button SB2 is released, the coil of relay KA1 can still be energized through its normally open contact (self-locking), and motor M1 continues to run. Relay KA3 is the relay for the extension stroke of motor M1, and relay KA5 is the relay for the extension stroke of motor M2. The normally closed relays KA3 and KA5 are connected in parallel and then connected in series with normally closed relay KA2. This means that motor M1 can only run forward when motor M1 has not reached its extension stroke, motor M2 has not reached its extension stroke, or motor M2 has not started. Similarly, the parallel connection of relays KA4 and KA6 realizes the linkage control of the stroke reduction.
[0026] Specifically, such as Figure 5As shown, the first push rod stroke control unit includes limit switches SQ11 and SQ12, relays KA3 and KA4 corresponding to the extension stroke of motor M1. The coils of limit switches SQ11 and KA3 are connected in series, and the coils of limit switches SQ12 and KA4 are connected in series. The second push rod stroke control unit includes limit switches SQ21 and SQ22, relays KA5 and KA6 corresponding to the extension stroke of motor M2. The coils of limit switches SQ21 and KA5 are connected in series, and the coils of limit switches SQ22 and KA6 are connected in series.
[0027] When the push rod of motor M1 extends to the target position, it triggers the limit switch SQ11 to close, energizes the coil of relay KA3, and opens the normally closed contact of relay KA3, thus cutting off the control circuit of contactor KM1 for forward rotation of motor M1, and M1 stops extending. When the push rod of motor M1 retracts to the target position, it triggers the limit switch SQ12, energizes the coil of relay KA4, and opens the normally closed contact of relay KA4, thereby causing contactor M1 to reverse. The control circuit of contactor KM2 is cut off, and M1 stops retracting. The limit switch SQ21 corresponding to the extension and retraction action of the push rod of motor M2 has the same logic as that of motor M1. It ensures that only its own motor circuit is cut off after a single push rod reaches its position. This solves the core problem that when two push rods have different speeds, the push rod that reaches its position first will trigger a stop, but the other push rod can still continue to run until it reaches its position. This also prevents the valve from leaking material because a push rod has not reached its position.
[0028] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.
Claims
1. A valve leakage prevention control circuit based on dual-channel relays and limit switches, characterized in that: It includes a main circuit unit and a control circuit unit. The main circuit unit includes a main power isolation switch QS1, a first motor control branch and a second motor control branch. The input terminal of the main power isolation switch QS1 is connected to a three-phase power supply, and the output terminals of the main power isolation switch QS1 supply power to the first motor control branch and the second motor control branch, respectively. The control circuit unit includes a control power module, a start control module, and a dual-path travel control module. The control power module is connected to a single-phase power supply and supplies power to the start control module and the dual-path travel control module. The start control module is used to control the on / off state of the first motor control branch and the second motor control branch. The dual-path stroke control module includes a first push rod stroke control unit and a second push rod stroke control unit. The first push rod stroke control unit is used for stroke control of the first motor control branch, and the second push rod stroke control unit is used for stroke control of the second motor control unit.
2. The valve anti-leakage control circuit based on dual-channel relays and limit switches according to claim 1, characterized in that: The first motor control branch includes a motor M1 for controlling the first push rod of the valve, a contactor KM1 for driving the motor M1 to rotate forward, a contactor KM2 for driving the motor M1 to rotate in reverse, and a thermal relay FR1 for overload protection of the motor M1. The contactors KM1 and KM2 are connected in parallel, and the main contacts of the contactors KM1 and KM2 are connected in series with the thermal relay FR1 and then connected to the power supply circuit of the motor M1.
3. A valve anti-leakage control circuit based on dual-channel relays and limit switches according to claim 2, characterized in that: The second motor control branch includes a motor M2 for controlling the second push rod of the valve, a contactor KM3 for driving the motor M2 to rotate forward, a contactor KM4 for driving the motor M2 to rotate in reverse, and a thermal relay FR2 for overload protection of the motor M2. The contactors KM3 and KM4 are connected in parallel, and the main contacts of the contactors KM3 and KM4 are connected in series with the thermal relay FR2 and then connected to the power supply circuit of the motor M2.
4. A valve anti-leakage control circuit based on dual-channel relays and limit switches according to claim 3, characterized in that: The control power module includes a fuse FU1 and an emergency stop button STOP, which are connected in series to a single-phase power supply.
5. A valve anti-leakage control circuit based on a dual-channel relay and a limit switch according to claim 4, characterized in that: The start control module includes a selection switch SA2, a start button SB1, a start button SB2 corresponding to motor M1, a start button SB3 corresponding to motor M2, and relays KA1 and KA2 for self-locking operation. The start button SB2 is connected in series with the coil of relay KA1 and the normally closed contact of relay KA2, and the start button SB3 is connected in series with the coil of relay KA2 and the normally closed contact of relay KA1. The normally closed contact of the selector switch SA2 is connected in series with the start button SB1. The start button SB2 is connected in parallel with the normally open contact of the relay KA1, and then connected in series with the normally closed contact of the relay KA3. The normally closed contacts of the relay KA3 and the relay KA5 are connected in parallel, and then connected in series with the normally closed contact of the relay KA2. The start button SB3 is connected in parallel with the normally open contact of relay KA2, the start button SB3 is connected in series with the normally closed contact of relay KA4, the normally closed contacts of relay KA4 and relay KA6 are connected in parallel, and the normally closed contacts of relay KA4 and relay KA1 are connected in series.
6. A valve anti-leakage control circuit based on a dual-channel relay and a limit switch according to claim 5, characterized in that: The first push rod stroke control unit includes limit switches SQ11 and SQ12 corresponding to the extension stroke of motor M1, relays KA3 and KA4. The coil of limit switch SQ11 is connected in series with the coil of relay KA3, and the coil of limit switch SQ12 is connected in series with the coil of relay KA4.
7. A valve anti-leakage control circuit based on a dual-channel relay and a limit switch according to claim 5, characterized in that: The second push rod stroke control unit includes limit switches SQ21 and SQ22 corresponding to the extension stroke of motor M2, relays KA5 and KA6. The coil of limit switch SQ21 is connected in series with the coil of relay KA5, and the coil of limit switch SQ22 is connected in series with the coil of relay KA6.
8. A valve anti-leakage control circuit based on a dual-channel relay and a limit switch according to claim 5, characterized in that: In the first motor control branch, the coil of contactor KM1 is connected in series with the normally closed contact of contactor KM2, the coil of contactor KM2 is connected in series with the normally closed contact of contactor KM1, the normally open relay KA1 and the normally closed relay KA3 are connected in series, and then connected in series with the normally closed contact of contactor KM2; the normally open relay KA2 and the normally closed relay KA4 are connected in series, and then connected in series with the normally closed contact of contactor KM1. In the second motor control branch, the coil of contactor KM3 is connected in series with the normally closed contact of contactor KM4, the coil of contactor KM4 is connected in series with the normally closed contact of contactor KM3, the normally open relay KA1 and the normally closed relay KA5 are connected in series, and then connected in series with the normally closed contact of contactor KM4, and the normally open relay KA2 and the normally closed relay KA6 are connected in series, and then connected in series with the normally closed contact of contactor KM4.
9. A valve anti-leakage control circuit based on a dual-channel relay and a limit switch according to claim 8, characterized in that: The normally closed contact of the thermal relay FR1 is connected in series to the end of the coil control circuit of contactor KM1 and contactor KM2, and the normally closed contact of the thermal relay FR2 is connected in series to the end of the coil control circuit of contactor KM3 and contactor KM4.