Design and application of low-power CNC contactors

Abstract: This paper describes a low-power CNC contactor, a basic switching element designed to adapt to the increasing complexity of new industrial automation control and low-voltage power supply control systems. The product uses a control system composed of interval energy storage, single-pulse trigger current, permanent magnet mechanism and program control technology to operate the contactor to close and open. It solves the three major technical problems of low-power starting, controllable motion and use in harsh environments for 100A to 800A contactors. Its main technical features are: (1) starting power 4.352 VA; holding power 0.116 VA; operating power consumption 0.0022 kW/h; (2) working voltage DC24V; coil temperature rise 6K; closing noise value not greater than 10dB; (3) the combination of power and electronics is realized on the same component. This gives the product the technical characteristics of energy saving, environmental protection and information technology.

Keywords: low-power CNC contactor, intermittent energy storage, low-power starting, single-pulse trigger current, program-controlled drive circuit and permanent magnet mechanism.

0 Introduction

AC contactors are widely used in industrial process automation and low-voltage terminal power supply, and have a solid market foundation [2]. However, existing AC contactors have major problems in industrial process automation: large starting power, poor motion controllability, and complex system structure. Note: This product passed the scientific and technological achievement appraisal hosted by the Sichuan Provincial Department of Science and Technology in December 2006 (Scientific and technological achievement appraisal certificate No. Chuan Ke Jian Zi [2006] 451, registration number: 9512007Y0011). Conclusion: Reached the international advanced level. This project has obtained a national invention patent, patent number ZL200510021642.0. Especially when a programmable logic controller (PLC) drives a large AC contactor, it needs to be amplified by an intermediate stage, and a dedicated control circuit is needed to reduce the power consumption during engagement, which restricts the application and development of the control system. This article introduces the low-power numerical control contactor proposed in reference [5], which is a basic switching element designed to adapt to the complexity of new industrial automation control and low-voltage power supply control systems. While adhering to the basic performance of AC contactors, low-power CNC contactors utilize programming code control technology to control the connection, load, and disconnection of the main circuit current, thereby significantly improving the core technical performance indicators of the product.

1. Technical Solution Concept

The technical solution for this project is conceived as follows: a low-power CNC contactor operates at a frequency of 1200 times per hour with a 3-second interval. During this interval, the energy storage capacitor is charged, and the accumulated energy is used to smooth the current surge during startup. The single-pulse contacting current generated by the discharge of the energy storage capacitor onto the excitation coil, combined with a permanent magnet mechanism, is precisely controlled by the control circuit to switch the contacting current at the zero-crossing point, completing the closure of the main contacts. Simultaneously, the permanent magnet retains the closed state. This concept achieves energy-saving, environmentally friendly, and information-based technical characteristics.

The main technical features of the product are:

(1) There is currently a gap in the domestic and international markets for high-capacity contactors with starting power less than 8VA. Reducing starting power is not only for energy saving, but its core function is to ensure compatibility with electronic circuits and enable information technology. The measured starting power of the SD-100 low-power CNC contactor is 4.352 VA.

(2) The product incorporates the design concept of transistors in its structural design, and sets up control terminals similar to the base, realizing the combination of "high voltage" and "low voltage" on the same component. The method of connecting to the electronic circuit and the power consumption are equivalent to an ordinary medium-power transistor. Its driving mode can be selected in a compatible or isolated form, which greatly facilitates the design of automated control systems and builds a feasible technical platform for information technology.

(3) Overcome the inherent "uncontrollable motion" of AC contactors. Ensure that the allowable error of contactor engagement and release time is no greater than ±1ms (the maximum measured time error is -0.335ms for engagement and -0.124ms for release).

(4) Breakthrough progress has been made in the environmental adaptability of the product. The environmental adaptability indicators such as temperature, tilt, sway, vibration, impact and electromagnetic compatibility have all reached the international advanced level[4]. For example: Low temperature: working temperature: -25℃ (uninterrupted working system), -40℃ (1h short-time working system). High temperature: working temperature: 55℃ (uninterrupted working system), 70℃ (1h short-time working system).

The table below compares some performance indicators of this product with those of the foreign GMC series products:

During the demonstration of the SD-180 low-power CNC contactor, only two 9V (6LR61) stacked batteries were used for power operation, ushering in a new era for next-generation contactors.

2. Project Technical Solution [5]

Technological innovation in low-voltage electrical appliances is achieved through trial and error. Based on scientific experimentation and rational verification, this is an effective way to study and reveal the inherent laws of motion of contactors.

2.1 Built-in drive circuit [5]

Figure 1 shows the schematic diagram of the drive circuit. The drive circuit is installed in the base of the low-power CNC contactor, forming a whole with the low-power CNC contactor. The power supply circuit in the figure has three relatively independent power branches, which are responsible for supplying power to the closing, opening, and control circuits respectively. The external power supply charges the energy storage capacitor C4 through a constant current source circuit composed of resistor R1, light-emitting diode D1, capacitor C1, transistor Q1, resistor R3, and resistor R4, forming the closing power supply; the external power supply charges capacitor C7 through diode D3 and resistor R5, forming the opening power supply; and the external power supply charges capacitor C3 through diode D2, forming the control power supply.

In the diagram, the excitation coil KIM's current direction is switched via changeover switches JK2 and JK3 to control the low-power CNC contactor's engagement, holding, and disengagement. The circuit operates as follows: After power is supplied, the two ends of the excitation coil KIM are grounded through the normally closed contacts of changeover switches JK2 and JK3, and the contactor is in standby mode. When control terminal C is "0" (low level), relay J1 engages, and the charging current of capacitor C5 causes relay J2 to engage. The excitation coil KM is energized through the normally open contact of changeover switch JK2, and the stored energy in capacitor C4 drives the low-power CNC contactor to engage. The LC circuit composed of capacitor C5 and relay J2 releases after a delay, de-energizing the excitation coil KIM. The residual voltage in KM is released through the normally closed contact of JK2, and the low-power CNC contactor remains engaged by permanent magnet force. When the control terminal C is "1" (high level), relay J1 releases, the charging current of capacitor C6 causes relay J3 to engage, and the excitation coil KM is energized in reverse through the normally open contact of switch JK3. The energy stored in capacitor C7 drives the low-power CNC contactor to disconnect. In the LC circuit composed of capacitor C6 and relay J3, after a delay, relay J3 releases, the excitation coil KM is de-energized, and the disconnected state is maintained by the support spring. Using relay J1 in the interface circuit improves the anti-interference capability of the low-power CNC contactor. Relay J1 can be directly driven by programmable logic controllers such as integrated circuits, microcontrollers, PLDs, LOGOs, and PLCs. In addition, the interface circuit can be used to extend the contactor's functions such as overheat protection, overload protection, and time delay, making the external function extension module of the low-power CNC contactor electronic.

2.2 Single-pulse trigger current

The opening and closing operation of a low-power CNC contactor requires applying a positive or reverse single-pulse tactile current to the excitation coil, and its dynamic process changes very complexly. A single-pulse tactile current refers to the tactile current flowing through the excitation coil as a pulse wave when the contactor is closed or opened. Figures 2 to 5 are a comparison of the tactile current waveforms of the SD-100 low-power CNC contactor and products from well-known foreign companies. Figures 2 and 3 are the tactile current waveforms flowing through the excitation coil when the SD-100 low-power CNC contactor is closed and opened, respectively. The technical information shown in the figures is as follows: (1) The tactile current flowing through the excitation coil is a single pulse. The slowing of the falling edge of the pulse is due to the influence of the reverse electromotive force generated during the movement of the moving iron core to the stationary iron core. This indicates that the movement process is complete. (2) The starting and ending points of the positive and reverse tactile currents are close to the zero-crossing point, realizing "arc-free" switching. (3) The rising edge of the pull-in and disconnection current waveform is steep, indicating that its motion control performance is excellent.

Figure 4 shows the trigger current waveform of the foreign GMC-100 AC contactor. Its characteristic is the use of high-frequency modulation technology to control the trigger current, avoiding inrush current during engagement. It automatically switches to the holding state after the contacts are fully closed. Figure 5 shows the trigger current waveform of the foreign LC1-D115 AC contactor, characterized by direct AC current starting. It automatically switches to the holding state after the contacts close. Its control circuit design is sophisticated, but it has the technical defect of not being able to suppress inrush current. From the above analysis, the technical advantages of low-power CNC contactor drive methods are clearly evident.

2.3 Permanent Magnet Mechanism

Figure 6 shows a schematic diagram of the core structure of the permanent magnet mechanism [1]. In the figure: 1 is the permanent magnet; 2 is the stationary core; 3 is the excitation coil; 4 is the moving core. The stationary core is an E-type structure and the moving core is an I-type structure. The core is made of silicon steel sheets and adopts a double permanent magnet structure. The permanent magnet is embedded in the middle of the bottom of the groove of the "E-type" stationary core (as shown in the right figure). When the contactor is in the "disconnected" state, the magnetic resistance of each branch from the position of the permanent magnet to the core connection is similar, and the static magnetic field distribution between the connection points is balanced. Since the permanent magnet is far away from the moving core, the attraction force on the moving core is relatively weak. Even if the moving core is subjected to a certain degree of external force interference, it will not cause malfunction. When the contactor is engaged, it can maintain a stable "engaged" state along the low magnetic resistance magnetic circuit generated when the core is closed.

The closing and opening of the main contacts of the low-power CNC contactor are accomplished by the movement of the moving iron core, which is supported by a spring. A permanent magnet is embedded in the stationary iron core, adding a magnetic source to the magnetic circuit, giving the low-power CNC contactor new technical characteristics in its closing, holding, and opening processes. Closing characteristics: The permanent magnet mechanism is still an electromagnetic operating mechanism; the magnetic attraction force on the moving iron core mainly comes from electromagnetism. The stationary iron core is composed of silicon steel sheets, which have better magnetic permeability than permanent magnets. Based on the anisotropy of permanent magnets, they are "embedded" in the stationary iron core. The silicon steel sheets on both sides of the stationary iron core remain intact, forming a special combination of soft and hard magnetic circuits within the stationary iron core. Through the superposition of multiple magnetic circuits, the dynamic magnetic attraction force is optimized. Combined with a capacitor-stored drive circuit and a single-pulse trigger current excitation method, the starting power of the low-power CNC contactor is significantly reduced. The "closing" of the main contacts is accomplished by the combined action of electromagnetic force and permanent magnet force. This composite magnetic force eliminates the drawback of contactor contact bounce.

Figure 7 shows a comparison of the magnetic circuit of this permanent magnet mechanism with that of the technical solution with non-magnetic clamps [3]. The superposition of multiple magnetic circuits in the E-type stationary iron core is very obvious. Holding characteristics: The holding process is divided into holding when attracted and holding when disconnected. When attracted, the excitation coil has no holding current and the magnetic force of the permanent magnet is used to maintain a stable attracted state, requiring the permanent magnet to be as strong as possible; while when disconnected, in order to avoid malfunction, the magnetic force is required to be as weak as possible. Practice has shown that the magnetized permanent magnet not only has residual magnetization intensity, but can also be magnetized by the external magnetic field to generate induced magnetization intensity. When the low-power CNC contactor operates, the magnetic properties of the permanent magnet will be repeatedly affected by the change of the magnetic field of the excitation coil when attracted or disconnected. The magnetic field generated by the current of the excitation coil during the attracting process is in the same direction as the magnetic field of the permanent magnet itself, thereby generating induced magnetization intensity, which is a magnetization process for the permanent magnet, enhancing the magnetic field strength of the permanent magnet. When the excitation current is eliminated, the permanent magnet will still maintain the moving iron core in the attracted state with a strong magnetic force. During disconnection, the magnetic field generated in the excitation coil undergoes a demagnetization process for the permanent magnet. The demagnetizing motive force causes the magnetic field strength of the permanent magnet to vary within the range of the recovery line. This alternating magnetization and demagnetization does not alter the magnetic stability of the permanent magnet, enabling it to possess a strong magnetic force when held in the closed state, while its magnetic force is relatively weak when held in the open state. Disconnection characteristics: Due to the embedding of the permanent magnet, a reverse current must be applied to the excitation coil during disconnection to overcome the attraction of the permanent magnet to the moving iron core. The use of a single-pulse current drive method provides good controllability for contactor disconnection.

2.4 Three-terminal wiring method

The symbols are shown in Figure 8. A1 - Power terminal, A2 - Common terminal, A3 - Control terminal.

The terminals are power terminals, control terminals, and a common terminal, similar to the collector, base, and emitter of a transistor. The power terminals and common terminal are connected to the power supply; when the power is on, the contactor is in standby mode. The control terminals and common terminal are connected to the signal source; the connection, carrying, and disconnection of the main circuit current are controlled by the effective level of the signal source. The addition of a control terminal, similar to the base of a transistor, significantly reduces the difficulty of operating the contactor. The control terminal's driving capability is shown in the table below:

3. Field industrial control systems[5]

Figures 9 and 10 show an industrial control system composed of a low-power CNC contactor and a transistor output PLC. This demonstrates that this connection method can reduce the complexity and cost of the control system, which is particularly important for improving the operational reliability of large-scale and complex control systems.

Figure 9 shows an application example of the low-power CNC contactor connected to the switching power supply and control signals in this project. The switching power supply of the control system is 50W. If a similar advanced product from abroad were used to build the control system, its power would be 1200W, and the PLC control of the AC contactor would require conversion via an intermediate relay. The programmable logic controller in this project is a PLC. The output terminals Q0, Q1, Q2, and Q3 are connected to the control terminals C of the four low-power CNC contactors, forming a commercial hardware platform. During use, corresponding control programs can be programmed according to specific circumstances to achieve the expected control objectives. Figure 10 shows an application example of this low-power CNC contactor in an industrial field. In the figure, 16 low-power CNC contactors and a PLC-226 are connected to a 300W switching power supply. The input terminals of the built-in decoding circuits of the low-power CNC contactors K1 to K16 are connected to the output terminals of the encoding circuit via connecting wires. The encoding circuit is then connected to the output terminals of the PLC. The technical advantage of this example is that the control system uses only three connecting wires, with a permissible length of 200 meters. A centralized power supply method is adopted to power multiple low-power CNC contactors, fully leveraging the safety and efficiency advantages of switching power supplies. The use of encoding and decoding control circuits greatly simplifies the wiring and connection of the control system, which has significant practical implications for the design of control systems in industrial settings. Coupled with a fieldbus interface, the complexity of the bus system design can be significantly reduced.

4. Conclusion

This paper presents a low-power CNC contactor that successfully solves three major technical challenges for contactors ranging from 100A to 800A: low-power starting, controllable motion, and operation in harsh environments. It also provides a new type of fundamental switching element for industrial automation control and low-voltage power supply control systems. The updating of fundamental components will fundamentally change traditional design concepts. The resulting product upgrades will rapidly improve the overall level of my country's equipment manufacturing industry and the core competitiveness of downstream product manufacturers, promoting technological innovation and development in the low-voltage electrical appliance industry.

References

[1] LIN Xin. Permanent magnet and vacuum breaker. Beijing: China Machine Press, 2002.

[2] WANG Renxiang. Common low-voltage apparatus and its controlling. Beijing: China Machine Press, 2001.

[3] Cai Yuanyu. Electrical circuit and magnetic circuit. Beijing: Higher Education Press, 1992.

[4] "The enterprise standard of Sichuan RongGao electrical corporation" Q/78014280-X.2-2006. Serial number: B51.1378-2006. Chengdu.

[5] Liu Jinping. Low voltage digital controlled contactor and its controlling system: China, ZL2005100216420. Authorization date: 2006-12-20. Liu Jinping (1952), male, university graduate, chief engineer, research direction is CNC low voltage electrical appliances. Email: [email protected] Liu Hao (1982), male, master, engineer, research direction is the stability of power system and its software design. Email: [email protected] Liu Yujie (1982), female, bachelor, engaged in electronic circuit design and development.