Abstract: Contactors are currently the most widely used low-voltage electrical products. Contact bounce during closure and arcing during opening are the main factors affecting their service life. This paper proposes a novel contactor contact control method, achieving closed-loop control of individual contacts. This is a completely new concept in the field of intelligent electrical appliances and control. This paper studies the structure of an AC contactor, establishes a mathematical model, corrects the air gap permeability using Ansoft electromagnetic analysis software, and studies the dynamic characteristics of the contactor using Simulink simulation. A novel closed-loop control method is derived, which can effectively eliminate contact bounce during closure and arcing during opening, thereby improving the contactor's service life.
Keywords: contactor, closed-loop control, Ansoft, Simulink
1. Introduction
Contactors are widely used automatic control electrical components, mainly used for frequently connecting and disconnecting AC load circuits. They have advantages such as large control capacity and remote operation [1]. Due to frequent operation, contactors must have a sufficiently long service life to ensure a certain service life. Under normal circumstances, the electrical life of a contactor is generally 1/5 to 1/20 of its mechanical life [2]. Therefore, improving the electrical life is the key to improving the service life of a contactor [3].
This paper combines a closed-loop control system with a contactor, achieving closed-loop control of the contactor's motion process through speed feedback. This enables contact closure without bounce and disconnection without arcing, improving the contactor's service life. Although closed-loop control is somewhat complex, it is highly adaptable and reliable.
2. Contactor Structure Analysis
This article takes the E-type iron core structure as an example for analysis. The dimensions and structure of the iron core are shown in Figure 1.
Figure 1 Schematic diagram of the electromagnet.
The maximum working air gap δ is 6mm and the minimum is 0.01mm. The number of coil turns is . The circuit equation, suction force equation and motion equation of the contactor are listed according to the size and structure of the contactor, and a mathematical model is established [4].
3. Air gap permeability optimization
Air gap permeability is a crucial parameter in electromagnetic force calculations. Traditionally, simplified magnetic circuit methods are used to derive the formula for air gap permeability. This paper employs a combined field-circuit approach to reduce model errors. First, the air gap permeability values for different air gaps are obtained through simulation using the finite element analysis software Ansoft Maxwell. Then, MATLAB is used to fit the corresponding curves and equations. Finally, the equations are substituted into Simulink for dynamic characteristic simulation. This combination of the accuracy of the Ansoft Maxwell field method and the convenience of the Simulink circuit method for studying motion processes significantly improves the accuracy of the model and motion control.
The Ansoft two-dimensional model of the contactor electromagnetic mechanism is shown in Figure 2.
Figure 2 Two-dimensional model of the contactor electromagnetic mechanism
In the image above, the pink portion represents the contactor's core and armature, while the two blue rectangles represent excitation coils pointing in opposite directions. By moving the position of the upper armature, the air gap permeability under different air gap lengths was calculated using Ansoft simulations. Observing the data distribution, it was found that the data closely conforms to an exponential distribution law. MATLAB was then used to fit the exponential function.
The final fitted function is:
(6)
4. Closed-loop control system
4.1 Control System Principle
This paper proposes an intelligent contactor control method based on motion process follow-up control, which performs closed-loop control of the contactor to achieve contactless closing. The schematic diagram of the control system is shown in Figure 3. It includes a drive circuit (4), a contactor (7), a speed detection (6), and a microcontroller, wherein the microcontroller includes speed setting (1), addition and subtraction operation (2), PID (3), and A/D conversion (5).
Figure 3 Schematic diagram of the control system
According to the hardware circuit diagram of the control system, this control system supplies voltage to the drive circuit via a microcontroller. After amplification and conditioning by the drive circuit, the voltage supplies power to the contactor coil, causing the iron core to move and initiating the opening and closing action. Speed detection typically uses a high-precision, fast-response speed sensor to detect the iron core's movement speed, obtaining the real-time motion state of the iron core. The sensor transmits the real-time motion state of the iron core to the microcontroller via an analog signal. After A/D conversion, the analog signal is converted into a digital signal that the microcontroller can recognize.
The speed setting is stored in the microcontroller. This speed setting is derived from the contactor's working air gap length, the mass of the contact system, and other factors, resulting in the optimal speed curve for the iron core's movement. The curve roughly follows this pattern: during closing, the contacts accelerate initially, then decelerate after reaching a certain position. This ensures that the contact speed is zero while the acceleration is still significant just before closing. The same principle applies during opening. The speed setting is compared with the received digital signal, and the difference is calculated through addition and subtraction. This difference is then used for PID control to obtain the optimal output voltage. After amplification and conditioning by the drive circuit, the voltage is supplied to the coil, forming a closed-loop control system.
When the actual speed is less than the given speed curve, the coil is energized, causing the armature to accelerate. When the actual speed exceeds the given curve, the iron core decelerates under the action of a reaction force, such as a spring force. This ensures that the actual speed of the iron core matches the given optimal speed curve. This achieves contactor-free opening and closing, eliminates secondary contact bounce, eliminates intermittent arcing, and breaks the circuit at the moment the current crosses zero, achieving arc-free breaking and significantly improving the contactor's service life.
4.2 Mathematical Model of Control System
This paper uses Simulink software to simulate the control system of an asynchronous combined contactor and studies its dynamic characteristics. Based on equations (3), (4), and (5), a closed-loop control system for the contactor is modeled. Figure 4 shows the Simulink model of the closed-loop control system. Subsystem1 is the circuit model, Subsystem2 is the magnetic circuit model, and Subsystem3 is the limit control module. The FF module represents the applied reaction force.
Figure 4 Simulink model of the closed-loop control system
The Simulink simulation results are shown in Figure 5.
Figure 5 Simulation results of the closed-loop control system during the opening and closing process.
As can be seen from the figure, the armature first undergoes constant acceleration during the release process. When the armature starts to decelerate at 0.3s, the armature speed is exactly 0 when it reaches the closed position (4mm), which means that the momentum of the armature is 0 when the circuit is closed, thus achieving a non-bounced closure. The total closing time is approximately 0.48s. During the release, the armature undergoes uniform speed change and turns at a speed of -24mm/s. The total release time is approximately 0.42s. This achieves a non-bounced closure, eliminates the arc generated during disconnection, and improves the electrical life of the contactor. The single-phase closing time is 0.5s and the release time is 0.42s, which is not much different from that of a conventional contactor [5] and does not affect the opening and closing speed during use.
5. Summary
This paper proposes a control method for asynchronous combined contactors, applying closed-loop control to intelligent contactors and exploring a new approach to achieve bounce-free opening and closing. Ansoft finite element analysis software is used to analyze and calculate the electromagnetic system of the asynchronous combined contactor. Based on the Ansoft simulation results, the air gap permeability is optimized using MATLAB data fitting, and a new air gap permeability formula is given and incorporated into the simulation model. Applying dynamic control of the motion process to the contactor eliminates many uncertainties affecting the contactor's movement time through closed-loop speed servo control. Staged control of the contactor contact movement process is achieved, eliminating contact bounce during contact closure and arcing during contactor opening, thereby improving the contactor's electrical life.
References
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