Current Status of Electrical Performance Testing Techniques for Contactors
Research into dynamic testing techniques for contactors and other switching devices featuring electrical contacts primarily focuses on the following aspects:
1. Automated Contact Parameter Detection Systems Utilizing a Computer as the Host and an A/D Sampling Board or DSP as the Slave Device
This approach employs a self-developed computer-based system for testing and controlling the electrical life of relays. This system captures overvoltage signals during three distinct phases of the relay's electrical life test: the beginning, the middle, and the end. Utilizing a proprietary A/D sampling board-or a high-speed data acquisition card centered around a DSP-the system samples key electrical parameters of the contacts, such as contact voltage drop, the voltage across open contacts, and the main circuit current. The control section employs a digital I/O board to drive solid-state relays, thereby switching the contactor or relay on and off. In terms of software, the system utilizes Visual Basic (VB) programming, incorporating interrupt service routines to execute functions such as data sampling and logic control. The data processing aspects described in the literature focus primarily on calculating the power grid frequency and power factor. By analyzing the captured voltage signals and applying the Fast Fourier Transform (FFT), time-domain signals are converted into frequency-domain signals. The results of this transformation are stored in separate arrays for the real and imaginary components; the position at which a peak value occurs corresponds to the power grid frequency, which is then calculated using a specific formula. By subjecting the collected data to a Fourier Transform-converting time-domain signals into frequency-domain signals-the phase angles of the voltage and current are determined, thereby enabling the calculation of the power factor.
2. Relay Parameter Detection Techniques Based on Microcontroller Control Technology
As the level of automation in electrical device testing continues to advance, microcontrollers are being increasingly applied to the testing and control of various electrical apparatus. By refining traditional test rigs used for switching (make and break) operations of AC contactors, a microcontroller is integrated as the control module to manage the switching actions of the AC contactor. The detection of contact electrical parameters is primarily accomplished through the use of voltage and current transformers, data acquisition cards, and a personal computer (PC). This system enables the real-time acquisition of dynamic waveforms-such as contact voltage and current-during the make and break processes of the contactor. Compared to traditional oscilloscope-based testing methods, this system offers superior accuracy in recording the voltage waveforms associated with contact arc burning. Visual C++ 6.0 software is utilized to develop the data acquisition program and the human-machine interface (HMI); the data processing module performs real-time, automated analysis of the collected data, thereby minimizing errors typically associated with manual waveform processing. This experimental scheme is simple and feasible, enabling the analysis of contact voltage and current waveforms during the dynamic make-and-break processes of AC contactors. In the literature, a relay electrical parameter testing device developed by Zhang Qiang et al. utilizes an enhanced 89C51 microcontroller as its core; by incorporating AC and DC voltage sources as well as contact detection circuits, it can test various electrical parameters-such as operating time, operating voltage, and contact resistance-for a wide range of AC and DC voltage relays. Regarding the measurement of operating time, the normally closed (NC) contact of the relay under test is connected to a high-level signal, while the normally open (NO) contact is grounded. Simultaneously with the application of the coil's rated voltage, a timer is initiated to begin counting, and a contact-level detection circuit is established to monitor changes in the contact's voltage level in real time. The operational status of the contacts is determined based on how their voltage levels change. When the voltage level transitions from high to low, the timer is immediately stopped; the elapsed time displayed by the timer at this moment represents the corresponding pickup time. The relay's dropout time can be determined using an analogous procedure. Furthermore, this testing device is capable of monitoring the contact resistance. Offering a high cost-performance ratio, this device serves as a valuable reference for the development of the experimental apparatus required for the current research project.
3. Application of Virtual Instrument Technology in the Parameter Testing of Switching Devices
With the advancement and maturation of virtual instrument technology, it is increasingly being applied to the testing of switching devices such as relays and contactors. Virtual instrument technology is a novel, software-centric measurement methodology that can significantly reduce the cost of test instrumentation. Measurement functions are primarily implemented through software programming; supported by a hardware platform centered on an industrial PC, the testing capabilities of the instrument are realized via programming within the LabVIEW software development environment. The LabVIEW application library contains a multitude of test and control modules designed for various purposes, allowing users to directly invoke relevant modules within a LabVIEW application to execute a wide array of testing functions. Compared to traditional text-based programming languages-such as Assembly, VB, and VC-writing software programs in LabVIEW is remarkably simple. Once the drivers for a data acquisition (DAQ) card have been installed within the LabVIEW environment, the card's functional routines can be invoked to facilitate control over the device, as well as to perform data acquisition, processing, and display tasks.
4. Methods for Obtaining Relay Time Parameters
The determination of relay time parameters has traditionally relied on analog testing methods, such as the use of electric chronometers and light-beam oscilloscopes. These conventional testing methods are characterized by slow measurement speeds, significant margins of error, and a lack of precision. With the advancement of computer technology, an increasing number of relay testing devices are incorporating microprocessors; the underlying principles of these devices are generally similar. One study describes a circuit for detecting relay timing parameters, the primary component of which is a single-chip microcontroller. The detection principle is as follows: when the relay contacts close, the voltage at the corresponding input channel of the microcontroller registers as 5V, setting the port state to "1"; conversely, when the relay contacts open, the corresponding voltage drops to 0V, setting the I/O port state to "0." When an excitation voltage is applied to the relay, the microcontroller samples the corresponding digital I/O port at a sufficiently short sampling interval; through subsequent data processing, the relevant timing parameters can be calculated. However, while this method yields generally accurate results when the relay is connected to a DC load, it proves problematic when connected to an AC load. Because AC voltage is alternating, the instantaneous voltage at the microcontroller port may be very low-or even close to zero-at the precise moment the relay contacts open. Consequently, when the circuit connected to the contacts is an AC circuit, relying on the magnitude of the instantaneous voltage across the contacts to determine their open or closed state introduces significant errors, thereby preventing the acquisition of accurate timing values. Another study describes a computer-based method for detecting relay timing parameters, utilizing a custom-developed data acquisition board comprising a microcontroller and its peripheral circuitry. This method enables the detection of various timing parameters, including relay operate time, operate bounce time, release time, and release bounce time. The microcontroller is integrated into the coil drive circuit to control the energization and de-energization of the excitation coil; simultaneously, it captures the contact states during the relay's closing and opening transitions and calculates the corresponding timing parameters. The detection principle is as follows: when the relay coil is energized, a specific "operate time" must elapse before the contacts close; therefore, the microcontroller initially captures a data value of "0," and only captures a "1" once the contacts have closed and stabilized. During this transition, the contacts typically undergo "bouncing" (momentary oscillations) before reaching a stable state; throughout this interval, the data captured by the microcontroller may fluctuate between "0" and "1." By setting the microcontroller's sampling interval to a specific value-for instance, 0.01 ms-the required operate time can be calculated by multiplying the address index (or sample count) of the captured data point by the sampling interval.
5. Dynamic Performance Testing Techniques and Comprehensive Evaluation Methods for Contactors
The assessment of the technical performance of electrical apparatus primarily relies on type testing. This method focuses on evaluating the mechanical and electrical service life of the device but is unable to provide a comprehensive assessment of its dynamic characteristics or the impact of these characteristics on its mechanical and electrical longevity. Consequently, research into the comprehensive performance evaluation of electrical apparatus-based on the detection of dynamic characteristics-holds significant practical guidance value for both the development and factory acceptance testing of such products. The cited literature describes a process involving the dynamic testing of AC contactors; by extracting parameters indicative of the contactor's mechanical and electrical properties from contact waveform data, and by subsequently establishing a comprehensive performance evaluation model, a complete system for assessing the dynamic performance of contactors is realized. The dynamic testing apparatus for AC contactors is built around a DSP core, incorporates various signal sensors, and facilitates data communication with a host computer via an RS232 interface. This testing apparatus is capable of measuring key electrical parameters, including the contactor's excitation current and voltage, as well as the coil power consumption during the pull-in process. This work provides practical guidance for the construction of the experimental apparatus required for the current research project; furthermore, the proposed contactor performance evaluation system offers significant reference value for the investigation into contactor performance degradation and reliability estimation that constitutes the focus of this study.
