Adaptive Holding Control Strategy of AC Contactor Based on Multiple Feedback Parameters

With the extensive use of AC contactors, energy consumption has become a problem that cannot be ignored, in which the coil holding energy consumption is the main source. In order to reduce the energy consumption, the reference value of the holding voltage or current must be as low as possible, and the control system should have the adaptive adjustment function to ensure that the effectiveness of the control is not affected by the aging of the AC contactor or the change of the mechanism characteristics. The closed-loop control mode with coil current feedback can effectively avoid the unreliable holding problem caused by temperature rise by controlling the duty cycle of the coil voltage in real time to keep the coil current at a constant value. However, the closed loop control mode usually uses several times of the critical holding current as the reference value, which will cause additional energy consumption and cannot achieve energy-saving operation in real sense. At the same time, the switching electronic devices in the coil excitation circuit need to be on and off constantly, that will bring additional switching losses. Consequently, giving consideration to both reliable holding and energy saving has become the research focus of holding process control. This paper proposes an adaptive holding control strategy based on multiple feedback parameters, which make the contactor have high holding stability even under low holding current. Through theoretical derivation, simulation and experiment, this paper analyzes the change rule of induced electromotive force when unreliable holding event occurs, and draws the conclusion that the generation of induced electromotive force is almost synchronous with the occurrence of unreliable holding events, and the time interval from the occurrence of induced electromotive force to the contacts breaking is more than 10ms. During this period of time, it is sufficient for secondary control of the contactor. The topology control circuit of the contactor coil is designed, which can adaptively adjust the holding voltage by using the numerical control switching power supply to meet the needs of AC contactor starting and adaptive holding in the closed-loop control mode. Taking coil current, contact current and coil induced electromotive force as feedback parameters, an adaptive holding control strategy for AC contactors based on multiple feedback parameters is proposed. Under normal conditions, the contact current and coil current are used as basic feedback parameters to adaptively adjust the holding voltage. If the moving iron core and the static one are separated abnormally, an induced electromotive force of the coil will be generated. When the induced electromotive force reaches a certain threshold, it will be detected by the single chip microcomputer in the control circuit. At this time, the coil is excited strongly by the circuit to make the moving iron core close again to ensure the reliability of the holding. Based on the control strategy, the control module is designed and tested. The results show that the control module can achieve the following functions. ① When the contact current increases, the holding voltage of the coil can change according to the change amplitude of the contact current, indicating that the control strategy can effectively prevent the unreliable holding of the contactor caused by overload. ② When the coil current drops to a certain threshold, the output voltage of the holding power supply rises, ensuring that the AC contactor can still operate reliably when the coil is powered on for a long time or the coil is heated due to the rise of the ambient temperature. ③ In case of aging of the contactor, change of mechanism characteristics, or unreliable holding of the contactor due to external vibration and other emergencies, the multiple feedback parameter adaptive holding control module will conduct secondary control on the coil based on the induced electromotive force, effectively prevent the separation of moving and static contacts, and ensure the normal operation of the main circuit. © 2023 Chinese Machine Press. All rights reserved.