Abstract: The automatic control device proposed in this paper can automatically switch the delta connection to a Y connection when the load of an asynchronous motor drops to 40% of its rated load. This improves the operating performance of the asynchronous motor and achieves energy saving. Keywords: squirrel-cage asynchronous motor, load, energy-saving current relay 1. Proposal of the Device As is well known, when a three-phase asynchronous motor uses Y-Δ starting, the starting current and starting torque can be reduced (see Figure 1). If the asynchronous motor is operating under light load (≤40 PN, where PN is the rated power of the motor), switching the delta connection to a Y connection can improve the power factor cos and efficiency η of the asynchronous motor, achieving a good energy-saving effect. This is because when switching from delta to Y connection, the stator winding voltage of the asynchronous motor decreases, which reduces both the excitation current I and the iron loss P, thus greatly improving the power factor cos and efficiency η of the asynchronous motor. Figures 2a, b, and c show the operating characteristics of an asynchronous motor in terms of efficiency η, power factor cos, and stator current I₁ in A-connection and Y-connection, respectively. Switching according to the load size of the asynchronous motor—when the load ≤ 40%Pn, changing the Δ connection to Y-connection; conversely, when the load exceeds the corresponding load power, changing back to Δ connection—is a better energy-saving measure. Manual switching is not only of poor quality (i.e., difficult to accurately switch to the predetermined power limit), but also presents significant challenges for operators. Based on analysis and research, a more complete automatic control device was designed. 2. Automatic Control Device and Working Principle The control principle of the automatic control device (see Figure 3) is as follows: Close the main circuit knife switch DK, press the start button QA₁ in the control circuit, and the line contactor C₁... The coil is energized and self-holding, while the coil of contactor C2 is energized and interlocked with the coil of contactor C3, causing the stator winding of the asynchronous motor M to be connected in a Y configuration for reduced-voltage starting. At this time, due to the energization of time relay SJ1, its contact delay action occurs. After the asynchronous motor has started for a certain period (i.e., the delay time of SJ), the normally closed contact of SJ opens C, and simultaneously the delay contact energizes C again and self-locks, thus the stator winding is switched to a delta connection for normal operation. Because C2 is energized and SJ is de-energized, the Y-Δ starting process ends. This is the control principle of Y-Δ starting. Based on this, the circuit can be modified to allow for Y-Δ reconnection according to the load size of the asynchronous motor, achieving energy saving. [b][align=center]For details, please click: Automatic Control Principle of Energy-Saving Reconnection of Asynchronous Motors[/align][/b]