Y-△ start circuit with instantaneous switching and uninterruptible power supply

Conventional motor Y-△ start, when the motor switches from Y-shape start to △-shape operation, in order to prevent phase short circuit, the motor windings will be in a momentary power-off state, which will inevitably cause a strong impact on the power supply network and the motor itself. secondary surge current. The circuit as shown in the figure can realize that the motor windings are not powered when the motor switches from Y-shaped start to △-shaped operation, and the motor winding voltage is not directly switched from the phase voltage during Y-shaped start to the line voltage during △-shaped operation. Instead, it goes through an intermediate voltage level transition, that is, a resistor group R is connected in series, thereby achieving the purpose of reducing voltage fluctuations and inrush currents in the power supply network and starting the motor smoothly. Principle: Press the start button ST, the AC contactor KM3 and KM1 coils are energized and closed, the time relay KT is energized for a delay, and the motor enters a Y-shaped step-down start. When the KT delay ends, contactor KM4 is closed and KM3 is cut off, the resistor group R is connected in series, and the motor enters the intermediate voltage level transition state before delta-shaped operation. During this switching process, since KM4 is turned on first and KM3 is turned off later, there is no power loss in the motor winding. After KM3 is cut off, the intermediate relay K is energized, the contactor KM2 is energized, KT is cut off, and finally KM4 is cut off, so that the resistance group R exits. At this point, the motor enters the delta-shaped normal operating state. Because KM2 is put in first and KM4 is cut off later, there is no power loss in the motor M winding during the switching process.