Mainly to reduce load current. In general, high-power motors are connected in delta. In the case of the same power, the rated current of the 380V coil is three times smaller than the rated current of the 220V coil (√3=1.732). The rated current is reduced, and the wire diameter of the coil enameled wire of the corresponding motor stator winding is also smaller. This is of great benefit to the manufacturing process of the motor and the heat generated during the operation of the motor. Therefore, motors below 4kW are usually connected in star connection, while motors above 4kW are connected in delta. The difference between star connection and delta connection of three-phase AC asynchronous motor is: Each phase winding of a three-phase asynchronous motor has two lead ends, one is called the head end and the other is called the end. According to the national standard, the first end of the first phase is represented by U1, and the end is represented by U2; as shown in the figure below:
The head and end of the second phase winding are represented by V1 and V2 respectively; the head and end of the third phase winding are represented by W1 and W2 respectively. These 6 terminals are respectively led out to the junction box of the motor casing, and the corresponding wiring for electricians is marked. By switching the first and last terminals of the binding post in the junction box, it can be conveniently connected into star connection Y or delta connection according to different power supply voltages. See the picture below: According to the connection shown in the figure above, short-circuit W1, U1, and V1 with copper connecting pieces to form a star connection. The characteristics of the star connection method are: it helps to reduce the withstand voltage of the winding coil (220V), reduces the insulation level, and reduces the starting current; the disadvantage is that the power of the motor is reduced, so most of the small-power motors below 4kW use star connection Law. The triangle △ connection method is shown in the figure below: Motors with a power of 4kW and above use a delta connection. Triangular connection method The triangle △ connection method is to short connect W1 and U2 together with a copper connecting piece to form a power input lead; then short connect U2 and V2 together to form a power input lead; finally connect V1 and W2 Short together for the third voltage input lead. Delta connection method: helps to increase the power of the motor, but the disadvantage is that the starting current is large, and the stator coil winding of the motor has to withstand voltage (380V), which increases the insulation level of the motor. friendly reminder:The beginning and end of the stator winding of the motor are pre-determined by the manufacturer, and the right and wrong cannot be reversed arbitrarily, but the beginning and end of the three-phase winding coils can be reversed together (such as U2, V2, W2 as the beginning, and U1, V1, W1 is used as the end, but it is absolutely impossible to reverse the beginning and end of a phase winding alone), which will cause wiring errors. The structure of three-phase asynchronous motor The main components of a three-phase AC asynchronous motor are shown in the figure below: It is mainly composed of two parts, the stator and the rotor. The stator refers to the stationary part of the motor. It is mainly composed of machine base, stator core, end cover, stator three-phase symmetrical winding and so on. The frame is usually made of cast iron or steel. The stator core is press-fitted in the frame. The stator core is a part of the magnetic circuit of the motor. In order to reduce iron loss, 0.5mm thick silicon steel sheets are stacked into a cylindrical shape and pressed into the machine base. On the inner circumference of the iron core, a number of evenly distributed slots are punched for embedding the three-phase winding of the stator. The stator windings of medium and small motors are generally made of enameled wire. The three-phase symmetrical winding of the asynchronous machine has a total of 6 outlets. The start and end of each phase winding are represented by U₁, V₁, W₁ and U₂, V₂, W₂ respectively, and they are usually drawn from the junction box on the base. The three-phase symmetrical stator windings can be connected in a star shape or in a delta shape, depending on the line voltage of the power supply and the rated voltage of the winding. For example, the line voltage of the power supply is 380V, and the rated phase voltage of the motor stator winding is 220V, then the winding must be connected into a star; if the rated voltage of the winding is 380V, the winding must be connected into a triangle. Only in this way can each phase winding be guaranteed to work under its rated voltage.
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