Delta connection and star connection are unique conversion relationships in three-phase motor wiring. Different connection methods for the same motor , and different connection methods for motors with the same rated voltage and power, involve interesting parameter relationships. There is a mysterious number involved, which Ms. Can will summarize and share with you.
Parameter relationships of star junction
Star connection and delta connection are unique wiring methods for three-phase motors. For the same motor, different wiring methods will correspond to different rated voltages. The most direct difference and the relationship between related parameters can be summarized as follows: When star connection is used, the line voltage is equal to 1.732 times the phase voltage, and the phase current is equal to the line current; when delta connection is used, the phase voltage is equal to the line voltage, and the line current is equal to 1.732 times the phase current.
For motors of the same power and under the same rated voltage, if designed with a star connection, the wire diameter is thicker, the phase voltage is lower, and the corresponding number of coil turns is less; conversely, if designed with a delta connection, because the line voltage is equal to the phase voltage, the corresponding number of coil turns is more, the current per turn is smaller, and the wire diameter is thinner. Therefore, the following conclusions can be drawn: (1) When changing from a star connection to a delta connection, the total cross-sectional area of the wire diameter in the original star connection divided by 1.732 equals the total cross-sectional area of the wire diameter in the delta connection; (2) When changing from a delta connection to a star connection, the total cross-sectional area of the wire diameter in the original delta connection multiplied by 1.732 equals the total cross-sectional area of the wire diameter in the star connection. Or, it can also be described as follows: (1) The conductive cross-sectional area in the delta connection is 0.57736 times that in the star connection. That is, the total cross-sectional area of the wire diameter in the delta connection divided by 0.57736 equals the total cross-sectional area of the wire diameter in the star connection; (2) The total cross-sectional area of the wire diameter in the star connection multiplied by 0.57736 equals the total cross-sectional area of the wire diameter in the delta connection.
Introduction to Three-Phase Asynchronous Motors
Compared to single-phase asynchronous motors, three-phase asynchronous motors have better operating performance and save materials. Three-phase asynchronous motors belong to the category of induction motors and are powered by three-phase alternating current power supplies with a certain rated voltage and a phase difference of 120 degrees. Because the rotor and stator rotating magnetic fields of a three-phase asynchronous motor rotate in the same direction but at different speeds, resulting in a speed difference, it is called a three-phase asynchronous motor.
The rotor speed of a three-phase asynchronous motor is always lower than the rotational magnetic field speed. The rotor winding generates electromotive force and current due to the relative motion between it and the magnetic field, and interacts with the magnetic field to generate electromagnetic torque, thus realizing the conversion of electrical energy into mechanical energy.
Based on their rotor structure, three-phase asynchronous motors can be divided into two types: squirrel-cage and wound-rotor. Squirrel-cage asynchronous motors are widely used due to their simple structure, reliable operation, light weight, and low cost; their main disadvantage is the difficulty in speed regulation. Wound-rotor three-phase asynchronous motors, like their stator, also have three-phase windings on the rotor, which are connected to an external rheostat via slip rings and brushes. Adjusting the rheostat resistance can change the speed corresponding to the maximum torque, improving the motor's starting performance and regulating its speed.
The windings of a three-phase asynchronous AC motor can be designed as single-layer windings, double-layer lap windings, and mixed single- and double-layer windings. A single-layer winding is a winding in which only one effective coil side is embedded in each stator slot, and the total number of coils is half the total number of slots in the iron core.
The advantages of single-layer windings are that they have fewer coils and are simpler to manufacture; there is no interlayer insulation, thus improving slot utilization; and the single-layer structure prevents phase-to-phase breakdown within the slot. The disadvantages are that the electromagnetic waveform generated by the winding is not ideal, resulting in higher iron losses and noise in the motor, and poor starting performance. Therefore, single-layer windings are generally only used in small-capacity motors.
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