Brief introduction of three-phase AC circuit
A circuit powered by a three-phase AC power supply. Referred to as three-phase circuit. Three-phase AC power supply refers to a power supply that can provide three voltages or currents with the same frequency but different phases. The most commonly used is a three-phase AC generator. The phases of the voltages of the three-phase generators differ from each other by 120 °. The order in which the voltage of each phase leads or lags between them is called the phase sequence. The three-phase motor rotates forward when the positive sequence voltage is supplied, and reverses when changed to the negative sequence voltage. Therefore, you must pay attention to the phase sequence when using a three-phase power supply. Some production equipment requiring forward and reverse rotation can control the forward and reverse rotation of the three-phase motor by changing the power supply phase sequence.
The three-phase power connection is commonly used in star connection (ie Y-shaped) and delta connection (ie △ -shaped). The three wires drawn from the three starting ends of the power supply are called end wires (commonly known as fire wires). The voltage between any two end wires is called the line voltage. In a star connection, the line voltage is twice the phase voltage; the phase difference between the three line voltages is still 120 °, which is 30 ° ahead of the three phase voltages. The star connection has a common point, called the neutral point. The line voltage is equal to the phase voltage when the triangle is connected, and the three power supplies form a loop. Only when the three-phase power supply is symmetrical and connected correctly, there is no circulating current inside the power supply.
Three-phase loads are divided into symmetric three-phase loads and asymmetric three-phase loads according to whether the three-phase impedance is equal. Three-phase motors, three-phase electric furnaces, etc. belong to the former; some three-phase loads connected by single-phase electrical equipment (such as household electricity and lighting loads) usually take one end line and a neutral line drawn from a neutral point Ground wire) for one phase user, take the other end line and neutral line to another phase user. In this connection, the loads on the three end lines cannot be completely equal, which is an asymmetric three-phase load. The connection mode of three-phase load is also divided into star and triangle.
The instantaneous power of a three-phase circuit (see power in an AC circuit) is equal to the sum of the instantaneous power of each phase. which is
P ï¼ PA ＋ PB ＋ PC
The subscripts in the formula indicate each phase. For symmetrical circuits,
At this time, UA = UB = UC = UP, where UP, IP, U, and I are the effective values ​​of phase voltage, phase current, line voltage, and line current, respectively. The average power of a symmetrical three-phase circuit is equal to its instantaneous power. The reactive power is UIsin and the apparent power is. The instantaneous power of the symmetrical three-phase circuit is constant. Therefore, the reaction torque suffered by the prime mover driving the three-phase generator during normal operation and the output torque of the three-phase motor are both stable.
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