A circuit powered by a three-phase AC power supply, referred to as a three-phase circuit. A three-phase AC power supply is a power supply capable of providing three voltages or currents having the same frequency but different phases, and the most common one is a three-phase alternating current generator. The phase voltages of the three-phase generators are 120° out of phase with each other. The order in which the voltages of the phases between them lead or lag is called the phase sequence. The three-phase motor rotates forward when the positive sequence voltage is supplied, and reverses when it is changed to the negative sequence voltage. Therefore, the phase sequence must be noted when using a three-phase power supply. Some production equipment that requires forward and reverse can control the forward and reverse of the three-phase motor by changing the power phase sequence.
Three-phase power connection:
Commonly used are star connection (ie Y-shaped) and delta connection (ie delta shape). The three lines drawn from the three beginnings of the power supply are called end lines (commonly known as FireWire). The voltage between any two end lines is called the line voltage. The star voltage is a multiple of the phase voltage when the star is connected; the phase difference between the three line voltages is still 120°, which is 30° ahead of the three phase voltages. A star connection has a common point called a neutral point. When the delta connection is connected, the line voltage is equal to the phase voltage, and the three power supplies form a loop. When only the three-phase power supply is symmetrical and the connection is correct, there is no circulation inside the power supply.
Three-phase load:
Divided into symmetric three-phase load and asymmetric three-phase load 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 (such as domestic electricity and lighting electrical loads) connected by single-phase electrical equipment are usually taken from one end line and the neutral line drawn from the neutral point (commonly known as The ground line is supplied to one phase user, and the other end line and the center line are taken to another phase user. The load on the three end lines of this type of connection cannot be completely equal, and it is an asymmetric three-phase load. The three-phase load is also connected by a star and a triangle. The instantaneous power of the three-phase circuit (see power in the AC circuit) is equal to the sum of the instantaneous powers of the phases. That is, P=PA+PB+PC where the subscripts indicate the phases. For symmetrical circuits, UA=UB=UC=UP at this time, 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. Its reactive power is UIsin, and the apparent power is. The instantaneous power of the symmetrical three-phase circuit is constant. Therefore, the counter torque of the prime mover that drives the three-phase generator in normal operation and the output torque of the three-phase motor are stable.
