Three-phase alternator principle

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Three-phase alternating current is generated by a three-phase Alternator. Figure 3.1 (a) is a schematic diagram of the principle of a three-phase alternator. It is mainly composed of two parts, the rotating part inside is called the rotor, and the direct current is passed through the coil of the rotor, so that a magnetic field distributed in a sinusoidal manner is generated in the space; Figure 3.1 Principle of three-phase alternator (a) Schematic diagram of three-phase alternator; (b) The part of the three-phase electromotive force that is fixed to the outside in the positive direction is called the stator, and three structures are completely embedded in the core slot of the stator. The same coils U1—U2, V1—V2, W1—W2, which are 120° apart from each other in space, are called three-phase stator windings. U1, V1, and W1 are called the beginnings of three windings, U2, V2, and W2. Called the end. When the engine drags the rotor to rotate at an angular velocity ω, the three-phase stator winding cuts the magnetic lines of force to induce an electromotive force. Since the magnetic field is distributed according to the sinusoidal law, the induced electromotive force is a sinusoidal electromotive force, and the three-phase windings have the same structure, the cutting magnetic lines have the same speed, and the positions are mutually different by 120°, so the electromotive force induced by the three-phase windings has the same amplitude and the same frequency. The phases are 120° out of phase. Such a three-phase electromotive force is called a symmetrical three-phase electromotive force, and the direction of each phase electromotive force is directed from the end to the beginning, as shown in Fig. 3.1(b). And with eU as the reference quantity, the instantaneous value expressions of the three electromotive forces are: The waveform and phasor diagrams are shown in Figure 3-2. At any instant, the sum of the symmetrical three-phase electromotive forces is zero, ie The order in which the three electromotive forces in the three-phase power supply reach the maximum value (or zero value) in sequence is called the phase sequence of the three-phase power source, and the phase sequence of the three-phase power source represented by the equation (3.1) is U-V-W-U, that is, V The phase lags behind the U phase, and the W phase lags behind the V phase, which is called the positive sequence. If the phase sequence is W-V-U-W, it is called reverse order. The general purpose of engineering is positive sequence. The U phase can be arbitrarily assumed, but once the U phase is determined, the V phase is 120° behind the U phase, and the W phase is 120° ahead of the U phase, which is not confusing. In the industry, the three-phase busbars of the alternator lead-out line and the power distribution device are usually coated with yellow, green and red colors to represent the U, V and W three phases.