An alternator typically consists of a rotor assembly, stator assembly, rectifier assembly, end covers, pulleys, and a fan. Today, we'll explain its working principle with illustrations to help you understand it better.
1. Generation of alternating electromotive force
The working principle of an AC generator is shown in Figure 1.
Figure 1 Working principle of AC generator
The three-phase windings of the AC generator stator are nested in the stator slots according to a certain pattern, with each phase differing from the others by 120° electrical degrees. When the rotor field winding is connected to a DC power supply, the rotor's claw poles are magnetized into N and S poles. The magnetic field lines start from the N pole, pass through the very small air gap between the rotor and stator into the stator core, and finally return to the S pole through the air gap.
Figure 2. Voltage and current waveforms in a three-phase bridge rectifier circuit.
a) Three-phase bridge rectifier circuit; b) Three-phase AC electromotive force; c) Output waveform of the rectified AC generator.
2. Working principle of rectifier circuit
The process of converting alternating current (AC) generated by an AC generator into direct current (DC) is called rectification. Common rectifier circuits include those for six-transistor AC generators and those for nine-transistor AC generators.
1) Rectifier circuit of a six-tube AC generator
The rectifier of a six-tube AC generator is actually a three-phase bridge rectifier circuit composed of six silicon rectifier diodes (see figure). The negative terminals of three diodes, VD2, VD4, and VD6, are connected to the beginning of the three-phase windings of the generator, and their positive terminals are connected together, forming a common anode group. The conduction principle of these three diodes is that the diode with the lowest negative potential at any given moment conducts. The positive terminals of three diodes, VD1, VD3, and VD5, are connected to the beginning of the three-phase windings of the generator, and their negative terminals are connected together, forming a common cathode group. The conduction principle of these three diodes is that the diode with the highest positive potential at any given moment conducts first. At any given moment, two diodes conduct simultaneously, one in the common cathode group and the other in the common anode group. The two diodes conducting simultaneously always apply the generator voltage to the load terminals (as shown in the figure).
When t = 0, the C-phase potential is the highest, while the B-phase potential is the lowest, and the corresponding diodes VD5 and VD4 are both forward-conducting. Since the internal resistance of the diodes is very small, the generator's output voltage is equal to the line voltage between the B and C windings at this time.
During the time interval t1-t2, phase A has the highest potential, while phase B has the lowest potential. Therefore, VD1 and VD4 are in forward conduction. Similarly, the output voltage of the AC motor can be considered as the line voltage between windings A and B.
During the time interval t2-t3, phase A has the highest potential, while phase C has the lowest potential; therefore, VD1 and VD6 are in forward conduction. Similarly, the output voltage of the AC motor can be considered as the line voltage between the A and C windings.
By repeating this process, a relatively stable DC pulsating voltage can be obtained on the load.
2) Rectification principle of a nine-tube AC generator
The characteristic of a nine-tube AC generator is that, in addition to the commonly used six rectifier diodes, three lower-power diodes are added. These three lower-power diodes are used to supply the magnetic field current to the AC generator, hence they are also called magnetic field diodes. With the magnetic field diodes, a charging indicator light is connected to the output terminal to indicate the generator's operating status. The circuit diagram of the nine-tube AC generator charging system is shown in Figure 3.
Figure 3. Circuit diagram of a nine-tube AC generator charging system
3. Excitation method of AC generator
The excitation method of automotive alternators represents a process of evolving from separately excited to self-excited generation. Because the residual magnetism of the rotor in an automotive alternator is relatively weak, it cannot generate electricity using the residual magnetism of the poles; therefore, an external DC power supply is required. Alternators can only generate electricity self-excitedly at relatively high speeds. At low speeds, alternators use separately excited methods.
