Automobile knowledge sharing-automotive generator structure and working principle by Asiautos Auto Parts(CHINA CAR Aftermarket Parts Expert)

The function of the rotor is to generate a rotating magnetic field.

The rotor consists of claw poles, a yoke, a field winding, a slip ring, and a rotor shaft.

Two claw poles are pressed on the rotor shaft. Each claw pole has six bird-beak-shaped magnetic poles. The claw pole cavity is equipped with a field winding (rotor coil) and a magnetic yoke. The collector ring consists of two copper rings insulated from each other. The collector ring is pressed on the rotor shaft and insulated from the shaft. The two collector rings are connected to the two ends of the field winding respectively.

When direct current is passed through the two collector rings (through the brushes), current flows through the field winding and generates axial magnetic flux, causing one claw pole to be magnetized as an N pole and the other to be magnetized as an S pole, thus forming six pairs of interlaced magnetic poles. When the rotor rotates, a rotating magnetic field is formed.

The magnetic circuit of the AC generator is: yoke → N pole → air gap between rotor and stator → stator → air gap between stator and rotor → S pole → yoke.

The function of the stator is to generate alternating current.

The stator consists of a stator core and a stator winding.

Each winding has several separate coils, which are placed in an iron bracket so that at the same time, the magnetic poles on each coil in the winding are the same. Since the windings are connected in series, the voltages induced in all windings are added to produce the required output voltage.

The rotor rotates freely in the stator support. As the rotor rotates, the claw arms on its pole pieces are positioned so that the magnetic field that changes between the north and south poles moves through the stator conductors. Since the magnetic poles that pass through the stator coils are changing, the induced voltage in each phase of the stator is an alternating current (AC) voltage.

The stator of the generator is connected in delta or star (Y) connection. The name delta comes from the triangle formed by the three phases in the stator.

To put it simply, a car generator is the application of the principle of electromagnetic induction in the car's electrical system.

Principle of electromagnetic induction:

When a magnetic field moves through a conductor, a voltage is generated.

A bar magnet, whose magnetic field rotates in a conductor coil. The rotating magnet is called the rotor, and the fixed conductor is called the stator. Then an induced voltage is generated in the stator.

A bar magnet, whose magnetic field rotates in a conductor coil. The rotating magnet is called the rotor, and the fixed conductor is called the stator. Then an induced voltage is generated in the stator.

As shown in the figure above, in the first half of the rotation of the bar magnet (rotor), the magnetic poles change position: the N pole moves directly below the upper conductor, and the S pole moves directly above the lower conductor. The induced voltage causes the current to flow in the opposite direction. The ends of the conductors are marked: A is the negative (-) pole and B is the positive (+) pole.

When the rotor completes the second half of the rotation, the N pole and S pole return to their original positions: A becomes the positive (+) pole and B becomes the negative (-) pole.

So the current starts flowing in one direction and then changes to the other direction, creating alternating current, and this alternating current is produced inside the generator. It is the origin of the word "alternator", which is sometimes applied to generators that use this principle. But the term recognized by the SAE is generator.

The alternating current generated at this time does not necessarily have a high voltage, and the next step is high voltage. There are three ways to increase the voltage:

1. Increase the number of coil turns in the stator. The more magnetic lines of force cut through the winding, the greater the current induced in the winding.

2. Increase the rotor speed, which causes the magnetic lines of force to be cut more frequently. Therefore, this leads to an increase in the voltage in the stator group. When the speed of the car engine increases, the rotor speed also increases. The belt connected to the engine crankshaft drives the rotor in the stator winding of the generator.

3. Increasing the rotor magnetic field strength and controlling the stator magnetic field strength are the most practical means of controlling the generator voltage output. This is because the magnetic field strength affects the induced voltage. The stronger the magnetic field, the higher the induced voltage. The weaker the magnetic field, the lower the induced voltage. The third method is achieved by replacing the rotor bar magnet with an electromagnet.

From a design perspective, the third method is to control the generator voltage when applied to an automobile.

For electromagnets, the amount of current flowing through the coil wire, the magnetic lines of force, or windings of the rotor, determines the strength of the rotor's magnetic field. Zero current produces zero voltage, medium current produces medium voltage, and maximum current produces maximum voltage output, or maximum magnetic field strength.

At any given moment, the maximum magnetic field strength can generate an induced voltage that is higher than the electrical system needs. The generator has a built-in voltage regulator that regulates the voltage and limits the voltage to a specified value by switching the field current on or off. This allows the generator, a single component, to both generate and regulate power, so the generator is considered a charging system.

Limiting voltage is important for many vehicle electronic components because unlimited voltage output can damage or shorten the life of batteries, light bulbs, external wiring harnesses, electronic modules and other electronic or electrical components.

The voltage regulator switches the field voltage at a fixed frequency of four hundred times per second. By varying the time that the field current is switched on and off, the voltage is controlled. Thus, at low speeds, the field may be on 90% of the time and off 10% of the time. This results in a relatively high average field current, which, combined with the generator speed, gives the desired voltage.

As the generator speed increases, the field current required to produce the required voltage decreases. This can be achieved by changing the duty cycle to reduce the average field current. For example, at high engine speeds, the voltage regulator may be on 10% of the time and off 90% of the time. As operating conditions change, the duty cycle will also change to provide the exact field current required for the system voltage.

Finally, how to convert the alternating current (AC) generated by the electromagnetic induction principle into direct current (DC) that can be used by automotive electronic components? This is where the rectifier comes in.

Generator rectifier (which contains diodes)

A series of diodes in a rectifier circuit, often called a diode bridge circuit, converts AC to DC voltage. A diode is a device that allows current to flow in one direction only, and will not conduct if connected in reverse.

A diode bridge circuit is a combination of diodes. Four diodes are connected to a loop conductor called AB to form a bridge circuit. When an AC voltage is induced in the AB loop, the voltage is converted to DC through the bridge circuit. Since all the AC voltage is converted to DC voltage, this diode bridge circuit is called a full-wave rectifier circuit.