Structure and working principle of automobile generator by Asiautos Auto Parts(vehicles Aftermarket Parts Expert)

The generator is the main power source of the car. Its function is to supply power to all electrical equipment except the starter when the engine is running normally, and to charge the battery at the same time. It is an indispensable part of the normal operation of the car.

Automobile generators can be divided into DC generators and AC generators. Since the performance of AC generators is better than that of DC generators in many aspects, DC generators have been eliminated. At present, automobiles generally use three-phase AC generators.

1. Rotor

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

The rotor consists of claw poles, yokes, magnetic field windings, collector rings, and rotor shafts.

Two claw poles are pressed on the rotor shaft, each of which has six bird-beak-shaped magnetic poles. The claw pole cavity is equipped with a magnetic field winding (rotor coil) and a 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 magnetic 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 the N pole and the other to be magnetized as the 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 the rotor and the stator → stator → air gap between the stator and the rotor → S pole → yoke.

2. Stator

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 bracket. When the rotor rotates, the position of the claw arms on its pole pieces causes the magnetic field that changes between the N pole and the S pole to move through the stator conductor. 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 in the generator is connected in a triangle or a star (Y) connection. The name triangle comes from the triangle formed by the three phases in the stator.

The automobile generator is simply the application of the principle of electromagnetic induction in the automobile electrical system.

Principle of electromagnetic induction:

Voltage is generated when a magnetic field moves through a conductor.

A bar magnet, whose magnetic field rotates in a conductor coil. The rotating magnet is called a rotor, and the fixed conductor is called a 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 a rotor, and the fixed conductor is called a 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 their positions: the N pole moves just below the upper conductor, and the S pole moves just 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 its rotation, the N and S poles return to their original positions: A becomes the positive (+) pole and B becomes the negative (-) pole.

Therefore, the current starts to flow in one direction and then changes to the other direction, generating alternating current. This alternating current is generated inside the generator. It is the source of the term "alternator", which is sometimes applied to generators using this principle. However, the term recognized by the SAE is generator.

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

1. Increase the number of coil turns in the stator. The more the magnetic lines of force cut 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. Increase the strength of the rotor magnetic field. Controlling the stator magnetic field strength is the most practical means of controlling the voltage output of the generator. This is because the strength of the magnetic field 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 magnets with electromagnets.

From a design perspective, the third method is the method of controlling the generator voltage when applied to automobiles.

For electromagnets, the magnitude of the current flowing through the coil wire, the magnetic lines of force, or the 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.

3. Voltage Regulator

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

Limiting voltage is very 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 changing the time of switching the field current, the voltage is controlled. So at low speeds, the field may be on 90% of the time and off 10% of the time. This creates a relatively high average field current, which, combined with the generator speed, produces the required voltage.

As the generator speed increases, the field current required to produce the required voltage decreases. This can be done by changing the duty cycle to reduce the average field current. For example: when the engine speed is high, 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.

4. Rectifier

Finally, how do you turn the alternating current (AC) generated by the principle of electromagnetic induction into direct current (DC) that can be used by the automotive electronics? This is where the rectifier comes in.

Generator Rectifier (with Diodes)

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

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

5. End cover

The end cover is generally divided into two parts (front cover and rear cover), which play the role of fixing the rotor, stator, rectifier and brush assembly. The end cover is generally cast with aluminum alloy, one is to effectively prevent magnetic leakage, and the other is that aluminum alloy has good heat dissipation performance. The rear cover is equipped with a brush assembly, which consists of a brush, a brush holder and a brush spring. The function of the brush is to introduce the power into the magnetic field winding through the collector ring.

6. Fan and bearing

The fan blades are installed between the front cover and the pulley (the two blades of the dual-fan generator are installed on the rotor inside the generator), and when the generator is working, it plays a role of forced ventilation and heat dissipation.