Automobile generator.
The automobile generator is the main power supply of the automobile, its function is to supply power to all electrical equipment (except the starter) when the engine is running normally, and charge the battery at the same time.
On the basis of the common alternator three-phase stator winding, increase the number of winding turns and lead out the terminal, add a set of three-phase bridge rectifier. At low speed, the primary winding and the extension winding are output in series, and at high speed, only the primary three-phase winding is output.
Working principle
The working principle of the whole alternator
When the external circuit energizes the field winding through the brush, a magnetic field is generated, so that the claw pole is magnetized into the N pole and the S pole. When the rotor rotates, the magnetic flux alternately changes in the stator winding, according to the principle of electromagnetic induction, the stator three-phase winding will produce alternating induced electromotive force. This is how an alternator generates electricity.
The prime mover (i.e. the engine) drags the DC excited synchronous generator rotor to rotate at the speed n(rpm), and the three-phase stator winding induction AC potential. If the stator winding is connected to the electrical load, the motor has AC power output, and the AC power is converted into direct current from the output terminal through the rectifier bridge inside the generator.
Alternator is divided into two parts of stator winding and rotor winding, three-phase stator winding is distributed on the shell according to the electric Angle of 120 degrees difference between each other, rotor winding is composed of two pole claws. When the rotor winding is connected to direct current, it is excited, and the two pole claws form the N pole and the S pole. The magnetic field line starts from the N pole, enters the stator core through the air gap and returns to the adjacent S pole. Once the rotor is rotated, the rotor winding will cut the magnetic force line, and generate sinusoidal electromotive force with a difference of 120 degrees electrical Angle in the stator winding, that is, three-phase alternating current, and then through the rectifier element composed of diodes into direct current output.
When the switch is closed, the battery first provides current. The circuit is:
Battery positive → charging light → regulator contact → excitation winding → lap iron → battery negative. At this time, the charging indicator light will light up due to the current passing through.
However, after the engine starts, as the generator speed increases, the generator terminal voltage also increases. When the output voltage of the generator is equal to the battery voltage, the potential of the "B" end and "D" end of the generator is equal, at this time, the charging indicator light is extinguished because the potential difference between the two ends is zero. Indicates that the generator is working normally and the excitation current is supplied by the generator itself. The three-phase AC electromotive force generated by the three-phase winding in the generator is rectified by the diode and outputs direct current to supply power to the load and charge the battery.
The alternator is generally composed of four parts: rotor, stator, rectifier and end cap.
(1) Rotor
The function of the rotor is to generate a rotating magnetic field.
The rotor consists of a claw pole, a yoke, a magnetic field winding, a collector ring and a rotor shaft.
Two claw poles are pressed on the rotor shaft, and each of the two claw poles has six bird-beak magnetic poles. A magnetic field winding (rotor coil) and a magnetic yoke are arranged in the cavity of the claw pole.
The collector ring consists of two copper rings insulated from each other. The collector ring is pressed on the rotor shaft and insulated with the shaft. The two collector rings are connected to both ends of the magnetic field winding.
When the two collector rings are passed into the direct current (through the brush), there is current through the magnetic field winding, and the axial magnetic flux is generated, so that one claw pole is magnetized to the N pole and the other is magnetized to the S pole, thus forming six pairs of interleaving magnetic poles. As the rotor rotates, a rotating magnetic field is created [1].
The magnetic circuit of the alternator is: yoke →N pole → air gap between rotor and stator → stator → Air gap between stator and rotor →S pole → yoke.
(2) The stator
The function of the stator is to generate alternating current.
The stator consists of a stator core and a stator coil.
The stator core is composed of silicon steel sheets with grooves in the inner ring, and the conductor of the stator winding is embedded in the groove of the core.
The stator winding has three phases, and the three phase winding adopts star connection or triangle (high power) connection, which can generate three phase alternating current.
The three-phase winding must be wound according to certain requirements in order to obtain the same frequency, equal amplitude, phase difference of 120° three-phase electromotive force.
1. The distance between the two effective sides of each coil should be equal to the space occupied by a magnetic pole.
2. The distance between the starting edges of the adjacent coils of each phase winding should be equal to or multiple of the distance occupied by a pair of magnetic poles.
3. The starting edge of the three-phase winding should be separated by 2π+120o electrical Angle (the space occupied by a pair of magnetic poles is 360o electrical Angle).
In the domestic JF13 series alternator, a pair of magnetic poles account for the spatial position of 6 slots (60o electrical Angle per slot), a magnetic pole accounts for the spatial position of 3 slots, so the position interval of the two effective sides of each coil is 3 slots, the distance between the beginning edge of each phase winding adjacent to the coil 6 slots, the beginning edge of the three-phase winding can be separated by 2 slots, 8 slots, 3 slots. 14 slots, etc.
(3) Rectifier
The role of the alternator rectifier is to change the three-phase alternating current of the stator winding into direct current. The rectifier of the 6-tube alternator is a three-phase full-wave bridge rectifier circuit composed of 6 silicon rectifier diodes, and the 6 rectifier tubes are respectively pressed (or welded) on two plates.
1. Characteristics of automotive silicon rectifier diodes
(1) Large working current, forward average current 50A, surge current 600A;
(2) High reverse voltage, reverse repeat peak voltage 270V, reverse non-repeat peak voltage 300V;
(3) There is only one lead. And some diode leads are positive, some diode leads are negative, the tube with a positive lead line is called a positive tube, and the tube with a negative lead line is called a negative tube, so the rectifier diode has a positive diode and a negative diode.
(4) End cover
The end cover is generally divided into two parts (front end cover and back end cover), which plays the role of fixing the rotor, stator, rectifier and brush assembly. The end cover is generally cast with aluminum alloy, which can effectively prevent magnetic leakage and has good heat dissipation performance.
The rear end cover is provided with a brush assembly composed of a brush, a brush holder and a brush spring. The role of the brush is to introduce the power supply through the collector ring into the field winding.
The connection between the magnetic field winding (two brushes) and the generator is different, so that the generator is divided into internal and external types
1. Internal lap iron generator: A generator with magnetic field winding negative brush directly lap iron (directly connected to the housing).
2. External-clad generator: A generator in which both brushes of the field winding are insulated from the housing.
The negative electrode (negative brush) of the magnetic field winding of the external iron-type generator is connected to the regulator, and then the iron is connected after passing.
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