alternator
An alternator is an electric generator - a three-phase generator is common these days - which is driven by a belt in vehicles with internal combustion engines. The alternator supplies the electrical power for the consumer and the starter battery .
The name alternator is historically based, since after the carbide lighting, the power generator only fed the vehicle lighting with electricity. The magneto ignition common at the time was independent of this. Since the engine was started with a hand crank for a long time , no accumulator was required.
Other names are alternator ( Switzerland and Slavic-speaking area ), generator , power generator or dynamo .
Mechanical drive
The alternator is driven by the running engine as an auxiliary unit or by a wheel of the vehicle via a friction wheel. The drive takes place in the automobile and sometimes in motorcycles usually with a belt drive such. B. Wedge or wedge ribbed belts. As is common with many motorcycles and gas turbines, the alternator can also be coupled directly to the crankshaft in cars . Their function can also be combined with that of the starter ( starter generator , Dynastart ).
A power generator converts mechanical energy into electrical energy (and heat), with the required mechanical power being approximately proportional to the electrical power output . Losses result from friction in bearings, depending on the design on the collector or the slip rings . Significant in three-phase alternators are copper losses in the windings (excitation winding in the rotor and stator windings), hysteresis losses in the stator laminated core, and losses in the rectifier. In addition, there are losses from the cooling fan and in the belt drive.
Automotive alternators are not very efficient because they have to be light. Only through synchronous rectification could the efficiency (under certain load and speed conditions) be increased to up to 80%.
Alternators with charge regulator
At low speeds, for example when the engine is idling , the electrical power generated by the alternator drops. If the power required in the vehicle for the consumers switched on is higher, the difference is taken from the vehicle battery; in the opposite case, the vehicle battery is charged. In order to keep the output voltage stable and thus also to prevent the starter battery from being overcharged, the alternator must be regulated.
With a constant magnetic field strength, the voltage of the alternator would fluctuate greatly with the speed and the connected load. In order to regulate the voltage, the excitation current is controlled in electrically excited alternators. An electronic charge regulator (until the 1970s, electromechanical regulators with contacts switched) compares the actual voltage in the on-board network with a built-in voltage reference and controls the excitation current so that the end-of- charge voltage of the starter battery is maintained. In the 12 V on-board network, this is around 13.8 ... 14.2 V. The excitation current required for this is inversely proportional to the speed and changes only slightly under load. Furthermore, the controller protects the alternator from overload and can communicate with the engine management in modern vehicles, e.g. B. to warm up the engine before full loading, to increase the idle speed if necessary or to regulate it at full throttle in favor of the vehicle's drive power.
With alternators excited by permanent magnets (in many motorcycles) the output voltage is kept constant in a combined rectifier / charge regulator . Thyristors make it possible, similar to a dimmer , to control the switch-on time in the conductive direction so that a more or less large part of the power made available by the alternator is rectified and fed into the on-board network. With this type of construction, the controller has to influence the output power of the power generator, which is much higher than the excitation power (over a wide speed range), which only became possible with the advent of power semiconductors. The advantage of this design is that it works without slip rings or collectors.
Unregulated alternators
On older motorcycles and bicycles, there are unregulated, permanently excited alternating current generators to power the on-board consumers. A certain voltage stabilization is achieved by the leakage inductance of the winding; with increasing speeds, a higher series resistance is formed in series with the consumer due to the increasing frequency. This means that a so-called "self-regulation" takes place.
In the case of motorcycles, the rotor with the permanent magnets can enclose the internal stator in a bell-shaped manner and thus at the same time contribute more to the flywheel. To supply the ignition system, a separate coil can be arranged in the alternator, either with an integrated high-voltage winding or with a separate, external ignition coil.
species
DC alternator
Until the 1970s, alternators were designed as direct current generators . In the stator , the magnet coils through which the excitation current flows, forms the excitation field in which the rotor rotates and generates alternating current. This is rectified by the collector arranged on the rotor shaft and diverted via carbon brushes . The disadvantage here is that the carbon brushes have to transmit the full output current of the generator and therefore wear relatively heavily. In addition, because of the collector and the centrifugal forces acting on its lamellas, the maximum permissible speed of a direct current alternator is lower than that of a three-phase alternator. Because of the lower transmission ratio of the drive by the vehicle engine, the consequence is that significant electrical power is only produced at a higher engine speed. In unfavorable operating conditions with a large number of electrical consumers switched on and often low speeds, this led to the vehicle battery being discharged.
The advantage of the direct current generator is that no additional rectification of the generated voltage is necessary. Before the availability of high-performance semiconductor diodes, this was decisive for their use in vehicle construction. In addition, it can be used as a motor for starting ( "Dynastart" ) without control electronics if it is designed accordingly . In this case it is directly coupled to the motor or turbine shaft.
DC alternators were predominant in cars, trucks and coaches until the late 1960s. Today you can still find them as a starting generator in aircraft turbines, small gas turbines and in some hybrid vehicles.
AC alternator
Some vehicle manufacturers such as B. Citroën in the 2CV equipped the vehicles with AC alternators .
Three-phase alternator
Three-phase generators have been used as alternators since the 1970s . Claw-pole generators are used for this today . Compared to the direct current version, the functions of the rotor and stator are reversed: The excitation field is generated by the rotor and induces the three-phase alternating voltage in the coils of the stator , which is available to the on-board electrical system after rectification.
Rectifier
The generated three-phase alternating current by the power semiconductor rectified , which are usually integrated in the generator. Since around the 1990s, three-phase generators have been protected from overvoltages by internal Zener diodes ( load dump ). Older versions without this protection always had to be connected to the vehicle battery when the engine was running in order to prevent damage to the rectifier diodes and the vehicle. The background to this is that the controller cannot regulate rapid, high load changes due to the high inductance of the field winding and the leakage inductance of the generator windings. The starter battery acts as a buffer to absorb voltage fluctuations. Nevertheless, automotive electronics must be able to withstand overvoltages of around 40 V.
Dissipation of the power loss
The alternator has losses - part of the mechanical drive power is converted into heat. If there is insufficient heat dissipation, the alternator would overheat and be destroyed as a result.
Active air cooling is used to dissipate the power loss, and some generators also have water cooling (e.g. Mercedes-Benz W210 ). The fan wheel is located on the generator shaft, either outside between the belt wheel and the alternator, or in the case of the so-called compact generator in the housing itself on both sides of the stator.
Rotor design as permanent magnet
This type of construction occurs in many motorcycles. Instead of the coil developments, permanent magnets are used here for excitation. The stator coils are connected in three phases and only 3 lines lead out of the housing. The controller is installed externally. The advantage over the usual three-phase alternator is the omission of slip rings. Regulation often takes place by periodically short-circuiting the stator coils. This results in high power losses. It results from the voltage drop across the effective resistance and the current. These generators often run in an oil bath on motorcycles, which provides better cooling.
Rotor design as an electromagnet
Here the excitation current is supplied via two smooth slip rings , its level and thus the voltage induced in the stationary stator is regulated. The excitation current is considerably smaller than the output current of the generator, which allows smaller dimensions of the necessary carbon brushes and results in a longer service life. In addition, a higher speed level is possible compared to the collector of a direct current generator, which is why considerable electrical power is available even at idle speed of the drive motor. Significantly less installation space is required than for a DC alternator of comparable power.
Charge control lamp on the three-phase alternator
The charge control lamp has two tasks:
- Indication of the correct functioning of the generator
- External excitation of the generator in the start-up phase
Normally (in the case of a motor vehicle) the charge control lamp lights up when the engine is stopped and the ignition is switched on and goes out even at low speed of the unit, at the latest after a one-time, brief increase in speed from idle, as there is no longer any voltage difference on the lamp. Any other behavior indicates defects in the generator (rectifier, carbon, regulator) or a defect in the lamp, provided the on-board battery is not discharged. The far more important function of the lamp is the transmission or provision of the excitation current. When the generator is stationary, there is no magnetic field in the currentless generator. Since this is necessary to generate electricity, the rotor must be supplied with electricity so that a field can build up in it. This flows from terminal 15 (ignition plus) via the charge control lamp through the generator winding via ground (terminal 31), back to the battery (minus) and is limited to about 300 mA by the incandescent lamp (4 W) (without a lamp, 2 to 5 A would flow ). When the rotor rotates, a voltage is then induced in the stator winding; the available current takes over a smaller part (above 2 to 5 A, depending on the speed), controlled by the charge controller, the supply of the excitation winding of the rotor and can be taken for the larger part as useful current at the output terminals (B +). If the charge control lamp is defective or if there is no battery / it is discharged, no external excitation can take place, so no voltage is generated even when the alternator is running. This is also the reason why you cannot push a vehicle with a completely discharged battery. With older alternators, a weak permanent magnetic field may have formed in the rotor during its service life, which also exists without voltage being applied. Such machines can also start without a charge indicator lamp and deliver electricity during operation. However, this is not an intended effect and it cannot be assumed that an alternator can be put into operation without a charge control lamp or without external excitation. With direct current alternators, however, self-excitation is not uncommon, since there the induction voltage does not have to overcome the forward voltage of the rectifier diodes before it can contribute to further excitation.
Charge regulator
- See main article: Charge controllers
The circuit diagrams show a "positive regulating" (the switching transistor is in the line from D + to the plus brush) switching regulator. There are also so-called "negative regulating" switching regulators.
The charge controller has the following tasks
- Regulation of the voltage generated by the alternator
- Protection against overload due to excessive output current (for direct current alternators)
- Protection against reverse current
Electrical power
The maximum output power of the alternators has increased steadily and is now more than 3 kW in many vehicles . This corresponds to a current of over 250 A for a 12 V on-board network and more than 125 A for a 24-V on-board network (truck). This power is required because many additional units have to be supplied, such as air conditioning compressors and heaters, but also units of modern engines such as high pressure diesel pumps, electric oil pumps or magnetic injection valves.
Terminal designations
The connections of the alternator in motor vehicles have the following names or terminal designations . The generator / charge controller unit has at least the following three terminals:
- B + battery plus, terminal 30 (positive charge current output)
- B− battery minus, also terminal 31 (ground or common negative reference potential)
- D + Dynamo Plus, terminal 61 (charge control lamp or positive pole switched with the ignition)
The D + connection does not necessarily have to be connected for the alternator to function, because a small permanent magnet in the rotor or its residual magnetization often ensures that the generator starts operating.
Generators with external controllers or older machines have the following connection designations:
- D− Dynamo Minus (if necessary, separate negative line for more precise voltage measurement on the battery)
- DF Dynamo field (connection of the excitation field winding)
DF1 (Dynamo Field 1) and DF2 (Dynamo Field 2) or J and K are field winding connections that are lead out floating.
Terminal W is used to connect a rev counter and supplies an alternating voltage directly from the stator winding for frequency measurement (required for diesel vehicles because there are no ignition pulses).
literature
Books
- Jürgen Kasedorf, Richard Koch: Service primer for vehicle electrics. 14th revised edition. Vogel, Würzburg 2001, ISBN 3-8023-1881-1 .
- Rudolf Hüppen, Dieter Korp: Car electrics - all types. Motorbuchverlag, Stuttgart 1968, ISBN 3-87943-059-4 .
brochures
- Bosch technical briefing on generators. Robert Bosch GmbH, Stuttgart, VDT-UBE 301/1 De (1.80).
- Bosch technical instruction for circuit symbols and circuit diagrams for vehicle electrics. Robert Bosch GmbH, Stuttgart, VDT-UBE 001/10.
Web links
Individual evidence
- ↑ https://de.bosch-automotive.com/de/parts_and_accessories/motor_and_sytems/starters_alternators_1/alternators_for_cars_1/starters_alternators_alternators Message from Bosch on their website, accessed on October 21, 2017
- ↑ http://nippon-classic.de/ratgeber/tipps-tricks/funktionsweise-der-lichtmaschine-am-motorrad/ Jens Schultze: Alternators with a rotor as a permanent magnet , accessed on October 20, 2017