What is the power in an alternator
Lexicon> Letter L> Alternator
Definition: the electrical generator in a vehicle
Alternative term: generator
English: electric generator, alternator
Categories: electrical energy, vehicles
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: June 23, 2013; last change: 05.02.2021
The power grid in a vehicle with an internal combustion engine, for example a car or truck, is usually fed by a generator, which is often driven by the engine via a belt. Since the operation of the headlights (i.e. the generation of light for the purpose of lighting) was originally the most important task of generating electricity on board, the term has become alternator naturalized for the generator. In the meantime, almost every car contains a large number of other electrical consumers, which in total require considerably more electrical energy than the headlights.
Other names for the alternator are generator and Power generator.
Since, on the one hand, the engine speed and, on the other hand, the electrical load are variable, a rechargeable battery is usually used as an energy store. The same battery serves as the starter battery, i. H. for starting the internal combustion engine and for supplying consumers when the internal combustion engine is switched off. The alternator includes one Charge regulatorwhich allows the battery to be charged properly (with adequate amperage and avoidance of overcharging). The electrical voltage is also limited at high engine speeds in order to avoid damage to consumers.
The regular operating voltage of the on-board network is quite low in the car: usually around 12 to 14 V, in trucks around twice that. Since the power in the car can already amount to several kilowatts, very high currents are required - sometimes over 200 A. That is why the alternator must be connected using very strong cables and the contact points must be clean.
If an alternator is defective, this can usually be recognized in the vehicle by the fact that the charge control lamp remains lit even when the engine is running.
DC and three-phase alternators
Old alternators, as they were used until the 1970s, were mostly electrically excited direct current generators. At low engine speeds, at least when idling, the electrical voltage generated was often too low to supply consumers or charge the battery.
Since the 1970s, three-phase generators have been used almost exclusively. They have a rotating exciter (fed by slip rings with direct current) that generates phase-shifted alternating currents (i.e. three-phase current) in the stator windings. The DC voltage required for the vehicle electrical system is generated with an integrated semiconductor diode rectifier.
In contrast to direct current alternators, three-phase alternators can deliver a considerable amount of power even at low speeds. The transmission ratio for driving a generator is often chosen so that around two thirds of full power can be achieved even at idle speed.
Increasing power requirements in vehicles
The alternators in today's cars are designed for a maximum output of sometimes over 5 kW, sometimes even for about twice that. In the future, the electrical power requirement is likely to increase even further, as not only more and more comfort functions are required, but also various functions that are important for the engine itself are increasingly electrified. For example, radiator fans, cooling water pumps, power steering and sometimes even compressors in air conditioning systems are increasingly being operated electrically. In spite of the energy losses in the alternator and electric motors, this can be energetically favorable because it enables operation that better corresponds to the respective requirements. In car air conditioning systems, for example, it is very annoying that a compressor driven directly by the engine draws a certain amount of power from it even when the air conditioning system is not being operated at all.
Modern starter generators can replace the alternator and the starter (starter); they enable the generation of higher powers with a higher degree of efficiency than traditional alternators.
With increasing performance, a low on-board power supply voltage of 12 V increasingly becomes a limiting factor; This is because very high electrical currents then occur (e.g. 200 A for an output of 2.4 kW), and correspondingly thick and heavy cables are required for this. Therefore, significantly higher on-board network voltages are likely to be used in the future, and of course alternators with a correspondingly higher nominal voltage.
Energy efficiency of alternators
The alternator is driven by the internal combustion engine and brakes it in the process; So it absorbs mechanical energy in order to be able to give off electrical energy. The mechanical power consumed can be significantly higher than the power output, since considerable energy losses also occur, especially at high engine speeds - for example through friction, for driving the fan (fan wheel) that cools the alternator, through ohmic losses in the coil windings and in the rectifier. Even when there is no electrical load, the drive of the alternator requires a certain amount of drive power, which increases accordingly with electrical load (as a result of magnetic forces).In today's cars, the energy efficiency of the alternator is important.
In old cars, the energy efficiency of the alternator was usually quite low; however, only relatively minor services were implemented anyway. The efficiency could easily be well below 50%. More efficient devices would have been possible, but were probably not implemented because of the cost pressure. In modern vehicles, much higher electrical outputs are required to supply a large number of consumers; several kilowatts may be required (see above). Here the efficiency of the generator becomes more important in order to keep the fuel consumption low and also not to let the cooling effort for the generator become too high: The power lost there becomes heat that has to be safely dissipated in order to avoid destruction. Unfortunately, there is also the problem with today's alternators that a high degree of efficiency is difficult to achieve for a wide range of speeds and loads. In particular, operating at high speed but with a low load is quite inefficient. Therefore, even a peak efficiency of z. B. 80%, which some modern alternators can achieve, lead to an average practical efficiency of probably only between 50 and 70%. Modern starter generators in connection with suitable power electronics could be a little better.
Recuperation with alternators
To a very limited extent, alternators can also be used for recuperation, i. H. for the recovery of braking energy in micro-hybrid vehicles. To do this, it must be possible to increase the power generated when braking in a targeted manner, and the additional power is fed into the vehicle battery. (At other times, less power can be generated so that the engine is less stressed and fuel is saved.) The regenerative power can be limited by the alternator or the battery, usually to a few kilowatts. In vehicles with a real hybrid drive, both units are designed to be much stronger, so that higher braking power can be used; one then no longer speaks of an alternator, but of a generator or possibly a powerful electric machine that can be used as a motor or generator. For the transport of corresponding services, considerably higher electrical voltages are then required (several hundred volts), which is why the normal on-board network cannot be used for this.
It is also possible to briefly reduce the power output of an alternator when there is a high demand for drive power (e.g. when accelerating). In this way, in particular, the acceleration can be improved somewhat without increasing the engine power, and indeed with a minimum of technical effort.
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See also: generator, hybrid drive, recuperation, combustion engine, starter generator
as well as other articles in the categories of electrical energy, vehicles
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