Car alternator: how it works and what functions it performs?
- 1. What does a car need an alternator for?
- 2. Design and construction of a car alternator
- Main parameters of an alternator
- 6. How does a car alternator work?
- 7. What is the voltage regulator for in an alternator?
- 8. Conclusion .
There are two sources of current to power the onboard network of the vehicle. And it is very important for the driver to understand the principles of operation of the car generator, which, along with the battery, is designed to provide energy for the electrical equipment of the car.
Strict requirements are imposed on the reliability and stability of devices of this kind.
In the Russian Federation electrical equipment produced and used must comply with GOST R 52230-2004. The document establishes general technical specifications, which also apply to starter batteries for automobiles. The mentioned national standard fully complies with international regulations, which allows using foreign-made components in domestic cars.
At the dawn of the automotive industry and up to the 60’s of the last century in on-board networks were used alternators of direct current – capricious and low-power. With the advent of semiconductor (selenium and silicon) rectifiers, alternating current units were put on cars. They are three times lighter in weight and provide higher output current stability for the same load.
Why do cars need an alternator?
The generator is used to maintain a certain voltage and current in the onboard network. The main purpose of the vehicle’s alternator is to provide stable power to the electrical equipment while the engine is running – in particular, to:
- Charging the battery.
- Power supply to all electric consumers under normal conditions.
- Power supply to consumers together with the battery under extreme operation.
Using a car alternator allows you to restore the battery charge, which is consumed to start the engine with the starter. In this case the voltage in the on-board network stays within the strictly defined limits, exceeding the electrochemical potential of the battery plates.
Having understood what the generator in the car is for, it is necessary to understand that in case of failure of the unit, the engine will run some more time at the expense of the battery. Prolong this period can be extended by switching off all secondary consumers: heater fan, air conditioner, audio system. When the battery charge is exhausted, the engine will stall.
Design and construction of automobile generator
Three-phase AC electric generators installed on modern cars can be of 2 types: standard and compact. The general structure of automotive alternators of the 2 types is the same – they consist of the following elements:
- A pulley with a shaft and bearings.
- The rotor with slip rings.
- The stator windings.
- Generator case.
- Voltage regulator.
- Brush unit.
Designs of automotive alternators differ only in layout features. With the same electrical parameters, the standard units are much larger than the small ones. Compactness is ensured through the use of modern materials and technologies.
Here is what an electrical generator consists of and what functions its components perform:
- The pulley transmits the rotation from the crankshaft to the rotor by means of a belt.
- The body of the generator has two covers (front, rear) and is needed to connect the elements into a single structure. On the outer surface there are brackets, by means of which the device is attached to the engine.
- The rotor is a shaft on which excitation windings and contact rings of electrical copper are installed.
- The stator includes a magnetic core made of a package of steel plates, in which shaped grooves are cut out. They contain three-phase windings of single-core copper wire, where the current is generated.
- The voltage regulator is made as a separate unit or combined with a brush unit. The main purpose is to control the operation of the alternator by changing the current in the field winding.
- Rectifier device according to Larionov scheme consists of two parts: aluminum heat sinks, in each of which three power diodes are pressed. The diodes provide conversion of alternating voltage into direct voltage, which is used in the onboard network for powering electrical equipment.
- Voltage transfer to the field winding is made through a special unit and cylindrical contact rings. The brushes are made of special grades of graphite and are mounted in a holder with guides made of dielectric materials. They are spring-loaded to ensure a tight contact, and voltage is supplied to them through a wire pressed into the base.
Dealing with the structure of the generator of a modern car, we should distinguish in it a mechanical and electrical part. The first (which includes the pulley and the two rotor bearings) ensures its rotation in the housing. The second part actually generates the electric current to power the on-board network. The described scheme of the automobile generator was first used in the products of the American company “Neville” in 1946. Such devices were equipped with military cars and buses.
The main parameters of the generator
The main nominal parameters are determined based on the technical requirements for the design of a particular model of vehicle:
- Voltage. In accordance with GOST 52230-2004 is selected from the range of 7.14 to 28 V.
- Recoil current.
- Frequency of excitation and self-excitation.
The current-rate characteristic determines the dependence of the generator rated current on its speed. The on-board voltage of cars, commercial vehicles and buses is 12 V, while the voltage of special-purpose machines is 24 V. The maximum output current is determined at a rotor speed of 6,000 min-1.
Another important characteristic of this unit is efficiency. For modern models, this indicator is at the level of 50-60%.
How does a car generator work?
The device begins to function only after the engine is started by the starter, which is powered directly from the battery. The key principle of the car alternator is to convert mechanical energy into electrical energy. On the crankshaft of the power unit there is a pulley, which unwinds the rotor mounted on maintenance-free bearings through a belt drive.
The excitation winding, located on the rotating armature, is powered from the battery through a brush assembly and contact rings. To protect the battery from self-discharge, the connection is made through a special rectifier consisting of three diodes. The voltage in this circuit is regulated by an electronic or electromechanical regulator, either integrated or as a separate device.
The rotating armature creates electromagnetic fields which induce an alternating current in the stator windings. This is fed to the rectifier, which is a block of diodes. It consists of six gates: three negative and three positive. They provide conversion of phase voltage into line voltage. The generator windings are connected in a delta or star arrangement. In the first case, the current is 1.7 times lower than in the second. Triangle is used on models of cars with higher power.
The described principle of operation of the automobile alternator provides maintenance of the voltage in the onboard network within the range of 13.9 to 14.5 V. The exact value depends on the speed of the crankshaft and the load level. Consumers (e.g. the battery) are connected to the generating set via the “B+” terminal.
What is the voltage regulator for in an alternator?
If the frequency of the crankshaft and, consequently, the rotor speed in the on-board network may cause voltage spikes, which adversely affect the operation of consumers. Spikes are eliminated by limiting the excitation current transmitted through the brushes from the voltage regulator to the rotor. This is controlled by varying the armature winding connection time depending on the on-board load.
If there is a malfunction of the AVR or damage to the brush unit and contact rings, the battery may be under or overcharged. Prolonged operation of the machine with such a defect will lead to battery failure.
Faulty alternator can be determined by the indicator on the instrument panel. Burning light of the battery charge lamp after start-up indicates insufficient voltage in the network, and flashing indicates an excess.
Even the most general understanding of the device and principles of operation of a car alternator can help avoid electrical equipment malfunctions. The alternator starts running after the engine is started and serves as the main power source in the vehicle.
During the operation of the vehicle, it is necessary to carefully monitor the tension of the drive belt, which affects the position of the alternator. On a number of modern vehicles, the unit is fixed firmly, and a worn V-belt or multi-ribbed belt should be replaced immediately. Keeping the alternator in good working order will help to avoid large expenditures on car overhaul.
Principle of operation and structure of a modern car alternator
In the standard version in the car, there are two power sources – the generator and the battery. The difference between them is that the battery stores electricity, and the car generator produces it. That is, this device converts the mechanical energy from the engine into electrical energy in order to further power all consumers and charge the battery.
When starting the engine, the starting current is supplied to the starter by the battery. But the battery itself does not generate energy, it only stores it and then gives it away. If you use only the battery to power all consumers, it will quickly run out of power. The car generator generates electricity, charges the battery and feeds the on-board network of the car during engine operation (when it reaches certain revolutions of the crankshaft).
The generator begins to produce electricity from idling speed, but on the optimal mode of operation it comes out when the engine reaches 1600-1800 rpm or more.
Types of alternators
There are two types of automobile generators:
- direct current;
- alternating current.
The first type of generators are no longer used today. Such devices were installed on older models of cars (GAZ-51, Pobeda, etc.). They have many disadvantages, such as:
- low power and efficiency;
- The need for constant monitoring and maintenance;
- short service life.
Now alternators of alternating current are used. Their main difference is that regardless of the engine operation mode, the car network is powered by direct current. This is achieved thanks to a semiconductor rectifier.
Design of an alternator of alternating current
The work of any alternator can be compared to an electric motor, which works in reverse mode, that is, it does not consume, but produces current. According to the type of design modern generators are divided into two types: compact and traditional. They have a common device, but differ in the layout of the housing, fan, rectifier unit and drive pulley. Also, modern devices have three phases.
A generator consists of the following basic elements:
- The drive with pulley, bearings, and shaft;
- The rotor with the field winding and the sliprings;
- The stator with the core and the winding;
- housing, consisting of two covers;
- voltage regulator;
- rectifier unit or diode bridge;
- brush assembly.
Let’s analyze each element of the device separately and in detail.
The housing contains all the main elements of the alternator. It consists of two covers (front and rear). The covers are connected to each other by bolts. The covers are made of lightweight aluminum alloys, which do not magnetize and conduct heat well. The covers have ventilation holes and mounting flanges.
The rear cover contains a diode bridge and a brush holder with brushes. Also in the rear cover is a lead contact that carries the current from the alternator.
Rotation from the crankshaft is transferred to the alternator pulley and rotates the rotor. The pulley speed is 2-3 times the crankshaft speed. The torque from the engine is transmitted by means of a belt drive. A V-belt and a V-belt can be used, depending on the design. V-belts are regarded as the most versatile and modern type.
On the rotor shaft is an excitation winding that creates a magnetic field and is essentially a conventional electromagnet. The winding is located between the two pole halves (cores) necessary to regulate and direct the magnetic field. Each of the halves has six triangular protrusions called beaks. Also on the rotor shaft are two copper contact rings. Sometimes they are made of steel or brass. The field winding is energized by the battery through the contact rings. The contacts of the winding are soldered to the rings.
On the front end of the rotor shaft there is a drive pulley and on the other end the fan impeller is mounted. There can be two of them. These are needed to cool the internal parts of the alternator. There are also maintenance-free ball bearings on both ends of the rotor.
Structurally, the stator is shaped like a ring. It is the main part that serves to generate alternating current from the rotor’s magnetic field. It consists of a winding and a core. The core, in turn, consists of interconnected steel plates in which 36 slots are formed. Three windings are wound into the slots, which form a three-phase connection. There can be two winding arrangements: “star” and “delta”. In the star circuit, the ends of each of the three windings are connected at one point. In the delta circuit, the ends of the windings are connected separately.
Rectifier box or diode bridge
The rectifier unit performs the task of converting the alternator alternating current into direct current, which is needed to power the on-board network of the car. In other words, it produces a voltage that is stable and of the same magnitude.
The unit is also called a diode bridge, which consists of two radiator plates (positive and negative) and diodes. There are two diodes per phase. The diodes themselves are hermetically embedded in the plates. The diode bridge is shaped like a horseshoe.
From the stator winding, the current flows to the diode bridge, then “rectifies”, and is fed to the lead pin on the back cover.
Only one direction of current flows through the diodes, and reverse polarity currents are cut off. The diode bridge may be in the case of the alternator, or it may be taken out of the case. But most often it is mounted on the inside of the back cover.
The voltage regulator maintains the alternator voltage within certain limits. Modern models use solid-state electronic voltage regulators. They are mounted on top of the brush holder assembly.
When the engine is running at high speed, the stator winding voltage can be as high as 16V. This voltage should not be fed into the on-board network. To avoid this, the voltage regulator, receiving current from the battery, will reduce its value. A small current in the rotor winding will create an equally small magnetic field. This means that the stator winding will have a lower voltage.
The brush assembly in modern alternators is combined with the voltage regulator into a single, non-disassembled mechanism. It transmits the excitation current to the rotor’s copper contact rings. It is a simple construction consisting of a brush holder, two graphite brushes and pressure springs.
Principle of operation
Now let’s take a closer look at the operation of the alternator in the car. When the ignition is turned on, the brush assembly is supplied with current from the battery. Through the brush assembly, it reaches the copper contact rings, and then to the excitation winding of the rotor. Recall that the rotor is essentially an electromagnet, which creates a magnetic field. The crankshaft begins to rotate the rotor through the pulley and belt drive. Around the rotor is the stator, which begins to generate alternating current from the rotation. When the rotation of the rotor reaches a certain frequency, the field winding is powered by the generator itself.
Through the diode bridge the alternating current is “rectified” and converted into direct current, which is necessary to power the on-board network. This is how the car alternator supplies power to consumers and recharges the battery. The voltage regulator changes the operation of the field winding when the rotor speed increases. In this way a stable load is maintained.
There is an alternator control lamp on the dashboard in the vehicle interior which indicates the status of the device. For example, the lamp may light up if the belt breaks. Then the mains power will go only through the battery. The duration of operation in this case will depend on the level of charge of the battery.
The operation of the generator is evaluated by several parameters:
- rated current and rated voltage;
- nominal excitation frequency;
- frequency of self-excitation;
- The efficiency factor (efficiency factor).
The nominal voltage for the vehicle’s onboard network from an alternator is 12V or 24V. The current-rate characteristic shows the dependence of the current intensity on the speed of rotation of the alternator.
The alternator voltage can be measured with a multimeter. With all consumers switched off and no load at idle speed, the multimeter should show a voltage between 14.3V and 15.5V. If the voltage after starting the engine is over 14V, it may indicate the discharge of the battery and its charging by the alternator. If you turn on the consumers one by one (headlights, heating, air conditioner, etc.) the voltage decreases by about 0,2 after each turn on. But as a result, the voltage should not drop below 12.8V. If the value is lower, the battery will start discharging. If, on the contrary, the voltage is very high (14V or higher), it can lead to battery failure. At the same time at the output of the battery itself the voltage must be within the range 12.6V – 12.7V.
The alternator voltage under load may differ from the nominal 12V. After turning on all current consumers the value should be in the range of 13.5V – 14V. If lower, it may indicate a malfunction of the device. The allowable limit is considered to be 13V.
The picture below shows a detailed connection diagram of the alternator in the car.
If you turn on all the power-consuming appliances in the car, the generator may not be able to cope with the load and part of the energy will give to the battery.
To calculate the power of the generator, just use a simple formula from the school course P = I * U, where P – power, I – current, U – voltage.
We have learned that the voltage at the output of the generator should be around 13.5V – 14.2V. The amperage can vary from model to model. On average it is from 80A to 140A. Let’s take the average value of 100A.
Using the formula we get 13.5V*100A = 1,350W or 1.35kW. This is the power of the alternator, which is measured in watts. You should also keep in mind that this is the maximum value that is reached at certain engine speeds, usually 3000 rpm or higher. At idle, the power output is equal to 75% of the maximum possible. It is considered that 80A is enough for a car. If you apply a more powerful autogenerator, the on-board network may not cope with the load. It is necessary to take this into account. High power is not always beneficial.
The device is quite reliable and should work for a long time, but some components can fail for various reasons. Faults can be mechanical or electrical in nature.
Since the vehicle’s alternator and battery operate inseparably from each other, a malfunction in either device will cause the battery discharge lamp to light up and the Check Engine light may illuminate. You can check the condition of the battery and diagnose a malfunction with the Rokodil ScanX Pro Universal Vehicle Scanner.
Faults related to the alternator or poor electrical connection in the control circuit are often indicated by errors P0620 and P0622.
The main possible failure could be a broken drive belt. In this case, the rotation from the crankshaft to the rotor will not be transmitted. All the load is taken by the battery, which will start discharging. This will be shown by the control lamp in the car interior. To avoid belt breakage, you should periodically check its condition and tension.
Also, simple wear and tear of the graphite brushes can happen. In this case, it is necessary to replace the entire brush assembly.
Malfunctions with the electrics in the alternator happen often, and they are difficult to notice. There can be a short circuit in the excitation windings of the rotor or stator, breakage of the winding. The voltage regulator can fail, which is fraught with big problems for all electronics and the battery. Also happens so-called diode bridge breakdown for various reasons. Do not disconnect the alternator or the battery while the engine is running. You also need to make sure the connections are secure, clean the terminals, etc.
Every driver needs to know the device and the principle of operation of the car alternator. This will help avoid many problems that can occur with the device. You need to monitor the components of the alternator on a regular basis. Check the tension and condition of the drive belt, the attachment of the device, the voltage, and more. If used properly, the unit will last for many years in good working order.