Tema VR Alternators

University of PitestiFaculty of Mechanical Engineering and TechnologyMaster: IAMD 2

ALTERNATORS

Supervisor: prof. PhD. BOROIU Alexandru

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Student: BICA Cornel

SUMMARY:

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2. Advantages over dynamos. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31.3. Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4. Field regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2. WHAT IS AN ALTERNATOR? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

3. ALTERNATOR COMPONENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. CONSTRUCTIVE SCHEME OF THE ALTERNATOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

5. UNDERSTANDING ALTERNATOR POWER OUTPUT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

6.THE LIFE AND DEATH OF AN ALTERNATOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.1. Bearing Failure: Causes and Cures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.2. Real values for times of failure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6.3. Statistical determination of the main indicators of reliability. . . . . . . . . . . . . . . . . 20 6.4. Identification of mathematical model for reliability equipment. . . . . . . . . . . . . . . 21

7. ALTERNATOR REPLACEMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

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1. INTRODUCTION

The alternator is a common equipment of the all today automobiles. Their are used not only on small cars but also in agricultural engineering, structural engineering, stationary generators, etc. They are produced in a variety of power and voltage levels and generally are always examined from many points of view, such as reliability, efficiency, dimensions, weight and costs. Special attention is paid to whole service life of alternator.

Fig. 1. Cutaway view of designed alternator (left) and its magnetic circuit (right)

1.1.History

The first car to use an alternator was an unusual system fitted to early Model T Fords. This entirely AC system was first used solely to power the trembler coil ignition system when the engine was running. When starting, a battery was used instead – cranking the engine was entirely manual. This system was sometimes used to also provide electric lighting. Being an AC system, there was no battery in this circuit. The starting battery was removed from the car for charging, a rare event as it was only needed when starting. The generator was usually described as a magneto, although this was not an ignition magneto(even though it was used to power the ignition) as it did not provide sparks itself. When the Model T was upgraded with electric lighting from the factory, a conventionaldynamo was installed instead. This then permitted battery charging as well.

1.2. Advantages over dynamos

Alternators have several advantages over direct-current generators. They are lighter, cheaper and more rugged. They use slip rings providing greatly extended brush life over a commutator. The brushes

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in an alternator carry only excitation current, a small fraction of the current carried by the brushes of a DC generator, which carry the generator's entire output. A set of rectifiers (diode bridge) is required to convert AC to DC. To provide direct current with low ripple, a three-phase winding is used and the pole-pieces of the rotor are shaped (claw-pole) to produce a waveform similar to a square wave instead of a sinusoid. Automotive alternators are usually belt driven at 2-3 times crankshaft speed. The alternator runs at various RPM (which varies the frequency) since it is driven by the engine. This is not a problem because the alternating current is rectified to direct current.

1.3. Operation

Despite their names, both 'DC generators' (or 'dynamos') and 'alternators' initially produce alternating current. In a so-called 'DC generator', this AC current is generated in the rotating armature, and then converted to DC by the commutator and brushes. In an 'alternator', the AC current is generated in the stationary stator, and then is converted to DC by the rectifiers (diodes). Typical passenger vehicle and light truck alternators use Lundell or 'claw-pole' field construction. This uses a shaped iron core on the rotor to produce a multi-pole field from a single coil winding. The poles of the rotor look like fingers of two hands interlocked with each other. The coil is mounted axially inside this and field current is supplied by slip rings and carbon brushes. These alternators have their field and stator windings cooled by axial airflow, produced by an external fan attached to the drive belt pulley.

Modern vehicles now use the compact alternator layout. This is electrically and magnetically similar, but has improved air cooling. Better cooling permits more power from a smaller machine. The casing has distinctive radial vent slots at each end and now encloses the fan. Two fans are used, one at each end, and the airflow is semi-radial, entering axially and leaving radially outwards. The stator windings now consist of a dense central band where the iron core and copper windings are tightly packed, and end bands where the windings are more exposed for better heat transfer. The closer core spacing from the rotor improves magnetic efficiency. The smaller, enclosed fans produce less noise, particularly at higher machine speeds. Larger vehicles may have salient-pole alternators similar to larger machines. The windings of a 3 phase alternator may be connected using either the Delta or Wye connection regime. set-up. Brushless versions of these type alternators are also common in larger machinery such as highway trucks and earthmoving machinery. With two oversized shaft bearings as the only wearing parts, these can provide extremely long and reliable service, even exceeding the engine overhaul intervals.

1.4. Field regulation

Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU). The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.

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The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.

Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A. Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.

In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator. Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed.This is similar to very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60%. Larger permanent magnet alternators can achieve higher efficiencies. Large AC generators used in power stations run at carefully controlled speeds and have no constraints on size or weight. They have much higher efficiencies, as high as 98%.

2. WHAT IS AN ALTERNATOR?

An automotive charging system is made up of three major components: the battery, the voltage regulator and an alternator. The alternator works with the battery to generate power for the electrical components of a vehicle, like the interior and exterior lights, and the instrument panel. An alternator gets its name from the term alternating current (AC). Alternators are typically found near the front of the engine and are driven by the crankshaft, which converts the pistons' up-and-down movement into circular movement. (To learn more about the basic parts of car engines, read How Car Engines Work.) Some early model vehicles used a separate drive belt from the crankshaft pulley to the alternator pulley, but most cars today have a serpentine belt, or one belt that drives all components that rely on crankshaft power. Most alternators are mounted using brackets that bolt to a specific point on the engine. One of the brackets is usually a fixed point, while the other is adjustable to tighten the drive belt.

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Alternators produce AC power through electromagnetism formed through the stator and rotor relationship that we'll touch on later in the article. The electricity is channeled into the battery, providing voltage to run the various electrical systems. Before we learn more about the mechanics of the alternator and how it generates electricity, let's look at the various parts of an alternator in the next section.

3. ALTERNATOR COMPONENTS

For the most part, alternators are relatively small and lightweight. Roughly the size of a coconut, the alternators found in most passenger cars and light trucks are constructed using an aluminum outer housing, as the lightweight metal does not magnetize. This is important since aluminum dissipates the tremendous heat generated by producing the electrical power and since the rotor assembly produces a magnetic field.

If you closely inspect an alternator, you'll find it has vents on both the front and back side. Again, this aids in heat dissipation. A drive pulley is attached to the rotor shaft on the front of the alternator. When the engine is running, the crankshaft turns the drive belt, which in turn spins the pulley on the rotor shaft. In essence, the alternator transfers the mechanical energy from the engine into electrical power for the car's accessories.

On the back side of the alternator you'll find several terminals (or connecting points in an electrical circuit):

S terminal - Senses battery voltage

IG terminal - Ignition switch that turns the voltage regulator on

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L terminal - Closes the circuit to the warning lamp

B terminal - Main alternator output terminal (connected to the battery)

F terminal - Full-field bypass for regulator

Cooling is essential to an alternator's efficiency. It's easy to spot an older unit by the external fan blades found on the rotor shaft behind the pulley. Modern alternators have cooling fans inside the aluminum housing. These fans operate the same way, using mechanical power from the spinning rotor shaft.

As we start to disassemble the alternator, we find the diode rectifier (or rectifier bridge), the voltage regulator, slip rings and brushes. The regulator distributes the power the alternator creates, and it controls the output of power to the battery. The rectifier bridge converts the power, as we'll learn in the next section, while the brushes and slip rings help conduct current to the rotor field winding, or wire field.

Opening the alternator reveals a large cylinder with triangular finger poles around the circumference. This is the rotor. A basic alternator is made up of a series of alternating finger pole pieces placed around coil wires called field windings that wrap around an iron core on the rotor shaft. Since we know the pulley attaches to the shaft, we can now visualize how the rotor spins inside the stator. The rotor assembly fits inside the stator with enough room or tolerance between the two, so the rotor can spin at high speeds without striking the stator wall. On each end of the shaft sits a brush and a slip ring.

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Alternators generate power through magnetism. The triangular finger poles fixed around the circumference of the rotor are staggered, so the north and south poles alternate as they surround the wire rotor field windings. This alternating pattern creates the magnetic field that in turn induces voltage into the stator. Think of the stator as the catcher's glove as it harnesses all the power created by the spinning rotor.

All these components work together to give us the power we need to run our vehicles. Tesla captured this electrical energy and used it to light up cities, but we only need enough volts to power our stereo, lights, windows and locks.

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4. CONSTRUCTIVE SCHEME OF THE ALTERNATOR

Fig2. AAK compact

Pos 1 -- PulleyPos 2 -- Drive end bearingPos 3 -- Drive end bracketPos 4 -- Stator with windingPos 5 -- RotorPos 6 -- Rear bracketPos 7 -- Rectifier with diodesPos 8 -- Protective coverPos 9 -- Terminals B+, D+, WPos 10 -- Rear bearingPos 11 -- Slip ringsPos 12 -- BrushPos 13 -- Brush holder with voltage regulatorPos 14 -- Rubber gaskets

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CHARACTERITICS

CONNECTION DIAGRAM

APPLICATIONS- for passenger cars- for commercial vehicles- for heavy-duty applications- for special applicationsFeatures- high specific power and efficiency- small dimensions

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- low weight- low noise level- higher protection against accidental contact- long life operation

DESIGN The alternator is a three-phase, 12-pole synchronous self-excited generator with two internal fans and built-in regulator and rectifier. The compact construction and carefully selected materials assure improved technical characteristics and long life, service free, operation even under the harshest conditions of high and low temperatures, salt spray, humidity, water, dust, vibrations, aggressive liquids.Stator The stator has a three-phase winding on a laminated pack. The selected design and high filling factor of the stator slots provides improved cooling, low noise and high output characteristics.Cooling Two internal fans positioned on the claw poles provide more effective cooling with lower noise and higher protection against accidental contact as well as higher output.Rotor Smaller slip rings provide higher brush durability, even at high speeds. Encapsulated slip rings offer increased durability of the alternator.Rectifier Sandwich construction of the rectifier with press fit Zener diodes provides the low temperatures of the rectifier diodes, high resistance to vibrations and protection of loads on the vehicle against alternator overvoltages. The installation of the rectifier on the outer side of the rear end bracket ensures flexible arrangement of all types of terminals.Regulator The regulator together with the brush holder is assembled on the rear end bracket. Regulators use microelectronic technology and are mono or multifunction. The highest quality of brushes ensure long life of the alternator.Brackets - Bearings - Pulleys Brackets, bearings and pulleys are made according to the customersÕ requirements. A range of special sealed bearings makes it possible to design alternators for specific installations, operating in the harshest conditions whilst achieving long, maintenance free life.

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5. UNDERSTANDING ALTERNATOR POWER OUTPUT

In the early days, cars used generators rather than alternators to power the vehicle's electrical system and charge the battery. That's not the case anymore. As automotive technology evolved, so did the need for more power. Generators produce direct current, which travels in one direction, as opposed to the alternating current for the electricity in our houses, which periodically reverses directions. As Tesla proved in 1887, alternating current became more attractive as it generates higher voltage more efficiently, something necessary in contemporary automobiles. But car batteries can't use AC power since they produce DC power. As a result, the alternator's power output is fed through diodes, which convert the AC power to DC power.

6.THE LIFE AND DEATH OF AN ALTERNATOR

As we saw in the beginning of the article, a failing alternator will kill a battery and ruin your day. But why did the alternator fail in the first place? Alternators have moving parts, get dirty and are subject to stress from heat and cold. As a result, the internal parts gradually wear out. One of the most common failures is bearing failure. The needle bearings that allow the rotor to spin freely inside the stator can break down from dirt and heat. When the bearings fail, the rotor will not spin efficiently and can eventually seize. Usually an alternator with bearings failure makes a loud grinding noise. If you suspect this problem, it's only a matter of time before the alternator gives up. Older vehicles with generators tend to require much more maintenance than newer models, but there's no hard and fast rule for how long an alternator will last. It varies from manufacturer to manufacturer. You can take several easy steps to diagnose whether your alternator is on the fritz. First, most cars today have a dashboard light that glows when the ignition is switched on. This light usually is represented with a symbol of a battery. Have you ever heard a buzzing noise when the key is on, but the car isn't running? That's the voltage from the battery running through the charging system. If this bulb is burned out, chances are the alternator won't work. The car may start, but as we learned, it's just a matter of time before the battery drains and the electrical system fails. As a rule, a three-phase alternator can operate with only one of the stator windings operational, although it's only one-third as efficient. To test whether your car had an issue with one of its stator windings, you'd need to use a voltmeter to check the voltage. (You can buy a basic voltmeter at an electronics store.) This is called a load test. Since the battery produces DC power, set the voltmeter to DC rather than AC. Connect the red lead (or wire) to the positive terminal and the black to the negative. With no accessories on, start the car and raise the RPM to around 1,000. The voltage should register around 14 volts. Anything less than 12 may indicate a problem. Next, turn on the headlights, air conditioner, radio and anything else that draws electrical power. Rev the engine and check the voltmeter. Again, the voltage should register around 14 volts. If you have a failing alternator, the voltage will be well below 14 volts. If so, it's time to replace the alternator. Before you decide you need to yank the alternator and replace it, make sure you check the serpentine belt. If the belt is worn or loose, the alternator won't function properly. A bad belt is easy to replace and won't set you back much, usually less than $20. But if you have to replace the alternator, you have options. Read on to learn how to go about replacing an alternator and what it may cost in the next section.

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6.1. Bearing Failure: Causes and Cures

Excessive Loads:• Excessive loads usually cause premature fatigue. Tight fits, brinelling and improper preloading can also bring about early fatigue failure.• The solution is to reduce the load or redesign using a bearing with greater capacity.

Overheating:• Symptoms are discoloration of the rings, balls, and cages from gold to blue.• Temperature in excess of 400F can anneal the ring and ball materials.• The resulting loss in hardness reduces the bearing capacity causing early failure.• In extreme cases, balls and rings will deform. The temperature rise can also degrade or destroy lubricant.

True Brinelling:

• Brinelling occurs when loads exceed the elastic limit of the ring material.• Brinell marks show as indentations in the raceways which increase bearing vibration (noise).• Any static overload or severe impact can cause brinelling.

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False Brinelling:

• False brinelling - elliptical wear marks in an axial direction at each ball position with a bright finish and sharp demarcation, often surrounded by a ring of brown debris – indicates excessive external vibration.• Correct by isolating bearings from external vibration, and using greases containing antiwear additives.

Normal Fatigue Failure:• Fatigue failure - usually referred to as spalling - is a fracture of the running surfaces and subsequent removal ofsmall discrete particles of material.• Spalling can occur on the inner ring, outer ring, or balls.• This type of failure is progressive and once initiated will spread as a result of further operation. It will always be accompanied by a marked increase in vibration.• The remedy is to replace the bearing or consider redesigning to use a bearing having a greater calculated fatigue life.

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Reverse Loading:

• Angular contact bearings are designed to accept an axial load in one direction only.• When loaded in the opposite direction, the elliptical contact area on the outer ring is truncated by the low shoulder on that side of the outer ring.• The result is excessive stress and an increase in temperature, followed by increased vibration and early failure.• Corrective action is to simply install the bearing correctly.

Contamination:

• Contamination is one of the leading causes of bearing failure.• Contamination symptoms are denting of the bearing raceways and balls resulting in high vibration and wear.• Clean work areas, tools, fixtures, and hands help reduce contamination failures.• Keep grinding operations away from bearing assembly areas and keep bearings in their original packaging until you are ready to install them.

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Lubricant Failure:

• Discolored (blue/brown) ball tracks and balls are symptoms of lubricant failure. Excessive wear of balls, ring, and cages will follow, resulting in overheating and subsequent catastrophic failure.• Ball bearings depend on the continuous presence of a very thin -millionths of an inch - film of lubricant between balls and races, and between the cage, bearingrings, and balls.• Failures are typically caused by restricted lubricant flow or excessive temperatures thatdegrade the lubricant’s properties.

Corrosion:

• Red/brown areas on balls, race-way, cages, or bands of ball bearings are symptoms ofcorrosion.• This condition results from exposing bearings to corrosive fluids or a corrosive atmosphere.• In extreme cases, corrosion can initiate early fatigue failures.• Correct by diverting corrosive fluids away from bearing areas and use integrally sealed bearings whenever possible.

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Loose Fits:

• Loose fits can cause relative motion between mating parts. If the relative motion between mating parts is slight but continuous, fretting occurs.• Fretting is the generation of fine metal particles which oxidize, leaving a distinctive brown color. This material is abrasive and will aggravate the looseness. If the looseness is enough to allow considerable movement of the inner or outerring, the mounting surfaces(bore, outer diameters, faces) will wear and heat, causing noise and runout problems.

Tight Fits:

• A heavy ball wear path in the bottom of the raceway around the entire circumference of the inner ring and outer ring indicates a tight fit.• Where interference fits exceed the radial clearance at operating temperature, the balls will become excessively loaded. This will result in a rapid temperature rise accompanied by high torque.• Continued operation can lead to rapid wear and fatigue.• Corrective action includes a decrease in total interference.

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6.2. For a batch size (30 + i), where i is the number in the group of student, to imagine, more realistic, in a full test, values for times of failure

For a batch size of 33 systems, we have the follwing values of failure times.

Piece

Failure time

Piece

Failure time

Piece

Failure time

Piece

Failure time

[hours] [hours] [hours] [hours]1 3250 11 3450 20 3234 30 32402 3290 12 3430 21 3035 31 32453 3260 13 3380 22 3120 32 30904 3420 14 3165 23 3340 33 32505 3480 15 3243 24 31656 3308 16 3233 25 32757 3254 17 3125 26 3288 8 3110 18 2980 27 3250 9 3500 19 3225 28 3480

10 2790 20 3423 29 3165

If the system work for more then 3250h it will be considered OK

Xi [h] fi 2500 2550 2600 2650 2700 2750 2800 xmin 2850 2 2900

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Xi [h] fi 2950

3000 1 3050 4 3100 3 3150 4 3200 7 3250 3 3300 4 3350 1 3400 1 3450 2xmax 3500 1

6.3. Statistical determination of the main indicators of reliability (mean m, reliability function R(t), unreliability function F(t), failure frequency f(t), failure rate z(t),

average m, dispersion D, standard deviation , coefficient of variation median Me, mode Mo, quantiles t10%, t50% and t90%.

- Mean m

- Reliability function R(t)

- Unreliability function F(t)

- Failure frequency f(t),

- Failure rate z(t)

- Average m

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- Dispersion D

- Standard deviation ,

- Coefficient of variation

6.4. Identification of mathematical model for reliability equipment

The dispersion of the failure for the cluch system

Failure after test

0

1

2

3

4

5

6

7

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2850 2900  2950 3000 3050 3100 3150 3200 3250 3300 3350 3400 3450 3500

Test hours

Nr o

f fai

led

syst

ems

Taking in count the form of the graph the mathematical model for the reliability equipment is the NORMAL LAW.

7. ALTERNATOR REPLACEMENT

For the most part, alternators are less expensive than say, a power steering pump or air conditioner compressor. Nevertheless, you have alternatives to forking out a lot of cash for a replacement alternator. Many automotive stores sell remanufactured or rebuilt alternators at a discounted price. Sometimes alternators are easily accessible and simple to replace for the amateur mechanic. With a modest amount of automotive experience and the proper tools, replacing an alternator in your garage can be done. But more and more cars don't have room under the hood, and the alternators can be

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difficult to reach without first removing several other components. In this case, it's best to take your car to an experienced technician who can do the job quickly. If you happen to own an alternator that has a repair kit available for sale, you can really save some money. Alternator repair kits run between $12 and $30, depending on which components you need to fix. Again, you need the proper tools and a little know-how, but if you're able to find the right kit and know what you're doing, you can rebuild an alternator for a fraction of the cost of even a remanufactured unit. One thing is certain: A bad alternator will ruin a good battery if you don't address it quickly. Batteries can only be recharged so many times before they'll lose their ability to hold a charge. For the most part, if the battery isn't relatively old, it should survive. But an older battery that is constantly drained and charged, drained and charged will have a shorter life span than a battery operated under normal conditions. The average life span of a battery is usually around 48 months.

BIBLIOGRAPHY:

1. www.wikipedia.org 2. www.howdostuffwork.com 3. www.iskra.com

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