Lecture 3 - Charging System

7/29/2019 Lecture 3 - Charging System

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Automotive Electrical Syste

Lecture by :

Jagwant SinghAssistant Professor

Department of Automobile Engineering

Gulzar College of Engineering , Khanna

Charging system


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Principle of generation of direct current

Whenever a conductor cuts magnetic flux, dynamically induced e.m.f. isproduced in it according to Faraday's Law's of Electromagnetic Induction. This

e.m.f. causes a current to flow if the conductor circuit is closed.

the basic essential parts of an electric generator are :

1. A magnetic field

2. A conductor or conductors which can cut the magnetic flux.

Basic DC machine is that of commutator type, i.e. it is an AC machine furnished

with a commutator such that it converts the AC to DC. In this respect we can saythat the commutator segments act as mechanical rectifiers.


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The charging system must meet the following criteria:

Supply the current demands made by all loads.

Supply whatever charge current the battery demands.

Operate at idle speed.

Supply constant voltage under all conditions.

Should be efficient.

Require low maintenance.


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Shunt Generator

A shunt generator is a method of generating electricity in which field windingand armature winding are connected in parallel, and in which the armature

supplies both the load current and the field current.

A direct current (DC) generator, not using a permanent magnet, requires a DC

field current.

The field may be separately excited by a source of DC, or may be connected to the

armature of the generator so that the generator also provides the energy requiredfor the field current.


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Externally Grounded Field Vs. Internally

Grounded Field


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Armature reaction in a DC Generator

In a DC machine, the main field is produced by field coils. In both thegenerating and motoring modes, the armature carries current and a magnetic

field is established, which is called the armature flux.

The effect of armature flux on the main field is called the armaturereaction.


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Cutout Relay

Sometimes called the circuit breaker, this device is a magnetic "make-and-break" switch.

It connects the generator to the battery circuit when the generator's voltagebuilds up to the desired value.

It disconnects the generator when it slows down or stops.

The relay has an iron core that is magnetized to pull down a hinged armature.

When the armature is pulled down a set of contact points closes and thecircuit is completed. When the magnetic field is broken (like when the

generator slows down or stops) a spring pulls the armature up, breaking thecontact points.


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Voltage Regulator

The voltage regulator keeps the voltage at a constant value and thereforecontrols the output in accordance with the requirements of the battery and

any accessories operating.

When the battery is low or power consuming items such as headlights are on,

the generator output is near maximum.

But when the demand for power is very low, the voltage regulator limits the

generator output so as to protect the battery from over charging and protectthe electrical system from high damaging voltages.


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Voltage Regulator

When the points are closed the field circuit takes the "easy" route to ground

but when the points are open the field circuit must pass through the resistorto get to ground.

When the generator is operating (battery low or a number of devices running)

its voltage may stay below that for which the control is set. Since the flow of

current will be too weak to pull the armature down the generator field will goto ground through the points.

However, if the system is fully charged the generator voltage will increase

until it reaches the maximum limit and current flow through the shunt coil

will be high enough to pull the armature down and separate the points.


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Current Regulator Even though the generator's voltage is controlled it is possible for its current

to run too high. This would overheat the generator, so a current regulator is

incorporated to prevent premature failure.

Similar in appearance to the voltage regulator's iron core, the currentregulator's core is wound with a few turns of heavy wire and connected in

series with the generator's armature.

In operation, current flow increases to the

predetermined setting of the unit. At this time,current flow through the heavy wire windings

will cause the core to draw the armature down,opening the current regulator points. In order to

complete the circuit the field circuit must passthrough a resistor. This lowers current output,

points close, output increases, points open,output down, points close, and so on. The

points, therefore, vibrate open and closed muchas the voltage regulator's points do, many times

every second.


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THIRD BRUSH Regulator

This type of generator uses three brushes instead of two. As a way of controlling thegenerator output, the field circuit is connected so the current sent to the field coil windingsis taken off of the commutator by this third brush.

The third brush is laced between the two main brushes and is adjustable. The closer thethird brush is to the main brush, the more output the generator will have. And as you havefigured out by now, the further away from the main brush the third brush is moved, the lessoutput the generator will have.

This third brush system works similarly to a voltage regulator. When the third brush is movedaway from the main brush, the current to the field windings is reduced and the output drops.

Third brush generators were used a lot on farm tractors and cars of the early days.

The advantage was that they did not need a voltage regulator. In car, for instance, when youturned on the lights at night, you also turned up the third brush in the generator toincrease the output.

The Disadvantage side was if you forgot to return the third rush to its original setting thenext morning, it would overcharge during the day and boil all of the water out of he battery.


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Third Brush Regulator


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Dynamo

A dynamo is an electrical generator that produces direct current with the useof a commutator. Dynamos were the first electrical generators capable of

delivering power for industry, and the foundation upon which many other

later electric-power conversion devices were based, including the electricmotor, the alternating-current alternator, and the rotary converter. Today, the

simpler alternator dominates large scale power generation, for efficiency,reliability and cost reasons. A dynamo has the disadvantages of a mechanical

commutator. Also, converting alternating to direct current using powerrectification devices


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Alternator

Without a commutator, a dynamo becomes an alternator, which is asynchronous singly fed generator. Alternators produce alternating current witha frequency that is based on the rotational speed of the rotor and the numberof magnetic poles.

Automotive alternators produce a varying frequency that changes with enginespeed, which is then converted by a rectifier to DC. By comparison,alternators used to feed an electric power grid are generally operated at aspeed very close to a specific frequency,

For the benefit of AC devices that regulate their speed and performancebased on grid frequency. Some devices such as incandescent lamps andballast-operated fluorescent lamps do not require a constant frequency, butsynchronous motors such as in electric wall clocks do require a constant gridfrequency.


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Principle of working

Alternators generate electricity using the same principle as DC generators, namely, when the magnetic fieldaround a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called therotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The fieldcuts across the conductors, generating an induced EMF (electromotive force), as the mechanical inputcauses the rotor to turn.

The rotating magnetic field induces an AC voltage in the stator windings. Often there are three sets ofstator windings, physically offset so that the rotating magnetic field produces a three phase current,displaced by one-third of a period with respect to each other.

The rotor's magnetic field may be produced by induction (as in a "brushless" alternator), by permanentmagnets (as in very small machines), or by a rotor winding energized with direct current through slip ringsand brushes. The rotor's magnetic field may even be provided by stationary field winding, with moving poles

in the rotor. Automotive alternators invariably use a rotor winding,[citation needed] which allows control ofthe alternator's generated voltage by varying the current in the rotor field winding. Permanent magnetmachines avoid the loss due to magnetizing current in the rotor, but are restricted in size, due to the cost ofthe magnet material. Since the permanent magnet field is constant, the terminal voltage varies directlywith the speed of the generator. Brushless AC generators are usually larger machines than those used inautomotive applications.

An automatic voltage control device controls the field current to keep output voltage constant. If theoutput voltage from the stationary armature coils drops due to an increase in demand, more current is fedinto the rotating field coils through the voltage regulator (VR). This increases the magnetic field around thefield coils which induces a greater voltage in the armature coils. Thus, the output voltage is brought backup to its original value.

Alternators used in central power stations may also control the field current to regulate reactive power and

to help stabilize the power system against the effects of momentary faults.


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Brushless alternators

A brushless alternator is composed oftwo alternators built end-to-end on one shaft.

Smaller brushless alternators may look like one unit but the two parts are readily identifiableon the large versions. The larger of the two sections is the main alternator and the smallerone is the exciter.

The exciter has stationary field coils and a rotating armature (power coils). The mainalternator uses the opposite configuration with a rotating field and stationary armature.

A bridge rectifier, called the rotating rectifier assembly, is mounted on a plate attached tothe rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts.

The main alternator has a rotating field as described above and a stationary armature (powergeneration windings).

Varying the amount of current through the stationary exciter field coils varies the 3-phaseoutput from the exciter. This output is rectified by a rotating rectifier assembly, mounted onthe rotor, and the resultant DC supplies the rotating field of the main alternator and hencealternator output. The result of all this is that a small DC exciter current indirectly controlsthe output of the main alternator.


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Brushless alternators


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Bridge Rectifiers

A bridge circuit is a type of electrical circuit in which two circuit branches(usually in parallel with each other) are "bridged" by a third branch connected

between the first two branches at some intermediate point along them.

A diode bridge Rectifier is an arrangement of four (or more) diodes in a bridge

circuit configuration that used for conversion of an alternating current (AC)input into a direct current (DC) output.

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Lecture 3 - Charging System