AUTOMOBILE ELECTRICAL SYSTEMS 7. CHARGING SYSTEM In the previous unit we studied that every motor vehicle is provided with a self starting system. Every time we start the engine the self starter draws current from the battery and in the process the battery gets discharged. Also the other electrical units in the vehicle draw current from the battery and the battery becomes weak. Hence a charging system is provided in the vehicle to recharge the battery so that it always has sufficient stored energy for the self starter to draw current for starting the engine and also for supplying current to the other electrical units in the vehicle.
7.0 i. ii. iii.
7.1
Objectives After going through this lesson, you will be able to: Realize the necessity of the charging system. Understand how the charging system works. Recognize the main components used in charging system.
Introduction
In this lesson we will study the charging system which is provided in the vehicle for recharging the lead-acid battery of the vehicle on continuous basis. The charging system comprises an alternator fitted with a pulley, which is driven from the engine using a V-belt and provides alternating current. This alternating current is converted into direct current by rectifiers for charging the battery. A voltage regulator is also used in conjunction with the alternator so that the output voltage does not exceed beyond the specified limit.
7.2
Charging System
The battery provides power for cranking the engine and to the other electrical equipment in the car such as lights, ignition etc. Hence, the battery gets discharged in the process and is needed to be recharged for further use. A charging system is therefore necessary and is provided in the vehicle for charging the battery. It comprises a generator fitted with a V-belt pulley and a regulator. The generator gets drive from the engine and is driven from the engine crankshaft pulley through V-belt as shown in Fig.7.1. The generator, therefore, draws mechanical energy from the engine and converts it into electrical energy. A regulator is
provided to control the output voltage of the system and generally electronic regulators are now provided whose setting cannot be adjusted. The negative terminal of the battery is grounded through the body of the vehicle.
Fig. 7.1: The charging system.
Self-check Questions 1. 2. 3. 4. 5.
7.3
Why is charging system necessary? What discharges the battery in the first place? How does the generator get its drive? Charger converts ___________ energy into ___________ energy. What is the function of regulator?
Types of Generators
7.3.1 DC Generator / Dynamo
Fig. 7.2: The dynamo and its working.
In earlier times, DC generators which are also called dynamos were used to provide direct current to charge the battery. In a dynamo, the rotor carries several conductors of copper wire in a laminated core made of soft iron which is mounted on a steel shaft. The rotor rotates within a yoke which has the field poles which are energized by a fraction of the current output of the generator. The voltage thus induced in the conductors of the armature is impressed across two carbon brushes through the commutator. The brushes convey the current to the external circuit when the circuit is closed. Self-check Questions 6. What is the other name of a DC generator? 7. What type of current is produced by generator? 8. The rotor of the generator carries several conductors of __________ wire in a laminated core made of _______ which is mounted on a _____ shaft. 9. The rotor rotates within a _______ which has the _________ that are energized by a fraction of the current output of the generator. 10. What is the function of carbon brushes?
7.3.2 Alternator In the modern automobiles, alternators are used. Alternators provide an alternating current and rectifiers are therefore required to convert the AC produced by the alternator into DC which is required to charge the battery.
Fig. 7.3: The alternator and its working.
• • • •
Alternators offer certain advantages over dynamos such as: a higher output can be obtained even at low speeds of the engine. alternators are capable of being run at higher speeds. alternators have a higher output to weight ratio. alternators require less maintenance.
Self-check Questions 11. Why are rectifiers used with an alternator? 12. List the advantages of an alternator over a dynamo.
7.4
Principle of Working
Generators work on the principle of electromagnetic induction. They consist of an electromagnet which produces a magnetic field which is cut by copper conductors, causing a difference in potential to be induced in the conductors. This difference in potential is applied across the external circuit and it makes current to flow in the circuit when it is closed.
Fig. 7.4: Components of a generator.
Fig. 7.5: Stator and brush of the alternator.
The main components of a generator are a rotor and a stator. In an alternator, the conductors are stationary and they form the stator while the electromagnetic field is rotated. Slip rings are used and brushes of copper alloy make contact with the slip rings and convey the current to the field winding of the alternator. Since an alternator supplies AC, rectifiers are required to be used for converting the AC into DC. This increases the initial cost of the alternator. 3-phase alternators are generally used so that ripples are reduced and a nearly pure DC can be obtained.
Fig. 7.6: Rectification (a) Diodes. (b) Reduction in ripples by using 3-phase.
Self-check Questions 13. What are the main components of a generator? 14. In an alternator, the ______________ are stationary and they form the stator while the ___________________ is rotated. 15. ____________ and ____________ are used to convey current to the field winding of the alternator. 16. ___________ alternators are used so that __________ are reduced
7.5
Class Assignment
1. Discuss the differences between alternator and dynamo in respect of construction and working. 2. Discuss the advantages of alternator over dynamos.
7.6
Summing Up
In this lesson we studied the necessity of the charging system. We also studied the types of generators used in motor vehicles, their construction, and working. Any generator works on the principle of electromagnetic induction. Whenever there is relative motion between a magnetic field and conductors, electromagnetic force (emf) or voltage is produced In case of a generator this is DC voltage and in case of alternator it is AC voltage which then has to be rectified to DC for battery charging. Advantages of alternators over dynamos now used in vehicles were also looked into. These arise primarily due to the fact that in an alternator, conductors are stationary and the field is rotated. Now in the next lesson we will study the alternator charging circuit and the functioning of the regulator.
7.7
Possible Answers to Self-check Questions
1. to charge the battery which gets discharged due to engine cranking and supply to other electrical equipment in the vehicle. 2. engine cranking 3. from engine crankshaft by V-belt 4. mechanical, electrical 5. to keep the output of the generator within the specified limit 6. dynamo 7. DC voltage 8. copper, soft iron, steel 9. yoke, field poles 10. they collect output and give to external circuit 11. because alternator gives AC and it is to be converted to DC 12. See section 7.3.2 13. stator and rotor 14. conductors, electromagnetic field 15. Brushes, slip rings 16. 3-phase, ripples
7.8
Terminal Questions
1. Discuss the necessity of the charging system and write down the types of generators used in automobiles and their main difference. 2. Discuss the working principle of generators. 3. Differentiate between the construction and working of a DC generator and an alternator. Give the advantages of alternators over DC generators. 4. Discuss the necessity of rectifiers and regulator in the charging system.
7.9
References 1. www.howstuffworks.com
7.10 Suggested Further Reading 1.
Kohli, P.L., 1987. Automotive Electrical Equipment. First Reprint. Tata McGraw-Hill Publishing Co. Ltd, New Delhi.
7.11 Glossary 3-phase alternator
Alternators with uniform phase lag to reduce the peaks and lows of output.
Alternating current
An electrical signal in which current and voltage vary in a repeating sequence.
Alternator
A device which produces ac (alternating current) which is
converted to dc (direct current) to charge the car battery. Commutator
The name for the copper segments of the armature of a starter or dc generator.
DC generator
A device which produces direct current (dc) necessary to charge the battery.
Direct current
Constant electric current that flows in one direction only.
Dynamo
A device that is driven by the engine and converts mechanical work into direct electrical current.
Rectifiers
A device (used primarily in alternators) that permits electrical current to flow in one direction only.
Regulator
A device which maintains the amperage and/or voltage levels of a circuit at predetermined values.
Ripples
Variations in output of alternator.