Generators

PPT Consists of DC and AC Generators

1. Automotive Electrical and Electronics Charging Systems By Mr. P. Karthik selvan , AP K L N College of Engineering

2. GENERATOR

3. Introduction A  generator  is a device that converts  motive power  ( mechanical energy ) into  electrical power  for use in an external  circuit . Sources of mechanical energy include  steam turbines ,  gas turbines ,  water turbines ,  internal combustion engines  and even hand  cranks .  Generators can be DC(Dynamo) and AC (Alternator) Generators provide nearly all of the power for  electric power grids .

4. All charging circuits operate in three stages: During starting, battery supplies all load current During normal operation, generator supplies all current and recharges the battery During peak operations, battery helps generator to supply current An AC Generator produce AC Power and DC Generator produces Direct power Both of these generators produce electrical power, based on same fundamental principle of Faradays`s law of electromagnetic induction.

5. Generator works based on : 1. Faradays law of Induction 2. Flemings right hand rule Faraday's law of induction  is a basic law of  electromagnetism  predicting how a  magnetic field  will interact with an  electric circuit  to produce an  electromotive force (EMF) —a phenomenon called  electromagnetic induction . It is the fundamental operating principle of  transformers ,  inductors , and many types of  electrical   motors ,  generators  and  solenoids .

6. DC Generator A generator which produces direct current is called as a dynamo. In the past the dynamo was the only convenient way to generate electrical energy Principle and Working: If a conductor cuts magnetic lines of force, electromagnetic force is generated in the conductor, and that the electromagnetic force produced will vary with the number of magnetic lines of force, number of conductors (number of coil windings), and the speed at which the conductor cuts the lines of magnetic. Produced current is collected by using external circuit

7. If a conductor is turned clockwise between the N and S poles of a magnet, electromotive force will be generated in the arrow direction in accordance with Flemings right hand rule

8. Construction The generator is comprised of the poles cores, filed coils, armature , brushes , pulley and frame. Pole cores- Poles cores become the N and S poles of magnet when the current flow through field coils. Most of generator have four poles Field coils- It is insulates wire wound in coil form and installed around the pole core. The current flows through the field coils Armature - It is a shaft rotates inside the pole cores and produce the electromotive force. It is comprised of a rotating shaft with the armature coil at the centre and the commutator at one end. The commutator serves to convert the AC to DC Brushes- The brushes contact against the commutator and serve to pass the current generated in the armature to the field coil and to the outside of the generator It is made up on carbon Pulley- It is provided with a fan to cool the generator

9. DC Generator Characteristics Electromagnetic force generated by the generator grows larger with greater amount of flux For these reasons, continued high speed operation will cause large current to flow through the battery, generator and other circuits and shorten their service life On the other hand , during low speed only small current will flow To prevent such condition, a regulator is provided

10. Control unit Regulator: It consists of three relays : 1. Voltage regulator to control the generated voltage 2. A current limiter to control the output current 3. A cut-out relay to prevent reverse flow of current from the battery

11. Voltage regulator: The purpose of this relay is to maintain the generated voltage at a constant value. When the voltage rises , resistance is inserted in series with the filed coil circuit to limit current flow through the field coils and lower the generated voltage. When the voltage drops, resistance is removed from the field coil circuit to allow the generated voltage to build up again The repetition of this action maintains the voltage at a constant value. Classified in to – Contact point type and carbon pile type

12. Contact point type When the voltage rises too high , the magnetization by the voltage coil attracts the iron piece to open the contact points. As a result, the current flow to the field coil is limited When the voltage drops too low , the contact points are closed by the tension of regulating spring to allow the voltage to recover This repeated opening and closing of the contact point maintains constant voltage

13. Current regulator The purpose of current regulator or limiter is to prevent excessive current from the generator Its construction and operation is very similar to those of voltage regulator When current rises the contact point closed, when current become less the contact point will open

14. Cut-out rely A voltage regulator has an additional component known as a cut-out relay, which prevents the battery from discharging back into generators field after key turned OFF or engine idling Without cut-out relay the generator would always in ON and lead to drain battery even without engine running When generator speed drops – Generating voltage becomes lower than battery voltage Cut-out relay – one form of Automatic switch

15. Construction When the generator is not working , contact points are held open , after generator starts coils get magnetized so that contact points are attracted and close Based on generator speed, closing and opening of contact points will be achieved The charging circuit is used to prevent the reverse flow of current

16. ALTERNATOR

17. Introduction The increasing numbers of accessories also consume a large amount of power we need to generate more power. But DC Generator capacity is only limited to over come this problem Alternator is used Alternator is the heart of the vehicles electrical system when the engine is running. It uses electromagnetism to change some of the engines mechanical energy into electrical energy Two types alternators – Conventional and separate voltage regulator type In recent, In built regulator alternator is used Amps rating- 40 to 80 amps

18. Construction The alternator is comprised primarily of the stator, rotor, bracket, rectifier(silicon diodes) and brushes. Stator- It has three sets of armature coils wound on it to induce a three phase current. Rotor- Rotors can be classified into the salient pole type and Randell type. At present randell is mostly used. In this type the number of poles is equal to the numbers of finers Bracket- The end bracket opposite to the pulley side has six diodes assembled into it that serve to rectify all of the three phase AC generated Brushes - It serves to pass a current of 2 to 3 amperes to the rotor coils. In alternator the main current does not flow through these brushes

19. Rectifier- Alternator produce three phase AC but the automobile charging system need DC supply. Converting AC to DC is called rectification. Rectification can be done in several ways but efficient way is using diode

20. Working An alternator is designed to change mechanical energy into electrical energy The working principle of an alternator is an electromagnetic induction (EMI) When the rotor spins, its strong magnetic filed cuts across the stator windings. This induces current in the stator windings If the stator winding are connected to a load(Ex- bulb), the load would operate Slip rings are used

21. Advantage: Rotor has high strength against speed Good rectifying characteristics Current limiter not required depending on the design Cut-out relay not required Charging is possible while idling