PHYSICS PROJECT WORK

Submitted By:- Akash KumarClass:- XIISection:- A

Topic:- A.c.

generator or

alternator or

dynamo

ACKNOWLEDGAEMENT

In the accomplishment of this project successfully, many

people have best owned upon me their blessings and the heart

pledged support, this time I am utilizing to thank all the

people who have been concerned with project.

Primarily I would thank god for being able to complete this

project with success. Then I would like to thank my principal

Mr. P.K. Singh and physics teacher Dr. Kumar Haemendra, whose

Valuable guidance has been the ones that helped me patch this

project and make it full proof success his suggestions and his

instructions has served as the major contributor towards the

completion of the project.

Then I would like to thank my parents and friends who have

helped me with their valuable suggestions and guidance has

been helpful in various phases of the completion of the project.

Last but not the least I would like to thank my classmates

who have helped me a lot.

CONTENTS 1. ACKNOWLEDGEMENT 2. INTRODUCTION 3. THEORY 4. APPARATUS REQUIRED 5. CONSTRUCTION 6. WORKING PRINCIPLE OF ALTERNATOR 7. USES OF AC GENERATOR 8. POWER RATING OF ALTERNATOR 9. EFFICIENCY10. MACHINE LOSSES

INTRODUCTIONAn electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric current to flow through an external circuit. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air, or any other source of mechanical energy. Generators provide nearly all of the power for electric power grids.

Most of the electrical power used aboard Navy ships and aircraft as well as in civilian applications is ac. As a result, the ac generator is the most important means of producing electrical power. Ac generators, generally called alternators, vary greatly in size depending upon the load to which they supply power. For example, the alternators in use at hydroelectric plants, such as Hoover Dam, are tremendous in size, generating thousands of kilowatts at very high voltage levels. Another example is the alternator in a typical automobile, which is very small by comparison. It weighs only a few pounds and produces between 100 and 200 watts of power, usually at a potential of 12 volts. Many of the terms and principles covered in this chapter will be familiar to you. They are the same as those covered in the chapter on dc generators. You are encouraged to refer back, as needed, and to refer 3-2 to any other source that will help you master the subject of this chapter. No one source meets the complete needs of everyone.

BASIC AC GENERATORS Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of (1) a coil cutting through a magnetic field, or (2) a magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. That part of a generator that produces the magnetic field is called the field. That part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts — a rotor and a stator. The Rotor is the part that Rotates; the Stator is the part that remains Stationary. In a dc generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator.

THEORY The strong magnetic field is produced by a current flow through the field coil of the rotor.

The field coil in the rotor receives excitation through the use of slip rings and brushes.

Two brushes are spring-held in contact with the slip rings to provide the continuous connection between the field coil and the external excitation circuit.

The armature is contained within the windings of the stator and is connected to the output.

Each time the rotor makes one complete revolution, one complete cycle of AC is developed.

A generator has many turns of wire wound into the slots of the rotor.

The magnitude of AC voltage generated by an AC generator is dependent on the field strength and speed of the rotor.

Most generators are operated at a constant speed; therefore, the generated voltage depends on field excitation, or strength.

APPARATUS REQUIRED

4pcs. - 1cm x 2cm x 5cm ceramic magnet

1pce. - Magnet wire 200ft spool

1pce. - Miniature Incandescent Lamp, 1.5V 25mA.1pce. - Cardboard strip, 8cm x 30cm

1pce. - Large nail, 8cm long or more

1. - Knife or sandpaper to strip the wires

1pce. - tape to hold wire down

Optional: hand drill or electric drill to spin it (hand drill is best)

ConstructionConstruction wise, an alternator generally consists of field poles placed on the rotating fixture of the machine i.e. rotor as shown in the figure above. Once the rotor or the field poles are made to rotate in the presence of armature conductors housed on the stator, an alternating 3 φ voltage represented by aa’ bb’ cc’ is induced in the armature conductors thus resulting in the

generation of 3φ electrical power. All modern day electrical power generating station use this technology for generation of 3φ power, and as a result the alternator or synchronous generator has become a subject of great importance and interest for power engineers of late.

An alternator is basically a type of a.c generator also known as synchronous generator, for the simple reason that the field poles are made to rotate at synchronous speed Ns = 120 f/P for effective power generation. Where f signifies the alternating current frequency and the P represents the number of poles. In most practical construction of alternator, it is installed with a stationary armature winding and a rotating field unlike in the case of DC generator where the arrangement is exactly opposite. This modification is made to cope with the very high power of the order of few 100 Mega watts produced in an ac generator contrary to that of a DC generator. To accommodate such high power the conductor weigh and dimension naturally has to be increased for optimum performance. And for this reason is it beneficial to replace these high power armature windings by low power field windings, which is also consequently of much lighter weight, thus reducing the centrifugal force required to turn the rotor and permitting higher speed limits.There are mainly two types of rotor used in construction of alternator, 1. Salient pole type.2. Cylindrical rotor type.

Salient Pole TypeThe salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths. The pole in this case are made of thick laminated steel sections riveted together and attached to a

rotor with the help of joint. An alternator as mentioned earlier is mostly responsible for generation of very high electrical power. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. To prevent these oscillations from going beyond bounds the damper winding is provided in the pole faces as shown in the figure. The damper windings are basically copper bars short circuited at both ends are placed in the holes made in the pole axis’s. When the alternator is driven at a steady speed, the relative velocity of the damping winding with respect to main field will be zero. But as soon as it departs from the synchronous speed there will be relative motion between the damper winding and the main field which is always rotating at synchronous speed. This relative difference will induce current in them which will exert a torque on the field poles in such a way as to bring the alternator back to synchronous speed operation.

The salient features of pole field structure has the following special feature-

1. They have a large horizontal diameter compared to a shorter axial length.2. The pole shoes cover only about 2/3rd of pole pitch.3. Poles are laminated to reduce eddy current loss.4. The salient pole type motor is generally used for low speed operations of

around 100 to 400 rpm, and they are used in power stations with hydraulic turbines or diesel engines.

The cylindrical rotor is generally used for very high speed operation and is employed in steam turbine driven alternators like turbo generators.

The cylindrical rotor type machine has uniform length in all directions, giving a cylindrical shape to the rotor thus providing uniform flux cutting in all directions. The rotor in this case consists of a smooth solid steel cylinder, having a number of slots along its outer periphery for hosing the field coils.The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of Ns = (120 × f)/P = (120 × 50) / 2 = 3000 rpm.Or 4-pole type running at a speed of Ns = (120 × f) / P = (120 × 50) / 4 = 1500 rpm. Where f is the frequency of 50 Hz.The a cylindrical rotor synchronous generator does not have any projections coming out from the surface of the rotor, rather central polar area are provided with slots for housing the field windings as we can see from the diagram above.

. Working Principle of Alternator

The working principle of alternator is very simple. It is just like basic principle of DC generator. It also depends upon Faraday's law of electromagnetic

induction which says the current is induced in the conductor inside a magnetic field when there is a relative motion between that conductor and the magnetic field.

For understanding working of alternator let's think about a single rectangular turn placed in between two opposite magnetic pole as shown above.

Say this single turn loop ABCD can rotate against axis a-b. Suppose this loop starts rotating clockwise. After 90° rotation the side AB or conductor AB of the loop comes in front of S-pole and conductor CD comes in front of N-pole. At this position the tangential motion of the conductor AB is just perpendicular to the magnetic flux lines from N to S pole. Hence rate of flux cutting by the conductor AB is maximum here and for that flux cutting there will be an induced current in the conductor AB and direction of the induced current can be determined by Fleming’s right hand rule. As per this rule the direction of this current will be from A to B. At the same time conductor CD comes under N pole and here also if we apply Fleming right hand rule we will get the direction of induced current and it will be from C to D.

Now after clockwise rotation of another 90° the turn ABCD comes at vertical position as shown below. At this position tangential motion of conductor AB and CD is just parallel to the magnetic flux lines; hence there will be no flux cutting that is no current in the conductor. While the turn ABCD comes from horizontal position to vertical position, angle between flux lines and direction of motion of conductor, reduces from 90° to 0° and consequently the induced current in the turn is reduced to zero from its maximum value.

After another clockwise rotation of 90° the turn again come to horizontal position and here conductor AB comes under N-pole and CD comes under S-pole, and here if we again apply Fleming’s right hand rule, we will see that induced current in conductor AB, is from point B to A and induced current in the conductor CD is from D to C.

As at this position the turn comes at horizontal position from its vertical position, the current in the conductors comes to its maximum value from zero. That means current is circulating in the close turn from point B to A, from A to D, from D to C and from C to B. Just reverse of the previous horizontal position when the current was circulating as A→ B → C → D → A.

While the turn further proceeds to its vertical position the current is again reduced to zero. So if the turn continues to rotate the current in the turn continually alternate its direction. During every full revolution of the turn, the current in the turn gradually reaches to its maximum value then reduces to zero and then again it comes to its maximum value but in opposite direction and again it comes to zero. In this way the current completes one full sine wave form during each 360° revolution of the turn. So we have seen how an alternating current is produced in a turn is rotated inside a magnetic field. From this, we will now come to the actual working principle of alternator.

Now we cut the loop and connect its two ends with two slip rings and stationary brush is placed on each slip ring. If we connect two terminals of an external load with these two brushes, we will get an alternating current in the load. This is our elementary model of alternator.

Having understood the very basic principle of alternator, let us now have an insight into its basic operational principal of a practical alternator. During discussion of basic working of alternator, we have considered that the magnetic field is stationary and conductors (armature) are rotating. But generally in practical construction of alternator, armature conductors are stationary and field magnets rotate between them. The rotor of an alternator or a synchronous generator is mechanically coupled to the shaft or the turbine blades, which on being made to rotate at synchronous speed Ns under some mechanical force results in magnetic flux cutting of the stationary armature conductors housed on the stator. As a direct consequence of this flux cutting an induced emf and current starts to flow through the armature conductors which first flow in one direction for the first half cycle and then in the other direction for the second half cycle for each winding with a definite time lag of 120° due to the space displaced arrangement of 120° between them as shown in the figure below. These particular phenomena results in 3φ power flow out of the alternator which is then transmitted to the distribution stations for domestic and industrial uses.

Different Stages of Generation of Electricity

Usages of AC generator1. Aircraft auxiliary power generation, wind generators, high speed gas turbine generators.

2. Hybrid electric vehicle (HEV) drive systems, automotive starter generators.

3. An ac generator, or 'alternator', is used to produce ac voltages for transmission via the grid system or, locally, as portable generators.

4. An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. And there are even hybrid diesel-gas units, called dual-fuel units. Many different versions of engine-generators are available - ranging from very small portable petrol powered sets to large turbine installations. The primary advantage of engine-generators is the ability to independently supply electricity, allowing the units to serve as backup power solutions.

5.  The main advantage of AC is ease of power distribution. It is more efficient to use high voltage to distribute power, but it is not safe to have high voltage at home. It is easy to step up (and step down) AC voltage using a transformer. 

6. Motor vehicles require electrical energy to power their instrumentation, keep the engine itself operating, and recharge their batteries. Until about the 1960s motor vehicles tended to use DC generators with electromechanical regulators. Following the historical trend above and for many of the same reasons, these have now been replaced by alternators with built-in rectifier circuits. 7. All of our household appliances run on ac current. Ex:

Refrigerator, washing machines, refrigerators, fan and etc.

Power rating of alternator

Power rating of alternator is defined as the power which can be delivered by an alternator safely and efficiently under some specific conditions. Increasing load, increases losses in alternator, this leads to temperature rise of the machine. The conductor and insulator parts of the machine have some specific over heating withstand limits. The power rating of an alternator is so specified, that at that maximum load, the temperature rise of different parts of the machine does not cross their specified safe limit.

The copper losses i.e. I2R loss varies with armature current and core losses vary with voltage. The temperature rise or heating of alternator depends upon cumulative effect of copper losses and core losses. As there is no role of power factor upon these losses, the rating of alternator generally given in VA or KVA or MVA. In other word, as the losses of alternator are independent of electrical power factor, hence power factor does not come into picture while power rating of an alternator is calculated or estimated. Although losses of alternator depends upon its KVA or MVA rating but actual output varies with electrical power factor.

The electrical output of an alternator is product of power factor and VA and output is expressed in KW. Sometimes alternators are also rated by its power instead of VA rating. That time electrical power factor of the alternator must be specified too.

In addition to KVA rating, an alternator is also rated with voltage, electric current, frequency, speed, number of phase, number of poles, field ampere,

excitation voltage, maximum temperature rise limit etc.

Efficiency of alternatorExpression for Instantaneous e.m.f. Produced:

Let position of the coil at any time t. It makes angle q with vertical. If w is uniform angular speed of the coil.

Then q = wt

B is the strength of magnetic field n is the number of turns in the coil and an area of the coil then magnetic flux with the coil in this position are given by:

f = nBA Cos q = nBA Cos wt.

Differentiate w.r.t. time

= nBA (-Sin wt) w

= -nBA w Sin wt

e = - (-nBA w Sin wt)

Maximum value of e.m.f. say E0

e = E0 Sin wt.

Efficiency of an AC generator is the ratio of the useful power output to the total power input.

Because any mechanical process experiences some losses, no AC generators can be 100 percent efficient.  

 

Efficiency of an AC generator can be calculated using Equation.

Efficiency =(Output /Input )x  100

Machine Losses1. Winding Losses (Copper Losses).

I2R stator loss

I2R rotor loss

*Eddy and circulating current loss in winding (parasitic currents induced in the winding).

2. Iron Losses.

Mainly stator losses due to hysteresis loss and eddy current loss in stator laminations

3. Parasitic Eddy Losses. Induced currents in all metallic components (bolts, frame, etc.)

Friction and windage loss

Losses in fans, rotor and stator cooling vents

Losses in bearings

4. Exogenous Losses.

Losses in auxiliary equipment

Excitation

Lubrication oil pumps H2 seal oil pumps H2 and water cooling pumps And so on... Iso-phase or lead losses

BIBLIOGRAPHY www.icbse.com www.sciencebuddies.com www.technopedia.com www.wikipedia.com NCERT Physics book