ALTERNATOR

An alternator is such a machine which converts mechanical energy from a prime mover to AC electric power at specific voltage and current and it is also known as synchronous generator.

Use of Alternator

The power for electrical system of modern vehicles produces from alternator. In previous days, DC generator or dynamos were used for this purpose but after development of alternator, the DC dynamos are replaced by more robust and light weight alternator. Although the electrical system of motor vehicles generally requires direct current but still an alternator along with diode rectifier instead of a DC generator is better choice as the complicated commutation is absent here. This special type of generator which is used in vehicle is known as automotive alternator.
Another use of alternator is in diesel electric locomotive. Actually the engine of this locomotive is nothing but an alternator driven by diesel engine. The alternating current produced by this generator is converted to DC by integrated silicon diode rectifiers to feed all the DC traction motors. And these dc traction motors drive the wheel of the locomotive.

Types of Alternator

Alternators or synchronous generators can be classified in may ways depending upon their application and design.
According to application these machines are classified as-

1. Automotive type - used in modern automobile.
2. Diesel electric locomotive type - used in diesel electric multiple unit.
3. Marine type - used in marine.
4. Brush less type - used in ELECTRICAL POWER generation plant as main source of power.
5. Radio alternators - used for low brand radio frequency transmission.

These ac generators can be divided in many ways but we will discuss now two main types of alternator categorized according to their design. These are-

1. Salient pole type 
   It is used as low and medium speed alternator. It has a large number of projecting poles having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron or steel of good magnetic quality. Such generators are characterized by their large diameters and short axial lengths. These generator are look like big wheel. These are mainly used for low speed turbine such as in hydel power plant.
2. Smooth Cylinderical type: It is used for steam turbine driven alternator. The rotor of this generator rotates in very high speed. The rotor consists of a smooth solid forged steel cylinder having a number of slots milled out at intervals along the outer periphery for accommodation of field coils. These rotors are designed mostly for 2 pole or 4 pole turbo generator running at 36000 rpm or 1800 rpm respectively.

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 according to which when a moving conductor is placed in the magnetic field then the current is induced.

Now let a rectangular peace(single turn loop) placed

 between two opposite poles of

 the magnets Say this single turn loop ABCD can rotate against axis a-b.Suppose this loop starts rotating clockwise.

After 90^o 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^o 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^o to 0^o and consequently the induced current in the turn is reduced to zero from its maximum value.

After another clockwise rotation

of 90^o 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 Flemming'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^o 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 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.

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 N_s 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^o due to the space displaced arrangement of 120^o between them as shown in the figure below. This particular phenomena results in 3φ power flow out of the alternator which is then transmitted to the distribution stations for domestic and industrial uses.

Construction of Alternator

Construction 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 3phase voltage represented by aa’ bb’ cc’ is induced in the armature conductors thus resulting in the generation of 3phase electrical power. All modern day electrical power generating stations use this technology for generation of 3phase power, and as a result the alternator or synchronous generator has become a subject of great importance and interest for power engineers.

An alternator is basically a type of AC generator which is also known as synchronous generator, for the simple reason that the field poles are made to rotate at synchronous speed N_s = 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 weight 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 Type

The term salient means protruding or projecting. The 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 covers only about ²/₃rd of pole pitch.
3. Poles are laminated to reduce eddy current losses.
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.

Salient pole alternators driven by water turbines are called hydro-alternators or hydro generators.

Cylindrical Rotor Type

The cylindrical rotor is generally used for very high speed operation and employed in steam turbine driven alternators like turbo generators. The machines are built in a number of ratings from 10 MVA to over 1500 MVA. 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 ofOr 4-pole type running at a speed ofWhere, f is the frequency of 50 Hz.
The 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. The field coils are so arranged around these poles that flux density is maximum on the polar central line and gradually falls away as we move out towards the periphery. The cylindrical rotor type machine gives better balance and quieter-operation along with lesser windage losses.

Armature Reaction

Every rotating electrical machine works based on Faraday's law. Every electrical machine requires a magnetic field and a coil (Known as armature) with a relative motion between them. In case of an alternator, we supply electricity to pole to produce magnetic field and output power is taken from the armature. Due to relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and hence there would be changing flux linkage with these armature conductors. According toFaraday's law of electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load is connected with armature terminals, there is a current flowing in the armature coil.

As soon as current starts flowing through the armature conductor there is one reverse effect of this current on the main field flux of the alternator (or synchronous generator). This reverse effect is referred as armature reaction in alternator or synchronous generator. In other words the effect of armature (stator) flux on the flux produced by the rotor field poles is called armature reaction.
We already know that a current carrying conductor produces its own magnetic field, and this magnetic field affects the main magnetic field of the alternator.
It has two undesirable effects, either it distorts the main field or it reduces the main field flux or both. They deteriorate the performance of the machine. When the field gets distorted, it is known as cross magnetizing effect. and when the field flux gets reduced, it is known as demagnetizing effect.
The electromechanical energy conversion takes place through magnetic field as a medium. Due to relative motion between armature conductors and the main field, an emf is induced in the armature windings whose magnitude depends upon the relative speed and as well as the magnetic flux. Due to armature reaction, flux is reduced or distorted, the net emf induced is also affected and hence the performance of the machine degrades.

Armature Reaction in Alternator

In an alternator like all other synchronous machines, the effect of armature reaction depends on the power factor i.e the phase relationship between the terminal voltage and armature current.
Reactive power (lagging) is the magnetic field energy, so if the generator supplies a lagging load, this implies that it is supplying magnetic energy to the load. Since this power comes from excitation of synchronous machine, the net reactive power gets reduced in the generator. Hence, the armature reaction is demagnetizing in nature. Similarly, the armature reaction has magnetizing effect when the generator supplies a leading load (as leading load takes the leading VAR) and in return gives lagging VAR (magnetic energy) to the generator. In case of purely resistive load, the armature reaction is cross magnetizing only.

Let's discuss in details
The armature reaction of alternator or synchronous generator, depends upon the phase angle between, stator armature current and induced voltage across the armature winding of alternator.
The phase difference between these two quantities, i.e. Armature current and voltage may vary from - 90^o to + 90^o
If this angle is θ, then,To understand actual effect of this angle on armature reaction of alternator, we will consider three standard cases,

1. When θ = 0
2. When θ = 90^o
3. When θ = - 90^o

Armature Reaction of Alternator at Unity Power Factor

At unity power factor, the angle between armature current I and induced emf E, is zero. That means, armature current and induced emf are in same phase. But we know theoretically that emf induced in the armature is due to changing main field flux, linked with the armature conductor.
As field is excited by DC, the main field flux is constant in respect to field magnets, but it would be alternating in respect of armature as there is a relative motion between field and armature in alternator. If main field flux of the alternator in respect of armature can be represented asThen induced emf E across the armature is proportional to, dφ_f/dt.Hence, from this above equations (1) and (2) it is clear that, the angle between, φ_f and induced emf E will be 90^o. 
Now, armature flux φ_a is proportional to armature current I. Hence, armature flux φ_a is in phase with armature current I.
Again at unity electrical power factor I and E are in same phase. So at unity pf, φ_a is phase with E. So at this condition, armature flux is in phase with induced emf E and field flux is in quadrature with E. Hence, armature flux φ_a is in quadrature with main field flux φ_f.
As this two fluxes are perpendicular to each other, the armature reaction of alternator at unity power factor is purely distorting or cross-magnetizing type.

As the armature flux pushes the main field flux perpendicularly, distribution of main field flux under a pole face does not remain uniformly distributed. The flux density under the trailing pole tips increases somewhat while under the leading pole tips it decreases.

Armature Reaction of Alternator at Lagging Zero Power Factor

At lagging zero electrical power factor, the armature current lags by 90^o to induced emf in the armature. 
As the emf induced in the armature coil due to main field flux thus the emf leads the main field flux by 90^o. From equation (1) we get, the field flux,Hence, at ωt = 0, E is maximum and φ_f is zero.
At ωt = 90^o, E is zero and φ_f has maximum value.
At ωt = 180^o, E is maximum and φ_f zero.
At ωt = 270^o, E is zero and φ_f has negative maximum value.
Here, φ_f got maximum value 90^o before E. Hence φ_f leads E by 90^o. 
Now, armature current I is proportional to armature flux φ_a, and I lags E by 90^o. Hence, φ_alags E by 90^o.
So, it can be concluded that, field flux φ_f leads E by 90^o.
Therefore, armature flux and field flux act directly opposite to each other. Thus, armature reaction of alternator at lagging zero power factor is purely demagnetizing type. That means, armature flux directly weakens main field flux.

Armature Reaction of Alternator at Leading Power Factor

At leading power factor condition, armature current I leads induced emf E by an angle 90^o. Again, we have shown just, field flux φ_f leads, induced emf E by 90^o.
Again, armature flux φ_a is proportional to armature current I. Hence, φ_a is in phase with I. Hence, armature flux φ_a also leads E, by 90^o as I leads E by 90^o.
As in this case both armature flux and field flux lead induced emf E by 90^o, it can be said, field flux and armature flux are in same direction. Hence, the resultant flux is simply arithmetic sum of field flux and armature flux. Hence, at last it can be said that, armature reaction of alternator due to a purely leading electrical power factor is totally magnetizing type.

Nature of Armature Reaction

1. The armature reaction flux is constant in magnitude and rotates at synchronous speed.
2. The armature reaction is cross magnetizing when the generator supplies a load at unity power factor.

When the generator supplies a load at leading power factor the armature reaction is partly de magnetizing and partly cross-magnetizing.

1. When the generator supplies a load at leading power factor the armature reaction is partly magnetizing and partly cross-magnetizing.
2. Armature flux acts independently of main field flux.

Parallel Operation of Alternator

Alternator is really an AC generator. In alternator, an EMF is induced in the stator (stationary wire) with the influence of rotating magnetic field (rotor) due to Faraday's law of induction. Due to the synchronous speed of rotation of field poles, it is also known as synchronous generator. Here, we can discuss about parallel operation of alternator. When the AC power systems are interconnected for efficiency, the alternators should also have to be connected in parallel. There will be more than two alternators connected in parallel in generating stations.

Condition for Parallel Operation of Alternator

There are some conditions to be satisfied for parallel operation of the alternator. Before entering into that, we should understand some terms which are as follows.

• The process of connecting two alternators or an alternator and an infinite bus bar system in parallel is known as synchronizing.
• Running machine is the machine which carries the load.

Incoming machine is the alternator or machine which has to be connected in parallel with the system.

The conditions to be satisfied are

1. The phase sequence of the incoming machine voltage and the bus bar voltage should be identical.
2. The RMS line voltage (terminal voltage) of the bus bar or already running machine and the incoming machine should be the same.
3. The phase angle of the two systems should be equal.
4. The frequency of the two terminal voltages (incoming machine and the bus bar) should be nearly the same. Large power transients will occur when frequencies are not nearly equal.

Departure from the above conditions will result in the formation of power surges and current. It also results in unwanted electro-mechanical oscillation of rotor which leads to the damage of equipment.

General Procedure for Paralleling Alternators

The figure below shows an alternator (generator 2) being paralleled with a running power system (generator 1).These two machines are about to synchronize for supplying power to a load. Generator 2 is about to parallel with the help of a switch, S1. This switch should never be closed without satisfying the above conditions.

1. To make the terminal voltages equal. This can be done by adjusting the terminal voltage of incoming machine by changing the field current and make it equal to the line voltage of running system using voltmeters.
2. There are two methods to check the phase sequence of the machines. They are as follows
   • First one is using a Synchroscope. It is not actually check the phase sequence but it is used to measure the difference in phase angles.
   • Second method is three lamp method (Figure 2). Here we can see three light bulbs are connected to the terminals of the switch, S1. Bulbs become bright if the phase difference is large. Bulbs become dim if the phase difference is small. The bulbs will show dim and bright all together if phase sequence is the same. The bulbs will get bright in progression if the phase sequence is opposite. This phase sequence can be made equal by swapping the connections on any two phases on one of the generators.

3. Next, we have to check and verify the incoming and running system frequency. It should be nearly the same. This can be done by inspecting the frequency of dimming and brightening of lamps.
4. When the frequencies are nearly equal, the two voltages (incoming alternator and running system) will alter the phase gradually. These changes can be observed and the switch, S1 can be made closed when the phase angles are equal.

Advantages of Parallel Operating Alternators

• When there is maintenance or an inspection, one machine can be taken out from service and the other alternators can keep up for the continuity of supply.
• Load supply can be increased.
• During light loads, more than one alternator can be shut down while the other will operate in nearly full load.
• High efficiency.
• The operating cost is reduced.
• Ensures the protection of supply and enables cost-effective generation.
• The generation cost is reduced.
• Breaking down of a generator does not cause any interruption in the supply.
• Reliability of the whole power system increases.