UNIT- III

ALTERNATORS ALTERNATOR

The machine which produces 3-phase power from mechanical power is called an alternator or synchronous generator.

An alternator operates on the same fundamental principle of electromagnetic induction as a D.C. generator i.e., when the flux linking a conductor changes, an e.m.f. is induced in the conductor.

Like a D.C. generator, an alternator also has an armature winding and a field winding.

But there is one important difference between the two. In a D.C. generator, the armature winding is placed on the rotor in order to provide a way of converting alternating voltage generated in the winding to a direct voltage at the terminals through the use of a rotating commutator. The field poles are placed on the stationary part of the machine. Since no commutator is required in an alternator, it is usually more convenient and advantageous to

place the field winding on the rotating part (i.e., rotor) and armature winding on the stationary part (i.e., stator).

Alternator

Advantages of stationary armature

The field winding of an alternator is placed on the rotor and is connected to D.C. supply through two slip rings.

The 3-phase armature winding is placed on the stator.

This arrangement has the following advantages: i. It is easier to insulate stationary winding for high voltages for

which the alternators are usually designed. It is because they are not subjected to centrifugal forces and also extra space is available due to the stationary arrangement of the armature

ii. The stationary 3-phase armature can be directly connected to load without going through large, unreliable slip rings and brushes

iii. Only two slip rings are required for providing D.C. supply to the field winding on the rotor. Since the exciting current is small, the slip rings and brush gear required are of light construction

Due to simple and robust construction of the rotor, higher speed of

rotating D.C. field is possible. This increases the output obtainable from a machine of given dimensions.

CONSTRUCTION OF ALTERNATOR An alternator has 3 phase winding on the stator and a D.C. field winding on

the rotor. Stator

It is the stationary part of the machine and is built up of sheet-steel laminations having slots on its inner periphery.

A 3-phase winding is placed in these slots and serves as the armature winding of the alternator.

The armature winding is always connected in star and the neutral is connected to ground.

Rotor

The rotor carries a field winding which is supplied with direct current through two slip rings by a separate D.C. source.

This D.C. source (called exciter) is generally a small D.C. shunt or compound generator mounted on the shaft of the alternator.

Rotor construction is of two types, namely; (i) Salient (or projecting) pole type (ii) Non-salient (or cylindrical) pole type

(i) Salient pole type

In this type, salient or projecting poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator.

The individual field pole windings are connected in series in such a way that when the field winding is energized by the D.C. exciter, adjacent poles have opposite polarities.

Low and medium-speed alternators (120-400 r.p.m.) such as those driven by diesel engines or water turbines have salient pole type rotors due to the following reasons:

(a) The salient field poles would cause an excessive windage loss if driven at high speed and would tend to produce noise. (b) Salient-pole construction cannot be made strong enough to withstand

The mechanical stresses to which they may be subjected at higher speeds. Since a frequency of 50 Hz is required, we must use a large number of poles on the rotor of slow-speed alternators. Low-speed rotors always possess a large diameter.

Salient Pole Rotor

(ii) Non-salient pole type

In this type, the rotor is made of smooth solid forged-steel radial cylinder having a number of slots along the outer periphery. The field windings are embedded in these slots and are connected in series to the slip rings through

which they are energized by the D.C. exciter. The regions forming the poles are usually left un slotted. It is clear that the poles formed are non-salient i.e., they do not project out from the rotor surface.

Non Salient Pole Rotor

High-speed alternators (1500 or 3000 r.p.m.) are driven by steam turbines and use non-salient type rotors due to the following reasons:

(a) This type of construction has mechanical robustness and gives noiseless operation at high speeds. (b) The flux distribution around the periphery is nearly a sine wave and hence a better e.m.f. waveform is obtained than in the case of salient-pole type. Since steam turbines run at high speed and a frequency of 50 Hz is required,

we need a small number of poles on the rotor of high-speed alternators (also called turbo alternators). We can use not less than 2 poles and this fixes the highest possible speed. For a frequency of 50 Hz, it is 3000 r.p.m. The next lower speed is 1500 r.p.m. for a 4-pole machine. Consequently, turbo alternators possess 2 or 4 poles and have small diameters and very long axial lengths. Alternator Operation

The rotor winding is energized from the d.c. exciter and alternate N and S poles are developed on the rotor. When the rotor is rotated in anti-clockwise direction by a prime mover, the stator or armature conductors are cut by the

magnetic flux of rotor poles.

Consequently, e.m.f. is induced in the armature conductors due to electromagnetic induction. The induced e.m.f. is alternating since N and S poles of rotor alternately pass the armature conductors. The direction of induced e.m.f. can be found by Fleming's right hand rule and frequency is given by;

The magnitude of the voltage induced in each phase depends upon the rotor flux, the number and position of the conductors in the phase and the

speed of the rotor.

Figure 3.4. Stator and Rotor Winding

When the rotor is rotated, a 3-phase voltage is induced in the armature winding. The magnitude of induced e.m.f. depends upon the speed of rotation and the D.C. exciting current.

The magnitude of e.m.f. in each phase of the armature winding is the same. However, they differ in phase by 120° electrical as shown in the phasor diagram

Frequency

The frequency of induced e.m.f. in the armature conductors depends upon speed and the number of poles.

Let N = rotor speed in r.p.m. P = number of rotor poles f = frequency of e.m.f. in Hz

No. of cycles/revolution = No. of pairs of poles = P/2 No. of revolutions/second = N/60 No. of cycles/second = (P/2)(N/60) = N P/120 But number of cycles of e.m.f.

per second is its frequency.

𝑓 =𝑃𝑁

120

The alternator must be run at synchronous speed to give an output of desired frequency. For this reason, an alternator is sometimes called synchronous generator.

A.C. Armature Windings

A.C. armature windings are always of the no salient-pole type and are usually Symmetrically distributed in slots around the complete circumference of the Armature.

A.C. armature windings are generally open-circuit type i.e., both end are brought out. An open-circuit winding is one that does not close on itself i.e., a closed circuit will not be formed until some external connection is made to a source or load. The following are the general features of A.C. armature windings: (i) A.C. armature windings are generally distributed windings i.e., they

are symmetrically distributed in slots around the complete circumference of the armature. A distributed winding has two principal advantages. First, a distributed winding generates a voltage wave that is nearly a sine curve. Secondly, copper is evenly distributed on the armature surface. Therefore, heating is more uniform and this type of winding is more easily cooled.

(ii) A.C. armature windings may use full-pitch coils or fractional-pitch coils. A coil with a span of 180° electrical is called a full-pitch coil. In this case, the two

sides of the coil occupy identical positions under adjacent oppositepoles and the e.m.f. generated in the coil is maximum. A coil with a span of less than 180° electrical is called a fractional-pitch coil. For example, a coil with a span of 150° electrical would be called a 5/6 pitch coil. Although e.m.f. induced in a fractional-

pitch coil is less than that of a full-pitch coil, fractional-pitch coils are frequently used in A.C. machines for two main reasons. First, less copper is required per coil and secondly the waveform of the generated voltage is improved.

(iii) Most of A.C. machines use double layer armature windings. In a double layer winding, one coil side lies in the upper half of one slot while the other coil side lies in the lower half of another slot spaced about one-pole pitch from the first one. This arrangement permits simpler end connections and it is economical to manufacture.

(iv) Since most of A.C. machines are of 3-phase type, the three windings of the three phases are identical but spaced 120 electrical degrees apart.

(v) A group of adjacent slots belonging to one phase under one pole pair is known as phase belt. The angle subtended by a phase belt is known as phase spread. The 3-phase windings are always designed for 60° phase spread. Winding Factors

The armature winding of an alternator is distributed over the entire armature. The distributed winding produces nearly a sine waveform and the heating is more uniform. Likewise, the coils of armature winding are not full-pitched i.e., the two sides of a coil are not at corresponding points under adjacent poles.

The fractional pitched armature winding requires less copper per coil and at the same time waveform of output voltage is unproved. The distribution and pitching of the coils affect the voltages induced in the coils

(i) Distribution factor (Kd), also called breadth factor (ii) Pitch factor (Kp), also known as chord factor

(i) Distribution factor (Kd)

A winding with only one slot per pole per phase is called a concentrated winding.

In this type of winding, the e.m.f. generated/phase is equal to the arithmetic sum of the individual coil e.m.f.s in that phase.

However, if the coils/phase are distributed over several slots in space (distributed winding), the e.m.f.s in the coils are not in phase (i.e., phase difference is not zero) but are displaced from each by the slot angle a (The

angular displacement in electrical agrees between the adjacent slots is called slot angle).

The e.m.f./phase will be the phasor sum of coil e.m.f.s.

Phasor Diagram

Pitch factor (Kp)

A coil whose sides are separated by one pole pitch (i.e., coil span is 180° electrical) is called a full-pitch coil. With a full-pitch coil, the e.m.f.s induced in the two coil sides an in phase with each other and the resultant e.m.f. is the arithmetic sum of individual e.m.fs. However the waveform of the resultant e.m.f. can be improved by making the coil pitch less than a pole pitch. Such a coil is called short-pitch coil. This practice is only possible with double-layer type of winding. The e.m.f. induced in a short-pitch coil is less than that of a full-pitch coil. The factor by which e.m.f. per coil is reduced is called pitch factor Kp. It is defined as:

For a full-pitch winding, Kp = 1. However, for a short-pitch winding, Kp < 1.

ALTERNATOR ON LOAD i) Voltage drop IaRa where Ra is the armature resistance per phase. (ii) Voltage drop IaXL where XL is the armature leakage reactance per

phase. (iii) Voltage drop because of armature reaction

Synchronous Reactance (Xs)

Phasor Diagram

Xs = XL + XAR

Synchronous impedance, Zs = Ra + jXs

E0 = OB2 + BC2

OB = Vcosφ + Ia Ra

BC = Vsinφ + IaXs

E0 = V + IaZs = V + Ia R + jXs

Phasor Diagram of a Loaded Alternator

Voltage Regulation

The voltage regulation of an alternator is defined as the change in terminal voltage from no-load to full-load (the speed and field excitation being constant) divided by full-load voltage

%Voltage regulation =No load voltage − Full load voltage

Full load voltage × 100

=𝐸0 − 𝑉

𝑉 × 100

Load Current vs Terminal Voltage Curve

Determination of Voltage Regulation

1. Synchronous impedance or E.M.F. method 2. Ampere-turn or M.M.F. method For either method, the following data are required:

(i) Armature resistance (ii) Open-circuit characteristic (O.C.C.) (iii) Short-Circuit characteristic (S.C.C.)

a. Open-circuit characteristic (O.C.C)

Like the magnetization curve for a D.C. machine, the (Open-circuit characteristic of an alternator is the curve between armature terminal voltage (phase value) on open circuit and the field current when the alternator is running at rated speed.

Figure 3.10. Open Circuit Characteristics

Open Circuit Characteristic Curve

b. Short-circuit characteristic (S.C.C.)

In a short-circuit test, the alternator is run at rated speed and the armature terminals are short-circuited through identical ammeters. Only one

ammeter need be read; but three are used for balance. The field current If is gradually increased from zero until the short-circuit armature current

Figure 3.12. Short Circuit Characteristics

Short Circuit Characteristic Curve

There is no need to take more than one reading because S.C.C. is a straight line passing through the origin. The reason is simple. Since armature resistance is much smaller than the synchronous reactance, the short-circuit armature current lags the induced voltage by very nearly 90°. Consequently, the armature flux and field flux are in direct opposition and the resultant flux is small.

c. Synchronous Impedance Method

In this method of finding the voltage regulation of an alternator, we find the synchronous impedance Zs (and hence synchronous reactance Xs) of the alternator from the O.C.C. and S.S.C. For this reason, it is called

synchronous impedance method.

Synchronous Impedance Method

Synchronous reactance,𝑋𝑠 = 𝑍𝑠2 − 𝑅𝑎

2

𝐸1 = 𝐼1𝑍𝑠

Drawback of Synchronous Impedance Method

It gives approximate results The combined effect of XL (armature leakage reactance) and XAR

(reactance of armature reaction) is measured on short-circuit. The current in this condition is almost lagging 90°. So the armature

reaction will provide its worst demagnetizing effect It is also called pessimistic method.

d. Ampere-Turn Method

This method of finding voltage regulation considers the opposite view to the synchronous impedance method. It assumes the armature leakage

reactance to be additional armature reaction. Neglecting armature resistance (always small), this method assumes that change in terminal p.d. on load is due entirely to armature reaction. The same two tests (viz open-circuit and short-circuit test) are required as for synchronous reactance determination; the interpretation of the results only is different. Under short-circuit, the current lags by 90° (Ra considered zero) and the power factor is zero. Hence the armature reaction is entirely demagnetizing. Since the terminal p.d. is zero, all the field AT (ampere-turns) are neutralized by armature AT produced by the short circuit armature current, (i) Suppose the alternator is supplying full-load current at normal voltage V It assumes the armature leakage reactance to be additional armature

reaction. This method assumes that change in terminal p.d. on load is due

entirely to armature reaction.

Power Factors

zero p.f. lagging

zero p.f. leading

PARALLEL OPERATION OF ALTERNATORS It is rare to find a 3-phase alternator supplying its own load

independently except under test conditions.

In practice, a very large number of 3-phase alternators operate in parallel because the various power stations are interconnected through the national grid.

Therefore, the output of any single alternator is small compared with the total interconnected capacity. For example, the total capacity of the interconnected system may be over 40,000 MW while the capacity of the biggest single alternator may be 500 MW.

For this reason, the performance of a single alternator is unlikely to affect appreciably the voltage and frequency of the whole system.

An alternator connected to such a system is said to be connected to infinite bus bars.

The outstanding electrical characteristics of such bus bars are that they are constant-voltage, constant-frequency bus bars.

Parallel operation of Alternator

Advantages of Parallel Operation of Alternators

The following are the advantages of operating alternators in parallel: (i) Continuity of service. The continuity of service is one of the important

requirements of any electrical apparatus. If one alternator fails, the continuity of supply can be maintained through the other healthy units. This will ensure uninterrupted supply to the consumers. (ii) Efficiency. The load on the power system varies during the whole day; being minimum during die late night hours. Since alternators operate most efficiently when delivering full-load, units can be added or put off depending upon the load requirement. This permits the efficient operation of the power system. (iii) Maintenance and repair. It is often desirable to carry out routine maintenance and repair of one or more units. For this purpose, the desired unit/units can be shut down and the continuity of supply is maintained through the other units. (iv) Load growth. The load demand is increasing due to the increasing use of electrical energy. The load growth can be met by adding more units without disturbing the original installation.

Conditions for Paralleling Alternator with Infinite Busbars

The proper method of connecting an alternator to the infinite busbars is called synchronizing. A stationary alternator must not be connected to live busbars. It is because the induced e.m.f. is zero at standstill and a short-circuit will result. In order to connect an alternator safely to the infinite busbars, the following conditions are met: (i) The terminal voltage (r.m.s. value) of the incoming alternator must be thesame as busbars voltage, (ii) The frequency of the generated voltage of the incoming alternator must beequal to the busbars frequency, (iii) The phase of the incoming alternator voltage must be identical with thephase of thebus bars voltage. In other words, the two voltages must be inphase with each other, (iv) The phase sequence of the voltage of the incoming alternator should be thesame as that of the busbars.

The magnitude of the voltage of the incoming alternator can be adjusted by changing its field excitation. The frequency of the incoming alternator can be changed by adjusting the speed of the prime mover driving the alternator.

Condition (i) is indicated by a voltmeter, conditions (ii) and (iii) are indicated by synchronizing lamps or a synchroscope. The condition (iv) is indicated by a phase sequence indicator.