What Is Alternator? - SKengineers

 

WHAT IS ALTERNATOR?

Ø  INTRODUCTION –

·        The emf induced in the conductors that are placed in the armature slots is A.C. but by the action of commutator and brushes, it is converted to D.C.

·        Due to the availability of electronic components D.C. generators can be replaced by rectifier-filter power banks.

·        But, it still becomes essential to use the A.C. supply to desired frequency in many circuits.

·        Similar to the D.C machines, the A.C. machines associated with alternating voltages are also classified as generators and motors.

·        The machines that generate A.C emf are called synchronous generators or A.C. generators. It is also called as an alternator.

·        An alternator can be run as a motor called synchronous motors. Both these machines i.e. the alternator and synchronous motor work at a specific constant speed.

·        Hence, they are called synchronous machines.

Ø  ALTERNATOR WINDINGS –

There are two windings of an alternator –

1.      Field winding

2.      Armature winding

The field winding is placed on the rotor i.e. it is a rotary winding whereas armature is placed on the stator i.e. armature is a fixed winding.

Ø  CLASSIFICATION OF ALTERNATORS –

Alternator can  be classified on the basis of different factors such as speed of operation, capacity, generated voltage and construction.

Let us discuss the classification.

1)     Classification based on speed –

Different types of prime movers are used for different speed ranges.

ALTERNATOR –

1)     LOW SPEED ALTERNATOR (60 – 500 RPM)

2)     MEDIUM SPEED ALTERNATOR (600 – 1000 RPM)

3)     HIGH SPEED ALTERNATOR (1500 – 3000 RPM)

Sr. No.	ALTERNATOR	PRIME MOVERS
1.	LOW SPEED	Water turbine, IC engine, steam engine.
2.	MEDIUM SPEED	IC engine, electric motor, high head water turbine.
3.	HIGH SPEED	Steam turbine, electric motors.

2)     Classification based on capacity and generated voltage –

·        The second factor used for the classification is the capacity and generated voltage.

·        The small capacity alternators produce voltages from 250V – 3.3KV.

·        ALTERNATOR –

1)     SMALL CAPACITY ALTERNATOR

2)     MEDIUM CAPACITY ALTERNATOR

3)     LARGE CAPACITY ALTERNATOR

·        Medium capacity alternators produce voltages from 3.3kv to 11kv whereas the large capacity alternator produce voltage from 6.6kv to 33kv.

3)     Classification based on construction –

·        Based on the construction, the alternators are classified into two categories –

·        ALTERNATOR –

1)     REVOLVING ARMATURE TYPE

2)     REVOLVING FIELD TYPE

·        In the revolving armature type alternators, the armature winding is put on the rotor which rotates while field winding is put on the stationary stator.

·        In the revolving field type alternators, the armature is stationary while the field winding is on the rotor.

·        The practical alternators are revolving field type machines.

Ø  CONSTRUCTION OF AN ALTERNATOR –

·        An alternator is a synchronous type machine.

·        In this type of machines, field windings are placed on the rotor and the armature winding is housed in suitable shaped slots in the stator.

·        The field winding (rotor) is connected to an external D.C source called exciter, through a pair of sliprings as shown in the figure.

·        The armature winding (stator) is a 3 phase winding and the induced voltage in this winding is applied to the load. This is a 3 phase A.C. voltage.

·        A prime mover is mechanically coupled to the shaft of the machine. The armature is rotated at a speed called synchronous speed (Ns).

·        The rotating field, fixed armature arrangement is preferred due to certain advantages.

·        The stator frame made from cast iron supports the stator core. The stator core has slots on its inner periphery in which the stator conductors are placed.

·        The rotor has alternate N and S poles placed on its outer rim.

·        These magnetic poles are excited from an external D.C. source (typically 125 to 600V).

·        When the excited rotor rotates, its magnetic field cuts the stationary stator ( armature ) conductors as a result of which a 3-phase A.C. voltage gets induced into the armature winding.

·        The frequency of this induced stator emf depends on the number of N and S poles moving past a conductor in one second and the direction of induced emf is given by Flemings right hand rule.

Ø  CONSTRUCTION OF A ROTOR –

TYPES OF ROTOR –

1)     SALIENT POLE TYPE ROTOR

2)     SMOOTH CYLINDRICAL TYPE ROTOR

Ø  SALIENT POLE TYPE ROTOR –

·        In this type of construction

the poles attached to the cylindrical rotor are of protruding or projecting type.

·        As the poles are projecting, the air gap between stator and rotor is non uniform. The poles are made up of thick steel laminations and they are bolted out to the rotor.

·        The field winding is provided on the pole shoe.

·        For such a construction it is possible to have a large number of poles (16,32).

·        The only disadvantages is that the size of rotor is big.

·        The mechanical strength of salient pole rotor is less due to the projecting poles. Hence, it is preferred for low speed alternators.

·        The prime movers that drive such rotors are water turbines and IC engines and steam engines.

Ø  SMOOTH CYLINDRICAL TYPE ROTOR –

·        They are also called as non-salient type rotor or non-projected pole type rotor.

·        In such a type of construction, the rotor is kept unslotted. The slots carry the field winding and the unslotted parts become poles. The air gap between the stator and the rotor is uniform.

·        The rotor is in the form of a smooth solid steel cylinder which has a large number of slots to accommodate the field winding.

·        The slots are covered at top, with steel or manganese wedges, the poles in this type of rotor are not projecting out and surface of rotor is smooth.

·        These rotors have small diameters and large axial lengths, in order to keep the peripheral speed within limits. These rotors are mechanically strong.

·        They are used for high speed alternators, which range from 750 – 3000 rpm. The prime movers that drive such rotors are steam turbines, electric motors.

Ø  DIFFERENCE BETWEEN SALIENT POLE AND SMOOTH CYLINDRICAL ROTOR –

Sr. No.	SALIENT POLE ROTOR	CYLINDRICAL ROTOR
1.	Salient pole rotor has poles which are projecting out from the surface.	Cylindrical rotor does not have poles that are projecting out.
2.	The air gap between stator and rotor is non-uniform.	The air gap between the stator and rotor is uniform.
3.	The salient pole rotor has large number of poles.	The smooth cylindrical rotor has small number of poles.
4.	Salient pole rotors are mechanically weak.	Cylindrical pole rotors are mechanically strong.
5.	They have large diameter and small axial length.	They have small diameter and large axial length.
6.	The prime movers used are water turbines, IC engines, and steam engines.	The prime movers used are steam turbines and electric motors.
7.	They are preferred for low speed alternators which range from 120-500 rpm.	They are preferred for high speed alternators which range from 750 – 3000 rpm.

 

Ø  STATOR CONSTRUCTION –

·        The stator is stationary armature. It consist of a core and slots to hold the armature winding. It uses a laminated construction, in order to keep the eddy current losses low.

·        The core is fabricated in a frame which is made of steel plates.

·        The core has slots on its periphery. The armature conductors are placed in these slots.

·        The frame provides support to the core. The frame does not carry flux.

·        Holes are cast in the frame, so that ventilation can be maintained.

Ø  EXCITATION SYSTEM –

·        The synchronous machines i.e. alternators and motors are separately excited machines.

·        They require a DC excitation for their operation. The field winding needs a DC supply of the order of 110V or 220V to produce the required flux.

·        This supply is obtained from a small DC shunt generator called an exciter. It is mounted on the same shaft as that of alternator. It is driven by the prime mover.

Ø  VENTILATION SYSTEM –

1)     Natural ventilation

2)     Closed circuit ventilation

A.     NATURAL VENTILATION –

·        In this method, a fan is mounted on the shaft and attached to one end of the machine. Air is the medium by which ventilation takes place, so that the heat of machine parts is carried away to atmosphere.

·        Natural ventilation is preferred for small machines.

B.     CLOSED CIRCUIT VENTILATION –

·        Sometimes the atmospheric air is harmful to elements like dust, moisture, acidic fumes which are harmful for the insulation of the winding.

·        In case of large capacity machine closed circuit ventilation is preferred for ventilation. The medium used for ventilation is hydrogen.

·        Hydrogen is circulated with the help of water cooled heat exchangers. Modern alternators prefer to use closed circuit ventilation with the help of hydrogen as the ventilating medium.

Ø  SLIP RING AND BRUSH ASSEMBLY –

·        In three phase alternators, the armature consists of a three phase winding and an A.C emf gets induced in these windings.

·        These winding are connected in either star or delta connection. The three phase supply is connected across these windings.

·        External load cannot be connected directly to these windings as they are continuously rotating. Hence, slip rings are made of conducting material are mounted on the shaft.

·        Each of the winding terminal is connected to an individual slip rings, permanently.

·        The brushes rest on the slip sings and just make a contact with the slip ring. The three phase supply is also available to the slip ring and the brush assembly.

·        Now, external load can be connected to the terminal that are available from the brushes.

·        The induced emf can be inserted into the rotating winding from outside with the help of a slip ring and brush assembly.

·        The external voltage that is applied across the brushes, gets applied across the rotating winding because of the slip rings.

·        the emf that is induced is the effect of relative motion which is present between the armature and the field.

·        So, in case of alternators it is possible to have –

1)     Rotating armature and stationary field

2)     Rotating field and stationary armature

·        Practically for the alternators we prefer rotating field type of construction rather than rotating armature construction due to certain advantages.

Ø  ADVANTAGES OF ROTATING FIELD AND STATIONARY ARMATURE –

1)     AC supply is widely used. Hence the voltage developed by the alternator is in the range of 5.5kv, 11kv, 33kv. If this high voltage winding is kept rotating then providing an insulation to the slip rings connecting stationary terminals with rotating windings is difficult. But, if the armature winding is kept stationary the connections can be made from the terminals to the winding. This arrangement also provides insulation.

2)     Field winding needs a low voltage supply of 110V or 220V just to produce the magnetic flux. Hence, even if the field winding is rotating is can be easily insulated, as the voltage required to field winding is supplied with the help of slip ring and brush assembly.

3)     The output power from any machine depends on the copper quantity placed inside it in the form of conductors. For more output more conductors must be accommodated in the armature slots. Hence, the armature slots must be made deep. With rotating armature, the slots will become mechanically weak. But, with stationary armature the slots can be made deep as the requirements to increase the output power of the machine.

4)     Higher voltage produces certain mechanical stresses. But with stationary armature these stresses are not dangerous. With rotating armature, the rotating stresses developed due to high voltage may reduce the life of the armature.

5)     The number of slip rings required for rotary armature will be 3. But if the field is rotating only two slip rings are sufficient. Thus it saves one slip ring.

6)     With stationary high voltage winding it is easy to provide better ventilation to the machine.

7)     As the voltage on the field side is less, it needs low inertia. This makes the overall construction of the rotating field simple. With simple, robust mechanical construction and low inertia of rotor, alternator can be driven at high speeds.

8)     Thus, overall it is seen that a robust, well insulated machine at lowest cost is possible with rotating field and stationary armature type of construction. But, in case of small alternators rotating armature and stationary field construction which is similar to DC machines is used.

Ø  PRINCIPLE OF OPERATION –

                   Let us now see the generation of the sinewave using the simplest type of alternator called single turn or single loop alternator. Here to make things simpler we assume that the armature is moving i.e. armature is rotor and flux is being produced by a permanent magnet.

CONSTRUCTION –

·        The construction of single turn alternator consist of a permanent magnet with two poles N and S, and a single turn rectangular coil made up of some conducting material (aluminum or copper).

·        The single turn coil is made from two conductors A and B. these conductors are connected to each other on one end whereas their other ends (C1 AND C2) are connected to the slip rings mounted on the shaft.

·        The coil can rotate around its own axis in clockwise or anticlockwise directions. The slip rings C1 AND C2 are connected to the coil.

·        The slip rings rotate along with the shaft. The brushes do not rotate. They are stationary and resting on the sliprings.