As-Salaam-Alaikum, Welcome to my blogger "World of Knowledge".Here we discuss the topic analysis of 3 Phase Alternator.The main idea of this article is to get familiar with the working, construction and about each and every part of our electrical machine element which is synchronous generator (alternator).

Points to be discuss in this Article are given below :

Abstract
Introduction
Working Principle
Synchronous Speed (Ns)
Classifications
By Excitation
Transformation and Rectification
Brushless Alternator
By Number of Phases
By Rotating Part
Cooling Methods
Use of Alternators
Automotive Alternator
Diesel Electric Locomotive
Bulk AC Power
Types of Alternators
Automotive Alternators
Diesel Electric Locomotive Alternators
Marine Alternators
Brushless Alternators
Radio Alternators
Construction
Stator
Rotor
Salient Pole Type
Smooth Cylindrical Type
Brushes and Slip Rings
Rectifier
Fan and Pulley

Abstract:

In this project we take a 3-phase alternator and study its working and construction. Also we later disassemble it to get to know about each of its components and then we assemble it again so that we learn it properly that how alternator can be assembled.

3-Phase Alternator

Table: Alternator Specifications

Voltage	12 Volts
Current	50 Amps
Stator Diameter	93 mm
Weight	2.35 kg (without pulley)
Regulator Type	2 wire (L, IG)
Max. Rotor Speed	18000 r.p.m.
Dimensions (A, B, C)	25, 20, 65
No. of Poles	10
Type of Winding	Delta Winding
Rotor	Salient Type

3-Phase Alternator

Introduction to Alternator

Introduction:

It is known that the electric supply used, now a days for commercial as well as domestic purposed, is of alternating type. Similar to d.c machines, the a.c machines associated with alternating voltages are also classified as generators and motors.

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. Alternators are also called synchronous generators. These machines work at a constant speed called synchronous speed and hence in general called synchronous machines.

All the modern power stations consist of large capacity three phase alternators. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 Hz three-phase alternators in power plants generate most of the world's electric power, which is distributed by electric power grids.

Working Principle:

The alternators work on the principle of electromagnetic induction. When there is a relative motion between the conductors and the flux, e.m.f. gets induced in the conductors. The only difference in an alternator and a d.c. generator is that in an alternator the conductors are stationary and field is rotating.

Working Principle

Principle of Operation

Synchronous Speed (Ns):

For fixed number of poles, alternator has to be rotated at a particular speed to keep the frequency of the generated e.m.f. constant at the required value. Such a speed is called synchronous speed of the alternator denoted as Ns.

Ns = 120 f / P

Where,

f = required frequency.

P = Number of Poles.

In our nation, the frequency of an alternating e.m.f. is standard equal to 50Hz. To get 50Hz frequency, for different number of poles, alternator must be driven at different speeds called synchronous speeds.

No. of Poles (P)	2	4	8	12	24
Synchronous Speed Ns in r.p.m	3000	1500	750	500	250

Classifications:

Alternators may be classified by method of excitation, number of phases, the type of rotation, cooling method, and their application.

By Excitation:

There are two main ways to produce the magnetic field used in the alternators, by using permanent magnets which create their own persistent magnetic field or by using field coils. The alternators that use permanent magnets are specifically called magnetos.

In other alternators, wound field coils form an electromagnet to produce the rotating magnetic field.

A device that uses permanent magnets to produce alternating current is called a permanent magnet alternator (PMA). A permanent magnet generator (PMG) may produce either alternating current or direct current if it has a commutator.

Transformation and Rectification:

This method depends on residual magnetism retained in the iron core to generate weak magnetic field which would allow a weak voltage to be generated. This voltage is used to excite the field coils for the alternator to generate stronger voltage as part of its build up process. After the initial AC voltage buildup, the field is supplied with rectified voltage from the alternator.

Brushless Alternator:

A brushless alternator is composed of two alternators built end-to-end on one shaft. With advancement in semiconductor technology, brushless alternators are possible. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on the rotor. Neither brushes nor slip rings are used, which reduces the number of wearing parts. The main alternator has a rotating field and a stationary armature (power generation windings).

By Number of Phases:

Another way to classify alternators is by the number of phases of their output voltage. The output can be single phase, or polyphase. Three-phase alternators are the most common, but polyphase alternators can be two phase, six phase, or more.

By Rotating Part:

The revolving part of alternators can be the armature or the magnetic field. The revolving armature type has the armature wound on the rotor, where the winding moves through a stationary magnetic field. The revolving armature type is not often used. The revolving field type has magnetic field on the rotor to rotate through a stationary armature winding. The advantage is that then the rotor circuit carries much less power than the armature circuit, making the slip ring connections smaller and less costly

Cooling Methods:

Many alternators are cooled by ambient air, forced through the enclosure by an attached fan on the same shaft that drives the alternator. In vehicles such as transit buses, a heavy demand on the electrical system may require a large alternator to be oil-cooled. In marine applications water-cooling is also used. Expensive automobiles may use water-cooled alternators to meet high electrical system demands.

Use of Alternators:

Automotive Alternator:

The power for the electrical system of a modern vehicle gets produced from an alternator. In previous days, we used DC generators or dynamos for this purpose, but after the development of alternator, we replaced the DC dynamos by more robust and lightweight alternator. Although the electrical system of motor vehicles requires direct current, still an alternator along with diode rectifier instead of a DC generator is a better choice as the complicated commutation is absent in alternator. This particular type of generator used in the vehicle is known as an automotive alternator.

Diesel Electric Locomotive:

The engine of this locomotive is nothing but an alternator, driven by a 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. These DC traction motors drive the wheel of the locomotive.

Bulk AC Power:

One of the primary uses of alternators is in the production of bulk ac power for commercial purposes. In thermal power plants, in hydel power plants, even in nuclear power plants, alternators only converts mechanical energy to electrical energy for supplying to the power system.

Types of Alternators:

Alternators or synchronous generators can be classified in many ways depending upon their applications and designs.

There are five different types of alternators:

Automotive Alternators: Used in modern automobiles.
Marine Alternators: Used in marine applications.
Radio Alternators: Used for low band radio frequency transmission.
Brushless Alternators: Used in electrical power generation plants as the main source of power.
Diesel Electric Locomotive Alternators: Used in diesel electric multiple units.

Construction:

Different parts of an alternator are enlisted below:

Stator
Rotor
Brushes and Slip Rings
Rectifiers Fan
Pulley Bearings

Alternator Mounted on an Automobile Engine

Stator:

The stator is a stationery armature. This consists of a core and the slots to hold the armature winding. The stator core uses a laminated construction. It is buildup of special steel stampings insulated from each other with varnish or paper. The laminated construction is basically to keep down eddy current losses. The choice of material is steel to keep down hysteresis losses. The entire core is fabricated in a frame made of steel plates. The core has slots on its periphery for housing the armature conductors. Ventilation is maintained with the help of holes cast in the frame.

Stator

Rotor:

Rotor is the rotating part in an alternator. It has permanent magnets that move around the stator iron plates to generate an Alternating current (AC).

Rotor

There are two types of rotors used in alternators:

Salient Pole Type

Smooth Cylindrical Type

Salient Pole Type:

This is also called projected pole type as all the poles are projected out from the surface of the rotor.

The poles are built up of thick steel laminations. The poles are bolted to the rotor.

The poles face has been given a specific shape. The field winding is provided on a pole shoe. These rotors have large diameters and small axial lengths. As mechanical strength of salient pole type is less, this is preferred for low speed alternators ranging from 125 r.p.m. to 500 r.p.m. The prime movers used to drive such rotor are generally water turbines and I.C. engines.

Salient Pole Type (1)

Salient Pole Type (2)

Smooth Cylindrical Type:

This is also called non-salient type or non-projected poly type of rotor. The rotor consists of smooth solid steel cylinder, having number of slots to accommodate the field coil. The slots are covered at the top with the help of steel or manganese wedges. The unslotted portion of the cylinder itself acts as the pole. The poles are not projecting out and the surface of the rotor is smooth which maintains uniform air gap between stator and the rotor. These rotors have small diameters and large axial lengths. This is to keep peripheral speed within limits. The main advantage of this type is that these are mechanically very strong and thus preferred for high speed alternators ranging between 1500 r.p.m. to 3000 r.p.m.