Alternator

What is Alternator?

An alternator is basically a machine that converts mechanical energy into electrical energy. Mechanical energy is provided by the prime movers. Prime Movers could be an engine, a turbine or a hydro-pressure system etc. An alternator is an electrical generator that transforms mechanical energy into electrical energy through the form of alternating current. For purposes of expense and efficiency, often alternators have a revolving magnetic field with a stationary armature.   

How does an  Alternator work?

A conductor moving relative to a magnetic field produces an electromotive force ( EMF) within it (Faraday's Law). This EMF reverses its polarity when it passes under opposite polarity magnetic poles. Usually a revolving magnet, called a rotor, transforms into a stationary series of conductors wrapped in coils on an iron block, called a stator.  The alternator produces power on the basis of Faraday's Electromagnetic Induction theory. As per Faraday's law of Electromagnetic Induction, where there is relative motion between the conductor and the magnetic field, the conductor is put in the magnetic field and the voltage or EMF-electromotive force is produced in the conductor. There are three methods for generating this relative motion.

1. The magnetic field is still standing and the conductor is moving within the field.
2. The conductor is still standing and the centre of the magnetic field is moving.
3. The conductor is still standing and the magnetic field source is still standing, but the magnetic field intensity varies.

Alternating current (AC) generators use electromagnetic induction to transform mechanical energy into electrical energy. Electromagnetic induction is characterized as the generation of electromotive force in a closed circuit by a variable magnetic flux (flow) through the circuit. The second choice is used with traditional and modern alternator sets. The strong conductor is put in the fixed stator. The magnetic circuit is found in a segment called the rotor. Rotor is connected to the prime mover. Rotor is placed in the middle of the stator. Instead, the rotor rotates around the prime mover. Since there is a magnetic current in the rotor, and the rotor rotates, so does the magnetic field is also rotating. In this case, the magnetic field in the rotor is fixed. So, how is the magnetic field produced in the rotor? Definitely, there is no magnet placed in the rotor for large application in the industry. Only a DC current is supplied in the rotor circuit to produce magnetic fields.

Components of Alternator  

Field poles

The construction of an alternator consists of field poles placed on the rotating fixture of the machine.

Rotor and Stator

An alternator is made up of two main parts: a rotor and a stator. The rotor rotates in the stator, and the field poles get projected onto the rotor body of the alternator. The armature conductors are housed on the stator. An alternating three-phase voltage represented by ‘aa’, ‘bb' or ‘cc’ is induced in the armature conductors thus resulting in the generation of three-phase electrical power. All modern electrical power generating stations use this technology for generation of three-phase power and as a result, an alternator (also known as a synchronous generator) has made itself a subject of great importance and interest for power engineers.

An alternator is basically a type of AC generator. The field poles are made to rotate at synchronous speed Ns = 120 f/P for effective power generator Where f signifies the alternating current frequency and the P represents the number of poles. For the most practical configuration of the alternator, it is mounted with a fixed frame and a revolving area, as in the case of a DC generator where the arrangement is precisely the opposite. Such change is made in order to deal with the very high power on the order of a few 100 megawatts generated in an AC generator, which is opposite to that of a DC generator. To satisfy the conductor with such high strength.

Types of Rotor

There are mainly two types of rotors used in the 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 poles, in this case, are made of thick laminated steel sections riveted together and attached to a rotor with the help of a joint.

 Cylindrical rotor type

• Uniform air gap.

• Used in high speed operations generally steam turbines or gas turbines.

• Due to uniform air gap the flux distribution is uniform and hence smooth and less noisy operation. 

Cylindrical rotors are used in high speed electrical machines, usually 1500 RPM to 3000 RPM. Windage loss as well as noise is less as compared to salient pole rotors. 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. When the alternator is driven at a steady speed, the relative velocity of the damping winding with respect to the 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 the 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 cylindrical rotor is generally used for very high speed operation and employed in steam turbine driven like turbogenerators. The machines are built in a number of ratings from 10 MVA to over 1500 MVA. The cylindrical rotor type machine has a 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 hosting the field coils.  

Slip Ring  

Slip rings are used as a means of providing DC excitation to the rotor of the alternator. In some excitation schemes the need of a slip ring arrangement may however be eliminated as with the brushless excitation system. The DC supply that we generate after rectification of the alternator output.

Bearings

These bearings are inside the alternator and are what the rotor rotates on to create the power. Many times, the bearings will squeak or make a grinding sound if they are going bad. If that is the case, it is likely that you will have to replace the alternator.  

Voltage Regulator

A voltage regulator controls the alternator voltage output to maintain a preset charging voltage for the battery. It also controls electrical power to the vehicle's different electrical systems. Without a voltage regulator, an alternator may put out up to 250 volts. The voltage regulator is located inside the alternator. When the voltage regulator fails the entire alternator is usually replaced. It should be on the alternator or internal to the alternator. There will be an oblong plug for it so just look for the connector.