Complete Basics of Alternator or Generator

 The generator or alternator is basically a machine that converts mechanical energy to electrical energy. Mechanical energy is produced from the prime movers. Prime Movers would be an engine, a turbine or hydro pressure system etc.

How generator or alternator generates electricity:

Generator or alternator generates electricity based on Faraday’s Law of Electromagnetic Induction. As per, Faraday’s Law of Electromagnetic Induction, if there is relative motion between a conductor and magnetic field, conductor is placed in the magnetic field then voltage or EMF- Electromotive force shall be generated in the conductor. This relative motion can be produced by three options-

1. The magnetic field is stand still and a conductor is moving within the field area.
2. The conductor is stand still and magnetic field source is moving
3. The conductor is stand still and magnetic field source is stand still but magnetic field intensity is varying

Alternating Current (AC) generators use electromagnetic induction to convert mechanical energy into electrical energy. Electromagnetic induction is defined as the generation of an electromotive force in a closed circuit by a varying magnetic flux (flow) through the circuit.

For conventional and modern alternator set, the second option is used. The heavy conductor is placed in the stator which is fixed. The magnetic circuit is placed in the section called rotor. Rotor is coupled with the prime mover. Rotor is placed in between the stator. Then through prime mover rotor rotates. As there is magnetic circuit in the rotor, and rotor is rotating, thus the magnetic field also rotating. In this case, the magnetic field in the rotor is fixed. So, how there is 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 field.

From where the electricity generated in a generator or alternator:

We already know that generator converts mechanical energy to electrical energy and electrical energy is basically comprises of flow of electron. With a continues operation of an alternator, fuel of prime mover is a concern, but where comes that much electron?

Basically, this electron is come from the ground. The strong magnetic forces in the alternator acts as a sweeping power to sweep and forces electrons to a specific direction. This massive flow of electron is called the electricity. The path of the electrons is through the neutral connection which is earthed and through the conductors then to the consumer end.

Construction of Alternator and Generator:

A basic AC generator is made up of a rotor and a stator:

Rotor: The rotor is a solid steel cylinder provided with machined slots. In the slots two, four or more (always in pairs) windings are inserted. As material for the windings copper or aluminum bars is used.
Stator: The stator is mounted into the generator casing is made of stapled laminated iron sheets. In the slots of the stator at least one – in technical application three – windings/coils are inserted.

Already we have discussed that, alternator generates electricity based on Faradays law of electricity, and as per the law three things are needed-

1. A conductor
2. Magnetic field
3. Relative motion between the conductor and magnetic field.

For a practical alternator which are mostly used in the industrial applications, the conductor is placed in the stator, and magnetic field circuit is placed in the rotor and relative motion is availed through a prime mover.
So, a stator is a circular housing, rotor is placed within it, rotor is mechanically coupled with a shaft, this shaft is connected with the prime mover – engine, turbine etc. The prime mover rotates the rotor within the stator, this way relative motion is availed.

How the magnetic field is created in alternator or generator?

Magnetic field is created in alternator or generator by simply supplying DC current to the circuit. In these case the DC current supplied is called excitation current and the circuit itself is called the excitation circuit. Off course, the DC excitation current is adjustable. By adjusting this excitation current, the induced voltage level and also, the amount of current supplied is controlled.

Alternating Current (AC) Generators

The generators for industrial applications are classified as depending on which component (stator, rotor) the DC excitation current is applied to

1. stationary
2. rotating field generators

1. Stationary Field AC Generator

In a stationary field AC generator DC is applied to the stator; the magnetic field does not rotate.

Design of stationary field AC generator
The rotor carries a winding inserted in the iron core and rotates inside the stator. The resulting voltage is taken from the rotor by an arrangement of slip rings and brushes. Each end of the winding is connected to one slip ring. The generator’s AC output voltage is transferred from the slip rings, over the carbon brushes, to an external electrical circuit.

Use of stationary field AC generators
The operation and maintenance of the slip rings and carbon brushes for larger currents is material and time consuming. For the required regular checks and overhauls the generator must be in standstill.
Therefore stationary field AC generators are in use for applications with lower power outputs, for example brushless exciter machines, low power emergency generators etc.

2. Rotating Field AC Generator

In a rotating field AC generator DC is applied to the rotor; the magnetic field is rotating.

Design of rotating field AC generator: In a rotating field AC generator the arrangement of slip rings and brushes is used to conduct the DC to the rotor winding thus creating the magnetic field. The DC power required for the magnetic field is significantly less than the generator’s AC output.

Use of rotating field AC generators: Most of the generators, which are in use for industrial applications with large currents (large power output), are rotating field AC generators.

Why Most AC generators are designed as 3-phase AC generators

Most AC generators are designed as 3-phase AC generators because of its compact size. Further a higher efficiency is obtained by this arrangement. In a 3-phase AC generator three sets of coils are arranged in the stator core, 120° displaced from each to the other.
With this design three voltages are generated simultaneously. In most of the technical applications the ends of three coils are connected as star connection. The maximum value of the induced voltage in each of the coils occurs 120° apart seen one rotation of the shaft.