What is an electrical generator?

A generator is a rotating electrical machine that transforms mechanical energy into electrical energy. It achieves this through the interaction of the two main elements of which it is composed: the moving part called the rotor, and the static part called the stator.

When an electric generator is in operation, one of the two parts generates a magnetic flux (acting as an inductor) for the other to transform it into electricity (acting as an armature).

Electric generators are differentiated according to the type of current they produce. There are two main groups of rotating electrical machines: alternators and dynamos.

Alternators generate electricity in alternating current. The inductor element is the rotor and the armature is the stator. An example is the generators in power stations, which transform mechanical energy into alternating electrical energy.

Dynamos generate direct current electricity. The inductor is the stator and the armature is the rotor. An example is the light on a bicycle, which is powered by pedalling.

Rotating electrical machines: generators

Electrical machines are devices capable of transforming electrical energy into any other form of energy. Electrical machines can be divided into:

  • Rotating electrical machines, which are composed of rotating parts, such as dynamos, alternators, and motors.
  • Static electrical machines, which have no moving parts, such as transformers.

We are going to look at the group of rotating machines, which are motors and generators. Rotating electrical machines are reversible, and can work in two different ways:

  • As an electric motor: Converts electrical energy into mechanical energy.
  • As an electric generator: Converts mechanical energy into electrical energy.

Electrical machines can be divided into rotating and static machines. In this case we are going to look at the group of rotating machines, which are motors and generators.

All rotating machines are made up of a fixed part called a stator, which is cylindrical in shape, and a moving part called a rotor. The rotor is mounted on a shaft that rests on two bearings. The air gap separating the stator from the rotor, which is necessary for the machine to rotate, is called the air gap.

Normally, both the stator and the rotor have windings made of copper conductors through which currents are supplied or transferred to an external circuit that constitutes the electrical system. One of the windings creates a flux in the air gap and is called an inductor. The other winding receives the flux from the first and is called the armature. Similarly, the inductor could be placed in the stator and the armature in the rotor or vice versa.

Losses and efficiency of rotating electrical machines

Like any machine, the output power offered by rotating electrical machines is less than the power supplied to them, the power delivered. The difference between the power output and the power supplied is the losses:

Pin - Pout = Losses

Therefore, the efficiency of an electrical machine determines the amount of useful work it can produce from the total energy it consumes.

Principle of operation of an electric generator: Faraday's Law

The principle of operation of generators is based on the phenomenon of electromagnetic induction.

Faraday's Law. This law tells us that the voltage induced in a circuit is directly proportional to the change in magnetic flux in a conductor or loop. This means that if we have a magnetic field generating a magnetic flux, we need a coil through which a current flows to generate the e.m.f. (electromotive force).

This discovery, made in 1830 by Michael Faraday, led to the creation of the Faraday disc a year later. The Faraday disc consists of a U-shaped magnet with a copper disc twelve inches in diameter and 1/5th of an inch thick in the middle placed on a rotating shaft inside a powerful electromagnet. By placing a conductive strip rubbing against the outside of the disc and another strip over the shaft, he proved with a galvanometer that electricity was produced by permanent magnets. This was the beginning of modern dynamos, i.e. electric generators that work by means of a magnetic field. It was very inefficient and had no use as a practical power source, but it demonstrated the possibility of generating electricity using magnetism and opened the door to commutators, direct current dynamos and finally to current alternators.

As can be seen in the chapter on electromagnetism, when we have a coil in a magnetic field through which an electric current is flowing, a couple of forces appear that cause the coil to rotate around its axis. In the same way, if we introduce a coil into a magnetic field and make it rotate, we will provoke an induced current. This induced current is responsible for the e.m.f. and will vary according to the position of the loop and the magnetic field.

The amount of induced current or e.m.f. will depend on the amount of magnetic flux (also called lines) that the loop can cut, the higher the number, the greater the flux variation generated and therefore the greater the electromotive force.

By rotating the coil inside the magnet, we will obtain a voltage that will vary according to the time. This voltage will have an alternating form, since from 180º to 360º the poles will be inverted and the voltage value will be negative.

The working principle of the alternator and the dynamo is based on the fact that the alternator maintains the alternating current while the dynamo converts the alternating current into a direct current.

Alternating current generator: the alternator

Alternating current generators or alternators are machines that transform mechanical energy, which they receive through the rotor, into electrical energy in the form of alternating current. Most alternators are synchronous alternating current machines, which are those that rotate at a synchronous speed, which is related to the number of poles that the machine has and the frequency of the electromotive force. This relationship makes the motor rotate at the same speed imposed by the stator through the magnetic field. This relationship is given by the expression:

Where f is the frequency at which the machine is connected and P is the number of pole pairs.

Its structure is as follows:

  • Stator: Fixed external part of the machine. It consists of a metal casing that serves as a support. Inside is the armature core, with a crown shape and longitudinal grooves, where the armature winding conductors are housed.
  • Rotor: Moving part that rotates inside the stator. It contains the inductor system and the friction rings, through which the inductor system is fed. Depending on the speed of the machine, there are two types of construction:
    • Pole impeller: Used for hydraulic turbines or heat engines, for low-speed systems.
    • Smooth pole rotor: Used for steam and gas turbines, these groups are called turbo-alternators. They can rotate at 3000, 1500, or 1000 r.p.m. depending on the number of poles.

The alternator is a synchronous rotating electrical machine that needs an excitation current in the inductor winding to generate the electric field and to operate. Therefore its operating diagram is as follows:

Switching in dynamos

Switching is the operation of transforming an alternating signal into a direct current signal and is also known as signal rectification. Dynamos do this switching because they have to supply direct current.

This switching in dynamos is carried out via the gas collector. The rings of the collector are cut because the current must always flow in the same direction outside the loop.

There are different problems with this switching. When the generator is operated with a load connected to its terminals, there is an internal voltage drop and a reaction in the armature.

The armature will create a magnetic flux that opposes the flux generated by the magnet. This effect is called counter-electromotive force, which will displace the neutral plane.

In order to solve this problem, several improvements can be made, such as:

  • Brush shifting: This method shifts the brushes to their new position correcting the plane offset, the problem is that the motor can work from 0% of its full load to 100%, so the plane can change.
  • Commutation or auxiliary poles: the function of these auxiliary poles is to compensate for the flux produced by the armature coils and to compensate it. This is a very useful and economical solution.

  • Compensation coils: When the generators are of high power, the commutation poles are not enough, in this case we use compensation coils.

Advantages of the alternator over the dynamo

The alternator has several advantages that make it a more widely used type of machine, not only because it produces electricity in alternating current, which is how it is consumed, but also because of other advantages in terms of use.

The advantages of the alternator over the dynamo are as follows:

  • In the electric alternator, a wider range of rotational speed can be obtained. The rotational speed can range from 500 to 7000 rpm. The dynamo at high rpm suffers from high wear and temperature rise in the commutator and brushes.
  • The rotor and stator assembly in the alternator are very compact.

  • Alternators have a single element as a voltage regulator.

  • Electric alternators are lighter: they can be 40 to 45% lighter than dynamos and 25 to 35% smaller.
  • The alternator works in both directions of rotation without modification.
  • The service life of the alternator is longer than that of the dynamo. This is due to the fact that the electric alternator is more robust and compact, due to the absence of the commutator in the armature, and withstands high temperatures better. 

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