Although you do not know very well what you are talking about when it comes to alternators, I am sure that with this article you will understand it quickly and easily.
The Alternator for generating set is similar to the device that moves the ceiling fan of your living room, but instead of consuming electricity to generate a mechanical movement, it uses a mechanical movement to generate electricity.
As simple as that!
We use the mechanical power from the diesel engine to move the electrical machine and generate current.
From the electrical point of view, it is not a simple machine to understand, therefore, we will not talk here about the ‘why’ it works, but only about how it is presented and its integration in the generating set.
What do you know about an alternator?
An alternator, which is also called a generator, is an electrical machine which operation is based on the properties of electromagnetic induction. This phenomenon causes that when a conductor moves inside a magnetic field, it generates a certain voltage at its ends. This voltage causes a current and in this way, we get electricity.
The diesel engine causes the relative movement of the conductor and the magnetic field, therefore:
Mechanical energy ⇒ electricity
A generator for generating set consists of a solid and hollow case, inside which is housed a kind of mobile cylinder. The fixed part is called STATOR and it is what you see, the internal part is the ROTOR, which is connected to the engine and turns because the latter transmits its rotation.
The magnetic field is caused in the part called ROTOR and induces the voltage that we mentioned before in the conductors that form the STATOR. Therefore, in the terminals of these drivers you can get a 230 V, 400 V voltage, or whatever you want to connect to the electrical appliances.
It is important to make a point between synchronous and asynchronous alternators. Technical details apart, it will be enough to know that the generating sets employ the synchronous ones. Asynchronous alternators are used for other applications, especially in fixed generation of high power connected to the electricity grid like wind or hydroelectric generators.
Synchronous alternators for generating sets are very efficient machines. Their efficiency varies from a minimum of 80% for the smallest sizes to 97% for the highest power.
Below, I’ll talk in detail about the most important characteristics of an alternator for a generating set.
Characteristics of an alternator for a generating set
The rotor is the set of parts that connected to the diesel engine, rotate inside the alternator generating a magnetic field.
It is composed of a mechanical shaft on which a number of copper windings are fixed. The type and number of windings determine the number of poles of the alternator, which is a very important feature, because it establishes the frequency of the electrical supply at a given engine revolutions. Each winding has two poles.
Running the engine to 1500 revolutions per minute and having 4 poles (2 windings) we will obtain an electric frequency of 50Hz. Running the engine to 1800RPM the frequency will be 60Hz.
The RPM and the frequency are related according to the formula – quiet, it is the only formula of the post :)
RPM = 60 * frequency / pair of poles
Where does magnetic field come from?
To generate the magnetic field in the rotor, this must be feeded with an electrical current called EXCITATION, alternatively permanent magnets can be used. Permanent magnet alternators are more expensive and they have a more complex construction.
There are many ways to excite a synchronous machine; however, I’ll just tell you what really are used today in generating sets.
Here are two types of excitement:
Brushless with diode bridge: the power is taken directly from the rotor itself, either through an auxiliary winding, or through a mini-alternator fitted on it. This way there is no need for electrical connections between the rotor and the stator. In old alternators there was this need and brushes were used.
Compound: the power comes through a transformer connected to the output voltage of the alternator. It is a special transformer because it controls both voltage and current with two different windings.
The stator is the set of parts that make up the case of the alternator and are fixed, static, while the rotor is spinning.
It is also made by a metallic structure and a series of copper windings. At the ends of these windings we will have the voltage induced by the magnetic field of the rotor.
The winding is done by winding a copper wire isolated around a metal structure, each turn is a loop. Once the winding is finished, it is impregnated with special resins.
The number of windings, the number of turns and their physical arrangement, determine the output voltage, while the frequency only depends on the number of revolutions of the main motor and the rotor. For them all manufacturers have different types of windings that identify with specific codes. The one that provides 400 Vac and 50 Hz three-phase with neutral is the most common but there are also more configurations such as single-phase, with neutral, without neutral, biphasic, and more voltages such as 380 V, 600 V or 690 V among others. Not to mention the medium voltage … Ok, I’ll stop here!
Arrangement and number of terminals identify the windings.
There are STAR, DOUBLE STAR, TRIANGLE, DOUBLE TRIANGLE, ZIGZAG windings and more. Depending on the desired configuration, they have 6 terminals or 12 terminals. That is, 6 ends for 3 coils, or 12 ends for 6 coils.
From the many electrical characteristic of the alternators, I will only mention a few here.
The short circuit current and the set of reactances of the electric machine (direct and indirect transient Z, direct and indirect subtransient Z, for example). These values depend on the constructive mechanical characteristics and the design and are fundamental parameters for the installation of the equipment. They are necessary to carry out the engineering calculations when connecting the generating set to an electrical network.
In fact, they are defined by the Grid Codes and must comply with given thresholds otherwisw the machine would not be compatible with the electrical network and this could generate serious damages and power cuts.
The isolation calss is another point. For generators we usually consider class H although F and B are also used in some cases.
This parameter indicates the quality of the impregnation of the windings -the insulation capacity between one conductor and another-. For example, ‘Class H insulation’ means that the windings are prepared to heat up to 165 ° C when the ambient temperature is 40 ° C.
However, class H insulation and performance may be required to be class F. This means that, although the winding is ready to work up to 165 ° C when the ambient temperature is 40 ° C, we will limit it to 145 ° C.
|Reference ambient temperature||40ºC||40ºC||40ºC|
|Maximum working temperature||165ºC||145ºC||120ºC|
Temperature values by insulation class.
The Automatic Voltage Regulator -AVR- is the device that regulates the voltage output and is responsible for keeping it controlled in all load conditions.
It can work in two ways: isochronous – fixed voltage – or droop – there is a controlled voltage drop between full load and no-load – depending on how the system is configured.
The AVR can be digital or analog and talk or not with other devices in the system. Important to note that depending on the type of load and the degree of precision you need, you can choose one that monitors all the phases of the circuit or just one at the same time. The accuracy of the regulation is usually between 0.5% and 1%, depending on the quality of the regulator.
The cooling system is normally air, but it can also be water with closed circuits. Water cooling is preferred in polluted, chemical or explosion hazards environments.
The harmonic distortion -THD- in the supply line that will depend on the load but also on the alternator and its reactance as seen above.
The alternator connections are made through the box placed on top of it. It is usually a simple box with a number of terminals and bars inside where you can screw the cables.
Accessories of an alternator for generating set
Among the most important accessories, it is worth to mention the windings’ temperature sensors that monitors the heat of the windings. Heat could damage the insulation and cause a short circuit.
Other sensors are the temperature of bearings, especially important in the application of continuous supply, a number of special connection boxes and mechanical adapters for the coupling to the diesel engine.
The PMG, or permanent magnets generator, is used to separate the supply of the excitation to the main electrical supply. It is especially useful when some electronic equipment are feeded. These devices may induce high harmonic distortions in the power line.
Do you want to learn more?
Much more could be said about generating set alternators but it is not this is not the place where to enter into technical details and engineering aspects.
However, if you like physics you can check Faraday’s law of electromagnetic induction; If you want to learn more, I recommend you start with this and other Wikipedia articles; and if you have not been clear what are the brushes you can find out here.
The Grid Codes in Europe are regulated by the European Commission and defined by a committee of experts. You can consult them here Europa.eu/Electricity Network Codes.
Photo Credit Wikipedia and own photos.