a. A three-phase alternator features two vital components, these being the stator and the armature. Another component that is required for the construction is the exciter, as this provides a source of DC to the system. The armature features a cooling system and is used to cover the internal components of the alternator. It also supports the core and windings of the alternator, in addition to the rotor. The cooling system consists of pipes that pass through the armature, dispensing a cooling gas. The ‘Iron’ core which is generally constructed out of numerous laminations of thin steel can are also present. They are laminated in order to prevent eddy currents from forming, which can cause excessive energy wastage due to heat being generated. The armature is wound in order to aid in the conduction of electricity. Copper bars are then covered with an insulating material and slotted into the armature, where they are held in place using wedges. The previously mentioned windings are then tied off at the opposite end of the armature. They are then fed through the housing to the base of the generator, where there are six terminals, two per phase. Slip rings are present in order to transmit power or signals between the component in motion, and the stationary component. Brushes are used in order to connect the windings and to ensure that the circuit is complete and that electricity can flow.
Excitation can be achieved by three different methods, these are static excitation, DC excitation and AC excitation. Static excitation involves taking a feed from the alternator and passing it through a stepdown transformer, this voltage is then used to power any other electrical equipment within the circuit. This type of circuit can be very useful in instances where reducing the cost of the circuit is imperative as it does not need any other starting equipment which can prove to be costly. DC excitation involves two separate exciters, the main exciter, and the pilot exciter. These can be used to limit the alternator power whenever a fault occurs within the system. However problems do exist when the turbo alternators are in operation, and as such, more exciters are required in case any back-ups are needed. AC excitation can come in two forms, brushless excitation and rotor excitation. Brushless excitation involves the removal of the commutator, collector and the brushes. This means that a very short time constant applies to the system, providing a much more dynamic output. Rotating thyristor excitation involves the use of an AC exciter to initiate the rotation of the armature used in the system, which helps to create a start-up voltage, this is then passed through a full wave rectifier and can be passed into the main windings. When this form of excitation n is operated manually, it is comparable to using a separate voltage adjuster.
b. A three phase alternator operates by using the rotational force involved in the turning of a shaft within a metal core, this in turn breaks the magnetic flux lines repeatedly, which results in a constant generation of an AC waveform due to the fact that the magnetic flux is still able to flow at all times, resulting in an AC waveform being produced.
Parallel operation of generators occurs when two or more generators or alternators are in operation at the same time. This can be beneficial due to the fact that they do not have to be located near to each other, and as such, their floor coverage can be reduced, in addition to this they can be used to balance the load during peak times and can be run so that there is a main one and a backup one.
Parallel operation can be synchronised using microcontrollers, however due to the advancement in technology, this can now be done digitally, and they ensure that the excitation current is sufficient so that the rotation speed can produce the same output voltage and frequency between the two generators. The microcontrollers can be referred to as ‘gensets’ and can be controlled with a relay that monitors the condition of the generators. It will generally monitor things such as rotational direction and speed, synchronisation within the other units, load balancing across the units. Due to the fact that they are digital, their readouts can be seen on computers in order to check the system for any faults or issues and also to see where the issue has occurred.
However if the generators are out of synchronisation, or have been done so to a poor standard, then the alternators can be damaged, and any equipment attached to them can be damaged due to spikes within the current.
For the generators to be synchronised, they must be operating at the same load and meeting the customers’ requirements, the correct excitation current must be used, and they must also be operating at the same frequency.
However when the alternators are operating in parallel conditions, it is important that they operate within the set tolerances for voltage and frequency, this helps to ensure that the system can operate safely in conjunction with the other equipment attached to it.
With regards to the variation in excitation and how it may control the power factor within an alternator, there are many points to take notice of.
The Vee curve opposite shows how that as the field current is increased, regardless of the load currently placed on the generator, the armature current will decrease, causing a lag within the power factor, however, if the field current keeps on increasing, then eventually a point will be reached where the power factor can be classed as being in a state of unity. This is the most desirable state for an electrical system to be in as it means that all of the power is being used by the circuit in a positive manner with minimal wastes in the system, thus allowing the system to work much more efficiently than if it were to be run with a higher or lower field current, otherwise this would result in either a leading or lagging power factor, both of which can be detrimental to the system as they will incur extra losses and the phase angles will change as a result of this.
The main purpose of this type of ‘Vee curve’ is to show changes in magnitude of the armature current when the excitation voltage of the equipment is changed. This is evident from the graph and the way in which the armature increases as a full load is placed on it, whereas with no load, the armature current undergoes a much larger scale of variation than when any form of load is applied to it.
An inverted vee curve can be used for a similar purpose, however it shows slightly different data. It shows how power factor and field current relate. It offers a different perspective to how altering the field current will in turn alter the field current. For example, by looking at the curve when a full load is applied to the alternator, it can be seen that the power factor increases with the field current until it reaches the stage of being in unity, thereafter it decreases back to its original value