Alternating Current and Direct Current Generators - Essay Example

Alternating Current and Direct Current Generators Number) December 17, (Faculty) Alternating Current and Direct Current Generators 1.0 Introduction Electricity can either be generated as direct current (DC) or alternating current (AC). Given that electricity is basically the movement of electrons through a conductor such as a wire, the difference between direct current and alternating current is obtained from the direction of flow of electricity (current). In a DC electrons keep flowing steadily towards one direction, we could term the direction as forward, while in AC electrons keep switching directions periodically, at one time they flow forward and then switches to backward. Electrical transition over large distances is done through alternating current. This is achievable through step-up and step-down by the use of transformers (Glenn 2006). An electric power generator can be built by spinning a wire loop within a uniform magnetic field (Portela et al 2008). Voltage between the loop terminals is induced by the spin. Portela et al (2008) further stipulates that generators which produce alternating current are known as alternators while those that produce direct current are dynamos. 2.0 AC Generator In this generator, an AC voltage is induced between the loop terminals of a wire loop rotating within magnetic field that is under the influence of a magnet. In this case a periodic change in voltage polarity is noticeable. This is caused by the change in position of the coil as it rotates through two different poles. The terminals to this loop are tied to a metallic ring attached to flexible brushes. An alternating current is therefore established as soon as the brushes get connected to electrical load. Generally, this is the principle of an AC generator operation as described by Portela et al (2008). There are various types of alternating current generators as we are about to discuss, one thing common about these generators is that they basically perform the same function. The most predominant ones are rotating armature, rotating field generators, and polyphone generators. Check Figure 1 for diagram 2.1 Rotating Armature Generator This generator has a rotor that acts as an armature and a stator that provides an electromagnetic field due to its staticness. As the rotor rotates in the field it cuts through lines of force and produce the desired output voltage in the end. This output voltage can be varied because the induced voltage into the armature is highly dependent on the rate at which magnetic lines of force get cut by the metallic ring (Keljik 2013). These generators are used in applications involving minimal power. 2.2 Rotating Field Generator Keljik (2013) discussion on the rotating field type generators reveal that voltage is induced into stationary conductors by the magnetic fields that move past them. This type of generator is so far the most widely used. One advantage of this set-up is that it enables the handling of large amount of current due to the absence of sliding contacts. Furthermore, the whole output circuit is insulated. The output power take from stationary windings enabled connection of output through fixed terminals. 2.3 Polyphase Generators Let us consider a polyphase generator as a three-phase AC generator. The generator is thus designed to produce three-phase AC power through the building of more coils in the stator around the rotor. Zarudi (1998) reveals that there might be a connection between the windings of a polyphase generator of not. The most used polyphase systems according to Zarudi (1998) are those that are connected due to certain advantages. The reason as to why most power is generated and distributed as three-phase rather than single-phase is that the cost of transmission is less in three-phase compared to single-phase while dealing the same amount of voltage. Another reason is that the three-phase has 180% greater capacity. 2.4 Generation of AC and its applications With reference to the learn.sparkfun.com website, an AC is produced using a device known as an alternator. This is a special type of an electric generator designed specifically for that. The generator has got a loop of wire spun inside a magnetic field in order to induce current along the wire. The wire can be rotated by wind, steam are even water. The alterations comes about as a result of the wire spin entering different polarities. Alternating current is used to deliver power to office buildings, homes, schools, and many other organizations. 3.0 DC Generators An alternating current generator can be transformed into a direct current generator. This is achievable by substitution the contact ring with a mechanical switch (Portela et al 2008). The metallic switch may be divided into two isolated segments in order to achieve a simple switch to be mounted in the axis. The type of commutater as such can be termed collector. A connection of each terminal of the loop is established on the collector segment which contributes to the induction of an AC voltage into the coil upon rotation of the loop. Interestingly, the AC voltage gets transformed into a DC voltage by the collector before it reaches the load. The collector in this case works as a mechanical rectifier. This is yet another operational principle by Portela et al (2008). Check figure 2 for diagram. 3.1 Construction of DC Generator "A DC generator is an electrical machine which converts mechanical energy into direct current electricity." (Daware 2012). The energy conversion is built on the principle of production of electromotive force (e.m.f). Basically, a DC generator consists two parts which are the stator and rotor. The constructional parts are described by Daware (2012) as follows: Yoke: This is the outer frame of a generator or motor. It is purposed to provide strength for the whole assembly of the generator. Poles: These are joined to the yoke by the help of screw and are meant to support windings. Pole shoe: This is an extended part of the pole which prevents field coils from slipping and also uniformly spread out the flux in air. Armature core: This is the rotor or generator. Slots are provided to carry armature windings here. Commutator and brushes: There is an established connection between the armature conductors and commutater. Conducting brushes on the other hand rest on commutater and have physical contact with armature conductors so that wires can be connected on them. 3.2 Excited DC Generators DC generators are classified as self-excited or partially excited generators depending on the excitation method used (Bakshi & Bakshi 2009). Direct current is used in the manufacture of battery cells (Traister 1979). 3.2.1 Self-Excited Generators A self-excited generator is one whose DC field magnet winding is supplied current from the output generator. Depending on the manner of field winding connection to the armature, there are three types of self-excited generators. They are: Series generator, shunt generator, and compound generator (Bakshi & Bakshi 2009). Field winding is connected in series with the armature winding in series generator so that the whole armature current flows through the field winding as well as load. The shunt generator has field winding connected in parallel with armature winding in a way that terminal voltage of the generator is applied across it. In the compound generator, two sets of field wind on each pole with one in series and the other in parallel with the arm. 3.2.2 Separately Excited Generators In this case a separate external DC supply is used to provide exciting current through field winding. This is to mean that when field winding is supplied from separate dc supply then the generator is termed as a separately excited generator (Bird 2014). 4.0 Conclusion Voltage induction occurs when a loop is spanned within a uniform magnetic field in a conveniently fashionable manner. The voltage amplitude and frequency is influenced by the speed at which the loop is rotating. Should an electric load get connected at the terminals of the loop then a current will be established in the circuit. Since currents generated by any basic electrical generator is always an AC then a collector has to be used when a DC is required. Appendix Figure 1 Courtesy of www.engineermaths.com Figure 2 Courtesy of www.tbub.com References Bakshi, U. A., & Bakshi, V. U. (2009). Electrical Technology (Rev ed.).: Technical Publications. Bird, J. (2014). Electrical Circuit Theory and Technology (5th ed.). : Routledge. Daware, K. (2012, December). Basic Construction and Working of a DC Generator. Retrieved from http://www.electricaleasy.com Engineer Maths. (November, 2010). Introduction to DC Generator. Retrieved from http://www.engineermaths.com Glenn, D. W. (2006). Electrical Power Industry in Nontechnical Language (2nd ed.).: Penn Well Books. Integrated Publishing. (2013, January). Industrial Carbon Brushes. Retrieved from http://www.tpub.com Keljik, J. (2013). Electricity 3: Power Generation and Delivery (10th ed.).: Cengage Learning. Partela, P., Sepulveda, J., & Esteves, J. S. (2008, June). Alternating Currenr and Direct Current Generator. The Hands-on Science Network, Vol. 1(No. 1), 345-405. Traister, R. J. (1979). DC Power Supplies: Application and Theory (2nd ed.).: Reston Publishing Company. Zarudi, M. (1998). Circuit Analysis for Power Engineering. Great Britain: Springer Science & Business Media. Alternating Current vs Direct Current. (213, July). Retrieved from http://www.learnsparkfun.com/tutorials Read More