EPM 202 Lecture 2 PDF

Basic Structure of Electric Machines Basic Structure of Electric Machines DC Machine Polyphase Synchronous Machine Polyphase Induction Machine Polyphase Induction Machine Polyphase Induction Machine Motor Control Section 1-2 Checkup Transformers  Introduction...

Basic Structure of Electric Machines Basic Structure of Electric Machines DC Machine Polyphase Synchronous Machine Polyphase Induction Machine Polyphase Induction Machine Polyphase Induction Machine Motor Control Section 1-2 Checkup Transformers  Introduction A transformer is a static device comprising coils coupled through a magnetic medium connecting two ports at different voltage levels (in general) in an electric system allowing the interchange of electrical energy between the ports in either direction via the magnetic field. The transformer is one of the most important component of a variety of electrical circuits ranging from low-power, low-current electronic and control circuits to ultra high-voltage power systems. Transformers are built in an astonishing range of sizes from the tiny units used in communication systems to monsters used in high-voltage transmission systems, weighing hundreds of tons.  Introduction Transformers are used extensively in ac power systems because they make possible power generation at the most desirable and economical level (11–33 kV), power transmission at an economical transmission voltage (as high as 230–1000 kV) and power utilization at most convenient distribution voltages (230/400 V) for industrial, commercial and domestic purposes but in industrial applications voltages may have to be as high as 3.3, 6.6 or 11 kV for large motors. In communication and electronic systems where frequency ranges from audio to radio and video, transformers are used for a wide variety of purposes. For instance, input/output transformers (used to connect the microphone to the first amplifying stage/to connect the last amplifying stage to the loudspeaker) and interstage transformers are to be found in radio and television circuits. Indeed, the transformer is a device which plays an important and essential role in many facets of electrical engineering.  Introduction A circuit model and performance analysis of transformers is necessary for understanding of many electronic and control systems and almost all power systems. The transformer being an electromagnetic device, its analysis greatly aids in understanding the operation of electromechanical energy conversion devices which also use magnetic field, but the interchange of energy is between electrical and mechanical ports. Applications  Working Principle of a Transformer A transformer, in its simplest form, consists essentially of two insulated windings interlinked by a common or mutual magnetic field established in a core of magnetic material. When one of the windings, termed the primary, is connected to an alternating- voltage source, an alternating flux is produced in the core with an amplitude depending on the primary voltage, frequency and number of turns. This mutual flux links the other winding, called the secondary. A voltage is induced in this secondary of the same frequency as the primary voltage, but its magnitude depends on the number of secondary turns. When the number of primary and secondary turns are properly proportioned, almost any desired voltage ratio, or ratio of transformation can be achieved.  Basic Types of Transformers If the secondary voltage is greater than the primary value, the transformer is called a step-up transformer; if it is less, it is known as a step-down transformer; if primary and secondary voltages are equal, the transformer is said to have a one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a circuit. Any transformer may be used as a step-up or step-down depending on the way it is connected.  Basic Types of Transformers Transformers are also categorized as electronic transformers and power transformers. Electronic transformer’s operating voltages are very low and are rated at low power levels. These are used in consumer electronic equipment like televisions, personal computers, CD/DVD players, and other devices. The term power transformer is referred to the transformers with high power and voltage ratings. These are extensively used in power generation, transmission, distribution and utility systems to increase or decrease the voltage levels. Various applications use wide variety of transformers including power, instrumentation and pulse transformers.  Basic Types of Transformers In order to ensure the largest and most effective magnetic linkage of the two windings, the core, which supports them mechanically and conducts their mutual flux, is normally made of highly permeable iron or steel alloy (cold-rolled, grain-oriented sheet steel). Such a transformer is generally called an iron-core transformer. Transformers operated from 25–400 Hz are invariably of iron-core construction. However, in special cases, the magnetic circuit linking the windings may be made of nonmagnetic material, in which case the transformer is referred to as an air-core transformer. The air-core transformer is of interest mainly in radio devices and in certain types of measuring and testing instruments.  Basic Types of Transformers In core-type construction shown in Fig. 3.2(a) the windings are wound around the two legs of a rectangular magnetic core, while in shell-type construction of Fig. 3.2(b), the windings are wound on the central leg of a three-legged core.  Core-type Transformer Though most of the flux is confined to a high permeability core, some flux always leaks through the core and embraces paths which partially lie in the air surrounding the core legs on which the coils are wound. This flux which links one of the windings without linking the other, though small in magnitude, has a significant effect on the transformer behavior. Leakage is reduced by bringing the two coils closer. In a core-type transformer this is achieved by winding half low-voltage (LV) and half high-voltage (HV) winding on each limb of the core as shown in Fig. 3.2(a). The LV winding is wound on the inside and HV on outside to reduce the amount of insulation needed. Insulation between the core and the inner winding is then stressed to low voltage. The two windings are arranged as concentric coils.  Shell-type Transformer In shell-type construction leakage is reduced by subdividing each winding into subsections (wound as pancake coils) and interleaving LV and HV windings as shown in Fig. 3.2(b). The core-type construction has a longer mean length of core and a shorter mean length of coil turn. This type is better suited for EHV (extra high voltage) requirement since there is better scope for insulation. The shell-type construction has better mechanical support and good provision for bracing the windings. The shell-type transformer requires more specialized fabrication facilities than core-type, while the latter offers the additional advantage of permitting visual inspection of coils in the case of a fault and ease of repair at substation site. For these reasons, the present practice is to use the core-type transformers in large high-voltage installations. In both core and shell-type transformers, the individual laminations are cut in the form of long strips, as shown in Fig.1. The assembly of the complete core for the two types of transformers is shown in Fig. 2.