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Note for Electrical Machines 1 - EM1 By balakrishna nandigam

  • Electrical Machines 1 - EM1
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ESWAR COLLEGE OF ENGINEERING Electrical Technology UNIT-I DC Machines Topics : Principle of operation of DC Machines- EMF equation – Types of generators, Magnetization and load characteristics of DC generators Introduction : Although a far greater percentage of the electrical machines in service are AC machines, the DC machines are of considerable industrial importance. The principal advantage of the DC machine, particularly the DC motor, is that it provides a fine control of speed. Such an advantage is not claimed by any AC motor. However, DC generators are not as common as they used to be, because direct current, when required, is mainly obtained from an AC supply by the use of rectifiers. Nevertheless, an understanding of DC generator is important because it represents a logical introduction to the behaviour of DC motors. Indeed many DC motors in industry actually operate as DC generators for a brief period. In this chapter, we shall deal with various aspects of DC generators. Principle of Operation of a DC Generator An electric generator is a machine that converts mechanical energy into electrical energy. All the generators work on a principle of dynamically induced e.m.f. This principle is nothing but the Faraday's law of electromagnetic induction. It states that, 'whenever the number of magnetic lines of force i.e. flux linking with a conductor or a coil changes, an electromotive force is set up in that conductor or coil.' The change in flux associated with the conductor can exist only when there exists a relative motion between a conductor and the flux. The relative motion can be achieved by rotating conductor with respect to flux or by rotating flux with respect to a conductor. So a voltage gets generated in a conductor, as long as there exists a relative motion between conductor and the flux. Such an induced e.m.f. which is due to physical movement of coil or conductor with respect to flux or movement of flux with respect to coil or conductor is called dynamically induced e.m.f. So a generating action requires following basic components to exist, i) The conductor or a coil ii) The flux iii) The relative motion between conductor and flux. In a practical generator, the conductors are rotated to cut the magnetic flux, keeping flux stationary. To have a large voltage as the output, the numbers of conductors are connected together in a specific manner, to form a winding. This winding is called armature winding of a DC machine. 1

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ESWAR COLLEGE OF ENGINEERING Electrical Technology The part on which this winding is kept is called armature of a DC machine. To have the rotation of conductors, the conductors placed on the armature are rotated with the help of some external device. Such an external device is called a prime mover. The commonly used prime movers are diesel engines, steam engines, steam turbines, water turbines etc. The necessary magnetic flux is produced by current carrying winding which is called field winding. The direction of the induced e.m.f. can be obtained by using Fleming's right hand rule. Fleming's Right Hand Rule : If three fingers of a right hand, namely thumb, index finger and middle finger are outstretched so that everyone of them is at right angles with the remaining two, and if in this position index finger is made to point in the direction of lines of flux, thumb in the direction of the relative motion of the conductor with respect to flux then the Outstretched middle finger gives the direction of the e.m.f. induced in the conductor. Visually the rule can be represented as shown in the following Fig. This rule mainly gives direction of current which induced e.m.f. in conductor will set up when closed path is provided to it. Construction of DC machines The d.c. generators and d.c. motors have the same general construction. Any d.c. generator can be run as a d.c. motor and vice-versa. A D.C. machine consists of two main parts : (i) Stationary part : It is designed mainly for producing a magnetic flux. (ii) Rotating part : It is called the armature, where mechanical energy is converted into electrical (electrical generator), or conversely, electrical energy into mechanical (electric motor). 2

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ESWAR COLLEGE OF ENGINEERING Electrical Technology The stationary and rotating parts are separated from each other by an air gap. The stationary part of a D.C. machine consists of main poles, designed to create the magnetic flux, commutating poles interposed between the main poles and designed to ensure sparkles operation of the brushes at the commutator (in very small machines with a lack of space commutating poles are not used) ; and a frame/yoke. The construction details and main parts of a DC machine are shown in the following fig. The essential parts of the DC machine are 1.Magnetic Frame or Yoke 2. Pole-Cores and Pole-Shoes 3.Pole Coils or Field Coils 4. Armature 5.Armature Windings or Conductors 6. Commutator 7. Brushes and Bearings 1) YOKE : The outer frame or yoke serves two main purposes. They are (i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine. (ii)It carries the magnetic flux produced by the poles. In small generators, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed. The modern process of forming the yoke consists of rolling a steel slab round a cylindrical mandrel and then welding it at the bottom. The feet and the terminal box etc. are welded to the frame afterwards. Such yokes possess sufficient mechanical strength and have high permeability. 3

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ESWAR COLLEGE OF ENGINEERING Electrical Technology 2) POLE-CORES AND POLE-SHOES : The field magnets consist of pole cores and pole shoes. The pole shoes serve two purposes (i) they spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of the magnetic path (ii) they support the exciting coils (or field coils). 3) Pole Coils : The field coils or pole coils, which consist of copper wire or strip, are former-wound for the correct dimension. Then, the former is removed and wound coil is put into place over the core as shown in the fig. When current is passed through these coils, they electro-magnetize the poles which produce the necessary flux that is cut by revolving armature conductors. 4) Armature : It is further divided into two parts namely, i) Armature core ii) Armature winding i) Armature core : Armature core is cylindrical in shape mounted on the shaft. It consists of slots on its periphery and the air ducts to permit the air flow through armature which serves cooling purpose. a) Functions : 1. Armature core provides house for armature winding i.e. armature conductors. 2. To provide a path of low reluctance to the magnetic flux produced by the field winding. b) Choice of material : As it has to provide a low reluctance path to the flux, it is made up of magnetic material like cast iron or cast steel. It is made up of laminated construction to keep eddy current loss as low as possible. A single circular lamination used for the construction of the armature core is shown in the Fig. 4

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