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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 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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ESWAR COLLEGE OF ENGINEERING Electrical Technology 5) Armature Winding : Armature winding is nothing but the interconnection of the armature conductors, placed in the slots provided on the armature core periphery. When the armature is rotated, in case of generator, magnetic flux gets cut by armature conductors and e.m.f. gets induced in them. a) Functions : 1. Generation of e.m.f. takes place in the armature winding in case of generators. 2. To carry the current supplied in case of DC motors. 3 To do the useful work in the external circuit. b) Choice of material : As armature winding carries entire current which depends on external load, it has to be made up of conducting material, which is copper. Armature winding is generally former wound. The conductors are placed in the armature slots which are lined with tough insulating material. 6) Commutator : The function of the commutator is to facilitate collection of current from the armature conductors. It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit. It is of cylindrical structure and is built up of wedge-shaped segments of highconductivity hard-drawn or drop forged copper. These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by means of a copper lug or strip (or riser). To prevent them from flying out under the action of centrifugal forces, the segments have Vgrooves, these grooves being insulated by conical micanite rings. A general appearance of the commutator is shown in the Fig. 7)Brushes and Bearings : Brushes are stationary and resting on the surface of the commutator. a) Function : To collect current from commutator and make it available to the stationary external circuit. 5

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