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Electrical Machine ll
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  IInnttr r ood d uuccttiioonn ttoo AACC MMaacchhiinneess Dr. SSuuaad d  II b br r aahhiimm SShhaahhll 1 ELECTRICAL MACHINES II Lecturer: Dr.   SSuuaadd IIbbrraahhiimm SShhaahhll   Syllabus I.   Introduction to AC Machine II.   Synchronous Generators III.   Synchronous Motors   IV.   Three-Phase Induction Machines   V.   Three-Phase Induction Motors   VI.   Induction Generators   VII.   Induction Regulators   Recommended Textbook : 1)   M.G.Say Alternating Current Machines Pitman Pub. 2)   A.S. Langsdorf Theory of AC Machinery McGRAW-HILL Pub.    IInnttr r ood d uuccttiioonn ttoo AACC MMaacchhiinneess Dr. SSuuaad d  II b br r aahhiimm SShhaahhll 2 I.   Introduction to AC Machines Classification of    AC Rotating Machines   • Synchronous Machines: •Synchronous Generators   : A primary source of electrical energy. • Synchronous Motors   :  Used as motors as well as power factor compensators (synchronous condensers). • Asynchronous (Induction) Machines:   • Induction Motors   :  Most widely used electrical motors in both domestic and industrial applications. • Induction Generators   :  Due to lack of a separate field excitation, these machines are rarely used as generators. •   Generators convert mechanical energy to electric energy. Energy Conversion •   Motors convert electric energy to mechanical energy. •   The construction of motors and generators are similar. •   Every generator can operate as a motor and vice versa. •   The energy or power balance is :  –    Generator: Mechanical power = electric power + losses  –    Motor: Electric Power = Mechanical Power + losses.  IInnttr r ood d uuccttiioonn ttoo AACC MMaacchhiinneess Dr. SSuuaad d  II b br r aahhiimm SShhaahhll 3 AC winding design The windings used in rotating electrical machines can be classified as    Concentrated Windings •   All the winding turns are wound together in series to form one multi-turn coil   •   All the turns have the same magnetic axis   •   Examples of concentrated winding are    –    field windings for salient-pole synchronous machines    –    D.C. machines    –    Primary and secondary windings of a transformer       Distributed Windings •   All the winding turns are arranged in several full-pitch or fractional-pitch coils   •   These coils are then housed in the slots spread around the air-gap periphery to form phase or commutator winding   •   Examples of distributed winding are    –    Stator and rotor of induction machines    –    The armatures of both synchronous and D.C. machines Armature windings, in general, are classified under two main heads, namely,    Closed Windings  •   There is a closed path in the sense that if one starts from any point on the winding and traverses it, one again reaches the starting point from where one had started •   Used only for D.C. machines and A.C. commutator machines    Open Windings  •   Open windings terminate at suitable number of slip-rings or terminals •   Used only for A.C. machines, like synchronous machines, induction machines, etc Some of the terms common to armature windings are described below: 1.   Conductor. A length of wire which takes active part in the energy-conversion process is a called a conductor.  2.   Turn. One turn consists of two conductors.  3.   Coil. One coil may consist of any number of turns. 4.   Coil –side.  One coil with any number of turns has two coil-sides.    IInnttr r ood d uuccttiioonn ttoo AACC MMaacchhiinneess Dr. SSuuaad d  II b br r aahhiimm SShhaahhll 4 The number of conductors (C)  in any coil-side is equal  to the number of turns (N)  in that coil. One-turn coil two-turn coil multi-turn coil 5.   Single- layer and double layer windings.    Single- layer winding •   One coil-side occupies the total slot area •   Used only in small ac machines one coil-side per slot       Double- layer winding   •   Slot contains even number (may be 2,4,6 etc.) of coil-sides in two layers •   Double-layer winding is more common above about 5kW machines Two coil –sides per slot   4-coil-sides per slot   Coil- sides Coil- sides Coil -sides Overhang Top layer Bottom layer  IInnttr r ood d uuccttiioonn ttoo AACC MMaacchhiinneess Dr. SSuuaad d  II b br r aahhiimm SShhaahhll 5 The advantages of double-layer winding over single layer winding are as follows: a.   Easier to manufacture and lower cost of the coils    b.   Fractional-slot winding can be used    c.   Chorded-winding is possible   d.   Lower-leakage reactance and therefore , better performance of the machine   e.   Better emf waveform in case of generators   6.   Pole – pitch. A pole pitch is defined as the peripheral distance between identical points on two adjacent poles. Pole pitch is always equal to 180 o 7.   Coil–span or coil-pitch. The distance between the two coil-sides of a coil is called coil-span or coil-pitch. It is usually measured in terms of teeth, slots or electrical degrees.   electrical.   8.   Chorded-coil.    If the coil-span (or coil-pitch) is equal    in case the coil-pitch is to the pole-pitch, then the coil is termed a  full-pitch coil  . less    if there are S  slots and  P  poles, then pole pitch  =     slots per pole   than pole-pitch, then it is called  chorded  ,  short-pitch  or  fractional-pitch coil       if coil-pitch  =    , it results in  full-pitch winding      in case coil-pitch  <     , it results in  chorded  ,  short-pitched    or  fractional-pitch   Full-pitch coil Short-pitched or chorded coil   N S Coil span Pole pitch N S Coil span Pole pitch
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