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UNIT - I D.C. GENERATORS D.C. GENERATORS-CONSTRUCTION & OPERATION • • • • • • DC Generators Principle of operation Action of Commutator Constructional details of DC Machine Types of DC generators EMF Equation DC Generator DC motor D.C. GENERATORS PRINCIPLE OF OPERATION DC generator converts mechanical energy into electrical energy. when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and e.m.f. produces in a generator and it is defined by faradays law of electromagnetic induction e.m.f. causes current to flow if the conductor circuit is closed. First Law : Whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it. or Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor. Second Law : The magnitude of the induced e.m.f. is equal to the rate of change of flux linkages. Faradays Law of Electromagnetic Induction A changing magnetic flux through a loop or loops of wire induces an electromotive force (voltage) in each loop. Lenz’s Law “The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..” OR “The direction of induced E.M.F in a coil (conductor) is such that it opposes the cause of producing it..” Fleming's Right Hand Rule E.M.F • The Thumb represents the direction of Motion of the conductor. • The First finger (four finger) represents Field. • The Second finger (Middle finger) represents Current Fleming's Right Hand Rule The following are the basic requirements to be satisfied for generation of E.M.F 1.A uniform Magnetic field 2.A System of conductors 3.Relative motion between the magnetic field and conductors • • • Magnetic field :Permanent Magnet (or) Electro Magnet (practical) Conductor :- Copper (or) Aluminum bars placed in slots cut around the periphery of cylindrical rotor Relative motion:By Prime Mover Turbine I.C Engine (Internal combustion) Simple loop generator Basic Generator Generators Simple loop generator with slip ring Generators Basic operation of the generator As the loop rotates, the magnetic flux through it changes with time This induces an e.m.f and a current in the external circuit The ends of the loop are connected to slip rings that rotate with the loop Connections to the external circuit are made by stationary brushes in contact with the slip rings Simple loop generator with split ring Simple loop generator with split ring Working Principle of D.C Generator Schematic diagram of a simple DC Generator 1st half cycle(00 to 1800 ) Path of current ABR1B1MLR2B2CD 2st half cycle(1800 to 3600) Path of current DCR2B1MLB2R1BA DC Generators, cont • The output voltage always has the same polarity • The current is a pulsating current • To produce a steady current, many loops and commutators around the axis of rotation are used – The multiple outputs are superimposed and the output is almost free of fluctuations Unidirectional current wave shape Resultant current wave shape when number of conductors used result current wave shape Constructional Details Of DC Machine Yoke: Rotor: Stator: Field electromagnets: Pole core and pole shoe: Brushes: Shaft: Armature: Coil: Commutator: Bearings: Construction details of DC generator Cross section view of dc machine N shaft S Main parts of a 4-pole d. c machine Practical Dc Machine 1)Yoke 1)Yoke:- - Acts as frame of the machine Mechanical support low reluctance for magnetic flux High Permeability -- For Small machines -- Cast iron—low cost -- For Large Machines -- Cast Steel (Rolled steel) Large DC machine Small DC machine 2)pole cores and pole shoes 2)Field Magnets:a) Pole core (Pole body) :- --Carry the field coils --Rectangle Cross sections -- Laminated to reduce heat losses --Fitted to yoke through bolts b) Pole shoe:- Acts as support to field poles and spreads out flux Pole core & Pole shoe are laminated of annealed steel (Of thickness of 1mm to 0.25 mm) 2)pole cores and pole shoes 2)Field Magnets:c) Field coils (Magnetizing coils):- -- Provide excitation (exciting coils) I . e field flux --Number of poles depends speed of armature on and the output for which the machine designed --Frame to used for design for exciting coils Different types of fields i) Separately Exciting ii) Self Exciting 3)Armature core 3)Conductor system:a) Armature core (Armature):-- To support armature windings --To rotate conductors in a magnetic field -- it is cylindrical or drum shaped is built --Made of high permeability silicon steel stampings (of 0.5 mm thick) -- Each stamping is separated from its neighboring one by thin varnish as insulation --Laminated to reduce eddy current losses -- A small air gap between pole pieces and armature so that no rubbing between them -- High grade silicon steel used to reduce i) Hysteresis loss ii) Eddy current loss -- Ventilating ducts are provided to dissipate heat to dissipate heat generated by above losses b) Armature Winding:Main flux cuts armature and hence E.M.F is induced --winding made of Copper (or) Aluminum --windings are insulated each other 4)commutator 4) Commutator:--Hard drawn copper bars segments insulated from each other by mica segments (insulation) -- Between armature & External circuit -- Split-Rings (acts like Rectifier AC to DC ) 5&6 Bearings and Brushes 5)Brushes and brush gear:- Carbon, Carbon graphite, copper used to Collects current from commutation (in case of Generator) 6)Shaft and bearings:Shaft-- Mechanical link between prime over and armature Bearings– For free rotation DC Machine Construction DC Machine Construction Rotor of a dc machine DC Machine Construction Cutaway view of a dc machine Armature Winding Armature Winding is classified into two types: Lap winding Wave windings Armature windings Lap Winding: are used in machines designed for low voltage and high current armatures are constructed with large wire because of high current Eg: - are used is in the starter motor of almost all automobiles The windings of a lap wound armature are connected in parallel. This permits the current capacity of each winding to be added and provides a higher operating current. No of parallel path, A=P ; P = no. of poles Wave winding: are used in machines designed for high voltage and low current their windings connected in series When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same are used is in the small generator. No of parallel path, A=2, Commutation process in D.C Generator Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. Generated EMF or EMF Equation of a generator Let = flux/pole in Weber Z =Total number of armature conductors =No. of slot × No. of conductors/slot P= No. of generator poles A =No. of parallel paths in armature N= Armature rotation in revolutions per minute (r. p. m) E= e.m.f induced in any parallel path in armature Generated e.m.f Eg= e.m.f generated in any one of the parallel paths i.e E Average e.m.f generated/conductor = d volt dt Now, flux cut/conductor in one revolution d = P wb No. of revolutions/sec=N/ 60 Time for one revolution , dt= 60 /N sec According to Faraday’s Law of electro magnetic induction E.M.F generated/conductor = d= PN volts dt 60 No. of conductors (in series) in one parallel path= Z / A E.M.F generated/path= PN × Z Volts 60 A Generate E.M.F, Eg= Z N × P Volts 60 A For i) Wave winding A = 2 ii) Lap winding A = P Generators D.C Generators A.C Generators (Alternators) Cummulatitave differentially Cummulatitave differentially Types of Generators 1)Separately excited generators 2)Self excited generators i) shunt wound ii) series wound iii) compound wound a) long shunt b) short shunt Clasifications of Generators Separately excited generators L G Ia=IL E=Vt+ IaRa +BCD VL shunt wound L G VL series wound L G VL compound wound long shunt short shunt L L G VL L G VL L The Practical DC Generator The actual construction and operation of a practical dc generator differs somewhat from our elementary generators Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator The main advantages of using electromagnetic poles are: (1) increased field strength and (2) possible to control the strength of the fields. By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled. Four-pole generator (without armature) D.C. Generator Characteristics The following are the three most important characteristics in a D.C. generator: 1. Open Circuit Characteristics (Eo/IF) 2. Internal Characteristics (E/Ia) 3. External Characteristics (V/Ia) Critical Resistance for shunt Generator Critical field resistance is a term that is associated with a DC Shunt generator. The value of resistance of shunt field winding beyond which the self generator fails to build up its voltage is known as " critical resistance at a given speed it is the maximum field resistance with which the shunt generator excite. Shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. How to Draw O.C.C. at Different Speeds? If we are given O.C.C. of a generator at a constant speed N1 then we can easily draw the O.C.C. at any other constant speed N2.Fig (3.11) illustrates the procedure. Here we are given O.C.C. at a constant speed N1.It is desired to find the O.C.C. at constant speed N2 (it is assumed that n1 < N2)For constant excitation, E α N. E2/E1=N2/N1 As shown in Fig. (3.11), for If = OH, E1 = HC. Therefore, the new value of e.m.f. (E2) for the same If but at N2i. E2=HC ×( N2/N1) = HD Critical Speed (NC) The critical speed of a shunt generator is the minimum speed below which it fails to excite. Therefore , Speed α Critical resistance In order to find critical speed, take any convenient point C on excitation axis and erect a perpendicular so as to cut Rsh and R’sh lines at points B and A respectively. Then, BC/AC =NC/N or NC = N ×(BC/AC) Conditions for Voltage Build-Up of a Shunt Generator The necessary conditions for voltage build-up in a shunt generator are: (i) There must be some residual magnetism in generator poles. (ii) The connections of the field winding should be such that the field current strengthens the residual magnetism. (iii) The resistance of the field circuit should be less than the critical resistance. In other words, the speed of the generator should be higher than the critical speed. Open circuit characteristics of Separately Excited D.C. Generator Internal and External Characteristics Characteristics of Shunt Generator Characteristics of Series Generator Compound Generator Characteristics Armature Reaction The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects: (i) It demagnetizes or weakens the main flux (ii) It cross-magnetizes or distorts. Commutation It is the process of converting A.C generated voltage in the armature conductors to D.C for external load. Commutation process in interpoles in DC machine Applications of D.C Generators Separately excited generators i) These are used for speed control of D.C motors over a large range. ii) These are used in areas where a wide range of terminal voltage is required Self excited generators i) shunt generators :- i) These are used as exciters for exciting the field of synchronous machines and separately excited D.C generators ii) These are used for battery charging because it’s terminal voltage are almost constant or can be kept constant. iii) Commonly used in ordinary lighting purposes and power supply purposes. i) ii) iii) iv) v) ii) series generators:These are used for series arc lighting Series incandescent lighting As a series booster for increasing the voltage across the feeder to compensate the resistance drop of the line. because of their rising characteristic. Special purposes such as supplying the field current for regenerative breaking of D.C locomotives (railway service). Constant current for welding. iii) compound generators:i) Compound generators are used where constant terminal voltages have to be maintained for different loading conditions. ii) Cumulatively compound generators:-These are for domestic lighting purposes and to transmit energy over long distance and for heavy power service such as electric railways. iii) Differential compound generator:- The use of this type of generators is very rare and it is used for special application like arc welding. Total losses in a D.C Machine Armature windings Armature windings Total losses in a D.C Machine The total losses in a dc machine are 1.Cu losses 2.Iron losses 3.Mechanical losses Cupper losses are mainly due to the current passing through the winding. 1.Armature cu losses (30 to 40% of full load losses) Cu losses 2.Shunt field cu losses(20 to30% of full load losses) 3.Series field cu losses Armature cu losses=Ia2 Ra Ra=Armature resistance Ia= Armature current --Losses due to brush contact resistance is usually include in armature cu losses Shunt field cu losses=Ish2Rsh Rsh=Shunt field resistance Ish=Shunt field current Series field cu losses=Ise2Rse Rse=Series field resistance Ise=Series field current Iron losses (Magnetic losses) (20 to 30% of full load losses) 1)Hysteresis losses 2)Eddy current losses Hysteresis losses (Wh):The losses is due to the reversal of magnetisation of the armature core Every portion of the rating core passes under N and S poles alternately. There by attaining S and N polarity respectively. The core undergoes one complete cycle of magnetic reversal after passing under one pair of poles. P=No. of poles N= Armature speed in rpm frequency of magnetic reversals f=NP 120 The losses depends upon the volume and B max and frequency of reversals. Hysteresis losses is given by steinmetz formula Wh=η B1.6maxf V wats V=Volume of the core in m3 η= Steinmetz hysteresis coefficient Eddy current losses:-(We) when the armature core rotates, it cuts the magenetic flux hence an e.m.f induced in in the body of the core according to faradays law of electro magnetic induction. This e. m.f through small sets up large current in the body of the core due to its mall resistance. This current is known as “Eddy Current” -These core laminations are insulated from each other by a thin coating of varnish. Due to the core body being one continuous solid iron piece (fig a) The magnitude of eddy current is large. As armature cross sectional area is large it’s resistance is small. hence eddy current losses is large. In (fig b) The same core has been split up in to thin cross section has very high resistance, hence magnitude of eddy currents is reduced considerably there by reducing eddy current losses. We=k B2 maxf2t2v2 watts Bmax=maximum flux densities f=Freequency of the magenetic reversals v=volume of the armaturecore t=Thick ness of lamination we∞t2 hence t should be kept as small as posible. Eddy current losses is reduced by laminated core but hysteresis losses can not be reduced by this way. Mechanical losses ( 10 to 20% of full load losses) 1.Friction losses 2.Windage losses Friction losses:Frictional losses due to bearings Windage losses:- Windage losses due to air gap between armature and pole shoe Stray losses(Rotational losses):magnetic losses and mechanical losses are collectively known as stray losses Losses are classified in to two types:i) Constant losses (standing losses)(Wc) --Field cu losses is constant --for shunt and compound generator are constant losses so, stray losses+ shunt cu losses are combined called “constant losses” ii) Variable losses:-The losses which varies with the load called “variable losses” -- Armature cu losses is know as “variable losses” -- In series generator shunt field cu losses also variable losses (IL=Ise=Ia) So, Total losses=Armature copper losses + WC =Ia2Ra+Wc=(I+Ish)2Ra+Wc Total losses=Variable losses+ Constant losses Efficiency of D.C Generator Efficiency of generator is defined as the ratio of output power to input power Efficiency (η) =output ×100 input input=output+ losses (or) output=input-losses For D.C generator input mechanical & output electrical Variation of η with load current