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D.C. GENERATOR Instructor: Dr. ABDAL-RAZAK SH. Materials covered from Chapter 2: Direct Current Generators Working principle of a generator Practical DC generators Energy conversion in a DC generator Armature reaction in a DC generator Classification of DC generators Equivalent circuit of DC generators No-load characteristics of DC generators Load-characteristics and voltage regulation of DC generators DC machines Dc machines are either motors or generators. DC motors applications DC motor applications Generating an AC Voltage Fig. shows an elementary ac generator composed of a coil that revolves at a constant rpm between the N, S poles of a permanent magnet The rotation is due to an external driving force, such as a motor, called prime mover The coil is connected to two slip rings mounted on the shaft The slip rings are connected to an external electric load by means of two stationary brushes x and y As the coil rotates, a voltage is induced according Electromagnetic principle This voltage appears across the load via slip ring and brush The amount of flux cut by the conductors depends on their position in between the poles The polarity voltage across the load changes as the conductors move from N pole to S pole Direct-current Generator How can we get constant polarity voltage? In order to get a constant polarity voltage, a commutator is used A commutator in its simplest form is composed of a slip ring that is cut in half, with each segment insulated from the other as well as from the shaft Now the voltage across the load is still fluctuating but always has same polarity Practical DC Generators In a practical generator, there are a number of conductors This conductors are housed in the slot of the armature The terminals of the conductor are connected to the terminals of other conductors such that the connected conductors create an winding in the armature. Sometime armature conductors are also called armature coils Usually there are two ways to connect the terminals – – One is called lap winding The other is wave winding The front terminals of the conductors are connected to the commutator’s segment Elements of Armature Winding Elements of an armature windings in DC Machines A turn – two conductors connected to an end by an end connector A coil – several turns connected in series A winding – several coils connected in series The angle between centers of adjacent poles is 180o (electrical) 180o electrical = 90o mech 360o electrical = 180o mech N S S N The angle between centers of adjacent poles is 180o (electrical) If coil sides are placed 180o electrical apart, the coil is said to be full-pitch N a a 180oelec S b N S b The most common ways of connecting coils for armature windings: Lap winding Wave winding Ends of the coils are connected to the commutator bars In DC machines most of the coils are full-pitch. Commutator pitch(Yc):Is the number of commutator segment spanned by each coil Of the winding is denoted by (Yc) Pole-pitch: Is the distance measured in terms of number of armature slots (or armature conductors) per pole 𝑁𝑜.𝑜𝑓 𝑠𝑙𝑜𝑡𝑠 𝑁𝑜.𝑜𝑓 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑜𝑟𝑠 Pole pitch=𝑁𝑜.𝑜𝑓 𝑝𝑜𝑙𝑒𝑠 or Pole pitch= 𝑁𝑜.𝑜𝑓 𝑝𝑜𝑙𝑒𝑠 Coil span or coil pitch(Ys): Is the distance measured in terms of number of armature slots (or armature conductors) spanned by a coil. FUTHER ARMATYRE WINDING TERMINOLOGY BACK PITCH(YB): Is the distance measured in in terms of armature conductors between two sides of a coil at the back of the armature. FRONT PITCH(YF): Is the distance measured in in terms of armature conductors between two sides attached to any one commutator segment. RESULTANT PITCH(YR): Is the distance measured in terms of armature conductors between the beginning of one coil and the beginning of the next coil to which it is connected. Progressive winding: YB >YF ; YC = +1 Retrogressive winding: YF >YB ; YC = -1 RELATIONS BETWEEN PITCHES FOR SIMPLEXLAP WINDING 1)𝑌𝐵 = 𝑌𝐹 ± 2 𝑌𝐵 = 𝑌𝐹 + 2 𝑌𝐵 = 𝑌𝐹 − 2 𝑓𝑜𝑟 𝑝𝑟𝑜𝑔𝑟𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑓𝑜𝑟 𝑟𝑒𝑡𝑟𝑜𝑝𝑟𝑜𝑔𝑟𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 2) Both YB and YF should be nearly equal to pole pitch. 3)Average pitch = 𝑌𝐵 +𝑌𝐹 2 . It equal to pole pitch(=Z/P) 4) Commutator pitch, YC = ±1 5) The resultant pitch (YR) is even being the arithmetical difference of two odd number 6) Pole pitch = Z/P YB = 𝑍 +1, 𝑃 YF= 𝑍 -1 𝑃 for progressive winding FURTHER ARMATYRE WINDING TERMINOLOGY Lap winding Commutator bar • One coil between adjacent commutator bars • 1/p of total coils are connected in series • No. of poles no. of brushes no. of parallel paths Wave winding • • • • p/2 coils in series between adjacent commutator bars ½ of all coils between brushes Regardless of no. of poles, there are always 2 parallel path The distance between end coils (commutator pitch) is 2(NC1)/p where NC is the no. of commutator bars SIMPLEX WAVE WINDING 1) Both pitches YB and YF are odd and are of the same sign 2) Average 3) YC=YA= 𝑍±2 pitch, YA= 𝑃 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑚𝑚𝑢𝑡𝑎𝑡𝑜𝑟 𝑠𝑒𝑔𝑚𝑒𝑛𝑡±1 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟 𝑜𝑓 𝑝𝑜𝑙𝑒𝑠 Value of the Induced Voltage: The voltage induced in a dc generator having a lap winding is given by the equation – – – – – Eg=ZnΦ/60 Volt Where Eg=voltage between the brushes [V] Z=Total number of conductors on the armature Φ=Flux per pole [Wb] Generator Under Load: The Energy conversion Process When a load (e.g., a resistor) is connected between twoterminals of a DC generator, some fundamental flux and current relationships take place The current delivered to the load also flows through all the armature conductors. The current in the conductors under N pole will have opposite direction of the current in the conductors under S pole Since these current carrying conductors are in a magnetic field, there will be force on conductors according to Lorentz’s law. The individual forces F on different conductors all act clockwise In effect they produce torque that acts opposite to the direction in which the generator is driven by a driver e.g., a turbine. Therefore, the prime mover or driver tends to slow down and need more torque to keep the generator going on in order to overcome this opposing torque. This is conversion process helps to convert mechanical energy to electrical energy Excitation and Classification of DC Generators Permanent Magnet Generator-PM is used to create magnetic fieldDifficult to have very strong PM and magnetic strength decays with time, usually used for low power and high efficient Gen. Excited Generator-Instead of PM, field coil is used to create the magnetic field – Self Excited-Current in the field coil comes from the generator itself – Shunt-The field coil is connected in parallel with the armature Series-The field coil is connected in series with the armature Compound-It has two field coils; one is connected in series with armature and the other is connected in parallel with the armature Differential Compound-In this case the series and shunt fields are connected as such the magnetic flux from these field coils cancel each other. Separately excited-Current in the field coil come from different source other than the generator itself Equivalent Circuits of DC Generators: A voltage source with a value of E0 which is determined by the rotational speed, number of conductors and flux per pole. Armature conductors or coils, commutators and Carbon brush are made of finite resistive element, there is also losses in these elements. Therefore, armature and its conductors are represented by a resistor Ra The magnetic field for excited generator is obtained from a field coil. The coil has a resistance, Rf Do not confused with winding and inductance in case of DC machines! Equivalent Circuits of DC Generators: Series Generator Shunt Generator Text Book has used Voltage source to show generator. Compound Generator LOSSES IN A D.C. MACHINE 1- COPPERLOSSES 2- IRON LOSSES ARMATURE Cu LOSS SHUNT FIELD Cu LOSS SERIES FIELD Cu LOSS HYSTERESIS LOSS EDDY CURRENT LOSS 3- MECHANICAL LOSSES FRICTION WINDAGE Hysteresis Loss Energy is spent when current changes its polarity every half cycle. Power loss due to Hysteresis is given by the equation: Ph= 𝐾ℎ V f 𝐵𝑛 𝑚𝑎𝑥 watts kh - is a constant relating to the loop area (Steinmetz coefficient) f - Frequency of magnetic reversals in Hz (NP/120) n = 1.5 to 2.5 depending on the material Bmax= Maximum flux density V = volume of the material. Eddy Current Loss Again, is a loss in the magnetic core, that is in the iron structure, caused by the induced voltages in the iron core. Iron cores are laminated to reduce this loss. Laminated cores offer a much higher resistance for the flow of eddy currents in the core. Pe= 𝑘𝑒(𝐵𝑚𝑎𝑥. 𝑡𝑓)2 *V watts ke-constant that depends on the material( electrical resistance) t -thickness of laminations. CONSTANT AND VARIABLE LOSSES 1- COSTANT LOSSES a- iron losses b- mechanical losses c- shunt field loss 2- VARIABLE LOSSES a- copper loss in armature winding b- copper loss in series field winding POWER STAGES CONDITION FOR MAXIMUM EFFICIENCY Armature Reaction In a loaded generator, the current flowing in the armature coils also creates a magnetic field or flux which interacts with the poles’ magnetic field. In fact the poles’ fields are distorted and weakened by the armature flux This m.m.f. in the armature is called as armature reaction End effects – – Sparking in the brush Resultant poles’ flux reduces and voltage generated by the generator decrease! Armature Reaction -- Generator Neutral Plane shifts in the direction of rotation 36 DEMAGNETISING AND CROSSMAGNETISING CONDUCTOR CALCULATING OF DEMAGNETISNG AMPERE-TURNS PERPOLE(ATd/POLE) Let Z=total number of armature conductors I=current in each armature conductor = Ia/2……..for simplex wave winding = Ia/p…… for simplex lap winding θm= forward lead in mechanical degrees Total demagnetising armature conductors= conductor in angles AOC and BOD 4𝜃𝑚 ×Z 360 1 4𝜃𝑚 𝑡𝑢𝑟𝑛𝑠 = 2 360 = ∴ 𝑇𝑜𝑡𝑎𝑙 𝑑𝑒𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 − × Z × 𝐼= These demagnetising ampere turns are due to pair of poles ∴ 𝑑𝑒𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 − 𝑡𝑢𝑟𝑛𝑠/𝑝𝑜𝑙𝑒 = i.e., ATd/pole = 𝜃𝑚 360 ×𝑍𝐼 𝜃𝑚 × 360 𝑍𝐼 2𝜃𝑚 360 ×𝑍𝐼 CROSS-MAGNETISING AMPERE-TURNS PER POLE(ATC/POLE) Total armature reaction ampere-turns per pole = 𝑍/2 𝑃 ×𝐼 = Demagnetising ampere-turns per pole is given by ; ATd/pole = 𝜃𝑚 360 ×𝑍𝐼 ∴ 𝐶𝑟𝑜𝑠𝑠 − 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑠𝑖𝑛𝑔 𝑎𝑚𝑝𝑒𝑟𝑒 − 𝑡𝑢𝑟𝑛𝑠/𝑝𝑜𝑙𝑒 are ATc/pole = 𝑍 2𝑃 × =𝑍𝐼 𝜃𝑚 𝐼×𝑍 360 1 𝜃𝑚 − 2𝑃 360 𝐼 𝑍 2𝑃 ×𝐼 COMPENSATING WINDING Zc = No. of compensating conductors/pole face Za = No. of active armature conductors Ia = total armature current Ia/A = current in each armature conductor Zc Ia = Za Ia/A Zc = Za/A AT/pole for COMPENSATING WINDING No. of armature conductors/pole = Z/P No. of armature turns/pole = Z/2P No. of armature turns under pole face = 𝑍 2𝑃 AT/pole required for compensating winding = 𝑍𝐼 2𝑃 = Armature AT/pole× × × 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒 𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒 𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ 𝑝𝑜𝑙𝑒 𝑓𝑎𝑐𝑒 𝑝𝑜𝑙𝑒 𝑝𝑖𝑡𝑐ℎ Effect of Armature Inductance on Commutation – No Load Consider Coil #2 Induced Voltage changes from clockwise to counter-clockwise as the coil passes from under the South pole, through the neutral plane, to under the North pole 43 Coil #2 Under the South Pole Clockwise voltage induced in coils under the South pole Counter-clockwise voltage induced in coils under the North pole 44 Coil #2 in the Neutral Plane Coil #2 is shorted out by the brush No voltage is induced 45 Coil #2 Under the North Pole Counter-clockwise voltage induced in the coil 46 Under “Loaded” Conditions Coils #2 and #3 supply current to the load through the commutator bar #3 and the brush to LOAD 47 Coil #2 in the Neutral Plane Current in Coil #2 wants to go to zero, but cannot change instantaneously! 48 Coil #2 Under the North Pole As soon as commutator bar #3 slides off the brush, the current in coil #2 is forced to zero A large emf is induced between commutator bar #3 and the brush, resulting in an “arc” 49 Ideal Commutation For Ideal Commutation, need the coil current in phase with the coil voltage (no delay) 50 METHODS OF IMPROVING COMMUTATION Resistance commutation E.M.F. commutation By brush shifting By using interpoles or compoles Interpoles – Generator Action Install narrow poles in the Neutral plane Only the coil undergoing commutation is affected A neutralizing voltage forces the reversal of current in each coil as it moves through the region 52 No-load operation and saturation curve: Voltage building up in a self excited DC shunt generator • • • • • Initially no voltage, no currents in the field However, there is a residual flux in the field-very important ! This generates a voltage, this voltage causes current flow in the field coil If the flux due to the current in the field is in the same direction, the total field flux increases-very important! The induced voltage then increases How much voltage will build up? Infinite amount Determined by the field resistance as shown the graph Controlling Voltage of Shunt Generator Recall E0=ZnΦ/60 Volt In order to change E0, we can change – – – No. of conductors, Z, it is not possible rotational speed, n which may not be feasible flux per pole. This is usually used in practice. A rheostat is connected in series with the field coil. If the value of the rheostat is increased, field current decreases and consequently the induced voltage E0 decreases. Voltage Regulation of a DC Shunt Generator: Terminal voltage of a self excited shunt generator decreases as the load current increases due to armature reaction and armature resistance drop. This is called as the voltage regulation. This is bad for most electrical loads. In a gold mine, you are driving motor and bulb from a generator. Voltage regulation is defined as (noload voltage-full load voltage)/full load voltage Compound Generators: In order to prevent compound generator is developed to prevent voltage regulation – – – – The series field’s flux add with shunt field flux If the load current increases, the shunt field current decreases and thus shunt field flux But the series field current increases and thus the series field flux Resultant flux remains almost same if number of turn in series coil is selected properly to compensate for voltage due to armature resistance drop Overcompound Generator-If the series coils has comparatively higher number of turns Differential compound generators-Series field’s flux works in opposite direction of shunt field flux. It is good for limiting short circuit current as terminal voltage falls very quickly in case of short circuit. Welding applications Load Characteristics of DC generators For differential compound terminal voltage falls abruptly For shunt terminal voltage falls quickly For separate excitation it falls but as not sharp as the shunt one For compound, it falls slowly, better voltage regulation and good for constant voltage application For over compounded, it increases Generator Specifications Generator Spec. – – – – – – – Power (how much it can deliver) Voltage (terminal voltage) Exciting current (shunt field current) Temperature rise Speed Type (series, shunt, compound..) Class (for insulation)