RGPV DIPLOMA EX305 UNIT II

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RGPV DIPLOMA EX305 UNIT II FOR III SEM DIPLOMA STUDENTS

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RGPV DIPLOMA EX305 UNIT II

  1. 1. 1 READING MATERIAL FOR III SEM DIPLOMA EX STUDENTS OF RGPV AFFILIATED COLLEGES SUBJECT ELECTRICAL MACHINES I Professor MD Dutt Addl General Manager (Retd) BHARAT HEAVY ELECTRICALS LIMITED Professor(Retd) in EX Department Bansal Institute of Science and Technology KOKTA ANANAD NAGAR BHOPAL Presently Head of The Department ( EX) Shri Ram College Of Technology Thuakheda BHOPAL Sub Code BE 305 Subject Electrical Machines I UNIT II D.C Generators
  2. 2. 2 RGPV DIPLOMA EX BRANCH SEMESTER: THIRD SCHEME: Jul.09 COURSE CODE: 305 PAPER CODE: 6232 NAME OF COURSE: ELECTRICAL MACHINE – I COMMON WITH PROGRAM (S): E01 Syllabus UNIT II D. C. Generator - Principle, construction, armature winding, types of winding, EMF equation, armature reaction and commutation, interpoles and compensating winding.Types of generators, characteristics and applications, losses and efficiency. Simple numerical. INDEX S No Topic Page 1 Principle 3 2 Construction details of DC machines 3,4,5,6 3 Types of winding, EMF equation 7,8 4 Armature reaction and commutation 9,10 5 Interpoles and compensating winding 11 6 Types of generators 12,13 7 Characteristics and application 14,15,16,17 8 Losses and efficiency 18,19,20,21 9 Simple numerical problems Separately 10 References 22
  3. 3. 3 Basic principle of operation of a D.C. Generator When the armature rotated in a magnetic field, an EMF force is generated in the armature conductors. Let us consider that. at speed N the armature is rotated in a magnetic field having field strength . The EMF generated is proportional to the field strength and rpm N . conductor are placed in a slot of armature and it is acted upon the magnetic field from the north pole of the motor. By applying R.H.S it is found that the conductor has tendency to generate EMF. Since the conductor is placed on the slot at circumference of rotor , The EMF generated is alternating in nature which converted to DC by the commutator action of commutator. Construction Details of DC Machines There are four main parts of a DC machine 1)Field Magnet
  4. 4. 4 2)Armature 3) Commutator 4)Brush and Brush gear Field System:- The purpose of field system is to create a uniform magnetic field within which armature rotates. It consists of following four parts. a) Yoke or frame b) Pole cores c) Pole Shoes d) Magnetizing coils Cylindrical yoke or frame is used which acts as frame and carries the magnetic flux produced by the poles. Poles are used to carry coils of insulated wires carrying the exciting current. The pole shoes acts as support to the coils and spread out flux over the armature periphery more uniformly. The magnetizing coils is to provide number of ampere turns of excitation required to give the proper flux through the armature to induce the desired potential difference. ARMATURE:- It is the rotating part of the DC machine and is built up in a cylindrical drum. The purpose of armature is to rotate the conductors in the uniform magnetic field , It consists of coils insulated wires wound around a iron core and is so arranged that the electric current are induced in these wires when
  5. 5. 5 armature is rotated in the magnetic field. The armature core is made from high permeability silicon sheet steel stampings. A small airgap exists so that armature can rotate freely without rubbing or touching poles. Armatures are LAP or WAVE wound. COMMUTATOR:- The commutator is a form of switch (rotating) placed between the armature and the external circuit so arranged that the input is fed (incase of motor) and the output is taken out (in case of generator) through commutator by brushes and brushgear. Two important functions it is doing in case of DC machine. 1) It connects the rotating armature conductors to the external circuit through brushes. 2) It converts the alternating alternating current induced in the armature conductors into unidirectional current to the external load circuit in generating action, where as it converts the alternating torque into unidirectional torque in motor action. The commutator is of cylindrical shape and is made up of wedge shaped hard drawn copper segments of mica. These segments are insulated from each other by thin sheet of mica . The segments are held together by two
  6. 6. 6 Vee rings that fits into the groove cut into the segments. Each armature coil is connected to the commutator segments through riser. BRUSHES :- The brushes usually made from carbon are pressed upon the commutator and from the connecting link between armature winding and external circuit. They are made from carbon because is conducting material and at the same time in powdered form provides lubrication effect on the commutator surface. The brushes are held in brush holders and brushgear on commutator. END HOUSINGS:- End housings are attached to the ends of the main frame and supports bearings. The commutator side supports brushgear assembly. Where as NCE side supports only bearing.
  7. 7. 7 BEARINGS :- The ball and roller bearings are fitted in the end housings. The function of bearing is to reduce the friction between the rotating part and stationary part. SHAFT :- The shaft is made of steel having maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, cooling fan etc are keyed to the shaft Armature coils can be connected to the riser of commutator to form either LAP or WAVE winding WAVE WINDING The ends of each armature coils are connected to commutator Segment some distance apart, so that only two parallel paths are provided between the positive and negative brushes. Thus wave wound machines have A=2, They are used for High voltage low current machines
  8. 8. 8 LAP WINDING The ends of each armature coil is connected to adjacent segments on the commutator so that total number of parallel path is equal to the total number pf poles, Thus for LAP A=P The Lap winding is used for low voltage high current machines. Generated E.M.F. or E.M.F. Equation of a Generator Eg = PNZΦΦΦΦ/60a Where: Φ = flux/pole in weber Z = total number of armature conductors 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. ARMATURE REACTION All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a DC generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called ARMATURE REACTION
  9. 9. 9 Armature reaction can be better understood with this figure:- UNDESIRABLE EFFECTS OF ARMATURE REACTION 1. Armature reaction causes a net reduction in the field flux per pole. Due to this net flux decrease, induced armature e.m.f. decreases and also the torque decreases. 2. Distortion of the main field flux along the air gap i.e. MNA axis shifted. Due to this there is a problem of commutation which results in copper losses, iron losses, sparking etc. COMMUTATION
  10. 10. 10 In the armature conductors of a d c generator are alternating in nature . The commutations process involves the change from a generated alternating current to an externally applied direct current . These induced current flow in one direction when the armature conductor are under north pole . They are in opposite direction when they are under south pole . As conductor pass out of the influence of north pole and enter the south pole , the current in them is reversed . The reversible of current takes place along the MNA or brush axis . When ever a brush spans two commutater segments , the winding element connect to those segments
  11. 11. 11 is short circuited . By commutation we mean the change that takes in a winding element during the period of short circuit by a brush. These changes are shown in figure. In position (a)The current I flowing towards the brush from L.H.S passes round the coil in a clockwise direction In position (b), this coil carries the same current in the same direction, but the coil is to short circuited by brush In the position (c) the brush makes contact with bars a and b, thereby short circuiting coil 1.The current is still I from L.H.S and I from R.H.S. It is seen that these two currents can reach the brush without passing through coil 1 In (d) shows that bar b has just left the brush and the short circuit of coil1 has ended. It is now necessary for the current I reaching the brush from the R.H.S in the anticlockwise direction. It is seen from above that during the period of short circuit of an armature coil by a brush the current in that coil must be reversed and also brought up to its full value in the reversed direction. The time of short circuit is known as period of commutation. METHODS OF IMPROVING COMMUTATION There are three methods for getting sparkles commutation. 1. Resistance commutation 2. Voltage commutation 3. Compensating winding Commutating or Inter poles Interpoles are narrow poles attached to the stator yoke, and placed exactly midway between the main poles. The compole windings are connected in
  12. 12. 12 series with the armature, because the interpoles must produce fluxes that are directly proportional to the armature current The armature and interpole mmf’s are affected simultaneously by the same armature current. Consequently the armature flux which tends to shift the MNA is neutralized. The interpoles must induce a voltage in the conductors undergoing commutation that is opposite to the voltage caused by the neutral plane shift and reactance voltage. i)For a generator, the polarity of the interpole must be the same as that of the next main pole further ahead in the direction of rotation. ii) For a motor, the polarity of the interpole must be the opposite as that of the next main pole in the direction of rotation. COMPENSATING WINDINGS Compensating windings are the most effective way for eliminating the problem of armature reaction and flash over by balancing armature mmf. The compensating windings are placed in the pole faces parallel to the rotor armature conductors. These windings are connected in series with the
  13. 13. 13 armature windings. The compensating winding produces an mmf that is equal and opposite to the armature mmf. The major draw back with compensating winding is that they are very costly. There use can be justified for the following special cases a) In large machines subject to heavy overloads or plugging b) In small motors subject to sudden reversal and high acceleration. Separately excitated DC machine. Ans:- There are two type of excitation , namely separately excited and self excited machine. he seethe field coils are energized by a separate DC source. The connections showing the separately excited d.c. machine is shown here below. Separately-excited generators are those whose field magnets are energised from an independent external source of d.c. current. Series wound DC Generator? Self-excited generators are those whose field magnets are energised by the current produced
  14. 14. 14 by the generators themselves. Due to residual magnetism, there is always present some flux in the poles. When the armature is rotated, some e.m.f. and hence some induced current is produced which is partly or fully passed through the field coils thereby strengthening the residual pole flux. Three types of self-excited DC Motors or Generators are there Shunt wound The field windings are connected across or in parallel with the armature conductors and have the full voltage of the generator applied across them Series Wound the field windings are joined in series with the armature conductors Compound wound DC Machines
  15. 15. 15 Compound Wound It is a combination of a few series and a few shunt windings and can be either short-shunt or long-shunt In general, three characteristics specify the steady-state performance of a DC generators: 1. Open-circuit characteristics: generated voltage versus field current at constant speed. This is also called magnetizing characteristics 2. Internal characteristic: It is plot between generated voltage versus load current
  16. 16. 16 3. External characteristic or Load characteristic: terminal voltage load current at constant speed. The terminal voltage of a dc generator is given by Open-circuit and load characteristics separately excited DC generator It can be seen from the external characteristics that the terminal voltage falls slightly as the load current increases. Voltage regulation is defined as the percentage change in terminal voltage when full load is removed, so that from the external characteristics, ( )[ ] aa mf aaat RI dropreactionArmatureIf RIEV − −= −= ω,
  17. 17. 17 EXTERNAL CHRACTERISTICS Self-Excited DC Shunt Generator Maximum permissible value of the field resistance if the terminal voltage has to build up. OPEN CIRCUIT CHARACTERISTICS 100× − = t ta V VE regulationVoltage
  18. 18. 18 Basic performance of DC Generator what are main characteristics Since the field current If in a shunt generator is very small the voltage drop IfRa can be neglected and V=Ea V=If Rf is a straight line and is called resistance line the figure gives below the voltage build up in DC shunt generator for various field circuit resistance. A decrease in resistance of the field circuit reduces the slope of the field resistance line resulting in higher voltage. If the speed is constant An increase in the resistance of the field circuit increases the slope of the field resistance line, resulting lower voltage If the field resistance is increased to Rc which is termed as the critical resistance of the field, the field resistance line becomes tangent to the initial part of the magnetizing curve, when the field resistance is higher than this value, the generator fails to excite . The figure above shows the variation of no load voltage with fixed Rf and variable speed of the armature. The magnetizing curve varies with the speed and its ordinate for any field current is proportional to the speed of the generator. The following conditions must be satisfied for voltage build up 1. There must be sufficient residual flux in the field poles.
  19. 19. 19 2. The field terminals should be such connected that the field current increases the flux in direction of residual flux 3. The field circuit resistance should be less than the critical field resistance. Characteristics of COPOUND DC GENERATOR Depending upon the number of series field turns, the cumulatively compound generators may be over compounded, flat compounded and under compounded. For over compounded generator the full load terminal voltage is higher than No load voltage, for flat compounded generator the full load terminal voltage is equal to the no load voltage. In an under compounded generator the full load terminal voltage is less than the no load voltage. LOSSES IN A DC MACHINES and efficiency Following are the losses in the DC machines
  20. 20. 20 1) Electrical or copper loss ( I²R Losses) 2) Core losses or Iron losses 3) Brush Losses 4) Mechanical losses 5) Stray load losses ELECTRICAL LOSSES:- Windings having resistance consumes certain losses, these are termed as copper losses because mostly windings are made of copper. i) Armature copper loss Ia²Ra ( Ia is armature current) ii) Shunt field copper loss Ish²Rsh iii) Copper loss in the series field Ise²Rse iv) Copper loss in the interpole winding which are in series with armature Ia²Ri v) Compound machines both series field and shunt field copper losses are also there vi) Copper losses are there in the compensating winding CORE LOSES:- The core losses are the hysteresis losses and Eddy current losses. Since the machine usually operates at constant flux density and speed, these losses are almost constant. These losses are about 20% of Full load losses. BRUSH LOSES:- There is a power loss at the brush contact with commutator and the carbon brushes. This loss can me measured by the voltage drop at the brush contact and armature current.
  21. 21. 21 Pbd = Vbd Ia The voltage drop is more or less remains constant over a wide range of Ia and it is assumed 2V ( approx) MECHANICAL LOSSES:- The losses associated with mechanical effect are called mechanical losses. These consists of friction losses at bearing and windage losses ( fan losses) . the fans are used to take away the heat produced due to I²R losses and iron losses inside the machine. STRAY LOAD LOSSES:- These are miscellaneous losses which are due to the following reasons:- 1) Distortion of flux due to armature reaction 2) Short circuit currents in the coils due to commutation. These losses are difficult to find out, However they are taken as 1 % of full load power output. EFFICIENCY
  22. 22. 22 Let us assume that the R = Total resistance I = Output current Ish = Current through the shunt field Ia armature current I +Ish V is the terminal voltage Power loss in the shunt field = V Ish InputPower Losses InputPower LossesInputPower InputPower OutputPower −−−−==== −−−− ==== ====ηηηη 1
  23. 23. 23 Mechanical Losses = Friction losses at bearings+ friction losses at commutator + windage losses Stray losses Core losses mechanical losses and shunt field copper losses are considered as combined fixed losses. ή = Output / Input = VI / ( VI + Ia²Rat +Pk =VbdIa Ia = I + Ish Since Ish compare to I is very small we can consider Ia ≡ I ή = VI/ ( VI+ I²Rat +Vbd I +Pk) LOAD for Maximum efficiency Ifl = Full load current at maximum efficiency Im = current at maximum efficiency Im ² Rat = Pk
  24. 24. 24 Im ² = Pk/ Rat Current at Maximum ή = F.L Current X( Pk/F.L Copper loss) ² 1. Basic electrical and Electronics Engineering By Pankaj Swarnakar and Shiv Shankar Mishra Tech India publication 2. Electrical & Electronics Engineering By RK Chaturvedi and SK Sahdev Dhanpatrai Publication. 3. Electrical & Electronics Engineering By JB Gupta KATSON Books 4. Basic Electrical Engineering by Vincent Deltoro 5. Basic Electrical Engineering by De an Sen TMH Publication 6.

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