1 / 112

Chapter one

Chapter one. DC Machines. Chapter Outline. Introduction Induced voltage Electromagnetic force, f Simple loop generator The voltage induced in a rotating loop The induced torque in the rotating loop Practical generator Armature winding Problems with commutation in real machines

esme
Télécharger la présentation

Chapter one

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter one DC Machines Electrical Machines Getu.G:getugaa@yahoo.com

  2. Chapter Outline • Introduction • Induced voltage • Electromagnetic force, f • Simple loop generator • The voltage induced in a rotating loop • The induced torque in the rotating loop • Practical generator • Armature winding • Problems with commutation in real machines • Solutions to the problems with commutation • The internal generated voltage and induced torque equations of real dc machines • Power flow and losses in dc machines Electrical Machines Getu.G:getugaa@yahoo.com

  3. Introduction • Electric machines convert electrical energy(power) to mechanical energy(power) or vice versa. • This process of conversion is known as electromechanical energy conversion. • An electric machine is therefore a link between an electrical system and a mechanical system. • In these machines the conversion is reversible. • If the conversion is from mechanical to electrical energy, the machine is said to act as a generator. • If the conversion is from electrical to mechanical energy, the machine is said to act as a motor. Electrical Machines Getu.G:getugaa@yahoo.com

  4. Introduction cont… • Conversion of energy from electrical to mechanical form or vice versa results from the following • When a conductor moves in a magnetic field, voltage is induced in the conductor. (Generator action) • When a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. (Motor action) Electrical Machines Getu.G:getugaa@yahoo.com

  5. Introduction cont… • These two effects occur simultaneously whenever energy conversion takes place from electrical to mechanical or vice versa. • In motoring action • the electrical system makes current flow through conductors that are placed in the magnetic field. • A force is produced on each conductor. • If the conductors are placed on a structure free to rotate, an electromagnetic torque will be produced, tending to make the rotating structure rotate at some speed. • If the conductors rotate in a magnetic field, a voltage will also be induced in each conductor. Electrical Machines Getu.G:getugaa@yahoo.com

  6. Introduction cont… • In generating action, • the process is reversed. • The rotating structure, the rotor, is driven by a prime mover (such as a steam turbine or a diesel engine). • A voltage will be induced in the conductors that are rotating with the rotor. • If an electrical load is connected to the winding formed by these conductors, a current i will flow, delivering electrical power to the load. • Moreover, the current flowing through the conductor will interact with the magnetic field to produce a reaction torque, which will tend to oppose the torque applied by the prime mover. • Note: In both motor and generator actions, the coupling magnetic field is involved in producing a torque and an induced voltage. Electrical Machines Getu.G:getugaa@yahoo.com

  7. Introduction cont… • DC machines are generators that convert mechanical energy to DC electric energy and motors that convert DC electric energy to mechanical energy. • Most DC machines are like AC machines in that they have AC voltages and currents within them • DC machines have a DC output only because a mechanism exists that converts the internal ac voltages to dc voltages at their terminals. Since this mechanism is called a commutator, dc machinery is also known as commutating machinery. Electrical Machines Getu.G:getugaa@yahoo.com

  8. Induced voltage • Voltage induced in a conductor moving at speed v in a magnetic field ,B is given as. eind = ( v x B) • I Where v is speed B is magnetic field I is length If all the three are perpendicular to each other as in this figure, eind = B lv Electrical Machines Getu.G:getugaa@yahoo.com

  9. Electromagnetic force, f • For the current-carrying conductor shown in Fig.3.3(a), the force (known as Lorentz force) produced on the conductor can be determined from the following equation: F = i(lxB) Where i is current B is magnetic field I is length If B is perpendicular to l as in this figure, f = Bli Electrical Machines Getu.G:getugaa@yahoo.com

  10. Simple loop generator Fig 1.1 Electrical Machines Getu.G:getugaa@yahoo.com

  11. Simple loop generator Fig 1.2 Electrical Machines Getu.G:getugaa@yahoo.com

  12. Simple loop generator • In the figure (fig 1.1 and 1.2)is shown a single turn rectangular copper coil ABCD moving about its own axis, a magnetic field provided by either permanent magnets or electromagnets. • The two ends of the coil are joined to two slip-rings or discs a and b which are insulated from each other and from the central shaft. • Two collecting brushes (of carbon or copper) (1 and 2)press against the slip rings. • Their function is to collect the current induced in the coil and to convey it to the external load resistance R. Electrical Machines Getu.G:getugaa@yahoo.com

  13. Working Theory • Imagine the coil to be rotating in clockwise direction • As the coil assumes successive positions in the field, the flux linked with it changes. • Hence, an EMF is induced in it which is proportional to the rate of change of flux linkages. • When the coil plane is perpendicular to the lines of the flux(position 1),flux linked with the coil is maximum but the rate of change of flux linkage is minimum. Electrical Machines Getu.G:getugaa@yahoo.com

  14. Working Theory cont.. Fig 1.2 • At this position 1, • the coil sides AB and CD run parallel to the line of the flux(slide along). • The induces emf=NdΦ/dt=0 • Take this as start position(angle of rotation θ=00) Electrical Machines Getu.G:getugaa@yahoo.com

  15. Working Theory cont.. • Until position 3,dΦ/dt increases. • At position 3: • Θ=900 • The coil is horizontal(parallel to flux lines) • AB and CD run perpendicular to the line of the flux • The flux linked with the coil is minimum • But rate of change of flux linkage is maximum • So,maximumemf is induced Electrical Machines Getu.G:getugaa@yahoo.com

  16. Working Theory cont.. • From 900 to 1800,the flux linked with the loop gradually increases but dΦ/dt decreases • So the emf decreases gradually and is zero at position 5(1800). • During this half cycle, • The emf is from A to B and C to D. • Direction of current ABMLCD • Current through R is M to L • Half revolution of the coil completes Electrical Machines Getu.G:getugaa@yahoo.com

  17. Working Theory cont.. • In the next half revolution(i,e 1800 to 3600) • The variations in emf are similar to those in first half • Maximum in position 7 • Minimum in position 1 • But the direction of induced current is reversed (D to C and B to A) • Path of current DCLMBA (reversed) • The current through R is L to M(reversed) • i.e The current through R is AC Electrical Machines Getu.G:getugaa@yahoo.com

  18. Commutation Fig 1.3 Electrical Machines Getu.G:getugaa@yahoo.com

  19. Commutation Fig 1.4 Electrical Machines Getu.G:getugaa@yahoo.com

  20. Commutation • For making the flow of current unidirectional in the external circuit, the slip rings are replaced by split rings which shown in Fig 1.3 and 1.4. • The split rings are made out of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulating material. • As before, the coil ends are joined to these segments on which rest the carbon or copper brushes. • In the practical generator which has more than two poles and more than one coil the split rings has not just two halves but has many parts as shown in Fig1.4 (b). Electrical Machines Getu.G:getugaa@yahoo.com

  21. Commutation • It is seen (Fig13.(a) to Fig1.3 (b)) that in the first half revolution current flows along ABXYCD i.e. the brush No. 1 in contact with segment a acts as the positive end of the supply and b as the negative end. Electrical Machines Getu.G:getugaa@yahoo.com

  22. Commutation • In the next half revolution (fig 1.5), the direction of the induced current in the coil has reversed. • But at the same time, the positions of segments a and b have also reversed with the result that brush No. 1 comes in touch with that segment which is positive i.e segment b in this case. • Hence, the current in the load resistance again flows from X to Y. Electrical Machines Getu.G:getugaa@yahoo.com

  23. Commutation • The waveform of the current through the external circuit is as shown in Fig1.6. • This current is unidirectional but not continuous like pure direct current. • It should be noted that the position of brushes is so arranged that the changeover of segments a and b from one brush to the other takes place when the plane of the rotating coil is at right angles to the plane of the flux lines. • It is so because in that position, the induced EMF in the coil is zero. Electrical Machines Getu.G:getugaa@yahoo.com

  24. Commutation Fig 1.5 Electrical Machines Getu.G:getugaa@yahoo.com

  25. Voltage o/p of simple loop gen Fig 1.6 Electrical Machines Getu.G:getugaa@yahoo.com

  26. The voltage induced in a rotating loop Fig 1.7 Electrical Machines Getu.G:getugaa@yahoo.com

  27. The voltage induced in a rotating loop • If the rotor of this machine is rotated, a voltage will be induced in the wire loop(fig 1.7). • The voltage on each segment isgiven by Equation eind = ( v x B) • I • Segment BA:eind = vBl under the pole eind = 0 beyond the pole adges • Segment BC and DA :eind = 0 under the pole eind = 0 beyond the pole adges • Segment CD:eind = vBl under the pole eind = 0 beyond the pole adges Electrical Machines Getu.G:getugaa@yahoo.com

  28. The voltage induced cont… • The total induced voltage on the loop eind is given by • When the loop rotates through 180°, segment ab is under the north pole face instead of the south pole face. • At that time(180°), the direction of the voltage on the segment reverses, but its magnitude remains constant. • The resulting voltage etot is shown as a function of time in Figure 8- 3. Electrical Machines Getu.G:getugaa@yahoo.com

  29. The voltage induced cont… Fig 1.8 Electrical Machines Getu.G:getugaa@yahoo.com

  30. The voltage induced cont… • Notice that the tangential velocity v of the edges of the loop can be expressed as v = rω • where r is the radius from axis of rotation out to the edge of the loop and w is the angular velocity of the loop. Electrical Machines Getu.G:getugaa@yahoo.com

  31. The voltage induced cont… • Notice also from Figure 8-4 that the rotor surface is a cylinder, so the area of the rotor surface A is just equal to 2πrl. • Since there are two poles, the area of the rotor under each pole (ignoring the small gaps between poles) is Ap = πrl. • Therefore, Electrical Machines Getu.G:getugaa@yahoo.com

  32. The voltage induced cont… • Since the flux density B is constant everywhere in the air gap under the pole faces, the total flux under each pole is just the area of the pole times its flux density: • Therefore, the final form of the voltage equation is Electrical Machines Getu.G:getugaa@yahoo.com

  33. The voltage induced cont… • Thus, the voltage generated in the machine is equal to the product of the flux inside the machine and the speed of rotation of the machine, multiplied by a constant representing the mechanical construction of the machine. • In general, the voltage in any real machine will depend on the same three factors: • The flux in the machine • The speed of rotation • A constant representing the construction of the machine Electrical Machines Getu.G:getugaa@yahoo.com

  34. DC voltage of single loop… • The induced voltage is converted in to DC in the external circuit by the commutator (split ring) and brushes Fig 1.9 Electrical Machines Getu.G:getugaa@yahoo.com

  35. DC voltage of single loop… Fig 1.10 Electrical Machines Getu.G:getugaa@yahoo.com

  36. The Induced Torque in the Rotating Loop Fig 1.11 Electrical Machines Getu.G:getugaa@yahoo.com

  37. The Induced Torque in the Rotating Loop • Suppose a battery is now connected to the machine • The approach to take in determining the torque on the loop is to look at one segment of the loop at a time and then sum the effects of all the individual segments. • The force on a segment of the loop is given by Equation • The torque on the segment is given by • where θ is the angle between r and F. • The torque is essentially zero whenever the loop is beyond the pole edges. Electrical Machines Getu.G:getugaa@yahoo.com

  38. The Induced Torque cont… • While the loop is under the pole faces, the torque is • Segment BA: • Segment BC and DA Electrical Machines Getu.G:getugaa@yahoo.com

  39. The Induced Torque cont… • Segment AC and DA • The resulting total induced torque on the loop is given by Electrical Machines Getu.G:getugaa@yahoo.com

  40. The Induced Torque cont… • By using the facts that Ap=πrl and ϕ = ApB, the torque expression can be reduced to • Thus, the torque produced in the machine is the product of the flux in the • machine and the current in the machine, times some quantity representing the mechanical construction of the machine (the percentage of the rotor covered by pole faces). • In general, the torque in any real machine will depend on the same three factors: • The flux in the machine • The current in the machine • A constant representing the construction of the machine Electrical Machines Getu.G:getugaa@yahoo.com

  41. The Induced Torque cont… • Example 1-1. (next figure) The next figure shows a simple rotating loop between curved pole faces connected to a battery and a resistor through a switch. The resistor shown models the total resistance of the battery and the wire in the machine. The physical dimensions and characteristics of this machine are: r=0.5m,R=0.3Ω,l=1.0m,VB=120v(battery voltage),B=0.25T • What happens when the switch is closed? • What is the machine's maximum starting current? What is its steady-state angular velocity at no load? Electrical Machines Getu.G:getugaa@yahoo.com

  42. The Induced Torque cont… • Suppose a load is attached to the loop, and the resulting load torque is 10 N· m. What would the new steady-state speed be? How much power is supplied to the shaft of the machine? How much power is being supplied by the battery? Is this machine a motor or a generator? • Suppose the machine is again unloaded, and a torque of 7.5 N • m is applied to the shaft in the direction of rotation. What is the new steady-state speed? Is this machine now a motor or a generator? • Suppose the machine is running unloaded. What would the final steady-state speed of the rotor be if the flux density were reduced to 0.20 T? Electrical Machines Getu.G:getugaa@yahoo.com

  43. The Induced Torque cont… Fig 1.12 Electrical Machines Getu.G:getugaa@yahoo.com

  44. Practical generator • The simple loop generator has been considered in detail merely to bring out the basic principle underlying the construction and working of as actual generator illustrated in Fig.3.13 which consists of the following essential parts: • Magnetic Frame or Yoke • Pole-cores and Pole-shoes • Pole Coils or Field Coils • Armature Core • Armature Windings or • Commutator • Brushes and Bearings Of these, the yoke, the pole cores, the armature core and air gaps between the poles and the armature core form the magnetic circuit whereas the rest form the electrical circuit. Electrical Machines Getu.G:getugaa@yahoo.com

  45. Practical generator cont… Fig 1.13 Electrical Machines Getu.G:getugaa@yahoo.com

  46. Practical generator cont… • Yoke The outer frame or yoke serves • Provides mechanical support for the poles and acts as a protecting cover for the whole machine • Caries the magnetic flux produced by poles • Pole cores and pole shoes • The field magnets consist of pole cores and pole shoes. • The pole shoes serve two purposes • Spread out the flux in the air gap and being of larger crossection ,reduce the reluctance of the magnetic path • Support field coils • They are made of lamminated steel Electrical Machines Getu.G:getugaa@yahoo.com

  47. Practical generator cont… • Pole coils or field coils • Consist of copper wires are former wound for correct dimension • The former is removed and the wound coil is put in to place over the core • When current passes through these coils, electromagnetic poles formed to produce flux to be cut by the armature conductors Electrical Machines Getu.G:getugaa@yahoo.com

  48. Practical generator cont… • Armature core • Houses armature conductors(coils) and cause them to rotate and hence cut the magnetic flux produced by the poles. • Slots to place armature coils • Lamminated steel discs(to reduce eddy current loss) and perforated (for cooling) Electrical Machines Getu.G:getugaa@yahoo.com

  49. Practical generator cont… • Armature windings • Usually former wound • Then put in the armature slots lined with tough insulating material • The conductors of the coil insulated from each other Electrical Machines Getu.G:getugaa@yahoo.com

  50. Practical generator cont… • Commutator • The function of the commutator is to facilitate collection of current from the armature conductors i.e. 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 high conductivity hard-drawn or drop-forged Electrical Machines Getu.G:getugaa@yahoo.com

More Related