Dc machines electrical machines – i

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Explains diagrammatically about DC machines - just one of my lectures

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  • Dc machines electrical machines – i

    1. 1. Electrical Machines – I DC machines
    2. 2. General Idea Electrical Machine Electrical Energy Mechanical Energy Losses – I2R, friction, etc
    3. 3. Types of electrical machines Types of electrical machines Rotating type Stationary type Transformers Motors and generators AC motors and generators DC motors and Generators
    4. 4. Magnets and field Permanent Magnet Electromagnet The magnetic field can be controlled by regulating the amount of current flowing
    5. 5. Faraday’s Coil and Magnet Experiment - Deflection The amount of deflection is proportion al to the amount of voltage induced
    6. 6. Faraday’s Experiment – Effect of speed of movement of the magnet The amount of deflection is proportional to the amount of voltage induced (electromotiv e force)
    7. 7. Faraday’s Experiment – effect of number of turns of the coil The amount of deflection is proportion al to the amount of voltage induced
    8. 8. Faraday’s Law • Faraday’s law of electromagnetic induction: - – The induced electromotive force (EMF or voltage) in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit. For a coil of wire of N turns EMF is given as: - – The negative sign indicates that the direction of EMF is opposite to the source that produces it – is the number of magnetic field lines that a conductor intersects per unit time dt d Ne   dt d dt d e  
    9. 9. DC Generators
    10. 10. Lorentz’s Single conductor Experiment in a magnetic field – a generator point of view I Conductor N W S E F B F B I
    11. 11. Lorentz force law - generators •When a conductor is moved in a magnetic field, the electrons experience a force which causes current to flow in the conductor and this current is mutually perpendicular to the direction of the field and the force applied. [For Generators] •Where θ is the angle between the wire and the magnetic field. • q = charge • v = velocity of electrons •B = magnetic field strength F = qvBsinθ
    12. 12. Lorentz’s Single conductor Experiment in a magnetic field – a generator point of view I Rotational direction Conductor N W S E F B F B I
    13. 13. Flux distribution at different positions N S B C D A dφ/dt = 0 dφ/dt = 0 dφ/dt = max dφ/dt = max •dφ/dt = number of lines that the conductor intersects (passes through) per unit time •dφ/dt = 0 because the direction of conductor movement is parallel to the direction of flux
    14. 14. Single loop in a magnetic field – position A N S V a b c d N W S E R1 R2
    15. 15. Single loop in a magnetic field – position B N S V -ve +ve F I F I a bc d N W S E R1 R2
    16. 16. Single loop in a magnetic field – position C N S V a b c d N W S E R1 R2
    17. 17. Single loop in a magnetic field – position D N S V-ve +ve a cb d N W S E R1 R2 I F I F
    18. 18. Single loop in a magnetic field – position A N S V a b c d N W S E R1 R2
    19. 19. DC from AC AC waveform DC waveform
    20. 20. The Idea of commutation POSITION B POSITION D R1+ve R2-ve R1-ve R2+ve
    21. 21. Slip rings and Commutator segments Slip Rings Commutator Segments Left Brush Right Brush
    22. 22. Single loop in a magnetic field – position A N S V N W S E a b c d CA CB 1 2
    23. 23. Single loop in a magnetic field – position B N S V +ve -ve N W S E d bc a CB CA 1 2 I I F F
    24. 24. Single loop in a magnetic field – position C N S V N W S E d c b a1 2 CB CA
    25. 25. Single loop in a magnetic field – position D N S V +ve -ve N W S E a cb d 1 2 CA CB I I F F
    26. 26. Single loop in a magnetic field – position A N S V N W S E a b c d CA CB 1 2
    27. 27. Converting Impure DC to Pure DC
    28. 28. Effect of number of poles N S S S N N polesofnumberp pE __  Eavg1 Eavg2Eavg2 > Eavg1
    29. 29. Effect of number of conductors N S N S Eavg2 Eavg1 Eavg2>Eavg1 zE  z = total number of conductors
    30. 30. Effect of field strength and Electromagnets • Another factor effecting the EMF is the field strength. The strength of Permanent magnets depend upon the material with which it is made. However, the strength of an electromagnet depends upon the amount of current passing through it. • Ampere’s Law – The magnetic field in space around an electric current is proportional to the electric current. E
    31. 31. The EMF equation zp aE n a znp E b b   60 60   Eb= generated EMF (volts) (back EMF - motors) p = number of poles n = speed (RPM) z = total number of conductors a = total number of parallel paths 1 parallel path 2 parallel path
    32. 32. Electromagnet I I N S I I NS
    33. 33. Electromagnet • Metals offer an easier path for the flow of magnetic flux Stator frame Field coils
    34. 34. Physical structure of DC machines
    35. 35. Physical structure of DC machines
    36. 36. DC Motors
    37. 37. Lorentz’s Single conductor Experiment in a magnetic field – a motors point of view F B I F B I Rotational direction Conductor N W S E
    38. 38. Lorentz’s force law - Motors • Lorentz force: - If a current carrying conductor (with current I A) is placed in a magnetic field (with field B T), it experiences a force (F N) in a direction mutually perpendicular to the magnetic field and the current. [For Motors] sinBILF  • B = Magnetic field intensity • I = current in the conductor • L = length of the conductor • θ = angle between the current (or conductor) and the field
    39. 39. Single loop in a magnetic field – position A N S N W S E a b c d CA CB 1 2 • Two brushes are shorted, hence no current flows through the conductors. +
    40. 40. DC motors + N S +N S N S N S A B B A A B A B F FF F
    41. 41. Electrical circuit of a DC machine A IPZ T TIE TPalso IEP RIIEVI RIEV RIEV RIEV a m maa mm aam aaaaa aaa aab aab     2 _ 2         •It is quite evident that the torque of the motor is directly proportional to the flux produced by magnets and the armature current. V Eb = Ea = armature voltage atm IkT 
    42. 42. Steady state characteristics – No load a pz k k V n nkV nkE a znp E e e e ea a 60 60        VE RIVE RIEV a aaa aaa    volts 250 500 speed 500 1000 RPM Ke = 2 • current drawn is very small under no-load or Ia is 0 or of the order of 1e-3 • no-load current basically supplies the frictional losses of the machine • We can also say that the no-load speed is directly proportional to the applied voltage Torque under no-load is the torque required to overcome the frictional losses which are extremely small
    43. 43. Steady state characteristics – When loaded lma t m a atm a m TTT k T I IkT A IPZ T       2 Torque speed RPM load motor 900 X • Acceleration and deceleration is governed by the sign of the accelerating torque • Under steady-state conditions the armature current is constant and the motor torque is equal to the load torque as there is no acceleration l tee aaa aaa T kk R k V n RIVE RIEV    900 time Transient characteristics lm TT 
    44. 44. Steady state characteristics – When loaded - Droop l te a e T kk R k V n  125V 250V 375V 500V RPM 500 1000 Torque Slope = Droop • Droop is the fall in speed as load is applied • Droop is directly proportional to the armature resistance •Machines with least amount of droop are chosen so that expensive controls can be avoidedΔn ΔT Full Load No-load Δn = 5 to 10%
    45. 45. Connections Stator frame Field coils
    46. 46. Types of Electric circuit of a DC machine • Permanent magnet excited • Separately excited • Shunt Excited – Constant Speed applications • Series Excited – Heavy torque
    47. 47. Shunt motors used for constant speed applications. Why? increasesT IkT increasesI increasesEV a znp EE R V I m atm a a ab sh sh       60  V • This increase in torque causes the machine to accelerate and hence prevent the speed from falling. • armature current control • armature current becomes extremely high
    48. 48. Series motors used for constant high torque applications. Why? current Flux lines Load added ase II  Φ α Ia • The Series motor is able to overcome high load torque as the flux is proportional to the load current. • armature current and flux increase • flux and current control • poor speed regulation due to high series resistance and saturation • higher voltage required due to voltage division
    49. 49. Speed control of DC motors
    50. 50. Control System DC motor PID Controller- Reference Speed (1750 RPM) + Load or Disturbances Δω ω Separately Excited machine rated 5Hp Ki/s Δω Kp sKd + V l te a e T kk R k V n  Proportional control Integral control Derivative control
    51. 51. PID control dt tde ku dtteku teku dd t ii pp )( )( )( 0     Proportional control sets the voltage proportional to the current value of error in speed integral control sets the voltage proportional to the accumulated error in speed over a certain time period Derivative controller sets the voltage proportional to the rate at which the error in speed is approaching zero Over many time steps the error accumulates and sums up. Seeking action.   d i p sk s k ksu )( Reduction of oscillations and snappy response but amplifies noise.
    52. 52. PID Control Parameter: Rise Time Overshoot Settling Time S.S.Error Kp Decrease Increase Small Change Decrease Ki Decrease Increase Increase Eliminate Kd Small Change Decrease Decrease None
    53. 53. The End

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