electrical engineering and instrumentation regulation : r2013 subject code : ee6352

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  • Slide 1
  • ELECTRICAL ENGINEERING AND INSTRUMENTATION Regulation : R2013 Subject code : EE6352
  • Slide 2
  • Syllabus UNIT I DC MACHINES 9 hours Three phase circuits, a review. Construction of DC machines Theory of operation of DC generators Characteristics of DC generators- Operating principle of DC motors Types of DC motors and their characteristics Speed control of DC motors- Applications.
  • Slide 3
  • Unit 1 DC MACHINES
  • Slide 4
  • Three phase circuits Three phase circuits is a polyphase system when 3 phases are used together from the generator to the load Each phase are having a phase difference of 120 If the load is single phase, then one phase can be taken from the three phase circuit and the neutral can be used as ground to complete the circuit.
  • Slide 5
  • Waveform
  • Slide 6
  • Why three phase preferred over single phase? Single phase system The conductor needed for a single phase circuit is very much less. The instantaneous power falls to zero when the single phase supply is disconnected. Three phase system The conductor needed in three phase circuit is 75% that of conductor needed in single phase circuit. The instantaneous power exists even after one phase is disconnected as the net power from all the phases gives a continuous power to the load.
  • Slide 7
  • Types of connections Star Connection Delta Connection
  • Slide 8
  • Star Connection V ph = V L / I ph = I L 3 phase wires and 1 neutral wire from the star point. Preferred for long distances.
  • Slide 9
  • Delta Connection V ph = V L I ph = I L / 3 wires from the phases are used. No Neutral wire Preferred for short distances.
  • Slide 10
  • Types of currents Balanced current Equal current flows through all the 3 phases. Unbalanced current - Unequal current flowing through all the 3 phases.
  • Slide 11
  • Y- arrangement Star-Star connection Delta-Delta connection Star Delta connection Delta Star connection
  • Slide 12
  • Simple loop generator
  • Slide 13
  • Principle of Operation Faradays Law of Electromagnetic induction:- When a current carrying conductor is placed in a magnetic field an EMF is induced. Generator :- Flemings Right hand rule Motor :- Flemings Left hand rule
  • Slide 14
  • DC Machines DC machines convert electrical energy to mechanical energy and vice versa. This process of conversion is called as electromechanical energy conversion. 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.
  • Slide 15
  • DC machines Construction Types Theory of operation Characteristics Speed control of DC motor Applications
  • Slide 16
  • DC machine Construction
  • Slide 17
  • Slide 18
  • Slide 19
  • Slide 20
  • Slide 21
  • Armature of a DC machine
  • Slide 22
  • Slide 23
  • Slide 24
  • Dismantled view of a DC machine
  • Slide 25
  • Slide 26
  • Slide 27
  • Parts of a DC machine Yoke Pole core & Pole shoes Pole coils Armature core Armature windings Commutator Brushes & Bearings
  • Slide 28
  • Yoke Outermost frame Purpose: Provides mechanical support Carries the magnetic flux produced by the poles. Materials used: Small machines Made of Cast Iron Larger machines Made of Cast steel or rolled steel
  • Slide 29
  • Pole cores & Pole shoes Spreads out the flux in the air gap Reduces the reluctance of the magnetic path Supports the exciting coils or field coils
  • Slide 30
  • Pole coil or field coil or exciting coil Copper wires or strips wound around the pole core. When current is passed thorugh these coils they produce magnetic flux surrounding the armature conductors. Materials used: Made up of copper
  • Slide 31
  • Armature core
  • Slide 32
  • Armature windings Usually former wound in the form of flat rectangular coils. Various conductors of the coil are insulated from each other Conductors are placed in the armature slots Slot insulation is folded over the armature conductors. Materials used: Made up of copper
  • Slide 33
  • Commutator Facilitates collection of current from the armature conductors The commutator & brushes arrangement converts AC to DC It is of cylindrical structure and built up of wedge-shaped segments of high-conductivity hard-drawn or drop forged copper. These segments are insulated from each other by thin layers of mica. The number of segments is equal to the number of armature coils. Each commutator segment is connected to the armature conductor by means of a copper lug or riser.
  • Slide 34
  • Brushes & bearings Collects current from the commutator They are housed in a brush holder. The brush holder is mounted on a spindle and can slide in the rectangular box open at both ends. The number of brushes per spindle depends on the magnitude of the current to be collected from the commutator.
  • Slide 35
  • Pole Pitch Distance between 2 adjacent poles. It is equal to the number of armature conductors (or armature slots) per pole. If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12.
  • Slide 36
  • Conductor The length of a wire lying in the magnetic field and in which an emf is induced, is called a conductor (or inductor) as, for example, length AB or CD in the following figure.
  • Slide 37
  • Coil & Winding element The two conductors AB and CD along with their end connections constitute one coil of the armature winding. The side of the coil is called the winding element. No. of winding elements = 2 x No. of Coils The coil may be single turn coil or multi-turn coil. Multi-turn coil may have many conductors per coil side. The group of wires or conductors constituting a coil side of a multi-turn coil is wrapped with a tape as a unit and is placed in the armature slot.
  • Slide 38
  • Coil span or Coil pitch (YS) Coil span is the distance between 2 sides of the coil.
  • Slide 39
  • Full pitched winding If coil span or coil pitch = pole pitch, the winding is called full pitched wdg. Full pitch means coil span is 180 electrical degrees. In full pitch, coil sides lie under opposite poles and their induced emf is scalar sum. So maximum emf is induced in full pitch winding
  • Slide 40
  • Chorded wdg or Short pitched wdg or Fractional pitched wdg The pitch of the winding is less than pole pitch
  • Slide 41
  • Single Layer Winding : Winding in which one conductor or one coil side is placed in each armature slot Double Layer Winding : Two conductors or coil sides per slot is placed in each armature slot
  • Slide 42
  • Types of windings Lap winding Wave winding
  • Slide 43
  • Lap winding Laps back with its succeeding coils. For Simplex lap winding, No.of parallel paths = No. of poles For duplex winding, No. of parallel paths = 2x No. of poles
  • Slide 44
  • Lap Winding
  • Slide 45
  • Different pitches for Lap winding Back Pitch (Y B ) No of coil sides or slots spanned by the back end connections. (Z/P) Front Pitch (Y F ) - No of coil sides or slots spanned by the front end connections. Resultant Pitch (Y R ) Distance between the beginning of one coil to the beginning of next coil to which it is connected. Commutator Pitch (Y C ) Distance between the segments to which the two ends of the coil are connected. For Lap, (Y C = Y B - Y F ) & for Wave, (Y C = Y B +Y F )
  • Slide 46
  • Equalizer rings or Equalizer connections A thick copper conductor connecting the equipotential points of lap winding for equalizing the potential of different parallel paths. Avoids unequal distribution of current at the brushes thereby heling to get sparkless commutation.
  • Slide 47
  • One Equalizer ring for a pole pair is used.
  • Slide 48
  • Interpoles or Compoles Small poles placed in between the main poles. As their polarity is same as that of main poles, they induce emf in the coil which helps in current reversal. The induced emf is called commutating emf or reversing emf which opposes the reactance emf thereby making commutation sparkless. Cross magnetising effect due to armature reaction is also neutralised.
  • Slide 49
  • Design of Lap Winding
  • Slide 50
  • Lap Winding Advantages: 1.This winding is necessarily required for large current application because it has more parallel paths.current 2.2. It is suitable for low voltage and high current generators.voltagecurrent Disadvantages: 1.It gives less emf compared to wave winding. This winding is required more no. of conductors for giving the same emf, it results high winding cost. 2. It has less efficient utilization of space in the armature slots.
  • Slide 51
  • Wave winding The end of one coil is not connected to the begi