1. DC Generator

2. Contents 1.Introduction 2.Construction Features 3.Principal of Operation 4.E.M.F Equation 5.Applications

3. Introduction Electric machines can be classified in terms of their energy conversion characteristics.  Generators convert mechanical energy from a prime mover to electrical form  Generators convert mechanical energy from a prime mover to electrical form.  Motors convert electrical energy to mechanical form  Motors convert electrical energy to mechanical form. There is no basic difference in the construction of a DC generator and a DC motor. In fact, the same DC machine can be used interchangeably as a generator or as a motor.construction of a DC generator and a DC motor

4. 4 A Motor/Generator are made of  Stator:  Stator: This is the stationary part  Rotor:  Rotor: This is the rotating part A Motor/Generator are made of  Stator:  Stator: This is the stationary part  Rotor:  Rotor: This is the rotating part Main parts of a DC machine:  Yoke:  Pole core and pole shoes:  Field winding:  Armature core and winding:  Commutator:  Brushes:  Shaft: Main parts of a DC machine:  Yoke:  Pole core and pole shoes:  Field winding:  Armature core and winding:  Commutator:  Brushes:  Shaft:

5. 1)Yoke:- - Acts as frame of the machine - Acts as frame of the machine - Mechanical support - Mechanical support - low reluctance for magnetic flux - low reluctance for magnetic flux - High Permeability - High Permeability -- For Small machines -- Cast iron—low cost -- For Small machines -- Cast iron—low cost -- For Large Machines -- Cast Steel (Rolled steel) -- For Large Machines -- Cast Steel (Rolled steel) Large DC machine Small DC machine

6. 2) Pole cores and pole shoes:- a) Pole core (Pole body) :- --Carry the field coils a) Pole core (Pole body) :- --Carry the field coils --Rectangle Cross sections --Rectangle Cross sections -- Laminated to reduce heat losses -- Laminated to reduce heat losses --Fitted to yoke through bolts --Fitted to yoke through bolts b) Pole shoe:- Acts as support to field poles b) Pole shoe:- Acts as support to field poles and spreads out flux and spreads out flux Pole core & Pole shoe are laminated of annealed steel Pole core & Pole shoe are laminated of annealed steel (Of thickness of 1mm to 0.25 mm) (Of thickness of 1mm to 0.25 mm) 3)Field (or Exciting)winding:- The pole is excited by a winding wound around the pole core. --Is prepared from copper. --Is prepared from copper.

7. 4) Armature Core:- a) Armature core (Armature):- a) Armature core (Armature):- -- To support armature windings -- To support armature windings --To rotate conductors in a magnetic field --To rotate conductors in a magnetic field -- it is cylindrical or drum shaped is built -- it is cylindrical or drum shaped is built -- Each stamping is separated from its -- Each stamping is separated from its neighboring one by thin varnish as insulation neighboring one by thin varnish as insulation --Laminated to reduce eddy current losses --Laminated to reduce eddy current losses -- A small air gap between pole pieces and -- A small air gap between pole pieces and armature so that no rubbing between them armature so that no rubbing between them -- High grade silicon steel used to reduce -- High grade silicon steel used to reduce i) Hysteresis loss i) Hysteresis loss ii) Eddy current loss ii) Eddy current loss b) Armature Winding:- b) Armature Winding:- Main flux cuts armature and hence E.M.F is induced Main flux cuts armature and hence E.M.F is induced --winding made of Copper (or) Aluminum --winding made of Copper (or) Aluminum --windings are insulated each other --windings are insulated each other

8. 5) Commutator:--Hard drawn copper bars segments insulated from each other by mica segments (insulation) other by mica segments (insulation) -- Between armature & External circuit -- Between armature & External circuit -- Split-Rings (acts like Rectifier AC to DC ) -- Split-Rings (acts like Rectifier AC to DC ) 6) Brushes:- The function of brush is to collect the current the current from the commutator and supply it to the external circuit of the DC generator or motor.

9. Working Principle of a DC Generator A DC generator operates on the principle of Faraday’s laws of electromagnetic induction. According to Faraday’s law, whenever a conductor is placed in a fluctuating magnetic field (or when a conductor is moved in a magnetic field) an EMF is induced in the conductor.

10. 10  When a current carrying conductor is placed in a magnetic field, the conductor experience a mechanical force.  Direction is given by Flemings left hand rule. F=B.I.L  Magnitude is F=B.I.L  Consider a motor with one pair of poles, an armature with a single conductor coil and a commutator with only two segments,  I f is field current supplied to the field winding to establish the main field between the poles N and S.  I a is armature current via the carbon brushes. This current produces magnetic fields around the armature conductors

12. Generated EMF or EMF Equation of a generator Let  = flux/pole in Weber Z =Total number of armature conductors =No. of slot × No. of conductors/slot 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 E g = e.m.f generated in any one of the parallel paths i.e E Average e.m.f generated/conductor = d  volt dt Now, flux cut/conductor in one revolution d  = P wb As the armature rotates, a voltage is generated in its coils. In the case of a generator, the emf of rotation is called the Generated emf or Armature emf and is denoted as Er = Eg. In the case of a motor, the emf of rotation is known as Back emf or Counter emf and represented as Er = Eb.

13. No. of revolutions/sec=N/ 60  Time for one revolution, dt= 60 /N sec According to Faraday’s Law of electro magnetic induction E.M.F generated/conductor = d  =  PN volts dt 60 No. of conductors (in series) in one parallel path= Z / A  E.M.F generated/path=  PN × Z Volts 60 A  Generate E.M.F, E g =  Z N × P Volts 60 A For i) Wave winding A = 2 ii) Lap winding A = P

14. Applications of DC Generators The applications of different types of DC generators include the following. The separately excited type DC generator is used for boosting as well as electroplating. It is used for power and lighting purpose using a field regulator The self-excited DC generator or shunt DC generator is used for power as well as ordinary lighting using the regulator. It can be used for battery lighting. The series DC generator is used in arc lamps for lighting, stable current generator, and booster. A compound DC generator is used to provide the power supply for DC welding machines.power supply Level compound DC generator is used to provide a power supply for hostels, lodges, offices, etc.generator Over compound, DC generator is used to reimburse the voltage drop within Feeders.

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