Direct Current Machines

Introduction

Faradays Law of Electromagnetic InductionFaraday found that the electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path.In practice, this means that an electrical current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it.Electromagnetic induction underlies the operation of generators, all electric motors, transformers, induction motors, synchronous motors, solenoids, and most other electrical machines.Faraday's law of electromagnetic induction states that:

E is the electromotive force (emf) in voltsB is the magnetic flux in webersN is the number of turns of wire

Right-hand Thumb Rule

Right-hand Thumb Rule

Flemings Left-hand Rule

Flemings Left-hand Rule

Flemings Left-hand Rule

Flemings Left-hand Rule

Flemings Right-hand Rule

Flemings Right-hand Rule

Current Reversal

Lenz Law"An induced current is always in such a direction as to oppose the motion or change causing it"

Construction of DC Machine

Magnetic Field Sources

Electromagnet

DC Generator Principle of Operation

DC Generator Principle of OperationA generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor

DC Generator Principle of OperationThe AMOUNT of voltage generated depends on theThe Strength of the magnetic field, Angle at which the conductor cuts the magnetic fieldSpeed at which the conductor is movedLength of the conductor within the magnetic field.The POLARITY of the voltage depends on the Direction of the magnetic lines of flux and Direction of movement of the conductor. To determine the direction of current or EMF in a given situation, the Flemings Right-Hand Rule For Generators is used.

Effect of Commutation

Effect of Additional Coils

Magnetic Circuit of DC Generator

Armature

Field windings on Pole Face

Brush Holder

Commutator Construction

Cutway of DC Generator

Armature Winding

Drum Type Armature

Four Pole Generator

Armature Coil

Coils Connection to Commutator

Construction of DC GeneratorYoke: Yoke is a outer frame. It serves two purposes. It provides mechanical support for the poles and acts as a protecting cover for the whole machine and It carries the magnetic flux produced by the poles and have high permeability.In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed. The feet and the terminal box etc, are welded to the frame afterwards.

Construction of DC GeneratorArmature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).

Construction of DC GeneratorYoke: Yoke is a outer frame. It serves two purposes. It provides mechanical support for the poles and acts as a protecting cover for the whole machine and It carries the magnetic flux produced by the poles and have high permeability.In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed. The feet and the terminal box etc, are welded to the frame afterwards.

Components of DC GeneratorStator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.Brushes:The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.

Applications of DC Generators1. DC shunt Generator The terminal voltage of DC shunt generator is more or less constant from no load to full load. Therefore these generators are used where constant voltage is required. For electro plating Battery charging For excitation of Alternators. 2. Series Generators: The terminal voltage of series generator increases with load current from no load to full load Therefore these generators are used as Boostersfor supply to arc Lamps

Applications of DC Generators3. Compound Generator: Differential Compound generators are used to supply dc welding machines. Level compound generators are used to supply power for offices ,hostels and Lodges etc. Over compound generators are used to compensate the voltage drop in Feeders. 4. Separately Exited Generator: As a supply source to DC Motors ,whose speed is to be controlled for certain applications. where a wide range of voltage is required for the testing purposes.

Commutator and Brushes

Component parts of D.C. Machine

Shunt Field Coil Construction

Series Field Coil

Compound Field Coil

Dismantled D.C. Machine

Armature laminations & Slots

Commutator construction

Commutator Connections

Brush Gear Assembly

Rectifying action of a Commutator

Rectifying action of a Commutator

Rectifying action of a Commutator

Output from a multi-coiled Armature

Types of DC GeneratorsGenerators are generally classified according to their methods of field excitation. (i) Separately excited D.C. generators (ii) Self-excited D.C. generatorsDC Shunt Generator DC Series GeneratorDC Compound Generator

Separately Excited D.C. Generators

DC Shunt Generator

DC Series Generator

DC Long Shunt Compound Generator Shunt field winding is in parallel with both series field and armature winding

DC Short Shunt Compound Generator Short Shunt in which only shunt field winding is in parallel with the armature winding

EMF EquationLet= flux/pole in Wb Z = total number of armature conductors P = number of poles A = number of parallel paths = 2 for wave winding = P for lap winding N = speed of armature in r.p.m. Eg= e.m.f. of the generator = e.m.f./parallel path

EMF Equation

Rotor Assembly of DC Machine

Eye Bolt

A Two pole DC Machine

Rotor spins inside a stator

Thank You

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