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Self Excited and Separately Excited DC Generator

  • Types of DC Generator:
    What is excitation?

    The current flows through the field winding for production of magnetic flux are called excitation.
    Depending upon the method of excitation D.C generators are classified as,

    (1).Separately excited D.C Generator
    (2).Self excited D.C Generator

    (1).Separately excited D.C Generator

    In separately excited D.C generator, the exciting field current supplied by a separate source as shown in figure. When current flows through the field winding, the main pole becomes magnet and the flux lines are produced.

    Applications:

    If it has constant d.c output voltage, these generators are used for

    1. Battery charging
    2. Supply source to d.c motor



    (2). Self Excited D.C Generator

    In self excited d.c generator, the emf induced in the armature supplies the exciting field current. Therefore, need not separate d.c supply for exciting field poles.

    Depending upon the way in which the field winding is connected with armature , self excited d.c generators are classified as,

    1. D.C shunt generator
    2. D.C series generator
    3. D.C compound generator

    1. D.C Shunt Generator

    In D.C shunt generator, field winding is connected in parallel with the armature winding as shown in figure. Its field winding has many turns of thin wire having high resistance. Then armature rotates emf is induced due to the residual magnetism. Due to induced e.m.f, field current (Il) flows and hence flux and voltage increases.

    D.C shunt generator, Ia = IL + Ish
    Where, Ia = armature current
    IL = load current
    Ish= shunt field current

    Applications

    1. It is used in battery charging
    2. It is used in electroplating.
    3. It is used as exciter for alternator.

    2. D.C Series Generator

    In d.c series generator, the field winding is connected in series with the armature winding as shown in figure. It carries a large current. Its field winding has few turns of thick wire having low resistance.

    Ia = IL = Ise

    Where,
    Ia = armature current
    IL = load current
    Ise = shunt field current

    For series generator no load characteristics is absent because the circuit is closed only when load is connected. The load current flows through the field winding and produces flux. In series generator, as the terminal voltage increases with the load current . Booster to compensate for load voltage drops.

    Applications

    1. It is used booster.
    2. It is used for supply to arc lamps.

    3. D.C Compound Generator

    Compound generator is a machine which consists of series field winding and shunt field winding. In d.c compound generator both series and shunt field winding are connected with the armature winding as shown in figure (a) & (b). The flux of the pole depends on the total flux of both field winding. 

    Depends upon the connection field winding d.c compound are classified into two types.

    1. Long shunt d.c compound generator
    2. Short shunt d.c compound generator

    1. Long Shunt Compound Generator

    In long shunt compound generator, the series field winding is connected in series with the armature winding and shunt field winding is connected in parallel with this arrangement as shown in figure.
    In long shunt compound generator, Ia = Ise
    Ia = Ish + IL

    2. Short Shunt Compound Generator

    In short shunt compound generator, the series field winding is connected in series with the armature winding and shunt field winding is connected is parallel with this arrangement as shown in figure.
    Ise = Ic
    Ia = Ish + IL

    Depends upon the connection of field winding the d.c compound generator is further classified into two types.

    1. Cumulative compound generator:
    2. Differentially compound generator

    1. Cumulative compound generator

    If the series field flux adds with shunt field flux it is called as cumulative compound generator.
    In cumulative compound generator, the flux produced by the shunt field winding and flux produced by the series field winding acts in the same direction. Hence the total field flux acting in this type of generator is sum of these two fluxes.

    Total flux, Φ = Φsh+ Φse

    2. Differentially Compound Generator:

    If the series field flux opposes with the shunt field flux, it is called as differential compound d.c generator.

    In differentially compound generator , the flux produced by shunt field winding and the flux produced by the series field winding acts in the opposite direction. Hence the total field flux acting in this type of generator is difference of these two fluxes.

    Applications:

    1. Differentially compound generators are used for D.C welding purpose.
    2. It is used in power offices and hotels.
    3. It is used where power is to be transmitted to a long distance.

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