Tutorial 02 (DC Generators)

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DC Generators

Transcript of Tutorial 02 (DC Generators)

  • DC Generators

    Engr. Muhammad Waseem Zeeshan Ashrafi

    Electrical Machines

    Tutorial Lecture No.02

  • EMF equation of DC generator

    Let,

    Z=Total no. of conductors

    = Flux /pole (Wb)

    P= No. of Poles

    A= No. of parallel paths (A=2 for Wave & A=P for Lap winding)

    N= Speed of Armature (in RPM)

    Eg= emf of the generator = emf/parallel path

    Flux cut by one conductor on one revolution of the armature,

    d = P (Wb)

    Time Taken to complete one revolution,

    dt = 60/N (Second)

    According to Faradays Law,

    (volts)

    Hence,

    emf of generator, Eg = emf per parallel path

    = (emf/conductor) x No. of conductors in series per parallel path

  • Types of DC generators:

    The magnetic field in a dc generator is normally produced by electromagnets

    rather than permanent magnets. Generators are generally classified according to

    their methods of field excitation.

    On this basis, DC generators are divided into the following two classes:

    (i) Separately-excited DC generators

    (ii) Self-excited DC generators

    The behavior of a d.c. generator on load depends upon the method of field

    excitation adopted.

  • Separately-Excited D.C. Generators

    A D.C. generator whose field-magnet winding is supplied from an independent

    external D.C. source (e.g., a battery etc.) is called a separately-excited generator.

    The voltage output depends upon the speed of rotation of armature and the field

    current (If).

    The greater the speed and field current, greater is the generated e.m.f.

    It may be noted that separately excited D.C. generators are rarely used in

    practice. The d.c. generators are normally of self-excited type.

    Following figure shows the connections of a separately-excited generator.

  • Self Excited D.C. Generators

    A D.C. generator whose field magnet winding is supplied current from the

    output of the generator itself is called a self-excited generator.

    There are three types of self-excited generators depending upon the

    manner in which the field winding is connected to the armature, namely;

    (i) Series generator

    (ii) Shunt generator

    (iii) Compound generator

    Q: How is self-excitation achieved?

    When the armature is rotated, a small voltage is induced in the armature

    winding due to residual flux in the poles.

    This voltage produces a small field current in the field winding and flux per pole

    to increase.

    The increased flux increases the induced voltage which further increases the

    field voltage which further increases the field current.

    These events take place rapidly and the generator builds-up to the rated

    generated voltage.

  • (1) Series Generator:

    In a series wound generator, the field winding is connected in series with

    armature winding so that whole armature current flows through the field winding

    as well as the load.

    Following Figure shows the connections of a series wound generator.

    Armature current, Ia = Ise = IL

    Terminal voltage, V = Eg - Ia(Ra + Rse)

    Power developed in armature = Eg.Ia

    Since the field winding carries the whole of load current, it has a few turns of thick

    wire having low resistance.

    Series generators are rarely used except for special purposes e.g., as boosters.

  • (2) Shunt Generator:

    In a shunt generator, the field winding is connected in parallel with the armature

    winding so that terminal voltage of the generator is applied across it.

    Following figure shows the connections of a shunt-wound generator.

    The shunt field winding has many turns of fine wire having high resistance.

    Therefore, only a part of armature current flows through shunt field winding and the

    rest flows through the load.

  • (3) Compound Generator:

    In a compound-wound generator, there are two sets of field windings on each Pole; one is in

    series and the other in parallel with the armature.

    A compound wound generator may be:

    (a) Short Shunt generator: in which only shunt field winding is in parallel with

    the armature winding [Fig. A]

    (b) Long Shunt generator: in which shunt field winding is in parallel with

    both series field and armature winding [Fig. B]

    Fig. A (Short Shunt) Fig. B (Long Shunt)

    Note:The two windings may be connected

    1- To aid each other (cumulative compounding) or

    2- To oppose each other (differential compounding).

  • Losses in a D.C. Machine:

    The losses in a DC machine (generator or motor) may be divided into three Classes:

    (1) Copper losses

    (2) Iron or core losses

    (3) Mechanical losses

    All these losses appear as heat and thus raise the temperature of the machine.

    They also lower the efficiency of the machine.

  • Power Stages:The various power stages in a d.c. generator are represented diagrammatically

    in Fig:

  • Characteristics of D.C. Generators

    Following are the three most important characteristics or curves of a d.c. generator :

  • These types of DC generators are generally more expensive than self-excited

    DC generators because of their requirement of separate excitation source.

    I. Because of their ability of giving wide range of voltage output, they are

    generally used for testing purpose in the laboratories.

    II. Separately excited generators operate in a stable condition with any variation

    in field excitation. Because of this property they are used as supply source of

    DC motors, whose speeds are to be controlled for various applications.

    Example- Ward Leonard Systems of speed control.

    Applications of Separately Excited DC Generators

  • The application of shunt generators are very much restricted for its

    dropping voltage characteristic.

    They are used to supply power to the apparatus situated very close to its

    position. These type of DC generators generally give constant

    terminal voltage for small distance operation with the help of field regulators from

    no load to full load.

    I. They are used for general lighting.

    II. They are used to charge battery because they can be made to give constant

    output voltage.

    III. They are used for giving the excitation to the alternators.

    IV. They are also used for small power supply.

    Applications of Shunt Wound DC Generators

  • These types of generators are restricted for the use of power supply because of their

    increasing terminal voltage characteristic with the increase in load current from no

    load to full load.

    They give constant current in the dropping portion of the characteristic curve. For

    this property they can be used as constant current source and employed for various

    applications.

    Applications of Series Wound DC Generators

    I. They are used for supplying field excitation current in DC locomotives

    for regenerative breaking.

    II. This types of generators are used as boosters to compensate

    the voltage drop in the feeder in various types of distribution systems

    such as railway service.

    III. In series arc lightening this type of generators are mainly used.

  • Applications of Compound Wound DC Generators

    Among various types of DC generators, the compound wound DC generators are

    most widely used because of its compensating property.

    We can get desired terminal voltage by compensating the drop due to armature

    reaction and ohmic drop in the in the line. Such generators have various

    applications.

    I. Cumulative compound wound generators are generally used lighting, power

    supply purpose and for heavy power services because of their

    constant voltage property. They are mainly made over compounded.

    II. Cumulative compound wound generators are also used for driving a motor.

    III. For small distance operation, such as power supply for hotels, offices, homes

    and lodges, the flat compounded generators are generally used.

    IV. The differential compound wound generators, because of their large

    demagnetization armature reaction, are used for arc welding where

    huge voltage drop and constant currentis required.