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Construction of DC Machines

  • Introduction

    D.C. generator is a electrical machine which converts mechanical energy (or power) into electrical energy (or power). The energy conversion is based on the principle of the production of dynamically or motionally. Induced e.m.f .

    Whenever a conductor cuts magnetic flux and e.m.f is produced in it according to Faraday’s laws of electromagnetic induction. This e.m.f causes a current to flow if the conductor circuit is closed. Mechanical energy is given on the generator by means of rotation of armature it is known as prime mover.

    e.g. prime mover-diesel engine, turbine wised mill.

    Construction of dc machines

    The construction diagram of DC machines is shown in figure. It consists of following main parts are

    1.         Yoke (or) magnetic frame
    2.         Magnetic poles
    3.         Field coils
    4.         Inter poles (or) Commutation poles
    5.         Commutator
    6.         Brushes
    7.         Bearing and end cover

    Yoke (or) magnetic frame

    1. It provides mechanical support for the machine and act as the magnetic flux produced the pole.
    2. It forms the portion of magnetic circuit. it carries the magnetic flux produced by the poles.
    3. It is made up of cast iron for smaller machine for larger machines it is made up of case steel.



    The field magnet consists of pole cores and pole shoes.

       (1) The spread out the flux in the air gap
       (2) They support the field coils.

    Pole cores and pole shoes are built with these laminations of steel. These laminations are held together using rivets. The cores are laminated to reduce the eddy current loss. The magnetic poles are fitted inside the yoke by means of screws.

    Field coils

    Field coils are usually wound with enameled copper wire. The magnetic field strength depends upon the current flowing through the coil. The north and South Pole depend upon the direction of the current flow through the field coil.

    Inter poles (or) Commutation poles

    1. The function of inter pole is to improve the commutation and to reduce the armature reaction.
    2 . The exciting coils on the interlopes are connected in series with the armature.

    Armature

    The armature core is keyed to the machine shaft and it rotates between the field poles. It consists of slotted steel laminations. These laminations are stocked to form a cylindrical core.

    The laminations are insulated from each other by thin coating of varnish. The purpose of the lamination core is reducing the eddy current loss. Armature winding divided into two,

    (1) Lab winding: for low voltage, high current machine
    (2) Wave winding: for high voltage, low current machine

    Commutator

    The commutator is made up of copper segments insulated from one another by mica sheets. The number of segments is equal to the number of armature coils. The segment is connected to armature conductor. Armature conductors are soldered to the commutator segment in a suitable manner to give rise to the armature winding.

    Functions of commutator

    1. Collection of current
    2. Current from the armature conductor.
    3. It converts alternating current induced in the armature conductors are to unidirectional current.

    Brushes

    Brushes are made up of carbon and rest on the commutator. The function of the brushes is to collect current from the commutator to the external stationary load. The brushes are put inside the brush holders. The brush holders are kept pressed against the commutator by a spring as shown in figure.

    Bearing and end cover

    Bearing

    Ball bearings (or) rollers bearings are fitted inside the end cover. Armature shaft is mounted over these brings.

    End over

    End over are made up of cast iron fabricated steel. They are fitted to both ends of yoke.

    Armature winding

    Type of winding

    (1) Lab winding
    (2) Wave winding

    Lab winding

    1. Lab winding is used in large output current and low voltage dock machine.
    2. In lab winding, the number of parallel paths (A) is equal to the number of poles (P).  i.e., A = P
    3. The number of brushes is made equal to the number of poles.

    Wave winding

    1. Wave winding is used in low output current and high voltage d.c machine.
    2. In wave winding, the number of parallel paths (A) =2
    3. Number of brushes = 2

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