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DC Motors - Operation, Types and Applications

  • Introduction

    An electric motor is a electrical machine which converts the electrical energy into mechanical energy. The parts and construction of D.C motor are as same as that of D.C generator.

    Principle of Operation (Or) Motor Action

    Whenever a current carrying conductor is placed in a magnetic field, a mechanical force is produced on the conductor. The direction of the force is given by ‘fleming’s left hand rule’.

    The magnitude of the force, F = B I L Newton

    Where, F = force in Newton
    B = magnetic flux density in web/m2
    I = current in ampere.
    L = length of the conductor in meter.

    To understand the principle of operation of d.c motor, Let us consider a two pole motor.


    Figure (a) shows a uniform magnetic field in which no current carrying conductor is placed. The direction of magnetic flux is from north to south pole. There is no movement of the conductor.

    In figure (b) there is no exciting current flow through the field winding and d.c current is sent through the conductor. Let the conductor carry the current away from the observer. It produces a magnetic flux lines around that in clockwise direction. There is no movement of the conductor.

    In Figure (c), the current carrying conductor is placed in magnetic field. The field due to the current in the conductor aids the main field above the conductor (flux strengthens) but opposes the main field below the conductor. (flux weakens). It is found that a force acts on the conductor, trying to push the conductor downwards as shown by the arrow (the conductor is pushed from high flux density to low flux density from high flux to low flux density).

    If the current conductor is reversed (current towards the observer), the strengthening of flux lines occurs below the conductor will be pushed upward as shown in the figure (d).

    Now consider a single turn coil carrying a current as shown in the fig. Figure (e). The coil side ‘A’ will be forced to move downwards, where as the coil side ‘B’ will be forced to move upwards. The force acting on the coil sides ‘A’ and ‘B’ will be same magnitude but their direction are opposite to one another. As the coil is wound on the armature coil, which is supported by the bearings armature will rotate. The direction of rotation is found out by “Fleming’s right hand rule”.



    Fleming’s Left Hand rule

    Keep the thump, fore finger and middle finger of the left hand mutually perpendicular to each other. If the fore finger points the direction of magnetic flux lines and the middle finger points the direction of current flow in the conductor, then the thump points the direction of rotation of conductors.

    Back e.m.f (or) Counter e.m.f

    When D.C supply is given to d.c motor its armature rotates. The armature rotates and cuts the magnetic flux lines. Therefore e.m.f is induced in the armature conductors (By faraday’s of electromagnetic induction). This induced e.m.f opposes the supply voltage (by Fleming’s Left Hand rule). Hence the e.m.f induced in the armature is called back e.m.f. (or) counter e.m.f.

    Back e.m.f, Eb = ΦZNP/60A
    Also, Eb = V - IaRa

    Where,
          Φ = flux per pole is Weber’s
          Z = number of conductors is the armature
          P = number of poles
          A = number of parallel paths
          V = supply voltage to the motor
          Ia = armature current
          Ra = armature resistance.

    Types of D.C Motor

    Depending upon the way in which the field winding is connected with armature winding the D.C motors are classified as,

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

    1. D.C. Shunt Motor

    In d.c shunt motor the field winding is connected in parallel with armature winding as shown in figure. The shunt field winding has a large number of turns and smaller cross section area and high resistance.

    Applied voltage, V    = Eb + IaRa
    Armature current , Ia = IL - Ish

    Where,
    Eb      = Back e.m.f
    Ia     = Armature current
    IL    = Line current
    Ish   = Shunt field current
    Ra = Armature resistance.

    2. D.C Series Motor

    In D.C series motor, the field winding is connected in series with the armature as shown in the figure. The series field winding has large cross sectional area , few number of turns and low resistance.

    In d.c series motor, IL = Ia = Ish

    Applied voltage, V= Eb +IaRa +Ise Rse
    = Eb +IaRa +Ia Rse
    = Eb +Ia (Ra + Rse) ( Ia = Ise)

    Where, Eb        = Back e.m.f
    Ia        = Armature current
    IL        = Line current
    Ise         = Series field current
    Rse       = Series field resistance
    Ra         = Armature resistance

    3. D.C Compound Motor

    In d.c compound motor both series field and shunt field windings are connected with the armature winding. Depending upon the connection of field winding, d.c compound motors are classified into two types,

    (i) Long shunt d.c compound m
    (ii) Short shunt d.c compound motor

    (i) Long Shunt D.C Compound Motor

    In long shunt D.C compound motor the series field winding is connected in series with the armature winding and the shunt field winding is connected in parallel with this arrangement. This is shown in fig.

    In long shunt compound motor,
    Ia = Ise
    IL = Ia + Ish

    (ii) Short- Shunt D.C Compound Motor

    In long shunt D.C compound motor the shunt field winding is connected in parallel with the armature winding and the series field winding is connected in series with this arrangement. This is shown in fig.

    In short shunt compound motor,
    IL = Ise
    IL = Ia + Ish

    Depending upon the way in which the shunt field flux and series field flux act with respect to each other, d.c compound motors are classified in to ,

    (a) Cumulative compound motor
    (b) Differentially compound motor

    Applications of D.C motors

    Shunt motors are used when constant speed is essential at low starting torque

    D.C. shunt motor

    1. It is used to drive centrifugal pumps
    2. It is used to drive highs machine tool
    3. It is used to drive wood working machine
    4. It is used to drive paper mills
    5. It is used to drive drilling machines etc.

    D.C series motor

    D.c series motors are used where high starting torque is required.
    It is used to drive
    1. Electric trains
    2. Cranes
    3. Lifts and Conveyors etc.

    D.C compound motor

    D.C compound motors are used where intermittent high starting torque is required.
    It is used to drive
    1. Rolling mills
    2. Printing machines
    3. Shears
    4. Punches etc.

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