DC Motor

DC Machines

  • Direct Current machines deal with conversion of one form of energy to another.
  • A DC machine can be:
  1. DC Generators: Converting Mechanical energy into electrical energy.
  2. DC Motors: Converting electrical energy into mechanical energy

Working Principle of DC Machine as a Generator:

  • A generator works on the principles of Faraday law of electromagnetic induction.
  • Whenever a conductor is moved in the magnetic field such that it cuts across the lines of flux, dynamically induced e.m.f is produced according to Faraday laws of electromagnetic induction.
  • The magnitude of this induced e.m.f in the conductor is given by the equation:
e= Blv sinθ

Where
l = length of the portion of the conductor within the magnetic field,
v = velocity of the conductor,
B = magnetic flux density and

θ = angle between direction of movement of the conductor and the direction of magnetic flux
  • The essential requirements are: 1. Conductor, 2. Magnetic field, and 3. Mechanical energy
  • This e.m.f causes a current to flow in the conductor if the conductor circuit is closed.
  • Thus, electrical power is developed in the conductor.
  • For the generation of e.m.f there should be relative motion between the conductor and the magnetic field.
  • This causes a change in the flux linking the conductor which causes a voltage to be induced in the conductor.
  • The direction of the induced e.m.f (or the current) in the conductor in the case of a dc generator is given by Fleming Right Hand Rule (or Generator Rule)
  • According to Fleming, the right hand is held with the thumb, first finger and second finger mutually perpendicular to each other (at right angles), as shown in the diagram.
  • The thuMb is pointed in the direction of Motion of the conductor.
  • The First finger is pointed in the direction of the magnetic Field. (north to south)
  • Then the seCond finger represents the direction of the induced e.m.f. or Current (the direction of the induced current will be the direction of
    conventional current; from positive to negative).

Working Principle of DC Machine as a Motor:

  • A DC motor in simple words is a device that converts direct current electrical energy into mechanical energy.
  • The very basic construction of a dc motor contains a current carrying armature conductor, connected to the supply through commutator segments and brushes and placed within the north south poles of a permanent or an electro magnet.
  • Fleming Left Hand or Motor Rule determines the direction of rotation of current carrying conductor in a fixed magnetic field.
  • Fleming left hand rule says that if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field (represented by the index Finger) is perpendicular to the direction of current (represented by the SeCond or middle finger), then the conductor experiences a force in the direction (represented by the thuMb) mutually perpendicular to both the direction of field and the current in the conductor.
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  • Force experienced by the conductor can be expressed as:
    F=BILsinθ

DC Motors

  • DC Motors convert electrical energy into mechanical energy.
  • They work on the principle that:
    • whenever the current carrying conductor is placed in the magnetic field, a force is set up on the conductor.
  • Due to this force a torque is produced, which is responsible for the rotation of armature of the machine.
  • This rotation produces the required mechanical power.
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Back e.m.f:

  • The current carrying armature conductors of a dc motor are placed in the magnetic field produced by the main poles of the motor.
  • The armature conductors produce their own magnetic field.
  • Thus there are two magnetic fields in the dc motor.
  • The interaction between the two fields causes the rotor to rotate.
  • This rotation makes the armature conductors to cut the magnetic field set up by the main poles.
  • Therefore an e.m.f is induced in the conductors.
  • According to Lenz law this induced e.m.f acts in a direction to oppose its cause of production, i.e., the rotation of the armature.
  • In turn it opposes the supply voltage given to the armature.
  • Therefore this induced e.m.f is called back e.m.f. Back e.m.f is also an induced e.m.f.
  • Back e.m.f is given by:
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Production of Torque and Torque Equations

  • The measure of causing the rotation of a wheel or the turning or twisting moment of a force about an axis is called the torque.
  • It is measured by the product of force and the radius at which this force acts.
  • Consider a wheel of radius R meters acted upon by a circumferential force F Newton, making it rotate at n rps.
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Shaft Torque:

  • The torque which is available at the motor shaft for doing useful work is called shaft torque (Tsh).
  • The total torque Ta developed in armature is not available at the shaft, as part of it is lost in overcoming the iron and frictional losses.
  • Therefore, shaft torque Tsh is somewhat less than the total armature torque Ta.
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Speed of a DC Motor:

  • speed is directly proportional to the back emf Eb and inversely proportional to the flux.
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Types of DC motors

  • Separately excited dc motor: the armature and field windings are connected to separate sources
  • Self excited dc motor: the armature and field winding are connected to same supply.
    Self excited motor are:
    i) Series dc motor: Field winding will have small number of turns of thick wire and is connected in series with the armature.
    ii) Shunt dc motor: Field winding will have large number of turns of thin wire and is connected in parallel with the armature
    iii) Compound dc motor: This motor will have both series and shunt field windings.
    Compound motor can be short shunt or long shunt.
    They can be Differential compound or Cumulative compounded.

Separately Excited DC Generators:

  • In this type of DC motor, the field winding is connected to a separate source.
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Self excited dc motors:

1. Series motors:

  • field winding connected in series with the armature.
  • It always carries the armature current.
  • The flux produced will be proportional to the armature current.
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2. Shunt DC Motor:

  • It takes small current from the supply voltage to produce the magnetic flux.
  • The flux produced almost remains constant.
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3. Compound DC Motor:

  • It has both series field and shunt field windings.
  • Based upon the connection of the field windings it can be of two types:
    (i)Short shunt – Shunt field is parallel to armature only
    (ii)Long shunt-Shunt field is parallel to Armature and Series field in series

Short Shunt:

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Long Shunt:

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Characteristics of DC aMotor

  • DC Motor characteristics depict the relationships between following quantities:
    i) Torque & armature current or Ta Vs Ia characteristic (also called electrical characteristic).
    ii) Speed & armature current i.e., N Vs Ia characteristic.
    iii) Speed & Torque or N Vs Ta characteristic (also called mechanical characteristic). This can also be ascertained from (i) and (ii) above.
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Characteristics of DC Series Motor:

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  • Applications of Series Motors: In this starting torque required is high. Like: 1. Traction 2. Hoists and Lifts 3. Crane 4. Rolling Mills 5. Conveyors

Characteristics of Shunt Motors:

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  • N Vs Ta is same as N vs Ia
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  • Applications of Shunt Motors: It is used mostly in constant speed applications, Like: 1. Lathe machine 2. Drilling machine 3.Grinders 4. Blowers 5. Compressors

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