DC Motor and Generator Basic Principle

Electrical Machines

An electrical machine is an Electro-mechanical energy conversion device.
The device which converts electrical energy into mechanical energy is called a motor. The device which converts mechanical energy into electrical energy is called a generator.

Electrical Machines divided into two.

    Direct Current Machines
    Alternating Current Machines(Ac)
Direct Current Machines(DC Machines Fundamentals And Operations)

DC Generator

A generator is a device that converts mechanical energy into electrical energy. Direct-current generator operates on the principle of magnetic induction. Whenever a conductor cuts magnetic flux , dynamically induced e.m.f is produced in it according to Faraday’s laws of Electromagnetic Induction. The direction of induced e. m. f is given by Fleming’s right hand roule.

The essential components of a generator are magnetic field,Conductor or group of conductor,motion of conductor w.r.t maganetic filed.

DC Machines Fundamentals-and Generator-basic -generator construction

Let us consider the coil is rotating in the clock – wise direction . an e. m. f is induced in the coil which is proportional to the rate of change of flux linkage. When the plane of the coil is in the direction of the field during its rotation, the maximum e.m .f is induced in the coil.

In order to get a unidirectional current in the external load resistance a split ring( ie commutator) arrangement is provided . The function of commutator convert the alternating current produced in the armature into direct current in the external circuit.

Emf Equation of a generator E=ΦZN*P/60A Volts
Where Φ =flux
Z=total nuber of armature conductors
P=No of poles
A=No of parallel paths
N=Speed of armature in r.p.m

DC Machines Fundamentals GeneratorCross - sectional view-

Main parts
  • Yoke:
  • hollow cylindrical frame, its made of cast steel or fabricated rolled steel.

  • Pole and pole shoe:
  • pole cores are usually made of steel sheets stacked and riveted together,
    Pole shoe spread out the flux in the air gap and reduce reluctance of the magnetic path, they support the field coils.

  • Armature core and windings:
  • Armature is a laminated cylinder and is mounted on a shaft. The air gap between armature and filed poles is kept very small to minimise the reluctance . Two types armature winding lap winding and wave winding. The laminations reduce eddy – Current loss.

    For a wave wound generator A=2
    E.M.F generated across armature = ΦZNP/120 Volts

    For a lap wound generator A=P
    E.M.F generated across armature = ΦZN/60 Volts

  • Commutator and Brushes:
  • Commutator is of cylindrical shape and is made up of wedge – shaped hard drawn copper segment. convert AC to DC .Armature coil is connected to the commutator segment through a riser. Brushes made of high grade carbon provides lubricating effect on the commutating surface.

    Types of dc generator
    Seperately excited d.c generator,Self –exited d.c generator.

    Notes
  • Armature reaction changes the position of the neutral plane. Inter pole help and Compensating winding correct armature reaction. Inter pole connect in series with the armature.
  • Series generator increase their output voltage as load added and shunt generator decrease their output voltage as load added.
  • Voltage regulation proportional to the resistance of the armature.
  • DC Motor

    A Motor is machine that converts electrical energy into mechanical energy. Principle D.C motor operation when a current carrying conductor is placed in a magnetic filed ,the conductor experience a mechanical force .The direction of this force is given by Flemings Left Hand Rule.

    DC Machines Fundamentals-dcmotor

    When the motor terminals are connected to a d c mains the armature conductor experience a force tending to rotate the armature .When the filed excited it develops alternate south and north poles .Armature conductors under N-pole carry currents in one direction while the conductor under s-pole carry currents in the opposite direction. The conductor experiences a force which to rotate the armature in the anti-cock wise direction .These forces collectively produce a driving torque which sets armature rotating.

    Notes
    Back e . m . f:

    Armature current Ia= V-Eb/Ra

    Where Ra is the armature resistance,Since V and Ra are usually fixed ,Ia depends on Eb. If the speed of the motor is high ,then back e.m.f .Eb is large and hence the motor will draw less armature current and vice –versa .

  • Necessity of motor starters:
  • At starting ,When the motor is stationary there is no back emf in the armature .If the motor is directly switched on,armature current Ia=V/Ra is very high.This high resistance current may result in

      Burning of armature due to excessive heating effect.
      Damaging the commutator and brushes due to heavy sparking .
      Excessive voltage drop in the line to which the motor is connected.

    In order to avoid heavy starting current a variable resistance known as starters.

  • Shunt motor:
  • Constant speed motor,Lathes, drills, boring mills, shapers, spinning and waving machines.

  • Series motors:
  • It is variable speed motor, speed law and high starting torque .Never start a series moor on no load .Electrical traction, cranes, elevators, air compressors, vacuum cleaner, hair drier, sewing machines.

  • Differential – compound motor:
  • Fairly constant speed is required with irregular loads or suddenly applied heavy loads.Presses,shears,reciprocating machines

  • The direction of rotation of direct current motor can be changed by reversing the connection of either armature or the filed leads.
  • Increasing the number of loops and turns increases output torque.
  • Reference

    Tony

    Hi, Myself Tony George. An Electrical Engineer by Education, a Software Developer by Profession and an Education Blogger by Passion. In the past, I had been working as an Engineer in KSEB. The knowledge that I have acquired I would like to share here with you. I will be happy if these articles can give you a way to your career growth. Feel free to contact me.

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