Investigate the operation of a simple DC motor

Investigate the operation of a simple DC motor


Current supplied produces interacting magnetic field which keeps the motor spinning

The function of the components of motor are as follows:

  • Armature: laminated (reduces eddy current) ferromagnetic cylinder, rotates on axle to give actual motion, concentrates magnetic field to increase
  • Coil: Wrapped around armature, medium for current, better motion with more coils at right angles
  • Split ring commutator: Changes direction of current to allow motion to continue
  • Brushes: Allows current flow into split ring
  • Magnets: Produce external magnetic field, curved magnets can improve torque (coil plane is always parallel)
  • Axle: provides centre of rotation


Torque production

  • Torque due to a force is defined as : torque-1
    • distance is the distance of OP and MN from the axis of rotation.
    • distance , where is the width of the conductor loop
    • so torque-2
  • Force on a current carrying wire :Force-on-a-current-carrying-wire
    • now conductor NO and MP are parallel to the magnetic field so no force acts on then sine-alpha
    • conductor MN and OP are always perpendicular to the magnetic field , so force on them is F = IlB where is the length of MN and OP.
  • So using torque-3 and F = IlB , we get
    • torque-4, for NM and OP
    • torque-5
  • If n loops are wounded, then the formula becomes torque-6, where
    • thetais the angle between direction of area (of loop) and direction of magnetic field.

Back emf in electric motor:

  • Energy must be conserved.
  • Therefore back EMF must opposite to supplied. Otherwise there would be an unchecked increase.
  • This reduces the efficiency of the motor.
  • A smaller back EMF gives a greater current under load.
  • Slower spin gives a lower back EMF
  • Vnet = Vsupply – EMFback
  • When starting there is a low back EMF and the high current could burn the motor out.
  • Variable resistance is used until more back EMF is produces

A steady speed is produced when Vsupply = EMFback

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)