Electric motors

      • Electric motors use force produced by current carrying conductors to create torque
      • Instead of single conductors, they use wires wound into coils

Permanent magnet DC motors

dc motor

      • Current flows from power supply into the stationary brushes
      • It then flows from the brushes into the split rings (commutator) – they are electrically isolated from each other and rotate with the armature
      • From the split rings, the current flows through the coil windings (the armature)
      • The current carrying coil experiences a torque when it is exposed to the magnetic field of the permanent magnet, called the stator
      • For the armature to continue to rotator in one direction, the direction of the current in the coil must be reversed every half turn
      • This is achieved via the split rings

Force on a loop

motor coil

      • The figure shows a loop of wire with a conventional current I.
      • The loop rotates about the axis O-O.
      • If we let L(m) equal the length of sides AB and CD, which are both perpendicular to the flux B, then the force on each of these sides of the coil is,

F = ILB

      • Note that there is no force in the other sides of the coil as they are parallel to B.

Force on a coil

      • In motors and speakers a coil is used to multiply the effects of a loop. If a coil is made up of N loops of wire then the total force on one side of the coil is,

F = NILB

Torque on a rotating coil

Torque that each force applies about O-O

τ = rF

Since there are two forces,

τtotal = 2rF

In a DC motor, N coils are used to multiply the torque and force

τtotal = 2rFN

Alternatively,

τtotal = INBA

Torque in a rotating coil

angled torque coil

τtotal = INBA cos α , where α is the angle the armature makes to the horizontal

τtotal= INBA sin β , where β is the angle between the force and the armature

Industrial Motors

Since the torque fluctuations are quite high, industrial motors try to reduce the torque ripple and increase the efficiency of the motor:

      • Increasing number of coils
      • Coils are wound around a soft iron core to maximise magnetic field
      • They have 4 or 8 poles instead of 2
      • Both the stator and rotor use electromagnets rather than permanent magnets
      • They use low friction bearings
2 pole and 4 pole motors
2 pole motors:
      • Have 2 poles on the armature and 2 poles on the stator
      • Current changes every half turn
      • Jerky movement as there are large torque fluctuations
4 pole motors:
      • Have 4 poles on the stator and 4 on the armature
      • Current changes direction every quarter turn
      • Smooths out torque fluctuations

Speakers

    • Speakers create sound by using magnetic forces created by a magnet and a coil to vibrate the cone
    • The cone is attached to the coil
    • When a current is passed through the coil, it experiences a force
    • AC is used to push the coil in both directions and vibrate it and thus the air.

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