Electric Motors

• Electric Motors

  • The motor effect can be used to create a simple d.c. electric motor
  • The force on a current-carrying coil is used to make it rotate in a single direction
  • The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field
  • The coil of wire, when horizontal, forms a complete circuit with a cell
  • The coil is attached to a split ring (a circular tube of metal split in two)
  • This split ring is connected in a circuit with the cell via contact with conducting carbon brushes
• Forces on the Horizontal Coil in a D.C. Motor: Forces acting in opposite directions on each side of the coil, causing it to rotate. The split ring connects the coil to the flow of current

  
• Forces on the Horizontal Coil in a D.C. Motor:

  • Current flowing through the coil produces a magnetic field
  • This magnetic field interacts with the uniform external field, so a force is exerted on the wire
  • Forces act in opposite directions on each side of the coil, causing it to rotate:
  • On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming's left-hand rule)
  • On the black side, current flows away from the cell so the force acts downwards
• Forces on the Horizontal Coil in a D.C. Motor:

  • Once the coil has rotated 90°, the split ring is no longer in contact with the brushes
  • No current flows through the coil so no forces act
• Coil in the Vertical Position: No force acts on the coil when vertical, as the split ring is not in contact with the brushes.

  
• Coil in the Vertical Position

  • Even though no force acts, the momentum of the coil causes the coil to continue to rotate slightly
  • The split ring reconnects with the carbon brushes and current flows through the coil again
  • Now the blue side is on the right and the black side is on the left
• Coil in the Vertical Position

  • Current still flows toward the cell on the left and away from the cell on the right, even though the coil has flipped
  • The black side of the coil experiences an upward force on the left and the blue side experiences a downward force on the right
  • The coil continues to rotate in the same direction, forming a continuously spinning motor
• Forces on the Coil when Rotated 180°: Even though the coil has flipped, current still flows anticlockwise and the forces still cause rotation in the same direction

  
• • Factors Affecting the D.C Motor
  • The speed at which the coil rotates can be increased by:
  • Increasing the current
  • Use a stronger magnet
  • The direction of rotation of coil in the d.c. motor can be changed by:
  • Reversing the direction of the current supply
  • Reversing the direction of the magnetic field by reversing the poles of the magnet
• • Factors Affecting the D.C Motor
  • The force supplied by the motor can be increased by:
  • Increasing the current in the coil
  • Increasing the strength of the magnetic field
  • Adding more turns to the coil