Electric Motors (Cambridge (CIE) O Level Physics): Revision Note

The DC Motor

• The motor effect can be used to create a simple d.c electric motor

• The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field:

Structure of a Simple D.C. Motor

A simple D.C. electric motor consists of a coil that rotates in a magnetic field, a split ring commutator and brushes

• This causes the coil to rotate since it experiences a turning effect

• The turning effect is increased by increasing:

  • The number of turns on the coil

  • The current

  • The strength of the magnetic field

Operation of a DC Motor

• When the current is flowing in the coil at 90^o to the direction of the magnetic field:

  • The current creates a magnetic field around the coil

  • The magnetic field produced around the coil interacts with the field produced by the magnets

  • This results in a force being exerted on the coil

  • The direction of the force can be determined using Fleming's left-hand rule

  • As current will flow in opposite directions on each side of the coil, the force produced from the magnetic field will push one side of the coil up and the other side of the coil down

• This will cause the coil to rotate, and it will continue to rotate until it is in the vertical position

  • In the vertical position momentum keeps the coil turning until the magnetic force takes over again

• The split ring commutator swaps the contacts of the coil

  • This reverses the direction in which the current is flowing every half turn

  • This keeps the current leaving the motor in the same direction (d.c)

• Reversing the direction of the current will also reverse the direction in which the forces are acting

  • As a result, the coil will continue to rotate

Force on Coil in a D.C. Motor

Forces on coil after commutator has reversed the direction of the current

• The split-ring commutator reverses the direction of the current in the coil every half turn

  • This will keep the coil rotating continuously as long as the current is flowing

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

  • Reversing the direction of the magnetic field by reversing the poles of the magnet

• 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