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9.2 Space
9.3 Motors & Generators
9.4 Ideas-Implementation
9.7 Astrophysics
9.8 Quanta to Quarks

9.3 Lab Work I
9.3 Lab Work II

 

CONTENTS

 

HOW TO BUILD A WORKING DC ELECTRIC MOTOR

One of the Assessment Tasks that my son had to do for his Year 12 Physics course was to build a working DC electric motor.  With his permission I include the basic details here in case they can be of use to other students.  Building the motor was one half of the task.  The rest of the task was to write a very detailed report on the design, theory and building of his motor.  I only include here basic details of the building of the motor.

Materials can be purchased from an electrical/electronics shop.  Jaycar Electronics had the best selection of materials.  You could also try Tandy Electronics & Dick Smith stores.

The motor my son built is a three coil DC motor, with a three-piece copper split-ring commutator and copper brushes.  The armature is a short length of dowel (broom handle) suspended between 2 metal shelf brackets by screws at each end.  The magnetic field is supplied by rare-earth magnets attached to metal shelf brackets.  Power supply is ten 1.5V size AA alkaline batteries.  Photographs of the finished motor follow.

View from Front

    

 

View from Above

 

Materials Required

        Resin coated copper wire - $7.65

        Wood board for base.

        4 Rare-Earth magnets - $30.00

        Shelf Brackets - $5.40

        Copper sheet (for use in commutator and for brushes)

        Insulated electrical wire - $4.50

        Battery pack - $2.30

        Dowel (old broom stick handle)

        Switch - $5.00

        Masking tape

        8 Self Tapper Screws (10g x 16mm) - to secure metal brackets to base board

        3 Wood Screws (8g x 40 mm) (Nickel Plated Brass) - to wrap coils around

        2 Pan Head Screws (10g x 25mm) - to use as axel supporting armature

        Phillips Head Screw Driver

        Electric Drill

        Tape Measure

        Wood Saw

        Scissors

        Ruler

        Paper

        Pencil

        10 x AA Batteries - $13.00 

Total Cost of materials - $67.85

 

Steps

  • Prepare wooden base board - 30cm x 30cm is plenty.
     
  • Attach 2 shelf brackets to base to hold dowel armature (broom handle).
     
  • Mark centres of ends of dowel & drill holes for screws.
     
  • Measure the circumference of dowel & mark three equidistant points for coil screws.  Drill these holes and insert screws.  Note that because the magnets used in this project are so strong, it is best to use non-magnetic screws for the coils.
     
  •     

     
  • Wind the three coils - 250 turns each, leaving plenty of wire at both ends to run through to the commutator, as shown above.  These wires will lie beneath the commutator plates.
     
  • Cut three equal-sized copper strips to use as commutator plates.  Measure these so that when placed onto the armature there will be a small (1mm) gap between consecutive plates.  Ensure the copper is clean and not oxidized.  Use soapy water and steel wool or vinegar and steel wool to clean the copper surface.
     
  • Strip the resin coating from the parts of the copper coil wire that are going to be placed under the commutator plates.  Just use the blade of a pair of scissors for this.  Stripping the resin ensures good electrical contact.
     
  • Arrange these coil wires so that each coil will be connected to a pair of oppositely placed commutator plates.  This is essential to enable a complete circuit to exist for each coil with the power source.  See circuit diagram (pdf file).  Note from the circuit diagram that Coil 1 will be connected to Commutator Plates 2 & 3.  Coli 2 will be connected to Commutator Plates 1 & 3.  Coil 3 will be connected to Commutator Plates 1 & 2.  See diagram below.
     
  • In this diagram of the finished motor you can see a wire from coil 1 and a wire from coil 3 coming up to and running under commutator plate 2.  The wires from coil 2 can be seen running away towards commutator plates 1 and 3.


        


     
  • Cover each pair of coil wires arranged as shown above with one of the copper commutator plates.  Tape each plate to the armature.  Insulation tape is best but duct tape or masking tape will do just as well for this project.  Ensure that the wires under each plate are not in contact with each other under the plate.  Ensure that there is a gap between each consecutive commutator plate.  Ensure that there is sufficient copper metal showing on each plate so that the brushes will have plenty of copper surface to contact.  Note that each commutator plate should line up with one of the coils.  So, for instance, commutator plate 1 should be in line with coil 1.  (In other words, a straight line drawn from the centre of commutator plate 1 back along the armature should meet coil 1.)
     
  •     

     
  • Mount the armature into the shelf brackets.  Ensure that it turns easily.  Coat the screw at each end and the screw hole in the bracket with Vaseline to reduce friction.
     
  • Attach the other 2 shelf brackets to the base board so that 2 rare-earth magnets can be supported by each bracket, as shown in the photographs of the finished motor above.  The magnets on opposite sides of the coils must be arranged so that a north-south magnetic field is set up through the area of the coils.  Also, for best results, arrange the shelf brackets holding the magnets, so that they are touching each other on the base board - see photograph.  This means that there is a complete magnetic circuit linking the pole in contact with one shelf bracket with the pole in contact with the other shelf bracket.  This intensifies the magnetic field around the coils.
     
  • Cut 2 copper strips to act as brushes.  Ensure the copper is clean and not oxidized.  Use soapy water and steel wool or vinegar and steel wool to clean the copper surface.  Attach these to the base board with a single screw each.  Ensure that the brushes contact the commutator plates as the armature rotates.  See diagram below.
     
  •     

     
  • Connect the electric circuit - one side of the battery pack, through the switch to one of the brushes - the other side of the battery pack to the other brush.  Note that attaching the conducting wires to the brushes as close as possible to the point of contact of the brushes with the commutator plates will ensure the most efficient delivery of current to the plates and therefore the most efficient operation of the motor.
     
  •     

     
  • Experiment with your motor.  It should work at this stage.  When you close the switch, the coils should spin.  If not check your circuit for breaks; check that the brushes are in contact with the split ring commutator; check you have sufficient lubrication on the axel screws.  Note that even a slight movement of the brushes or increase in contact of the brushes with the commutator plates can make a huge difference to the rate of spin of the motor.  If the coils move just a little and then stop, you may have to give the coils a little push in the direction they look as if they are trying to turn.  They should then spin.  All the best.

     

Take a look at this brief video (9.6Mb) captured on a mobile phone camera of the motor working.  This is before the switch & fixed electrical wiring was added.  (Depending on the speed of your connection, this video may take about a minute to load the first time you view it.  If it does not load or takes forever to load, please let me know.)

 

This brief video (7.1Mb) shows the finished motor working.  (Again, there may be a brief delay before starting, the first time you view this video.)

 

Also, take a look at: http://www.youtube.com/watch?v=h6Ev64h49wg which shows a very well-designed working DC electric motor in action.  This motor was the inspiration for the one built here.

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Last updated:

Robert Emery Catholic Schools Diocese of Maitland-Newcastle 2002 - view the Terms of Use of this site.