HOW TO BUILD A WORKING DC
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
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
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
Insulated electrical wire - $4.50
Battery pack - $2.30
Dowel (old broom stick handle)
Switch - $5.00
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
Phillips Head Screw Driver
10 x AA Batteries - $13.00
Total Cost of materials - $67.85
- 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
- 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
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
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.)
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:
which shows a very well-designed working DC electric motor in action. This
motor was the inspiration for the one built here.