Blair Tech Ed Single Pole Pulse Motor

Montgomery Blair High School Technology Education Department 1999

Students in Exploring Technological Concepts built a small electric motor using plans developed by Mr. Wizard and adapted by the Tech Ed staff here at Blair. It works on the pulse concept, so it is actually only running 1/2 of the time, the other half it is coasting with no magnetic field. While these can be built from smaller parts and still run, the staff felt that enlarging some pieces and building to a plan better taught some difficult concepts. As you know magnetism is invisible so sometimes students don't quite understand what they (can't) see. Here are some pictures so you can build your own motor.

Parts

These are motor mounts. You need two 24cm pieces of solid #14 bare wire shaped like this (8cm spread) You can fake these with any wire, or even paper clips, but we wanted to teach wire gauges and such.

This is the armature/commutator. The + and - bars are 3/32" brass 5cm long. These fit inside a coffee stirrer 5cm long. They can be welding rod or coat hanger with the paint removed. The tiny insulator is wood (round toothpick) or plastic and serves to separate the + and - poles. (3/32" is about 2mm but we bought it to fit so 3/32" it is.)

The magnet wire (#20 or so) is about 90cm long and the ends must be sanded to remove the lacquer insulation. If you have to you can use bell wire with plastic insulation, just strip off the plastic from the ends.

Here is a very nice coil, the ends are wrapped tightly around the brass rods. Perhaps now you understand why there must be an insulator inside the coffee stirrer to separate the rods. To make the coil wrap the wire around a dowel or broomstick. Put tape on the coil so it won't unravel (it won't hurt the magnetism). Put the armature in the middle and wrap the bare ends of the coil on the + and - brass rods. Magic Step: On one of the brass rods you must paint out 1/2 of the connection that will touch the motor mount. If you don't the motor will merely move into alignment with the permanent magnet and just sit there. You need to shut off the electromagnetism for about 1/2 turn. You do this by shutting off the electricity. Nail polish works best.

Magic Step Continued: The brass rod is shown above with crosshatching. You must cover a portion of the rod so it does not conduct. The place to do this is where the rod will touch the motor mount, that's just outside of the magnet wire connection to the brass rod. On the illustration below the - rod (on the right) is painted with white-out. We use nail polish for durability.

Place the motor mounts on a base 6cm apart. This is the perfect size for a "D" cell. Touch the "D" cell to the motor mounts (they act as brushes) and place the magnet underneath. Nudge the motor along, reverse polarity on the "D" cell or the magnet if the motor does not spin. Keep trying, the motor will spin when everything is balanced and lined up.

Troubleshooting

brass rods move	tighten wires turned around rods
magnets move	tape them down
no spin	are armature wires bare? is circuit complete?
lazy or incomplete spin	flip battery or magnets, move magnets, spin in opposite direction, reverse armature in motor mounts, center coil in armature for balance, clean brass rod, see below
slow or weak as above	change timing of motor - gently twist painted brass rod a bit so the electricity will be on/off at a different time, also, enlarge painted area or scrape some paint off. We don't yet know the "optimum" timing of the motors.

Tech Ed Challenge: Determine the speed of the motor. Tech Ed Challenge: Determine the strength of the motor, in Watts or horsepower.