Intro: DC Brush Motor for Learning Fun
A friend has a young son who is newly fascinated by motors and electrical things. This Instructable replicates a DC electric demonstration motor I made about two decades ago for my own children. It is offered here for the sake of parents and teachers like my friend who want to help young people build such things and learn, even though an electric motor may not be part of the previous experience of the adult helping the young person. I am purposely using only common hand tools so anyone could make this for his or her own children. The materials used are also very easy to obtain. The final product is designed to work reliably for a long, long time.
If you would like to read a bit about how electric motors work, go to step 19. I did not put that information in the earlier steps because some important concepts must be discussed first.
3 inch finish nail or thin steel rod
1/4 inch steel rod (a 1/4 inch carriage bolt from the hardware store would do and costs very little)
Plastic tubing for a fish tank or medical equipment
Enameled magnet wire about 24 gauge (Spools of magnet wire can be bought at Radio Shack, or magnet wire can be salvaged from a number of devices that no longer work, like old transformers or motors. Be careful about going into an old television set. While you can find wire there, you can also encounter a lethal dose of stored electricity.)
#12 copper wire (from some old household electrical cable)
Thin stranded wire to connect the batteries
Spring clothes pins for holding the connecting wires in contact with the motor
Dowel rod (1/2 inch)
Wood for a base
Thin brass sheet or brass hobby tubing
Two ceramic magnets
Batteries or a 3 -- 6 volt AC/DC power supply (2 "D" batteries, a battery holder is suggested)
Motor oil (just a few drops)
Pan head sheet metal screws (two)
Electric hand drill and drill bits (a 1/2 inch spade bit is the best, least expensive option for the larger holes)
Center punch and hammer
Grinding wheel or a concrete face for grinding
Solder gun or iron
Hot glue gun
Fine-toothed handsaw for wood
Files for metal
Multi-meter (helpful, but not necessary)
Step 1: The Armature Shaft
Begin by selecting a finish nail about 3 inches long. Try fitting it to holes in a drill index to find a bit approximately the same size as the diameter of the finish nail.
Step 2: Prepare to Drill for the Shaft
Use 1/4 inch rod for the magnet core on the armature shaft. Make a dimple with a center punch so the drill bit does not skate, but bores a hole where you wish. I made the dimple at what looked like an appropriate distance from one end of the 1/4 inch rod. I did not cut the rod yet.
Step 3: Drill the Magnet Core
The magnet core will be fitted onto the armature shaft. A drill press could be used to drill a hole accurately, but with a little care, a properly aligned hole can be made with a vise and a handheld drill. I centered the dimple from the center punch between the jaws of the vise. Notice the two yellow squares for mentally aligning the drill while boring. Stop and check your progress regularly. Make corrections as needed. I rested the heel of my hand on the top of the vise while drilling to steady the drill in its proper position.
Step 4: Hone the Shaft to Fit
I cut the head from the finish nail and gently honed it to reduce the diameter of the nail just enough that the nail will fit into the hole. The finish nail will have a very slight taper and the magnet core will wedge on it to lock firmly. Tap it lightly with a hammer to wedge the two parts together securely.
In the event you do not have a grinding wheel available, go outside and hone the finish nail against the edge of a concrete sidewalk. It will take a little longer, but will work if nothing else is available.
Step 5: Cut the Magnet Core to Length
The distance from the shaft to the end of the magnet core is 7/8 inch. I measured 7/8 inch from the other side of the armature shaft to mark for sawing the magnet core to length. The cut should be placed fairly precisely, but the ends of the magnet core will be ground so the steel part of the armature (shaft plus magnet core) is nicely balanced for smooth running.
Step 6: Balance the Armature
The armature should spin very nicely and will likely need to be static balanced. I cradled it on the shaft between the jaws of my vise. If the armature is the least bit out of balance, one end of the magnet core will quickly dip downward. Grind or file on it until neither end shows a tendency to dip, even when slightly nudged. Spin the armature in your fingers. It should have a feel of good balance when it spins.
Step 7: The Commutator
A direct current (DC) motor needs a way to transfer electrical current to wire coils on the armature so they energize and make magnetism that propels the motor. The commutator is a split metal collar brushed by carbon or metal conductors (brushes). The brushes touch the commutator and current flows while the armature spins.
I decided to use clear plastic tubing from a fish tank or from medical equipment for the commutator sleeve. Its internal diameter is too large to fit snugly on the shaft, so I built up the shaft by wrapping it with black plastic electrical tape. When I was sure I had enough, I cut the tape and began to fit the tubing over it. I had to remove tape by half turns until the tubing would slip over the tape, but still be snug and not move. The second photo shows the plastic tubing in place on the commutator.
Step 8: Wind the Armature Coils
I wrapped each end of the magnet core on the armature with one layer of black plastic electrical tape for extra insulation. Then I began the process of winding approximately 200 turns of enameled wire around the magnet core ends. I left plenty of wire on both ends for soldering to the commutator sleeve sections later. The turns on the right side appear loose because two hands are needed to wrap the wire and to hold it tight. I was using one hand to operate the camera. When winding these coils be sure that both are wound in the same direction. If the coil on one side were wound in opposite directions one would cancel the effect of the other. You do not want that.
Step 9: Balance the Armature Again
When winding coils by hand it is not easy to be as even on one side of the armature as on the other. My armature was out of balance and one end dipped considerably when placed across the jaws of my vise. Grinding the end of the magnet core yields only so much improvement before there is a serious risk of grinding into the coil wire. I unwound a number of turns on the heavy side and moved them to the other side. This brought the armature into balance again. The frosted tape is to keep the coil from unwinding until I can solder the ends to the commutator sleeves and mount them.
Step 10: The Rest of the Commutator
The first photo shows the finished armature with the complete commutator in place. Pieces of brass have been glued to the clear plastic tubing (step 7).
The commutator needs conducting segments so current can flow through the coils and make magnetism. It is time to add the conductor segments. I planned to cut some pieces of brass from a small sheet obtained at a hobby store and bend them to fit the contour of the commutator. But, I discovered that I had some brass hobby tubing just the right internal diameter to slip over the fishtank tubing. (second photo) I cut a piece of brass tubing just a little shorter in length than the core of the commutator. Then I cut it lengthwise to make two pieces that cover just a little more than 1/4 the diameter of the commutator each.
I am trying to use the most simple tools for this, so I used a hacksaw and a vise, not a Dremel tool that many who read this will not have.
The third photo shows the conductor segments of the commutator held in place. I used a file to smooth rough edges left from sawing the brass. The conductor sections will be glued onto the commutator with epoxy glue. I used some sandpaper to roughen the surface of the plastic tubing and the underside of the brass pieces so the epoxy will bond better.
Before gluing, I cleaned one end of each brass piece with steel wool. Then I tinned each with a little solder. I cut the coil leads to length and scraped away the enameled insulation at the ends. Use a knife for this and make the ends shiny and bright. I soldered the coil leads to the brass conductor segments and tested for good connections with a multi-meter. (When soldering do not rest the brass pieces directly on the plastic tubing. It can melt. I used several layers of paper to protect the plastic tubing.
Epoxy can be a sticky mess that coats conductors and creates problems later. I put a small piece of masking tape over each brass piece before gluing. Then I wrapped the whole commutator with masking tape until the epoxy cures. See the fourthphoto. The frosted cellophane tape used in step 9 to keep the coils from unwinding can be removed now. The masking tape can be removed after the epoxy has cured.
Work on the armature is finished. It can be set aside for now.
Step 11: Make a Base for the Motor
A piece of scrap wood will do for the base of the motor. If you do not have many tools, a piece of paper will work as a square to mark your cut.
I began with this piece from some crating lumber, but had to select a larger piece. I needed more space at the ends of the armature coils to mount the field magnets. There will be more about them later.
Step 12: Motor Bearings
I decided to use wooden bearings for my motor. Using a hole in a steel angle bracket is very noisy. I remember farm machinery that ran hard all day and had oil impregnated wooden bearings that lasted several seasons before they needed replacement. My wooden bearings are holes drilled into the side of 1/2 inch dowel pins.
Make the dowel pins long enough so the armature can spin freely without crashing into the motor's base. The shaft of the motor should be parallel to the top of the base when mounted, too.
One of the dowels is drilled only part of the way through. The other is drilled all the way through. The holes are big enough that there is a little play for the shaft to move very freely.
Notice I also marked the base for drilling. The dowels will be glued to the base.
(This is still the first base I tried. It was replaced with one slightly larger.)
Step 13: Preparing the Field Magnets
A motor has two parts: an armature and a field. This motor will use two ceramic permanent magnets for the field magnets.
It is very important that they not present like poles. Unlike poles attract. Allow the two magnets to attract each other and stick together. Use small pieces of tape to mark the faces that stick together. These faces will each point toward the armature when mounted on the motor.
Suitable magnets like these can be purchased at Radio Shack. Or you can salvage magnets from some junk equipment, like an old hard drive. You can even take them from refrigerator magnets no longer needed.
Step 14: Mounting the Field Magnets
The field magnets mount on 1/2 inch dowel pins. Cut them so the center of the magnets is even with the height of the armature shaft. Allow some extra for gluing into the base.
I drilled a hole about 3/16 inch in diameter through the center of each dowel where the center of the magnet would be. This allowed a socket for hot glue to make a big rivet. In the photo you can see the back of one field magnet and the front of the other.
Step 15: Mounting the Field Magnets on the Base
In the photo you can see the motor bearing dowels mounted on the base. Hold a dowel for the field magnets so there is a little clearance between the magnet and the end of the armature coil. Try both armature coils, in case one is longer than the other. Mark around the dowel with a pencil. Drill. Glue the dowel to the motor base. Do this also for the other field magnet. If you have not yet, you may remove the tape from the magnet faces.
Step 16: The Commutator Brushes
In the photo you can see that both field magnets and their dowel pins have been mounted.
Strip the plastic insulation from a piece of #12 household copper wire. Bend it as you see in the photo. The "U"-shaped bend will fit around a mounting screw. The tension provided by this wire against the commutator can be adjusted by bending the wire a little. The brushes should have enough tension against the commutator that there is never a loss of contact. But, they should not be so tightly against the commutator that they make it difficult for the armature to turn. Mark the location for the brush mounting screw. Drill a hole for it. Fasten the brush to the motor base with the screw. The upward bent end of the brush near the screw is for making electrical contact with the motor.
Step 17: Lubricate the Motor Bearings
Add a drop or two of oil to the motor bearings and let it soak into the wooden bearings.
In this photo you can see the completed motor, including the brushes mounted to the motor base. If there is ever a problem with the brushes, it is very easy to loosen one screw or the other and bend or replace the brush. Dust and corrosion will appear between the commutator and the brushes. If the motor begins to perform poorly, remove the brushes. Clean them and the commutator. Also, bend the copper wire for the brushes just a little so there is a little more tension against the brass conductor strips. The tension can become weak, even though they appear to make good contact.
Step 18: It Runs!
The motor works. The armature is barely visible because it is a blur, but you can see it.
I used some jumper wire to connect to two "D" cells from a flashlight. 3 volts is about the minimum voltage on which this motor will operate. I expect you could use up to about 6 volts with no problem. I would not want to exceed 6 volts, though. Running it a long time could cause the commutator to become hot, and that could cause the plastic tubing under the brass sleeves on the commutator to loosen.
Regular use of this motor would make a battery holder a very desirable thing to have. It appears to draw quite a bit of current, but I have not checked for an exact number with an ammeter. I did notice the flashlight batteries I have been using seem to be going down more rapidly than I would have expected. I want to try powering it with a 6 volt auto battery charger.
If you want the motor to run in the opposite direction, reverse the battery connections. When you start the motor, it works best if the armature is almost vertical.
Step 19: How This Motor Works
The graphic shows two bar magnets. The red ends represent the north pole of each magnet. The gray ends represent the south poles. Like poles repel each other. Unlike poles attract each other. If like poles are placed near enough to one another, they will physically move apart. Harnessing this movement can make it useful in a motor.
The magnet at the left shows a black spot in its center. This is to represent an axle about which the magnet can revolve. The two red ends are pushing away from each other. Because the magnet at the left has an axle, the direction it moves is in an arc, which turns the axle. This is already a very simple motor. But, it is also a very limited motor. As soon as the gray end swings around to be opposite the red end of the right magnet, the two unlike poles will lock onto each other and the motor will freeze.
But, suppose that by some magic the polarity of the left magnet could change just as the gray end was about to lock onto the red end of the right magnet and become a like pole instead of an unlike pole. The left magnet would be repelled by the end of the right magnet again and the left magnet would continue to turn for another half turn. Suppose the ends of the left magnet reversed their polarity each time one was about to lock onto the magnet at the right. The left magnet would continue to spin on its axle.
It is impossible to change the polarity of a permanent magnet rapidly as hypothetically suggested in the last paragraph. But, it is possible to make magnets that can be reversed very rapidly and at will. Electricity flowing through a wire makes a weak magnetic field around the wire. By wrapping many turns of wire around a steel core, this weak magnetic field can be concentrated in the steel core to make a very powerful magnet. Suddenly changing the direction of the current flow in the wire reverses the magnetic polarity so that the north pole of the magnet becomes the south pole, etc. That is what the commutator does. As the motor's armature turns, the brushes and the commutator constantly reverse the current flow in the armature coil, which reverses the magnetic polarity just before the magnetic poles would lock onto each other and freeze. The result is that the armature continues to spin and spin. The efficiency of the motor increases when another permanent magnet is added to the left side of the graphic. In addition, the magnets do not only repel each other. Unlike magnets can also pull toward each other during part of the rotation, just as like magnets push away from each other during other parts of the rotation. A motor becomes even more powerful and efficient by adding more pole sections to the armature with more segments on the commutator and more magnets around the circumference of the field. Although this motor is very simple and not very powerful, it illustrates how very large and powerful motors work.
Note: Most large electric motors run on alternating current and are designed a bit differently than this motor so that the current flow reversals that are characteristic of alternating current naturally cause the magnetic polarities in the motor poles to reverse many times per second without the use of a commutator. An exception is the many direct current electric motors used in an automobile to raise and lower the windows, to adjust the seats and mirrors, and to start the car's engine. Those motors work very much like the motor described in this Instructable.