posted on 26 Jan 2013 by guy
last changed 7 Jan 2016
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ages: 12 to 99 yrs
budget: $1.00 to $5.00
prep time: 0 to 60 min
class time: 10 to 45 min
Building a simple motor is much easier than most people realize. All it takes is a battery, a magnet and a piece of wire. This lesson reviews some simple motors and demonstrates a minimalist design constructed from a button cell battery, some wire, and a small but strong magnet. The motor takes a delicate hand to adjust properly, but it is superb as a demonstration for a small group of observers or as a hands-on activity for more dextrous and patient students.
subjects: Engineering, Physics
keywords: motor, homopolar, Neodymium, magnet, magnetism, electromagnetism
Commercial motors can sometimes seem to be fairly sophisticated. Most contain several standard components:
- electromagnets made out of coils of wire,
- permanent magnets that exert a magnetic force on the coils, causing the motor to move,
- a fixed axle that is free to rotate (the "rotor"), to which one set of magnets is attached,
- a nearby stationary structure (the "stator") to which another set of magnets is attached, and
- a rotary switch (the "commutator") that changes the current in the electromagnets as the motor turns, thereby changing the forces in the motor to keep optimum power supplied.
Simple motors often do not contain a commutator switch, and these motors are called "homopolar" motors. Sometimes they also do not have a fixed rotating axle or wire coils. Instead, an ingenious arrangement allows one piece of the motor to circle around the center of the motor. The motor shown here, like all motors, uses an electric current flowing in a wire to generate a magnetic field, which interacts with a separate permanent magnet to generate a force that drives the motor. (See A Survey of Simple Electric Motors for more background on the operation of motors.)
The earliest electric motors were all homopolar motors. The first motor, designed by the English scientist Michael Faraday in 1821, used a single straight wire that revolved around a central magnet (see our modern reproduction in Faraday Motor). In 1822, another English scientist by the name of Peter Barlow designed and built another homopolar motor that used a rotating conducting wheel.1 A modern homopolar design somewhat akin to the Barlow wheel is demonstrated in An Electric Screw Motor.
The Simplest Mini Motor by alexs095.
the button cell motor
One extremely simple and well-known modern design uses a battery, a disk magnet, and a shaped piece of stiff copper wire. The most elegant implementation I've seen of this design is shown in the video above (from alexs095 on YouTube). This particular motor uses a button cell battery, a Neodymium disk magnet, and some stiff uncoated copper wire. This design takes some patience and a delicate touch, but a very similar more robust version can be made using a D cell battery (see below).
Button cell batteries come in a variety of materials, sizes, and voltages. One of the most common (and one of the cheapest) is shown in Figure 1. Neodymium magnets are made from a Neodymium-Iron-Boron alloy (Nd2Fe14B) and are the strongest type of permanent magnets produced. Neodymium disk magnets in a range of sizes and strengths can be obtained from a number of suppliers. WARNING: Neodymium magnets are very strong and must be kept away from electronics chips and credit cards. They are also brittle and can shatter when dropped. For this motor, choose a disk magnet roughly the same dimensions as your button cell battery, at least 5 mm thick (but thicker is better) . You will also need 10 centimeters or more of solid core copper wire, roughly 22 to 26 gauge (0.6 to 0.4 mm diameter). The wire can be bare copper or it can be insulated, although you will have to remove the insulation in places to make proper electrical contact. It's important to use copper wire (or at least non-magnetic wire) so that the metal itself is not attracted to the magnet.
Fig. 1: A common button cell battery. This LR44 alkaline battery is 11.4 mm in diameter and 5.2 mm in height, and supplies 1.5 Volts. In the image at the left, the positive terminal is facing up; it extends across the top and down the sides of the battery. In the image at the right, the negative terminal is facing up; it is surrounded by the black insulator that separates it from the positive terminal.
Fig. 3: Side view of a button cell motor with the field lines of the magnet drawn in. In this diagram, where the north pole of the magnet is facing upwards, the field lines come out the top of the magnet, go down around the outside of the magnet, and back up through the bottom of the magnet. The field is strongest right above and right below the magnet.
how does it work?
When the wire is connected, electrons flow from the negative terminal of the battery up through the prong of the wire and towards the outside (left and right on both sides), then down the side legs of the wires to the ends, through the magnet and back to the positive terminal of the battery. The magnetic exerts forces on the currents in the wire called Lorentz forces, which are always perpendicular to both the direction of the current and the direction of the magnetic field lines. The precise direction of the force is determined by a useful mnemonic called the right hand rule. (See A Survey of Simple Electric Motors and Classroom Rail Gun for background discussion on field lines and Lorenz forces.) In Figure 3, the electrons flowing along the top leg to the right side feel a force pointing into the page, while the electrons flowing along the top to the left side feel a force pointing out of the page. Both these forces cause the motor to turn counter-clockwise (as viewed from above). As the electrons flow down the side legs of the wire, they feel very little magnetic force, partly because the field in those regions is very small and partly because the electrons are flowing in almost the same direction as the field lines, which does not produce a Lorentz force. Finally, as the electrons flow along the bottom legs towards the magnet, they feel a weak force pulling them towards the center of the magnet, helping slightly to hold them against the magnet as they turn.
Set the button cell battery with the negative terminal facing up on top of the disk magnet. The positive terminal is the more magnetic surface on the battery and will stick better to the magnet. Now bend the copper wire to fit. Needle-nose pliers will help. Start at the middle of the wire to make the prong that pokes down from the top. The wire will rest on top of the battery at this point. Next bend the wire down on both sides with room to clear the battery without touching. Finally, bend up the bottom ends in the horizontal direction and then around the battery. The bottom legs should be just barely tight enough to touch the magnet at all times. Trim off any excess wire with wire cutters or scissors.
The challenge in this design is to make the bottom legs tight enough to touch the magnet without providing so much friction that the motor does not turn. One technique that seems to work reasonably well is to loosen the bottom legs until the wire balances and teeters on the prong, not quite touching the magnet with the bottom legs. Then gently squeeze the legs closer together until they just make contact with the magnet.
Depending on precisely how you shaped the wire, the legs may slide around the magnet more smoothly in one direction than the other. If you want to make the motor go in the other direction, just flip the magnet over so that the other pole is facing up (as discussed below under "questions to ponder").
Finally, all contacts must be very clean in order for the current to flow. If you are not getting any movement even when contact appears to be good, clean all contact surfaces with alcohol to remove any oil or dirt that might have been deposited.
Slip the wire over the battery and magnet until the prong of the wire is resting on the battery and the bottom legs of the wire are clamped around the magnet. Watch it take off.
version with D cell battery
If you don't have a button cell battery available, or you're getting frustrated trying to adjust your button cell battery just right, it's also possible to make the same kind of motor using a flashlight battery (AA, C or D cell). This version tends to be a little easier to work with than the button cell version because the larger battery can provide larger current (and more torque), and because slightly thicker wire can be used (16 to 20 gauge is good). Invert the battery on top of the disk magnet so that the negative terminal is facing up and the small positive terminal balances on the magnet. Some batteries have a dimple in the center of the negative terminal, which will help keep the wire prong in place as it spins. Construction is analogous to the button cell version. See one in action in Amy Lingard's construction at http://www.youtube.com/watch?v=fkWsTATK9U4.
questions to ponder
- What if we turned the magnet over, flipping it upside-down so that the north pole was on bottom?
Naturally, if we turned the magnet, we reverse the direction of the magnetic field lines and we reverse the direction of all the forces. The motor will run backwards.
- What if we turned the battery over so that the negative terminal was on bottom?
First of all, it may be difficult to center the magnet on the battery. For many button cell batteries, the negative terminal is much less magnetic than the positive terminal. This means the magnet will try to slide around towards the positive terminal of the battery. If it is possible to reverse the battery, the current in the wire will be reversed, which reverses the Lorentz forces and causes the motor to run backwards.
- What happens if the wire brushes against the side of the battery as it spins?
In this case the current will short circuit. Electrons emerging from the negative terminal will travel along the wire only until it touches the positive terminal of the battery, where they will enter the battery again. There will be no current in the remainder of the wire. This condition may not be a problem however, since the majority of the torque on the wire comes from the upper legs of the wire, where the field is strongest and where the current is nearly perpendicular to the field direction. As long as current is flowing through these legs, the motor should continue to turn.
- What could we do to provide more power to the motor?
In principle, there are two methods of increasing the power to the motor: increase the current in the wire or increase the strength of the permanent magnetic.
First of all, we could stack two batteries on top of each other to get twice the voltage, and force more current through the circuit.
Note for geeks: Precisely predicting the current in the circuit is a little tricky. It is determined by the resistance in the circuit, the voltage supplied by the batteries, and the induced voltage in the hanging wire from moving through the magnetic field (see Faraday's Law of Induction), which works against the supplied battery voltage. The current is calculated as the difference between the supplied voltage and the induced voltage, divided by the resistance (Ohm's Law). When we stack two batteries on top of each other to get twice the voltage, we also get twice the resistance from the batteries. The extra resistance in the circuit may or may not have a big effect depending on how much induced voltage is generated by the moving wire. That voltage depends on how fast the wire is moving, which depends on the friction in the system ... which is starting to get pretty complicated. For reference, a typical LR44 button battery has an internal resistance of about 10 ohms, and a maximum current of about 150 mA. A typical alkaline D cell battery has an internal resistance of about 5 ohms, and a maximum current of about 300 mA. The D cell generally makes for the more powerful motor.
The other way to make a more powerful motor is to use a stronger permanent magnet, or to stack more than one magnet. For thin magnets, the strength of the magnetic field scales roughly linearly with the thickness of the magnet, so using two magnets stacked one on top of the other will roughly double the strength of the magnetic field, and therefore double the strength of the motor.
Introducing students to the operation of a homopolar motor works best after they have seen and understood electromagnets and can appreciate the interaction between a magnet and a current in a wire. Check out Basic Electromagnets for some ideas. Classroom Rail Gun also demonstrates the Lorentz force in a very simple way that helps prepare students to figure out the operation of more complex motors.
More precise understanding of the Lorentz force (the force of a magnet on a current carrying wire) can be achieved if the students already know about magnetic field lines. See Iron and Magnets for an introductory lesson.
The button cell motor is a very tiny device that only works well as a demonstration for small groups of students. It is more successfully used as a take-away kit that students can play with by themselves after seeing it work, or seeing the video above. When bought in bulk, the materials can cost less than $1 per motor, making it possible to outfit an entire class at relatively low cost. If you can afford to let the students take the kit home to keep, it will be a lesson they will never forget. WARNING: Neodymium magnets are very strong and must be kept away from electonics chips and credit cards. They are also brittle and can shatter when dropped.
After the basic motor is working, try to get students to think about other possible wire shapes. It's not necessary to have wires going down both sides of the magnet in order to get the motor to run. Each side by itself produces an electromotive force as long as the wire stays in contact with both the battery and the magnet. For a single-wire spiral design, check out the video by Anand Vyas at http://www.youtube.com/watch?v=ac2oBK6IHWM.
John Jenkins has compiled a beautiful visual history of motors showing a number of early homopolar motors at: http://www.sparkmuseum.com/MOTORS.HTM