Motors that use direct current (DC) voltage are found in many familiar items, such as electric shavers, battery-powered drills, and the fans in laptop computers. DC voltage does not alternate with time. For example, a 9-volt (V) battery has a constant voltage difference between its positive and negative terminals of 9 V. A graph of the voltage at the positive terminal vs. time would be a straight line at 9 V. Alternating current, on the other hand, fluctuates between positive and negative values. The current that powers the outlets in your house is AC current. If you graphed the voltage supplied by the outlet, it would be a sine curve, alternating between positive and negative voltages about 60 times per second (sec). The number of times the AC current changes from positive to negative in 1 sec is called its frequency. The unit for frequency is the hertz (Hz), where 1 Hz is 1 cycle per sec.
In this science fair project, you will make a simple DC motor. The key parts of the DC motor are an electromagnet, a rotating shaft that has attached permanent magnets, and a reed switch. Let’s go over each of these separately. In this science fair project, the electromagnet is made by wrapping wire around a nail. When a current passes through the wires that are wrapped around the nail, the nail becomes a magnet. When the current is turned off, the nail loses its magnetism. The strength of the electromagnet depends on the number of times the wire is wrapped around it and on the level of the current. When the electromagnet is turned on, it pushes against the permanent magnets that are attached to a rotating shaft. Permanent magnets’ magnetism does not depend on electric current.
The trick to getting the shaft to spin is to turn the electromagnet on in such a way that it pushes against the permanent magnets, causing the shaft to turn, and then turning the electromagnet off, so that the permanent magnet can freely pass by the electromagnet. This cycle is shown in Figure 1.
Figure 1. This diagram shows the principles of operation of a simple DC motor. The electromagnet switches on and off. When it is on, it pushes against the permanent magnets that are attached to the rotating shaft (A and C in the diagram). When the electromagnet is off, the magnets are free to rotate past the electromagnet (B and D in the diagram). The electromagnet is switched on and off by the reed switch. When a magnet is near the reed switch, it causes the switch to close. When the switch is closed, current flows through the wires around the electromagnet, turning it on. When the permanent magnet rotates away from the reed switch, the switch opens, shutting off current to the electromagnet. The cycle repeats continuously. There can be more than two permanent magnets on the rotating shaft. Note that the reed switch is placed somewhat below the midpoint of the rotating shaft so that the impulse given by the electromagnet occurs slightly after the permanent magnet has passed.
This design is well-suited for learning about how electric motors work because of its simplicity. The reed switch responds to nearby magnets. When a magnet gets near it, the reed switch closes. When the reed switch is closed, the electromagnet is turned on. So the magnets attached to the rotating shaft are doing double duty: they close the reed switch when they pass near it, and they respond to the push from the electromagnet when it is switched on.
The electromagnet is set up so that the side near the rotating shaft has the same polarity (north or south pole) as the side of the permanent magnet that faces out from the rotating shaft. In the kit you will buy, the permanent magnets have their south poles facing outward. The electromagnet’s south pole is at the end near the rotating shaft. When two magnets are brought close to each other, opposite magnetic fields attract each other and identical magnetic fields repel each other. So when the electromagnet is turned on, it repels the magnet attached to the rotating shaft, providing the force to keep the motor working. The reed switch then opens when the permanent magnet on the opposite side of the shaft rotates away from the reed switch. With the switch open, the permanent magnet approaching the electromagnet can pass by the electromagnet with out being repelled. The cycle of opening and closing of the reed switch is timed so that the electromagnet provides a push, at just the right time, to the passing magnet on the rotating shaft, to keep the shaft spinning.
The experimental procedure is based on a kit you can buy that has all of the parts ready-made. This will allow you to make the motor and start your experiments fairly quickly. Once you have the motor working, you will test how changing the voltage affects the speed of the motor. Making the motor is one goal of this science fair project. The other goal is to determine how the voltage affects the spin rate. To do that, you will need a way to measure how fast the motor is turning. There are a number of ways to do this. The method outlined in the experimental procedure involves using an inexpensive optical tachometer. The tachometer measures the rate at which a spinning object blocks a bright light. By attaching a cardboard “propeller” to your motor, you can measure the spin rate of the rotating shaft.
Terms and Concepts
- Electric motor
- Direct current (DC)
- DC voltage
- Alternating current (AC)
- Sine curve
- Hertz (Hz)
- Permanent magnet
- Reed switch
Materials and Equipment
- Electric motor Kit #1; available from www.simplemotor.com, click “Ordering Information” on the left, and select “Kit 1.”
- Newspaper, scrap
- Ruler, metric
- Cardboard (1 small piece)
- Safety goggles
- AA batteries, new (4)
- Optical tachometer; available from online retailers, such as Tower Hobbies at www.towerhobbies.com, model # LXPT31
- Lab notebook
- Graph paper
- This involves super glue, so first cover the work surface with newspaper.
- Build the motor following the directions that come with the kit.
Figure 2. Picture of an assembled reed switch motor and the battery case.
Measuring the Motor Speed
- Attach a piece of cardboard to the shaft to create your propeller. Try a 10-cm x 3-cm rectangle to begin with, but feel free to experiment with other sizes and shapes. The cardboard propeller should be free to rotate.
- Put on the safety goggles. It is a good idea to wear eye protection any time you are working with rapidly spinning objects.
- Add all four batteries to the battery holder (for a total of 6 V).
- Use the tachometer to measure the rate of rotation.
- Follow the directions that come with the tachometer.
- The tachometer is sensitive to the 60 Hz (3600 cycles per minute) flicker in artificial light. Make your measurements in a room lit with sunlight.
- Remember that the propeller will be counted two times for every one turn of the motor shaft.
- Record the spin rate and voltage in your lab notebook.
- Repeat the measurement of spin rate at 4.5 V, 3 V, and 1.5 V.
- Follow the directions in the motor kit instructions to change the voltage. This involves removing batteries from the battery holder. One battery has 1.5 V, two batteries have 3 V, and 3 batteries have 4.5 V.
- You can also try using a variable voltage source.
- Repeat your readings so that you have data from at least three trials.
- Average your results and record the results in your lab notebook.
- Graph the voltage vs. the spin rate.