EXPERIMENT

IN

LINEAR INDUCTION

By:

HP Inge

McCulloch Middle School, 7th Grade

University Park, Texas

How do you make a magnetically levitated car, and how can it be made to move with an electromagnetic force?

The original purpose of my project was to levitate an object, with it being held in place magnetically. It would be suspended in midair, and therefor be frictionless. A series of electromagnetic coils would be laid in a straight line, and would function similarly to the stator of a motor, producing a magnetic field that would cause the armature (the car) to slide down the track, rather than spin as does the shaft (armature) of a typical motor.

The car was made to move using a polyphase [2 phase] alternating current passing through two separate sets of electromagnetic coils. The effect of the alternating current causes the poles for each electromagnet to continuously reverse position as the direction of the current changes. The polyphase current causes the position of the north and south poles to move in a linear fashion up and down the coils. Magnetic fields created in the coils induce a magnetic field in the aluminum plates wrapping around the outside of the coil, which follow or are pushed by the moving poles of the coils.

Keeping the car on the tract magnetically turned out to be impossible with fixed magnets. One retaining wall had to be used to balance the car between the wall and the top or high point of the magnetic field of the tract. This has been demonstrated in the attached pictures. However, two walls have been used to keep viewers from knocking it off the track.

From this experiment I concluded that efficiently levitating and moving an object is very difficult. My car weighed 12.5 to 14.5 ounces, depending on the size and thickness of the plates mounted on the top of the car, and it required a minimum of 95 volts in one set of coils to start it moving easily. However, it does appear that high rates of speed are possible as the car does accelerate rapidly once it has started moving.

To magnetically levitate an object and then move it from point A to point B, being a minimum of 40 cm., using an electromagnetic force with few or no moving parts other than the object itself.

A series of electromagnetic coils can be used to create a moving magnetic field which will induce a magnetic field in a conducting material causing that the material to move along the coils.

Lego building blocks were used when working with different configurations for the track and on the car. This material was ideal in working on the car as well as the track, because it is easily moved and changed into different shapes. It is ironic that at certain distances the ridges or locking pins were perfectly spaced to hold the magnets.

Many different configurations for both the car and the track were tried in trying to float the car without a retaining wall. However, this proved impossible, or at least with the materials that were available. The most stable configuration turned out to be with two retaining walls because the magnetic field resembles a dome and the car tries to slide off the track. The retaining wall holds it in place, while two walls are best, only one wall is necessary.

There is nothing magic about the width of the track or the car, except the width of each must be approximately the same. Strong duct tape had to be used to hold the magnets in place because they always tried to repel each other. The magnets had to be constantly pushed back in place. When the magnets did break loose, banging would occur when the magnets popped together. I had to purchase two sets of magnets. I think that during the experiments, all the banging and being forced together caused the magnets to weaken.

The magnets used each weighed 1 oz. and measured 4.4 cm. by 1.9cm. and .6 cm. thick, with the poles on the large flat sides. An older magnet could lift 4.8 times its own weight (4.8 oz.). in contrast, a new magnet could lift 5-6 times (5-6 oz.) its own weight. to lift the car I used three pairs of magnets. together they are able to lift approximately 2 lbs., including the weight of the car.

When experimenting with the track and car design, I tried to use magnets of different sizes at the same time. This did not work because the field of the bigger magnet went around the opposing field of the smaller magnet and reacted with the attracting magnetic field of the smaller magnet, so that the two magnets were always pulled together.

For the body of the car a tennis ball can was used. This material was chosen because it is extremely light weight and is also clear, enabling you to see through it to all that is going on. The ring proved to be of no consequence because it is a nonferrous material; a ferrous king would have been attracted to the coil causing the car to stop.

Lexan was used to mount the magnets on the car because it is clear and light weight, easy to cut. This material was also used in the walls.

The aluminum reaction plate on top of the car was the result of many tests with different lengths and thickness. The best length was one that extended a little past both ends of one coil. Thicker material reacted better/ but was harder to bend. Aluminum was used because it was lighter and cheaper than copper. Smaller plates tended to get stuck in the nulls in the center of each coil.

Electromagnetic induction was discovered in the 1800's by Nicola Tesla. The system used in this model is very basic and is a simple version of his original discovery. It consists of 4 electromagnetic coils over a single 60 cm. iron core.

Thin insulated sheets of soft iron, called laminations, are usually bonded together in blocks to make cores. However, this material was not available so the core is made of "tie-wire", the kind used with reinforcing bars in concrete. Without laminations strong electrical currents can cause unexpected or unpredictable changes in the magnetic field in the core. This is called the Hysteresis Effect and greatly reduces the efficiency of the coils.

The coils are made of four layers of narrow gage coated copper wire. In each layer there are approximately 200 turns. This means that there are 800 turns per coil. Each layer is separated from the next by a layer of tape. These layers are mounted on a 15 cm. length of p.v.c. The coils could have been longer or shorter; this length simply worked best (60 cm. a cm.) with the length of the wooden base (shelf). In the first working model, each coil had 600 turns. It was hoped that adding additional turns would increase the magnetic field strength and reduce heat in the coils. However, little or no improvement occurred, probably due to the Hysteresis Effect. This is probably also the reason for greater vibration in the car at certain points along the core. Loss of efficiency required more power, so the heat problem remains.

The coils were mounted above the car mainly because the strong magnetic field in the coils causes a strong vibration in the fixed magnets. The fixed magnets position and distance from the coils is important. A distance of 8.8cm., or more reduces the vibration greatly. So does turning the flat side of the magnet to the coils.

An alternating current passes through the coils, producing a magnetic field in and around each coil, creating a north and south pole at the end of each coil. The effect of the alternating current causes the poles for each electromagnet (coil) to continuously reverse position as the direction of the current changes. The polyphase (2-phase) current causes the position of the north and south poles to move in a linear fashion up and down the coils. The current in the coils induces a current in the aluminum plate mounted on the car. The current in the aluminum plate creates magnetic fields in the plate. These fields have one or more sets of north and south poles, which follow or are pushed by the moving poles of the coils.

A minimum of two phases of electrical current is required to create this effect. Since the power coming out of the electrical outlet is single (one) phase, it was necessary to divide the current into two paths. One path goes into a transformer then through two coils. Because of the resistance of the coils the field induced in the coils lags behind that of the power supply. The second path goes into a transformer and then through a series of capacitors before passing through the other two coils. The capacitors work to speed up, or overcome, the resistance of the coils, resulting in alternating magnetic field in those two coils, which is close in timing or sequence with the current that is available in the outlet. Since two coils are producing strong magnetic fields in time with the original power supply and two coils are producing magnetic fields slightly behind the timing of the power supply, there are now two waves or phases of current at work.

The greater resistance in the series of coils without the capacitors requires greater voltage be applied - 95 volts compared with 85 volts in the other series of coils. With smaller voltages the car will move, but is very difficult to start at some positions along the stator (line of coils).

The coils are arranged in a line, with alternating coils being powered by the same transformer. The switches on the table determine which coils are powered by which transformer, and the order or direction of the phase shift and direction of movement of the magnetic field along the coils.

My car weighed 12.3 to 14.5 ounces, depending on the size and thickness of the armature (reaction plate) mounted on the top of the car, and it required a minimum of 95 volts in one set of coils to start it moving easily.

The car accelerates rapidly once it has started moving. However, because the field only cycles every 60th of a second, a maximum speed of about 30 mph can be achieved with this set-up. Greater speeds could be achieved using transistors to increase the current's frequency.

The core in the coils is supposed to strengthen the magnetic field created by the coils. The strongest position is at the end of each coil. However, because the aluminum reaction plate only passes along the side of the end of the coil, the reaction is weak. "U" shaped coils would have worked much better. Heat is a problem with these coils. They get hot very quickly, limiting the length of time that the power button can be pushed. This has proved to also be problem for full-scale vehicles. The core material is also important. Laminations are supposed to increase the strength of the magnetic field by preventing eddies or unpredictable currents that can occur in a solid soft iron core. This may be another reason for the large amount of power required and the vibration in the car at some positions along the stator (coils).

The different reactions from different metals were interesting. Iron was pulled to the coils, because it is a ferrous material and is attracted by a magnetic field; it did not want to move along the coils. Aluminum was not attracted by the coils, but did want to be moved along them.

From this project I concluded that a magnetically levitated car can be made to move using an electromagnetic force. The future potential for this type of transportation and propulsion is great. It can be used for rides like roller coasters' transportation for people and freight between big cities. Since these trains can travel over 300 mph, and stations can be located inside the cities, it can easily compete even with the airlines over short distances.

I believe investing in this future method of travel would be a rewarding business, and would also benefit our nation. Each of the problems that were encountered can be corrected with research and in time, and many already have been.

The greatest problem was the lack of efficiency. A large amount of current was required to move a small weight. Also, large currents generate a lot of heat, which reduces the strength of the electromagnet. Full-scale trains have overcome this with super cooled electromagnets which are stronger, yet need less power, and do not have the heat problem.

Speed is limited in this model to approximately 30 mph because the car cannot exceed the speed of the pole shift. An adjustable frequency current is necessary to be able to adjust the speed. On this model there is a retaining wall which is necessary to keep the car on the track. This problem has not yet been completely solved even for the full-scale models. They too need a retaining wall of some kind. There are test models without walls except around the turns. This is a safety method to keep the train on track while turning at high speeds. Someday this problem will be solved as well.