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Using Magnetic Fields to Change Voltages

Chris Greacen

©1993 Chris Greacen

At the turn of the century a battle raged over whether the country would be run on alternating current (ac) electricity, or direct current (DC) electricity. Thomas Edison was the big proponent of DC electricity. The eccentric inventor Nikola Tesla, backed by George Westinghouse, was sure the future lay with ac electricity. When the dust cleared, the utilities chose ac electricity. The determining factor was that ac electricity can easily be changed from one voltage to another using transformers. This meant that power could be produced at a voltage convenient for the power plant (hundreds of volts), transported great distances at a higher voltage (hundreds of thousands of volts), and eventually reduced to lower, safer voltages for use in homes and businesses.

Historically, homes powered by renewable energy were 12 Volt homes. Energy was produced, stored, and used as 12 Volt DC. We're finding, though, that renewable energy systems can be cheaper and more flexible if we play some of the voltage transforming games which the utilities have long been doing on a grand scale. This allows power producers (solar, wind and hydro) to harvest Nature's energy offerings efficiently. It allows low-loss transportation, and convenient and safe storage and use.

Technology has progressed since the days of Tesla and Edison. Nowadays it's possible to "transform" DC electricity from one voltage to another using DC to DC converters. These circuits are more complex than ac transformers, because they contain an oscillator and switching circuitry. But properly designed, DC to DC converters can be smaller, more flexible, while just as efficient as the best transformers. PowerStar and Statpower inverters and Todd chargers are common renewable energy equipment which use DC to DC converters in places which had previously been the exclusive territory of 60 Hertz ac transformers. Bobier Linear Current Boosters and Solarjack pump controllers are DC to DC converters which "transform" the power from photovoltaics to better match the current and voltage needs of DC electric motors.

How Does This Stuff Really Work?

Let's get into the nitty gritty, starting with how transformers work. My last article (Home Power#35, page 77) discussed magnetic fields created by electrical current, and electrical current made by changing magnetic fields. Here's a recap of the two laws of physics at work.

Ampere's Law: When current flows in a wire it creates a magnetic field wrapping around the wire.

The "right hand rule" predicts the direction of the magnetic field: point the right thumb in the direction of

Figure 1: the right hand rule.

the current, and the fingers wrap around in the direction of the magnetic field.

If you wrap the wire into a coil, a current flowing in the wire creates a magnetic field (stronger than one made by a straight wire) inside the coil. A coil of wire like this is called an inductor.

Figure 2: Magnetic Fields in an inductor.

The more turns of wire, the greater the magnetic field. If you stick a hunk of iron inside the coil the magnetic field can increase thousands of times.

To convince yourself Ampere's law is for real, make an electromagnet. Wrap a small (22 gauge to 26 gauge) wire 100 or more times around a big nail. Connect the wire ends to the positive and negative sides of a flashlight battery. Pick up washers, small bolts. Try more wire wraps, or another battery in series. (Note: this experiment is pretty hard on batteries)

Faraday's Law: A changing magnetic field produces a voltage in a nearby wire.

The current (if any) which flows as a result of this voltage will be in the direction such that the magnetic field it produces will counteract the original magnetic field change. If this sounds confusing, read on, it will make more sense as we look at how a transformer works.

Transformers

Figure 3 shows a simple transformer. The "primary" winding (on the left hand side) is where you put in the ac electricity you want to "transform". You attach the load to the "secondary" winding (right hand side). A soft iron ring magnetically couples the primary and secondary. In this picture the primary winding has 15 turns, and the secondary has three. This was easy to draw, but let's pretend they have 150 and 30 turns respectively, as a real transformer might.

Figure 3

Imagine applying direct current (not ac) to the primary. The current flowing through the primary coil creates a magnetic field (pointing downward in the picture). This magnetic field travels through the iron core (counterclockwise) and inside the loops of the secondary coil. For the instant that the magnetic field grows from zero to its full strength, it induces a voltage in the secondary coil, and the bulb lights for a moment. But the instant the magnetic field stops growing (when the primary current reaches a steady state — Figure 4), the bulb goes out (secondary current is zero).

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