(offtopic) Transformer Basics – by Alvenh Channe

I just wanted to answer some of the questions you’ve had earlier about the electronics stuff. I’m sorry I didn’t have the time to sit down to explain. So, let us begin with a discussion of transformer basics…

What is a transformer?

There are many different types of transformers, but basically, a transformer is simply a device that uses electricity (AC) to generate electricity. By doing that, they accomplish one or more of the following tasks: convert high voltage AC to low voltage AC (called step-down transformers), jack-up low voltage AC to high voltage AC (called step-up transformers), isolation of one circuit from another (isolation transformers), impedance matching, removal of DC component from mixed AC/DC signals, and other tasks.

As you might have learned in your high school physics class, moving charges (i.e. electrons or protons in motion) generate magnetism, thus if you have an electric current running thru a wire, a magnetic field is created around the wire. The strength of the magnetic field is determined by the amount of current, which is given by the equation F = IlB, where F is magnetic force, I is current, l is length of wire, and B is magnetic field. DC results in a non-changing magnetic field; AC results in an alternating magnetic field (one in which north and south keeps changing back and forth).

Now, going the other way, if a wire is placed in a CHANGING field, a current is generated or INDUCED in the conductor (the wire) such that its magnetic field acts to oppose the change. If the wire is placed is a NON-changing magnetic field, there is no current generated because there is no need to oppose any changes; however, if you MOVE the conductor in a way such that it “cuts” the lines of magnetic forces, this motion mimics the magnetic activity of an alternating field and a current will be generated in the conductor. That’s how basic mechanical electric generators work by converting kinetic energy (energy of motion) to electrical energy.

On the other hand, you can apply an electric current to a conductor that’s already in a magnetic field and the conductor will start to move (like poles between B-field of wire and magnet repel; opposite polarities attract). That’s how electric motors work. And that’s why electric motors with permanent magnets can be used as generators and vice versa.

Now, what if you have coil of wire (with an electric current running thru it) wrapped around an iron core? You’d have an electromagnet. But what if you ran AC into the coil? You’d have an electromagnet with alternating magnetic field (north and south poles keeps swapping at a rate of whatever the AC frequency is). As I have mentioned above, if a conductor is placed in an alternating magnetic field, an alternating current is induced (generated) in the conductor. So, what if I took a second coil of wire (with no current running thru) and placed it onto the same core (the AC electromagnet)? Well, in this case, as you might have guessed, an AC voltage is generated in the second coil. Also, as you might have noticed, the second coil is isolated from the first. This device (coils plus iron core) is called an induction coil, commonly known as a TRANSFORMER.

In a transformer, the coil in which you apply AC is called the primary winding (or simply the primary). The other coil in which an alternating current is generated is called the secondary winding (or simply the secondary). Voltage in the secondary can be of the same voltage as in the primary or can be higher or lower. Although, all transformers with separate primary and secondary windings perform the task of isolation, the transformers I have mentioned are known respectively as isolation, step-up and step-down.

If there are MORE windings in the secondary than in primary, the output voltage will be GREATER than that of the input voltage as in the case of a step-up. If the secondary has LESS turns of wire than the primary, the output voltage will be LOWER than that of the input voltage, as in the case of a step-down transformer. If a transformer is 100% efficient, the TURNS RATIO is equal to the VOLTAGE RATIO. Thus, a transformer that’s used to step down 120 volts to 60 volts would have a turns ratio of 2:1.
(Note: Transformers with an efficiency of less than 85% are considered dangerous, as the rest of the input energy is converted to heat.)

So, you may be wondering, can I take a 2:1 step down (like the one described above), reverse the wiring and use it as a step-up transformer to convert 120 volts to 240 volts? Theoretically speaking, it is possible…BUT…in reality, the coils, WITH RESPECT TO the core material, are NOT rated to handle those voltages. If there are not enough turns, the windings will not be able to handle higher voltages since there would be more magnetic flux than the core could hold. This results in flux offset known as core saturation. Current in wire will increase to dangerous levels, and your transformer will go up in smoke and melt.

The transformer doesn’t know if it’s a step-down or step-up, but it does know the voltage ratings of the windings. Thus, you can take what is normally a 120-volt-to-6-volt step-down transformer and use it to step-up 6 volts to 120 volts by applying 6 volts to the 6 volt windings. In this case, what is normally the secondary is now used as the primary. As I have mentioned, 1:1 transformers also exist.

So, why would anybody want to convert 120 volts to 120 volts when you can get that right out of the wall without having to spend extra? The answer is isolation, and transformers used to perform such tasks are called isolation transformers.

Isolation is important when you want to work with live circuits. On the unisolated primary (house wiring), one side of the 120 volt line is connected to ground and it’s called neutral. The other side of the 120 volt line is known as the “hot” side, which measures at 120 volts WITH RESPECT TO THE GROUND YOU ARE STANDING ON. Thus, if you come in contact with the “hot” side of the wiring, you will receive a shock. An isolation transformer gives you 120 volts completely UNGROUNDED on the secondary. Therefore, should you come in contact with any one side of the 120 volt secondary, you will not get shocked. However, if contacting both sides of the secondary is equally as dangerous as contact with both sides (or one hot side) of the primary.

Transformers that are built into power supplies to provide power to equipment are known as “power transformer”. The Hammond 269AX used in the AECA transmitter is an example of a power transformer.  If you go to a store like Future Shop to buy a modern stereo system, you’ll notice that the device has a power cord for you to plug into the wall so that there is electricity to run the system.

Although the power that comes out of the wall outlet is 120 volts AC (really 117 VAC at 60 cycles in Canada, US, and Mexico), the stereo system does NOT run on 117 volts AC. Instead, a step-down power transformer that is built into the system converts 117 VAC to 12 or 24 volts or so on which your system is designed to use. Also, the 12 or 24 (or whatever) volts from the transformer is AC, and will need to be converted to DC before it can be used by the system.

An example: that big block that you plug into that wall to recharge you Nokia cell phone contains a step-down transformer. However, in the olden days when tubes were used, 117 volts from your wall was often insufficient for equipment. Therefore, step-up power transformers were often used to convert 117 volts AC to HIGHER voltages. Household tube devices usually require 200 to 600 volts to run. The AECA transmitter is an example of a device that uses a step-up transformer to provide 250 volts for the tubes.

Step-up and step-down transformers are also used a lot by the power company (BC Hydro). The power created by water turbine generators at the dams create about 10,000 to 100,000 volts of electricity. Step-up transformers boost up the voltage to anywhere between 100 kV (medium distance transmission lines) to 1 MV (long distance transmission lines) for greater efficiency in transmission. Power substation around town contain step-down transformers to supply electricity to medium voltage local feeders (about 6 kV to 60 kV), which in turn, is stepped down further into to low voltage feeders (around 10 kV or less) around residential areas. Wherever, electricity is needed for houses or buildings, step-down transformers mounted on the poles tap power from the feeder cables and is stepped down, providing 230 volts (center tapped) to buildings.

Center tapped means the middle of the secondary winding has a tap. Voltage at one end of the windings is 117 volts with respect to center tap; voltage at one end is 234 volts with respect to the other end. [234 volts CT is also known as 117-0-117] The power company grounds the center tap, thus, a 117 volt outlet has one hot side while a 234 volt outlet has BOTH sides hot. If you accidentally touch one side of the 234 volt wiring in your house, the shock is really a 117 volt shock, not 234 volts.

Anyways, that’s the best I can do in explaining the basics. I hope this helps. Please do not hesitate to e-mail me if you have any questions.  We’ll talk about impedence matching transformers next time.

By: Alvenh Channe (achanne@sfu.ca), Jan. 16, 2004.

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