A transformer is a basic component in electronic circuits that steps voltage up or down. This is accomplished through two copper wire windings, the primary and secondary coils, around a continuous magnet, called the core. Transformer losses refer to the electrical energy that is lost during the stepping up or stepping down of voltage.
Another way that one can look at this is that nothing comes without a cost in electronics that run at normal operating temperatures. The amount of power put into the primary transformer winding always comes out lower in the secondary winding. The primary coil does not physically touch the secondary coil, as one would expect in other types of electrical connections. The connection is actually done by the magnetic field and it interaction with electrons. This connection is known as induction, which makes sense because the magnetic field induces, or causes, the electricity to move from the primary coil to the secondary.
Transformer losses are a direct result of magnetic induction and can be mathematically predicted. To understand this, one can consider what a magnetic field looks like. If iron filings are scattered on a stiff piece paper placed over a magnet, the iron filings form into curved lines. Electricity is lost in transformers because the curved magnetic lines take some of the energy into the open air and surrounding materials rather than directly to the secondary coil.
When people are first introduced to transformer losses, the reaction might be that transformers are too inefficient to be any good. The engineering challenge, however, is to reduce transformer losses to amounts that are unimportant within the rest of the circuit. Transformers vary in size from the very small to be found on computer motherboards to the very large in use at industrial power plants. The large transformers can afford to lose more energy than their smaller counterparts.
Heat energy is an important result of transformer losses. The lost electrons interact with materials around them, including some gases in the air, and this is where the heat comes from. If the heat is not removed fast enough, the transformer could pop and, in larger models, explode. Popping and exploding can also occur if a relatively large electrical power spike is pushed into the primary coil. This is why the mathematics need to be run first to determine the operating limits of a particular transformer design.