A transformer takes in electricity at a higher voltage and lets it run through lots of coils wound around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core is an output wire with fewer coils. The magnetism changing back and forth makes a current in the wire. Having fewer coils means less voltage. So the voltage is “stepped-down.”

The following are three different kinds of transformers:

l       Three-phase transformers.

A basic 3-phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.

The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the wye, in which all three non-polarity (or polarity) ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or wye-wye), or differently (delta-wye or wye-delta). It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30? phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.

l       Autotransformers.

An autotransformer is a transformer with an electrical connection between the primary and secondary windings. Autotransformers have considerably more MVA capacity per pound of core iron and winding conductor than standard power transformers, but are limited to small turns rations — ideally 2:1.

Although the designs of different transformers vary extensively, their basic operation remains the same.

l       Special transformers.

Transformers can have more than two windings per phase. These designs help reduce fault current levels. Other transformers have been built to operate at relatively low voltages but extremely high currents. Arc furnace transformers fall into this category, and can have secondary current ratings in excess of 150,000A. Regulating transformers are designed to maintain their secondary voltage within specific limits as the primary voltage fluctuates. Transformers can also be built to shift phase a specified amount to control the flow of real power in a networked system.

An electric plug is a male electrical connector with contact prongs to connect mechanically and electrically to slots in the matching female socket.

Wall sockets are female electrical connectors that have slots or holes which accept and deliver current to the prongs of inserted plugs. To reduce the risk of injury or death by electric shock, some plug and socket systems incorporate various safety features. Sockets are designed to accept only matching plugs and reject all others.

Electrical plugs and their sockets differ by country in shape, size and type of connectors. The type used in each country is set by national standards legislation. As is known to all, each type is designated by a letter designation from a U.S. government publication, plus a short comment in parentheses giving its country of origin and number of contacts. Subsections then detail the subtypes of each type as used in different parts of the world.

IEC Classes are assigned to electrical devices depending on whether or not they are earthed and the degree of insulation they incorporate. Class I, for example, refers to earthed equipment, while class II refers to unearthed equipment protected by double insulation.

Special purpose sockets may be found in residential, industrial, commercial or institutional buildings. These may be merely labeled or colored, or may have different arrangements of pins or keying provisions. Some special-purpose systems are incompatible with general-purpose lighting and appliances. Examples of systems using special purpose sockets include:

• “clean” ground for use with computer systems,
• emergency power supply,
• uninterruptible power supply, for critical or life-support equipment,
• isolated power for medical instruments,
• “balanced” or “technical” power used in audio and video production studios,
• theatrical lighting
• outlets for electric clothes dryers, electric ovens, and air conditioners with higher current rating.

Depending on the nature of the system, special-purpose sockets may just identify a reserved use of a system (for example, computer power) or may be physically incompatible with utility sockets to prevent use of unintended equipment which could create electrical noise or other problems for the intended equipment on the line.