Transformer

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling, without requiring relative motion between its parts. A transformer comprises two or more coupled windings, and, in most cases, a magnetic core to concentrate magnetic flux. A changing voltage applied to one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

A transformer can be likened to a mechanical gearbox, which transfers mechanical energy from a high-speed, low torque shaft to a lower-speed, higher-torque shaft, but which is not a source of energy itself. A transformer transfers electrical energy from a high-current, low-voltage circuit to a lower-current, higher-voltage circuit.

Transformers are adapted to numerous engineering applications and may be classified in many ways:

• By power level (from fraction of a voltampere (VA) to over a thousand MVA),
• By application (power supply, impedance matching, circuit isolation),
• By frequency range (power, audio, radio frequency (RF))
• By voltage class (a few volts to about 750 kilovolts)
• By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.)
• By purpose (distribution, rectifier, arc furnace, amplifier output, etc.).
• By ratio of the number of turns in the coils

• Step-up: The secondary has more turns than the primary.

• Step-down: The secondary has fewer turns than the primary.

• Isolating: Intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).

• Variable: The primary and secondary have an adjustable number of turns, which can be selected without reconnecting the transformer.

An ideal transformer would have no energy losses, and would therefore be 100% efficient. In practice, energy is dissipated due both to the resistance of the windings known as copper loss, and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are, in general, highly efficient: large power transformers (over 50 MVA) may attain an efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient.

The windings of high-power or high-voltage transformers are immersed in transformer oil — a highly refined mineral oil, that is stable at high temperatures. The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. 

Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability and toxic effects of PCB by-products, and its accumulation in the environment, it is no longer used in new equipment. Old transformers that still contain PCB is recommended to be examined on a weekly basis for leakage. If found to be leaking, it should be changed out, and professionally decontaminated or scrapped in an environmentally safe manner. Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.

Outdoor electrical distribution systems are subject to lightning surges. Even if the lightning strikes the line some distance from the transformer, voltage surges can travel down the line and into the transformer. High voltage switches and circuit breakers can also create similar voltage surges when they are opened and closed. Both types of surges have steep wave fronts and can be very damaging to electrical equipment . To minimize the effects of these surges, the electrical system is protected by lighting arresters but they do not completely eliminate the surge from reaching the transformer. The basic impulse level (BIL) of the transformer measures its ability to withstand these surges. All 600 volt and below transformers are rated 10 kV BIL. The 2400 and 4160 volt transformers are rated 25 kV BIL.

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