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See the respective unrelated articles about the toyline and related comics and animated television series from the 1980s onwards, Transformers, and the glam rock album by Lou Reed, named Transformer, for those respective topics.
A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling. It is often used to convert between high and low voltages and accordingly between low and high currents.
A simple single phase transformer consists of two electrical conductors called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field of voltage around the conductor. According to the principle of mutual inductance, which is a special case of electromagnetic induction applied to two coupled conductors, the secondary, which is placed in this varying magnetic field, will develop a potential difference called an electromotive force or EMF. If the ends of the secondary are connected together to form an electrical circuit, this EMF will cause a current to flow in the secondary. Thus, some of the electrical power fed into the primary is delivered to the secondary.
Transformers cannot do the following:
Consider the following two conservation of energy, the power delivered by a transformer cannot exceed the power fed into it.
It follows from the above two laws that if the transformer is used to change power from one voltage to another, the magnitudes of the currents in the two windings must also be different, in inverse ratio to the voltages. The high-current low-voltage windings have fewer turns of wire. The high-voltage, low-current windings have more turns of wire.
(The amount of current a wire can carry is limited by its thickness. So most transformers have thicker wire on the high-current winding, thinner wire on the low-current winding).
Transformers can be classified into various types according to the ratio of the numbers of turns in the coils, as well as whether or not the primary and secondary are isolated:
In most practical transformers, the primary and secondary conductors are coils of wire (usually copper), because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor. The EMF developed in the secondary is proportional to the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. Hence, the transformer equation:
Where <math>V_p<math> is the voltage in the primary coil, <math>V_s<math> is the voltage in the secondary coil, <math>N_p<math> is the number of turns of wire on the primary coil, and <math>N_s<math> is the number of turns of wire on the secondary coil. This leads to the commonest use of the transformer: to convert power at one voltage to power at a different voltage.
The difference between the power output and the power input is called the loss. An ideal transformer would have no loss, and would therefore be 100% efficient. Real transformers are often more than 98% efficient; the remaining 2% (or less) of the input energy is lost to:
Those credited with the invention of the transformer include:
Standard symbols
| Transformer with two windings and iron core. | |
| Transformer with three windings. The dots show the adjacent ends of the windings. |
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| Step-down or step-up transformer. The symbol shows which winding has more turns, |
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| Transformer with electrostatic screen, which prevents electrostatic coupling between the windings. |
Transformer designers optimize the wire sizes so that each winding will have the lowest resistance while keeping the winding size as small as possible, in an effort to minimize resistive power dissipation (commonly called copper losses). Some transformers have equal numbers of windings on both coils. These "isolation" transformers are used to prevent direct current flow between electric circuits, while transferring power. In transformers designed to operate at low frequencies, the windings are usually formed around an iron core. This helps to confine the magnetic field within the transformer and increase its efficiency, although the presence of the core causes energy losses.
Transformers often have silicon steel cores to channel the magnetic field. This keeps the field more concentrated around the wires, so that the transformer is more efficient. The core also keeps the field from being wasted in nearby pieces of metal. The core of a power transformer must be designed so that it does not reach magnetic saturation. Carefully designed gaps are sometimes placed in the magnetic path to help prevent saturation.
Laminated cores are made of many stamped pieces of thin steel. The high resistance between layers reduces eddy currents in the cores that waste power by heating the core. These are common in power and audio circuits. A typical laminated core is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". The best efficiencies of laminated-core transformers are around 85%.
In higher frequency circuits, powdered iron cores are sometimes used. These are common, for instance, in switching power supplies. At even higher frequencies (radio frequencies typically) other types of core made of nonconductive magnetic materials, such as various ceramic materials called ferrites are common.
High-frequency transformers in low-power circuits where moderate losses are acceptable may have air cores. These save weight and cost.
The voltage difference between parts of the primary and secondary windings can be quite large, and layers of insulation are sometimes required between windings to prevent arcing.
Although an ideal transformer is purely magnetic in operation, the close proximity of the primary and secondary windings can create a mutual capacitance between the windings that sometimes can not be ignored in analyzing the circuit behavior. Sometimes an electrostatic shield is placed between windings to minimize this effect. This is common, for instance, in transformers designed to achieve high electrical isolation between primary and secondary circuits.
The higher-voltage transformers are bathed in nonconductive oil that is stable at high temperatures. This used to be polychlorinated biphenyl, the famous toxic waste, "PCB". Nowadays, nontoxic, very stable fluorinated hydrocarbons are preferred. The oil cools the transformer, and helps prevent short circuits. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire.
Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel wound into a coil. This construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance, and eliminates the air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference.
Toroidal transformers are more efficient (around 95%) than the cheaper laminated EI types. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantage is higher cost. Another problem, significant in larger transformers of more than a few hundred watts output, is the higher inrush current (the extra current that flows for a short period when the transformer is first switched on), which can cause mains fuses to blow unless current-limiting circuitry is added.
When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork.
An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are commonly used as spark coils in internal combustion engines in automobiles, and as high-voltage flyback transformers in television sets and computer monitors.
Variac is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. However, the name "variac" is so commonly used, even to describe competing brands such as Powerstat (TM), that it has become the standard term for variable autotransformer, in much the same way that Xerox has become synonymous with photocopiers by all makers. A sliding contact on a variac determined what fraction of the winding was connected across the output; a common configuration provided for 120 V as input and percentages of that voltage as high as about 110%. Typical variacs thus go from 0 to 140 volts when supplied with 120 volts input. They are thus stepacross transformers (neither properly considered stepdown, nor stepup). More compact semiconductor light dimmers have displaced variacs in many applications, such as theatrical lighting, but such dimmers reduce lamp voltage by rapidly turning the lamp on and off, creating a "buzz" that shortens lamp life and introduces harmonics. Accordingly, variacs still enjoy use in certain niche applications where a pure sinewave is desired.
The simple transformers mentioned above are for transforming a real-valued input to a real-valued output. For transforming complex electrical signals or complex power, multiple transformers can be used, e.g. one to transform the real part, and one to transform the imaginary part, for two-phase signaling or two-phase power. For three phase power, three separate transformers can be used. Three-gang variacs are commonly used as stepacross transformers for three phase power. The gangs work together in much the same way as a stereo potentiometer gangs two pots on one shaft. Alternatively, there also exist three phase transformers (one transformer that handles all three phases).
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