A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits. A varying current in the primary winding creates a varying magnetic flux in the transformer’s core and thus a varying magnetic flux through the secondary winding.
This varying magnetic flux induces a varying electromotive force (EMF), or “voltage”, in the secondary winding In electrical engineering, two conductors are referred to as mutual-inductively coupled or magnetically coupled  when they are configured such that change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction. The amount of inductive coupling between two conductors is measured by their mutual inductance.
Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stagemicrophone to huge units weighing hundreds of tons used in power plant substations or to interconnect portions of the power grid. All operate on the same basic principles, although the range of designs is wide. The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).
Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil. An ideal transformer is shown in the figure below. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.
If a load is connected to the secondary winding, the load current and voltage will be in the directions indicated, given the primary current and voltage in the directions indicated (each will be alternating current in practice). Ideal power transformer If a load is connected to the secondary winding, current will flow in this winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. Transformers may be used for AC-to-AC conversion of a single power frequency, or for conversion of signal power over a wide range of frequencies, such as audio or radio frequencies.
In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows: If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient. All the incoming energy is transformed from the primary circuit to themagnetic field and into the secondary circuit.
If this condition is met, the input electric power must equal the output power: giving the ideal transformer equation Hysteresis losses Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.  Eddy current losses Ferromagnetic materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length.
Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.  Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.
Magnetostriction related losses Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformersthat can cause losses due to frictional heating. This buzzing is particularly familiar from low-frequency (50 Hz or 60 Hz) mains hum, and high-frequency (15,734 Hz (NTSC) or 15,625 Hz (PAL)) CRT noise. Stray losses
Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer’s support structure will give rise to eddy currents and be converted to heat..  There are also radiative losses due to the oscillating magnetic field but these are usually small. Mechanical losses In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings.
These incite vibrations within nearby metalwork, adding to the buzzing noise and consuming a small amount of powe Laminated steel cores Transformers for use at power or audio frequencies typically have cores made of highpermeability silicon steel.  The steel has a permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. General Types of Transformers
Transformers are more than just extraterrestrial robots—they’re also highly useful devices for transferring energy between circuits. By using inductively coupled electrical conductors as the main agent of transfer, a change in current in the first circuit is carried over to the second circuit, which subsequently assumes the new charge. Each end of the circuit carries the charge within a winding—either primary or secondary—which is constituted of electrically conductive wire wound around opposite ends of the transformer core, which has high magnetic permeability, making the transfer possible.
In an ideal situation, the change in voltage is proportional, with the second circuit receiving voltage in relation to the number of turns in the primary winding. The voltage is therefore manipulated by altering the number of turns in the primary winding to be larger or smaller than the number of turns in the secondary winding, which either increases or decreases the amount of electricity received. Transformers are essential when it comes to the national power grid and are responsible for transmitting large amounts of high voltage power over long distances.
This is not to say that all transformers are large—they come in many different sizes—and some are certainly not designed for high levels of output. Depending on the intended function and the amount of power needed, transformers can be as small as a fingernail or weigh several hundred tons. Common Types of Transformers Autotransformers Autotransformers are different from traditional transformers because autotransformers share a common winding. On each end of the transformer core is an end terminal for the winding, but there is also a second winding that connects at a key intermediary point, forming a third terminal.
The first and second terminals conduct the primary voltage, while the third terminal works alongside either the first or second terminal to provide a secondary form of voltage. The first and second terminals have many matching turns in the winding. Voltage is the same for each turn in the first and second terminal. An adaptable autotransformer is another option for this process. By uncovering part of the second winding and using a sliding brush as the second terminal, the number of turns can be varied, thus altering voltage (see image on right).
Polyphase Transformers This type of transformer is commonly associated with three phase electric power, which is a common method of transmitting large amounts of high voltage power, such as the national power grid. In this system, three separate wires carry alternating currents of the same frequency, but they reach their peak at different times, thus resulting in a continuous power flow. Occasionally these “three-phase” systems have a neutral wire, depending on the application. Other times, all three phases can be incorporated into one, multiphase transformer.
This would require the unification and connection of magnetic circuits so as to encompass the three-phase transmission. Winding patterns can vary and so can the phases of a polyphase transformer. Leakage Transformer Leakage transformers have a loose binding between the primary and secondary winding, which leads to a large increase in the amount of inductance leakage. All currents are kept low with leakage transformers, which helps prevent overload. They are useful in applications such as arc welding and certain high-voltage lamps, as well as in the extremely low-voltage applications found in some children’s toys.
Resonant Transformer As a type of leakage transformer, resonant transformers depend on the loose pairing of the primary and secondary winding, and on external capacitors to work in combination with the second winding. They can effectively transmit high voltages, and are useful in recovering data from certain radio wave frequency levels. Audio Transformer Originally found in early telephone systems, audio transformers help isolate potential interference and send one signal through multiple electrical circuits.
Modern telephone systems still use audio transformers, but they are also found in audio systems where they transmit analog signals between systems. Because these transformers can serve multiple functions, such as preventing interference, splitting a signal, or combining signals, they are found in numerous applications. Amplifiers, loudspeakers, and microphones all depend on audio transformers in order to properly perform. A transformer has a primary coil of 100 turns and a secondary coil of 300 turns. What will be the voltage out of the secondary coil if we apply 20 V to the primary?
Power factor The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit, and is a dimensionless number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor. The devices for correction of the power factor may be at a central substation, spread out over a distribution system, or built into power-consuming equipment Power factor correction of linear loads
A high power factor is generally desirable in a transmission system to reduce transmission losses and improve voltage regulation at the load. It is often desirable to adjust the power factor of a system to near 1. 0. When reactive elements supply or absorb reactive power near the load, the apparent power is reduced. Power factor correction may be applied by an electric power transmission utility to improve the stability and efficiency of the transmission network.
Individual electrical customers who are charged by their utility for low power factor may install correction equipment to reduce those costs. Power factor correction brings the power factor of an AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load, respectively. For example, the inductive effect of motor loads may be offset by locally connected capacitors. If a load had a capacitive value, inductors (also known as reactors in this context) are connected to correct the power factor.
In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the energy is just moving back and forth on each AC cycle ————————————————- Measuring the power factor The power factor in a single-phase circuit (or balanced three-phase circuit) can be measured with the wattmeter-ammeter-voltmeter method, where the power in watts is divided by the product of measured voltage and current. The power factor of a balanced polyphase circuit is the same as that of any phase. The power factor of an unbalanced polyphase circuit is not uniquely defined.