A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called mutual induction.
Or we can write “A transformer transfers electrical energy between two circuits. It usually consists of two wire coils wrapped around a core. These coils are called primary and secondary windings. Energy is transferred by mutual induction caused by a changing electromagnetic field. If the coils have different number of turns around the core, the voltage induced in the secondary coil will be different to the first.”
Transformers are based on the theory of electromagnetic induction, which was discovered by Michael Faraday in 1831. It was not until 1836 that the first device, an induction coil, was invented. William Stanley, who designed the first commercial model, introduced the term "transformer" in 1885.
Working principle of a XFR: 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 .The secondary current arises from the action of the secondary EMF on the (not shown) load impedance.
An ideal transformer is shown in the adjacent figure. 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).
Induction law:
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:
where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is the magnetic fluxthrough one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals
Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping down the voltage
Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the case of step-up transformers, this may sometimes be stated as the reciprocal,Ns/Np. Turns ratio is commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.
Ideal Power equation :
Fig: The ideal XFR as a circuit element
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 incoming electric power must equal the outgoing power:
giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable approximation.
If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by thesquare of the turns ratio.[29] For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.
Types of Transformers:The two major types of transformer are Laminated cores and Toroidals
Laminated cores are those common cube-shaped transformers, which are used in power adapters. They are stronger and cheaper than toroidals.
Toroidals are smaller and lighter, for the same power rating. They also produce less electrical noise and are more efficient. The secondary winding can be joined in series to double the voltage or joined in parallel for higher current.
Other types of transformers include the variac, audio, and balun.
Variacs have a movable carbon brush that connects to the winding, providing a large range of voltages.
Audio transformers are used to amplify signals and drive speakers.
Baluns are short windings that convert impedances, such as those between a television and an antenna.
Use of the Transformer:
Transformers are mainly used to convert one voltage to another. The process of increasing the voltage is called "stepping up", while decreasing the voltage is called "stepping down". Most electronic equipments need a transformer to lower the mains voltage to a usable level. Transformers are also found in power adapters and battery chargers. Inverters are transformers which step-up a low voltage to a higher voltage, allowing a mains powered equipment to run on a battery. Additional circuitry is required to change the battery's direct current into alternating current. Transformers are used for electricity distribution to minimize energy loss over long distances. Higher voltages allow for lower currents, which reduces the losses caused by resistance.