A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling without using any moving parts. It is often used to convert between high and low voltages and for impedance transformation. The transformer was an important element in the development of high-voltage electric power transmission and central generating stations.
A simple single phase transformer consists of two electrical conductors called the primary winding and the secondary winding. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field around the conductor. According to the principle of mutual inductance, the secondary, which is placed in this varying magnetic field, will develop an electromotive force or EMF. If the ends of the secondary are connected together to form an electric 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. In practical transformers, the primary and secondary conductors are coils of conducting wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor.
Transformers alone cannot do the following:
However, transformers are components of the systems that perform all these functions.
Consider the following two laws:
- According to the law of conservation of energy, the power delivered by a transformer cannot exceed the power fed into it.
- The power dissipated in a load at any instant is equal to the product of the voltage across it and the (in phase) current passing through it (see also Ohm's law).
It follows from the above two laws that a transformer is not an amplifier. 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, inversely proportional to the voltages. If the voltage were to be stepped down by the transformer, the secondary current available to the load would be greater. For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.
- P = E·I (power = electromotive force · current)
50 W = 2 V · 25 A in the primary circuit
- Now with transformer change:
50 W = 25 V · 2 A in the secondary circuit.
The high-current, low-voltage windings have fewer turns of (usually) thicker wire. The high-voltage, low-current windings have more turns of (usually) thinner wire.
The electromotive force (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. Neglecting all leakage flux, an ideal transformer follows the equation:
Where is the voltage in the primary coil, is the voltage in the secondary coil, is the number of turns of wire on the primary coil, and is the number of turns of wire on the secondary coil. This leads to the most common use of the transformer: to convert power at one voltage to power at a different voltage.
Neglecting leakage flux, the relationship between voltage, number of turns, magnetic flux intensity and core area is given by the universal emf equation:
Where is the sinusoidal root mean square (RMS) voltage of the winding, is the frequency in hertz, is the number of turns of wire, is the area of the core (square units) and is magnetic flux density in webers per square unit. The value 4.44 collects a number of constants required by the system of units.
In normal operation, a transformer winding should never be energised from a constant DC voltage source, as this would cause a large direct current to flow. In such a situation, in an ideal transformer with an open circuit secondary, the current would rise indefinitely as a linear function of time. In practice, the series resistance of the winding limits the amount of current that can flow, until the transformer either reaches thermal equilibrium or is destroyed. DC is occasionally applied to large power transformers in order to "bake out" water prior to adding the c