Basic principles
A simple single phase(1φ) transformer consists of two electrical conductors called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed direct current) 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 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.
In practical transformers, the primary and secondary conductors are coils of wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor.
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, 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.
Transformers alone cannot do the following:
Convert DC to AC or vice versa
Change the voltage or current of DC
Change the frequency (the "cycles") of AC.
However, transformers are components of the systems that perform all these functions.
Electrical laws
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 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, in inverse ratio to the voltages. Thus if current were to be brought down by the transformer, voltage would go up. If voltage were to be brought down by the transformer, current would go up. For example, suppose 50 watts is fed into a transformer with a ratio of 25:2.
P = I*E (Power = Current * Electromotiveforce)
50 W = 25 A * 2 V in the primary circuit
Now with transformer change:
50 W = 2 A * 25 V in the secondary circuit.
The high-current low-voltage windings have fewer turns of wire. The high-voltage, low-current windings have more turns of 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: Vp/Vs = Np/Ns
Where Vp is the voltage in the primary coil, Vs is the voltage in the secondary coil, Np is the number of turns of wire on the primary coil, and Ns 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.
Again, neglecting leakage flux, the relationship between voltage, number of turns, magnetic flux intensity and core area is given by: E = 4.44 * F * n * a * b
Where E is the sinusoidal root mean square (RMS) voltage of the winding, F is the frequency in hertz, n is the number of turns of wire, a is the area of the core (square units) and b is magnetic flux density in webers per square unit. The value 4.44 collects a number of constants required by the system of units.
