A transformer is a static electrical device which transfers AC electrical energy from one (or more) electrical system to another (or others) through electromagnetic induction, without changing the frequency. It consists of two or more coils wound around a common magnetic core, galvanically separated from each other. The coil, which is energized, is called primary winding, and the voltage – input voltage. The voltage which is obtained at the output is referred to as an output voltage, and the coil –the secondary winding.
Principle of operation
The operation of the transformer is based on two main principles:
1. The time-varying electric current in the primary winding creates a time-varying electromagnetic field.
2 . The electromagnetic field generates alternating electric current in the secondary winding by electromagnetic induction. The electromagnetic induction in the secondary winding is
and respectively in the primary winding it is:
U2 is the voltage of the secondary winding;
N2 is the number of turns of the secondary winding;
Ф is the cumulative magnetic flux through one turn;
If we divide the secondary voltage to the primary voltage, we get:
In order to reduce losses, the coils are wound around a magnetic core of ferromagnetic material. There are transformers operating at high and ultra-high frequencies that don’t have a magnetic core. In the ideal transformer all magnetic power lines go through all turns and the variable magnetic field generates one and the same electromotive force in each turn, so the total electromotive force is proportional to the number of turns of the coil. Moreover, in the ideal transformer the whole primary power is converted without any losses into an electromagnetic field and then into energy in the secondary circuit. In the ideal transformer the input power is equal to the output power and it also equals to the product of the current and the voltage of the primary side, as well as to this product of the secondary side:
P1 and P2 are respectively the instantaneous values of the power in the primary and in the secondary circuit.
From the latter two equations it follows that:
The latter ratio shows that if we increase the voltage of the secondary side, this will reduce the current.
In the real transformer there is also idle current when the transformer is not loaded (the secondary circuit is open). This generates peak inrush current in the primary circuit, which is several times larger than the rated current, and it must be taken into account when circuit protections of transformers are designed, when selecting switchgear, etc. There is also inter-coil, inter-winding and inter-layer capacitance, because in the presence of conductors which are separated by a dielectric and are close enough to each other, there is always parasitic capacity. Except for the idle mode, there is also a short-circuit mode. In the short-circuit mode small voltage is fed to the primary side, while the secondary side is short-circuited, in order to measure the losses in the windings of the transformer. It is usually used when measuring the losses of current transformers. Of course there is a loading mode which is the normal operating mode of the transformer. When connecting a load to the secondary winding, this leads to the flow of secondary current which generates a magnetic field directed opposite to the direction of the magnetic field of the primary winding. As a result the equation between the induction EMF and the power supply EMF is broken, leading to an increase in the current in the primary winding, while gradually the magnetic flux reaches its previous value.
It is important to specify the transformer losses and try to reduce them because their size is in inverse ratio to the performer’s efficiency. The losses in the transformer are mainly losses from the heating of the magnetic core, hysteresis losses and eddy current losses.
If the magnetic core was made of a solid iron unit, the eddy current losses would be significant, that is why it is made of electromagnetic steel lamellae with added silicon, bonded in the shape of the magnetic core. The transformer steel has become very popular due to the great necessity of steel with special electromagnetic properties and low losses for the production of transformers. The shape of the magnetic core is also essential for reducing the losses in the transformer, but we will discuss this when we talk about the types of transformers.
It is no secret that the size of a transformer depends on its power and with quality transformer steel and optimal shape of the magnetic core this size is reduced to a minimum. If you are wondering how the transformer’s dimensions depend on its power, here is a practical formula:
Types of transformers
According to the number of phases they are: single-phase, three-phase, poly-phase.
Depending on the magnitude of the output voltage compared to the input voltage they are: power-enhancing, power-reducing and separating.
According to the cooling they are: dry and oil.
autotransformers (used to change the voltage within definite limits),
measuring (for measuring current and voltage when their values are unsuitable for direct measuring).