Transformers

Single phase transformer: structure

Generalities

A single phase transformer essentially consists of two inductors, called primary and secondary windings, that are equipped with high magnetic coupling, generally assured by a high permeability structure, called ferromagnetic core.
In particular this is always present in the power transformers; anyway there are also transformers with air magnetic circuit, where the coupling between windings is assured by their geometric disposition, they are mainly used in signal electronic devices.

Ferromagnetic core

Ferromagnetic cores are made of bars to avoid the flow of high parasitic currents, and as a consequence to avoid the relative losses.
Low loss laminations are used, having a thickness generally included between 0,3 mm and 0,5 mm, that are cut and packed to obtain the wished useful sections in the different sections. The most used shapes are:

column core:

shell or armored core:

Fig. 1

A	=	core
B	=	high voltage winding
C	=	low voltage winding
D	=	yoke
E	=	column

Fig. 2

The sections of ferromagnetic core, forming the windings are called columns; the connection sections are called yokes.
For manufacturing reasons the core quite always presents a certain number of joints that, building up some air gaps, have a minimum thickness to minimize the reluctance of the magnetic circuit. Therefore these air gaps can be not only of facing type (Figure 3-a) but of intercalated type too (Figure 3-b or Figure 3-c).
The first type is, on the manufacture point of view, simpler than the other ones, but these ones have air gaps of lower equivalent thickness and therefore give rise to lower reluctance values.

Fig. 3

The columns have a rectangular, square or joggled section, in a more or less high number (Figure 4).
The latter solution, more expensive, allows a better use of the space available within the windings and a better anchorage of these ones, to support the electrodynamic forces. The yoke section is generally rectangular or equal to the column one. The laminations are strongly pressed among them through presspackages or in-and-out bolts, suitably isolated not to cause electric contacts among the different limitations.

Fig. 4

Windings

The windings are made of copper or aluminium, owing to their high conductivity; they are carried out in coils made of more turns; the primary and secondary coils are mounted coaxially among them on one or more columns. The commonest arrangements of the coils are:

concentric arrangement, where the coils setting up the primary and secondary winding have different diameters (Figure 5).
This arrangement is preferred in the transformers for high voltages, since it allows a better isolation of the high voltage winding (h.v.), that is set externally to the low voltage one (l.v.), by interposing between the two ones and towards the tube core made of isolating material with high dielectric strength.

Fig. 5

A = h.v. coil

B = l.v. coils

C = core column

alternating arrangement, where the coils setting up the primary and the ones setting the secondary have the same diameter and they are mounted intercalated each other (Figure 6).
This arrangement, preferred for not very high voltages, allows a modular manufacturing of the coil setting up the windings.

Fig.6

The coils are isolated among them and as to any other part of the transformer, at the rate of the voltages that can be present in conditions of normal operation and even in fault condition. Normally tapings impregnated of epoxy resins and/or impregnated cards are used.

Cooling

The transformer dissipates power both in the ferromagnetic core, owing to the hysteresis and to the parasitic currents, and in the windings, for Joule's effect.
To avoid that the following overheating leads to the damages of isolations, the produced heat has to be disposed of; at this purpose among the windings (and for big transformers even in the core) there are suitable channels for the passage of cooling fluids.

For the transformers of small power the cooling fluid is generally natural or forced ventilation air (air transformers).
The transformers of big power are plunged in mineral oil (oil transformers), presenting in the same time excellent thermal and dielectric properties; we foster therefore the electric isolation too. At this purpose the transformer is set in a tank full of oil, through whose walls the removed heat is disposed of.
The tank can be:

with smooth walls, for modest rated powers;
with finned walls or equipped with cooling tubes, for powers included between 10 kVA and 100 kVA;
with external radiators cooled with natural convention, for powers included between 50 kVA and 500 kVA;
with external heat exchangers cooled with air or water forced conventionone, for powers greater than 500 kVA.

We have a larger and larger diffusion, especially in indoor installations, of capsulated transformers with resin solid isolation, which don't require mineral oil and therefore they don't involve fire risks. Usually epoxy resins are used, loaded with quartz, because of their dielectric, thermal and mechanic properties.
The transformers of this type have also the advantage of requiring little maintenance.

Rating plate

The main parameters characterizing the operation of a transformer are called rating plate because they are shown in a "plate" applied to the transformer itself.
It typically includes:

	manufacturer, serial number and manufacture year;
	prescribing conformity brands (such as the Eupean brand CEE),
	the main rated values (P_n, U_ln, U_2n, fn, u_cc%, cosj_cc )
	cooling type (shown with initials such as ONAN = Natural Oil Natural Air),
	service type,
	weight.