Although a transformer is a relatively simple piece of electrical equipment, there are numerous factors which must be taken into account in the design and manufacture to ensure that it will give reliable service, perform the task intended and be safe in operation.
Whilst many of these considerations are left to the manufacturer, a transformer properly specified, designed, manufactured, and installed should give many years of reliable service.
As is commonly known, a transformer essentially consists of a magnetic core, upon which are wound two or more separate coils of insulated copper wire, suitably located in respect to one another and usually termed the primary and secondary respectively. The ratios of these windings in respect to the input voltage determine if it is a step up or step down transformer.
Individual laminations in iron cored transformers are given insulating coatings to develop adequate magnetic insulation between layers in the stacked core, to reduce inter-lamination eddy current losses to an acceptable level, which keeps the total watts loss of the stack to a minimum. C-cores and E-cores take advantage of their shape, and the excellent magnetic properties of silicon iron when rolled in the direction of the grain of the material. These cores can therefore be used at higher flux densities, thus giving higher efficiency, smaller size, or reduced weight, although usually at a higher cost than that of a conventionally constructed transformer.
Stamped laminations come in a variety of shapes and sizes, the most popular being ‘E’ and ‘I’ laminations. This shape gives excellent economy with acceptable losses per kilo of material being used for a given transformer design. There are several grades of this type of lamination available, which can be used to either reduce physical size or improve efficiency.
When a voltage is applied to the primary windings with the secondary open circuit a relatively small current will flow, which magnetizes the core and supplies the iron loss. This is called the no load current. The iron losses are constant whether the transformer is at no load or full load. The losses that vary with load are the copper losses. Maximum efficiency of a given transformer is usually, although not always obtained when the iron losses and copper losses are balanced. This should usually occur at full load.
When a load is connected to the secondary, increased current flows through the primary winding as well as the secondary and copper losses become more significant. The ability of the surface area of the transformer, in respect to the sum of the iron losses and copper losses determine the eventual temperature rise. Other losses are present such as stray eddy currents in core clamps, bolts, di-electric loss in insulation, etc., although these are normally of a smaller nature.
On the following pages we have explained some of the common terminology encounterd when specifying transformers.