Obviously, the alternating flux 4), linking winding H as well as L, will induce a voltage in H proportional to its turns: into L sufficient to maintain the flux 4. This additional current as also the current being produced by the load, are called the load currents of the windings, to distinguish them from the no-load current which flows only in the excited winding and which is called exciting current.
To neutralize the demagnetizing effect of IH, it is necessary and sufficient that the magnetomotive force or ampere-turns associated with be equal and opposite to that associated with I.
That is, the load currents in the two windings are inversely pro portional to their respective turns, and thus is obtained current transformation. It may be noted that current and voltage trans formation are in inverse ratio, so that when one is "stepped up" the other is "stepped down" in the same ratio.
Energy Relationship.—That winding of the transformer on which voltage is impressed and into which energy flows is called the primary; and the other winding, from which energy is drawn out, is called the secondary. Either the higher or the lower voltage winding may be used as the primary, and the other as the sec ondary, so that the operation of a transformer is reversible.
The energy flowing into a circuit being proportional to the product of current and voltage, for the load circuit it will be IH and for the primary circuit By the law of con servation of energy, these two must be equal to each other in an ideal transformer. Comparing the values of Ill EH and from the foregoing, they are seen to be equal and opposite to each other; that is, as much additional energy flows into the primary as is drawn out from the secondary. In an actual transformer, some energy is lost within the transformer, so that the energy output is always slightly less than the energy input.
Rating of a Transformer.—The type of service for which a transformer is suitable is expressed by its rating, covering usually the following items : (I) Kv-a (i.e., kilovolt-amperes), represent ing the power capacity; (2) voltages of the various windings; (3) frequency; (4) temperature rise. The rating plate usually gives information also with reference to the method of cooling, type of construction, and the proper connections to make.
(I) Exciting Current.—It has been indicated already that the voltages of the various windings of a transformer are induced by an alternating magnetic flux in its core, and that sufficient exciting current flows into the primary winding to produce the necessary flux from instant to instant. For a given frequency, terminal volt age and the number of turns, the necessary flux is completely de termined by the formula given above. The necessary exciting cur rent is then determined with the aid of the characteristic curve of the material used in the magnetic circuit. Thus, given the total flux 4), and cross-section A of the magnetic core, the flux density B in the core follows as: By referring to the characteristic curve of the core material, the value of the necessary exciting ampere-turns corresponding to the flux density B is obtained. This divided by the turns of the excited winding gives the necessary exciting current in amperes. Finally, if this exciting current is multiplied by ioo and divided by the rated load current of the winding, the per cent exciting current is obtained. Obviously, the exciting current does not flow into a useful load, and is in a sense wasted. It is therefore desirable that it be limited to as small a value as possible. In commercial power transformers it is limited to between 1% and i o% of the rated useful load current of the transformer. In miniature transformers, it may greatly exceed the latter limit.