CONSTANT CURRENT TRANSFORMER BASIC OPERATION TUTORIALS

The operation of low-voltage lamps in parallel on a constant-voltage system necessitates a prohibitive expenditure for conducting material when the area to be lighted is extensive and the lamps are widely separated. For such service it is the common practice to operate the lamps, which are connected in series, with a constant current.

The constant-current transformer is a special form of transformer which converts alternating current at a constant voltage to a constant (alternating) current with a voltage varying with the load. It consists of a primary coil upon which the constant voltage is impressed, a secondary coil (or coils) movable with respect to the primary, and a core of low magnetic reluctance.

It depends for its regulation upon the magnetic leakage between the primary and secondary coils. Consider first the primary coil; with the constant emf impressed upon this coil the total magnetism within the coil will be practically constant under all conditions.

The emf generated in the secondary will depend upon the strength of the field which it surrounds. In all types of stationary transformers the secondary current is opposite in general time direction to the primary, so that there is not only a repulsive thrust between the two coils but also a considerable tendency for the magnetic lines from the primary to be forced out into space without penetrating the secondary.

In the ordinary constant-voltage transformer the repelling action between the two currents is prevented from producing motion of the coils by the rigid mechanical construction, while the proximity of the primary and secondary coils limits the magnetic leakage.

In the constant-current transformer, however, the repelling action is utilized to adjust the relative positions of the primary and secondary coils; when the coils are widely separated, the paths for the leakage lines are increased and the lines which the secondary surrounds are fewer than when the coils are quite close together.

The counterweights mechanically attached to the movable coil (or coils) are so arranged that when the desired current exists in the secondary coil (independent of its position along the core), the weights are just balanced. An increase in the current increases the repulsion and causes the coils to separate.

With any current less than normal, the repelling force diminishes, and the primary and secondary coils approach each other, thereby restoring the current to normal. The primary can be wound for any reasonable voltage (say, as high as 10,000 V), while the secondary can be wound for the voltage required for operating the number of lamps in the circuit—from 15 to 200 or more lamps.

THREE PHASE TRANSFORMER APPLICATION BASIC AND TUTORIALS

For central stations of medium size, three-phase transformers are rarely superior to single-phase, except when the large sizes can be applied, in which case the transformers are normally installed in substations or central stations.

The chief reason for this is the nonflexibility of a three-phase transformer. It is usually purchased for a particular size and type of load, and if that load should be changed, the transformer, representing a comparatively heavy investment, remains on the hands of the central station, whereas a single-phase transformer of one-third the size could usually be adapted for some other service.

This feature becomes of less importance as the central station increases its size, and three-phase transformers for purely power service are now being used by a considerable number of the large central stations in the United States.

The three-phase transformer costs less to install, and the connections are simpler, points that are of importance in connection with outdoor installations. The fact that a failure of a three-phase transformer would interrupt service more than the failure of one single-phase transformer in a bank of three is of little importance because of the comparatively few failures of modern transformers.

On the other hand, especially for 2200-V service, the single-phase transformer has been carried to a high degree of perfection and is manufactured in much larger quantities, so that better performance is usual and in some cases initial cost is lower.

Three-phase distribution transformers are used extensively in underground city network service on account of the smaller space required by them in the manhole, their higher efficiency, and their lower initial cost.

For overhead service for pole or platform mounting, three single-phase units are more common on account of the ease of handling and mounting the smaller-sized units.