In 1866, Werner Siemens, using an electromagnet, created a machine that transformed mechanical energy into electric called a DC generator or dynamo. It had the ability to self-induce because it removed from the rotor’s own winding the voltage needed to generate the magnetism.
When a continuous electrical voltage is applied at its terminals, the rotor is in motion, similar to that of the electric motor.
The dynamo was developed by the Italian Antonio Pacinotti, however, according to some historians, he presented the invention to a workshop to obtain the patent and the owner, the Belgian Zenobe-Theophile Gramme, patented it with his name in 1871, after making some improvements.
The large ring at the top of the Gramme machine is a permanent magnet and at the bottom there is a magnet and, between the magnet poles, a disc attached to a handle. Next to this crank are coils consisting of copper wires with coils around a circular shaped iron core.
By turning the crank, the copper coils cross the uniform magnetic field, thus generating electric current. It is known that, by the principle of electromagnetic induction, this current will be alternated, but, with the use of the commutator, the current obtained was continuous.
Current transformers must be specified according to the load to be connected to their secondary. In this way, NBR 6586/92 standardizes secondary loads.
For a current transformer, the secondary load represents the ohmic value of the impedances constituted by the different apparatus connected to its secondary, therein including the interconnecting conductors.
By definition, nominal secondary load is the impedance connected to the secondary terminals of the TC, whose value corresponds to the power for the guaranteed accuracy, under rated current.
It should be noted that when the nominal secondary current is different from 5A, the values of the loads shall be multiplied by the square of the relation between 5A and the corresponding nominal secondary current in order to obtain the desired values of said parameters.
The load of the devices to be connected to the current transformers must be carefully dimensioned to choose the compatible standardized load current transformer. However, since the devices are connected to the CTs by means of wires, usually of great length, it is necessary to calculate the power dissipated in these conductors and to add it to the power of the corresponding apparatuses.