Inverters transform direct current into alternating current. Direct current produces a current flow in just one direction, while alternating current changes rapidly the direction of the flow of current from one side to the other. The frequency of the alternating current in Spain is usually 50 cycles.

Each cycle includes the movement of the current first in one direction and then in the other. This means that the direction of the current changes 100 times per second. Alternating current supplied by an electric company or by a diesel or petrol generator is (or should be) as the one shown in the black graph. Changes in the magnitude of the current follow a senesoidal law and therefore, current is also a senesoidal wave.

The conversion of alternating current in to direct current may be done in different ways. The best way depends on how much does it have to look like the ideal senesoidal wave in order to carry out a proper operation of the alternating current charge:

• Square wave inverters:Most of the inverters work by sending direct current through a transformer, first in one direction and then in the other. The switching device that changes the direction of the current must act very fast.
  
  As the current goes through the primary face of the transformer, polarity changes 100 times per second. As a consequence, the current that comes out from the secondary of the transformer alternating in a frequence of 50 complete cycles per second.
  
  The direction of the current flow through the primary face of the transformer changes suddenly and so, the shape of the wave of the secondary is “square", as represented in the graph. Square wave inverters are less efficient. They produce too many harmonics that generate interferentes (noises). They are not apt for induction engines. If alternating current is needed only to feed a television set, a computer or a small electrical appliance, this type of inverter can be used. Its power will depend on the nominal power of the specific appliance.
  
  
• Senesoidal or modified wave inverters:they are more sophisticated and use broadness of impulse modulation techniques. The output broadness is modified in order to bring it near as much as possible to a senesoidal wave. The output is not an authentic senesoidal wave yet , but it is quite close.
  
  The harmonics contents is less than in the square wave. In the graph it is represented in blue, they are the most cost effective ones for the connection of light, television or frequency variators.
  
  
• Senesoidal wave inverters: cwith more accurate electronics, a pure senesoidal wave can be obtained. Lately, new inverters have been developed with a 90% efficiency or more, depending on the power. The incorporation of latest generation micro-processors allows for the increasing of the inverters performance with value added services, such as tele-control, consumed energy counter, etc.