Later, at Budapest, he highlighted the principle of the rotating magnetic field and developed plans for an induction motor that would become his first step toward the successful utilization of alternating current.

Then Tesla went to work in Paris for the Edison's electric company in Europe, the Continental Edison Company and in after-work hours he constructed his first induction motor. In 1884 he sailed for US and found immediately employment with Thomas Edison thanks to his good reputation, but the two inventors were too different and their separation was inevitable.

A year later George Westinghouse, head of the Westinghouse Electric Company, purchased the Tesla's polyphase system and other 40 patents that contributed to breaking the Edison's monopoly on electric power. There were therefore two types of electrical supplying system, but the Westinghouse built power plants and transmission lines proving once and for all that AC system was better then the DC one.

The first hydroelectric power plant was built on Niagara Falls.

Later Tesla used his money to start the Tesla Electric Company in New York where spent his time inventing new types of electrical devices. He died in 1943.

The major advantage of AC electricity over DC is that AC voltage can be step up and step down by use of a transformer. This means that the high voltage used to carry electricity over long distances from the power station then could be reduced to a safer voltage for domestic use. So: high voltage, little current line, less heating losses, smaller conductor size, less costs and more efficiency. On the contrary DC system limited its distribution distance to about a mile.

Thanks to Tesla's inventions we have different types of electric and electromechanical machines such as the three-phase transformer, generator and motor.

A machine used for the production of alternating current is the 3-phase synchronous alternator.

This machine is reversible and can be run as a synchronous motor.

In its simplest form it consist of a set of poles, produced by a winding which is energized by a d.c. generator, and one or more windings in which a.c. is generated. When the set of poles is rotated, the relative motion between the armature winding and the magnetic field produced by the poles causes the armature conductors to cut the pole flux. It means that voltage is induced in the armature winding.

Since an armature conductor which previously was under the influence of the north pole, comes under that of the south pole, the direction of the e.m.f. induced in the conductor reverses periodically. And when the ends of the armature windings are connected to an external circuit an alternating current is generated. The frequency depends upon the number of its poles and the rotative speed of the rotor.

So the basic construction of a d.c. field winding and a 3-phase a.c. armature winding.

The field structure may be of either the salient- or non-salient pole type. This latter is more feasible for the higher-speed machines and is used in steam-turbine-driven generators. The salient-pole construction is suitable for multipolar medium- or low-speed machines. It is therefore commonly used for water-turbine or engine-driven alternators and for synchronous motors.

The 3-phase synchronous motor is not widely used but it has some advantages. In fact the constant speed can be useful in specific applications, it's not deeply affected by different voltage values and its performance is more affective than the one of asynchronous motor of equal power. The disadvantages are: it needs both a.c. for the stator and d.c. to be wound; the load cannot be suddenly increased without consequences; its speed cannot be varied and this is a negative aspect for ordinary applications and as the starting operation is difficult to perform, an expert is needed. Finally 3-phase synchronous motors are not constructed for a wide range of solutions.