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An inverter, also defined Variable Speed Drive (VSD, speed variator), is an electronic device that permits the variation of the frequency of the current power supply of an electric motor and its rotation speed.

The speed of synchronism of an asynchronous three-phase electric motor depends on the frequency of the power supply voltage (f in Hz) and of the number of couple of poles. The motor will tend to get close to the speed of synchronism but it won't reach it. The speed of synchronism is directly proportional to the power supply current frequency so it will be enough to change this one to modify the speed of rotation of the motor.

So the possibility to take action on the speed of rotation of the electric motor can bring many benefits: first of all, the energy saving and the improvement of efficency of the system by reducing the consumption; the reduction of the noise level, expecially for fans and pumps, to have a better working environment; reduction of mechanical solicitations on the machines to extend the duration; adjust the speed/the torque/the power of frequency converter to process requirements.

Through the inverter is possible a gradual start-up of the system: the inverter permits to hold the start-up current of the motor and this permits to avoid high inrush currents and high starting torques with related mechanical stress in addition to energetic peaks that would affect on consumption.