What is a three-phase variable frequency drive? 

A variable frequency drive is used to vary the frequency speed of an electric motor but also to control the speed of a motor's rise and fall when starting and stopping. Variable frequency drives are found mainly in industry, in machines where the speed of an electric motor needs to be adapted with soft start, soft or fast braking, and direction change.

The three-phase variable frequency drive is supplied from a 380v/400V three-phase network, providing three phases. Three-phase inverters are used in industry for machines with electric motors ranging from 0.4 kW to 320 kW (centrifugal pumps, drills, lifting equipment, heat pumps, etc.).

The difference between a variable speed drive and a variable frequency drive 

There are many terms used to describe variable speed drives: variable speed drive, variable frequency drive, power converter, etc. All these terms refer to the same device. The frequency inverter or variable speed drive controls the speed of an asynchronous electric motor, and thus varies the supply frequency of the electric motor (50 Hz). 

There are two possible operating modes. For constant loads, the constant torque mode is used, keeping the u/f ratio (voltage/frequency) constant. For variable loads, the flux vector or quadratic control mode is used. This allows the control loop and the sensor to be optimised for speed feedback.