The inverters can be classified into two types: scalar inverter and vector inverter. Both have the same operating structure, differing only in the way the torque is controlled.
In the scalar inverter, the V / f curve is fixed (parameterized) based on the type of working regime in which the inverter will operate. There is, however, a problematic condition, which is precisely the critical point of any AC drive system: low rotations.
The AC system does not achieve satisfactory torque at low speeds due to the AC motor’s own performance. To compensate for this phenomenon, the vector frequency inverter was developed, which, although much more expensive and complex than the scalar, does not work with a preset V / f curve (parameterized).
In fact, it varies voltage and frequency in order to optimize the torque for any rotation condition (low or high). And as if every millisecond a new V / f curve was parameterized for each new situation. The vector inverter controls V / f by means of the magnetizing and rotoric currents of the motor. Generally, a tachometer or an encoder are used as speed sensors, forming a “closed loop” of speed control.