AC IM Scalar Drives
In scalar drives normally only the frequency and the magnitude of the voltage fed to the motor are controlled (current feedback not mandatory).
For 3Φ Induction Motors in particular, scalar drives typically control the voltage-to-frequency (V/F) ratio in open or closed speed loop (motor speed feedback mandatory only in the latter case). If motor speed information is known, slip regulation can be additionally implemented resulting in an optimized motor efficiency.
In spite of its simplicity (it can be executed by both 8 and 32 bit microcontrollers), V/f control is more than sufficient for most applications.
AN1904: ST7MC three-phase AC induction motor control software library
AN4363: How to select the right Triac
Design Notes & Tips
DN0005: A three phase induction motor drive using a V/F control