A typical inverter circuit using IGBTs in ac motor control system is illustrated in Figure below. I don’t know correctly, I hope following brief description can be useful for you! In addition to being suitable for high switching frequencies, these transistors virtually retain the sinusoidal waveform of the motor currents. The motor current now contains lesser harmonics, and causes less heating of the motor windings. It also causes less pulsation in torque and low motor noise. The motor also runs smoother, even at lower speeds. Now V and f can both be varied through this single device and a fixed voltage power diode bridge rectifier is sufficient to obtain a fixed d.c. voltage, rather than to use a phase-controlled thyristor rectifier to obtain a variable d.c. voltage. However, they may generate switching surges. Although moderate, they have caused failure of motor insulation in some cases. Depending upon the type of installation, a surge protection, in the form of dv/dt protection through chokes, may become mandatory with such drives, particularly when the cable length from the drive to the motor is too long or when the motor is rather old and may not possess a sound dielectric strength. * Uncontrolled line side diode bridge rectifier. When a variable d.c. is required, it can be replaced by thyristors. ** Mechanical braking or non-regenerative braking: For small brake power, resistance unit is small and can be located within the main enclosure. But for higher power that may call for large resistance units and have to dissipate excessive heat, it is mounted as a separate unit. The resistance units are short-time rated depending upon the duty they have to perform. Also a typical bi-phase chopper scheme with IGBT is illustrated in Figure below.