AC drives can be classified according to the following generic topologies:
Voltage-source inverter (VSI) drive topologies : In a VSI drive, the DC output of the diode-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The vast majority of drives are VSI type with PWM voltage output.
Current-source inverter (CSI) drive topologies : In a CSI drive, the DC output of the SCR-bridge converter stores energy in series-reactor connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output.
Six-step inverter drive topologies : Now largely obsolete, six-step drives can be either VSI or CSI type and are also referred to as variable-voltage inverter drives, pulse-amplitude modulation (PAM) drives, square-wave drives or D.C. chopper inverter drives.
In a six-step drive, the DC output of the SCR-bridge converter is smoothed via capacitor bus and series-reactor connection to supply via Darlington Pair or IGBT inverter quasi-sinusoidal, six-step voltage or current input to an induction motor.
Load commutated inverter (LCI) drive topologies: In an LCI drive (a special CSI case), the DC output of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal six-step current output of a second SCR-bridge’s inverter and an over-excited synchronous machine.
Cycloconverter or matrix converter (MC) topologies : Cycloconverters and MCs are AC-AC converters that have no intermediate DC link for energy storage. A cycloconverter operates as a three-phase current source via three anti-parallel-connected SCR-bridges in six-pulse configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage to an alternating voltage at a variable load frequency. MC drives are IGBT-based.
Doubly fed slip recovery system topologies: A doubly fed slip recovery system feeds rectified slip power to a smoothing reactor to supply power to the AC supply network via an inverter, the speed of the motor being controlled by adjusting the DC current.
VFD Working Principle