In a 3-phase voltage source inverter used for speed control of induction motor, antiparallel diodes are used across each switching device. The main purpose of diodes is to:
Right Answer is:
Allow the motor to return energy during regeneration
In regenerative braking, the motor, instead of being disconnected from the supply, remains connected to the supply and returns the braking energy to the supply line.
VSI fed IM drive has a voltage source inverter at the machine end and a six-pulse converter at the source end. When the induction machine is motoring, the source end converter operates as a rectifier, and the machine end converter operates as an inverter.
With a single 6 pulse converter at the input side, regeneration is not possible since the electrolytic capacitor does not allow reversal of the voltage Polarity and the SCRs do not allow the reversal of currents.
Regeneration is possible by having an antiparallel 6 pulse converter at the input side. When regeneration takes place, firing pulses to converter 1 are stopped and firing pulses are given to the machine end converter such that it operates as an inverter. The motor operates as an induction generator since it converts mechanical power into electrical power. The diode rectifier part of VSI converts AC to DC. Converter II converts DC power into AC power and is fed to the AC mains. The disadvantage being that an additional six-pulse converter is required for regeneration.