Increasing Motor Capacity on Same Voltage by VFD

How is it possible? You purchase a VFD rated for 900kW at 690V. Maybe I'm not understanding your question?

We have a conveyor motor system ( right now at engineering stage) , in which 11/0.720/0.720 KV converter duty transformer is feeding to 900 KW VFD, which supply to 900 KW rating motor at 690 V. My question is how is it possible to limit current in such a high rated motor at 690V?

what is the actual voltage before 690v ?

Actual voltage before 690V is 11 KV (On converter duty transformer primary side)

From discussion it is clear

1. The motor (Conveying system) is in design stage
2. The input is 11 KV to converter
3. Motor terminal voltage is 690 Volts.

Motor of higher rating can run on 690 Volts with VFD or sift starter.

This is matter more of financial viability and selection of technology then of engineering viability.

As I interpret this, you have a 900kW 690V motor and a 900kW VFD that has its own input transformer which has an 11kV input and a 720V output, and you are asking how it is that the VFD can limit the VOLTAGE (although you keep saying current) to 690V for that motor. Is that correct?

If so, you have to understand that a VFD uses the line supply power simply as a source of voltage. It converts the AC to DC, charges a bus, and uses the charged bus to re-create an AC sine wave of varying frequency AND VOLTAGE on the output side. That RMS voltage level can be anything LOWER than the input line voltage; this is inherent in the design of VFDs because to make the motor produce constant torque, you must vary the RMS voltage and frequency at the same rate.

So if you are asking "how does it do this? You have to study the concept of PWM (Pulse Width Modulation) AC theory. It's too involved to explain here if you don't already know.