Dear Unmesh,

I confess I have some stomach-ache, when I think of you and the 900 Volts!
Please could you make at least some measurements first? How high is voltage between each wire and earth?

Also, there are even better transformers arround. Take a safety transformer with tested isolation voltage of 5000V.

You need to have double isolation in your front end electronics. High voltage isolation is often made with expensive silicon cables. PVC would dry out after some time, becomes brittle and inside wire is touchable.
You need also high isolation connectors, which do not allow to simply cut the line by pulling the wire.

Take an optocoupler with highest isolation voltage you can get. OPI110A or OPI1264 (Optek) are good ones, having +-10kV.

Don't think that you have only 'handsome' 900 Volt. With such an inductive load voltage spikes of some thousands of Volt must be expected! These spikes can dangerously erode all isolation measures, even when using expensive ones. Think about transistor ignition cables in modern car. After sometime isolation becomes brittle and cables have to be exchanged.

There is one point which still could make trouble. If your signal does not see any connection to earth, means, if it is highly isolated from everything, how to handle electrostatic charge which is produced in the motor? If there's no path for electrostatic charge, it will discharge across your optocoupler (when your secondary electronics has a safety earth connection). Then even a device with highest isolation voltage will fail.
As long as I do not know how your 900 Volt are produced, I cannot give further help in this point.

Bye,
Kai