What should be the specification of ESC that I should use for a 320KV 1820W brushless DC Motor?. Is it necessary to use an 80A ESC?, Basically, How can I choose the required ESC for a motor?
It depends on the voltage.
Say you are running 12V:
1820W / 12V = 151A
With a 24V system:
1820W / 24 = 75A
There are a couple motor metrics that will help you decide:
Motor voltage: Your motor has a range of voltages that it will run effectively at. Pick an ESC whose output voltage(usually the same as battery voltage) is in this range.
Stall current: This is the current the motor will draw when the load is so high it can't spin. Most motors will list this specification; you want your ESC's current rating to be higher than stall current.
Kv: This determines rpm per volt of the motor at 0 load. It is usually inversely proportional to torque; the higher the Kv the faster the motor will spin, and the less torque it can provide at that rpm.
You haven't given the motor voltage and stall current, so there's no saying what exact ESC you should pick, but the information above will help you decide.