How can i calculate torque by a motor

[aa u ss]

I want to drive a trike but using dynamo at pedaling and the electrical power given to 400 watt motor
So any suggestion what i can do best

 

[billy_joule]

Why? What problem are you trying to solve?
Pedal > chain > wheel is much more efficient (up to 99% ) than pedal > dynamo > motor > wheel ( could be lower than 50 %)
It gets even worse if you include a battery.