With the required rpm and high torque, there are many factors that may affect this. The magnetic capacity is one. There is also the type of winding, number of turn and etc. If you look at the diagram of a motor, http://2.bp.blogspot.com/-I4qci8WncqM/T6AWyZm.../7ZNwQ2EFHLA/s1600/AC+Motor+basic+parts+2.JPG, there are various parts to consider. Making one is easy but making a good and efficient one is a different story.
