To choose an engine, you should first look for the type of gate you want to automate. The motors are chosen depending on the weight and / or size of the gate.
It is always advisable to choose an engine that has superior strength, since the gate drive conditions are not always the same, depends on the hardness of the course, the effort required to start moving it, etc.
For example, for a sliding gate should take into account your weight, but it is also very important to consider whether the sliding gate occurs easily. If it is difficult to push the gate and does not slide well, you choose to apply to a motor support more weight, so that it has sufficient force to move the gate.
A swing gate is the size of sheet to be taken into account, as well as the wind hitting against the sheet exerting a counterforce.