Though an e-bike looks simple, it's quite complicated in its construction and operation. These complexities are the reason why they cost in thousands of dollars. If you don't want to spend that much, then you can get a conversion kit. One of the main components of the kit is a controller.
Basically, controller sends a signal to the motor hub in various voltages. These signals help to determine the orientation of rotor relative to the stator coil. And as rotor must direct the rotation of rotor, it is very important for controller to function properly. In order to do so, it employs several different mechanisms. Some of them use Hall Effect sensors to detect the rotor orientation while several other mechanisms measure the back EMF in undriven coils to sense the rotor orientation. Such controllers are often called as sensor less controllers.
As controller sends signals to motor hub in various voltages, there is a certain range of operation. The available voltage range is 24V, 36V, 48V and 72V and the available current range is 12A, 15A, 20A, 35A, 40A and 48A. These controllers are available in various ranges because, there are certain implications related with them. As you go on increasing the voltage, motor speed will also increase. And if you increase motor current, motor torque will also increase. Now it's up to us to optimise so as to get the best results. We recommend 12A-15A design for lithium ion battery and 20A design for NiMH and SLA batteries.