A peek inside an electric bike motor
October 20, 2010
I have started on a new project and decided to use electric bike motors. I though some of you would be interested to know what's inside. These motors have good power and torque so no gearbox is required and with all those small windings inside they allow fairly precise control of speed and direction without any mechanical play.
As I want these motors for a robotic project I did not buy the speed controller normally used as it would not suit my needs. Electric bike controllers do not normally have reverse. Now I am building my own.
I have attached a datasheet for a typical controller IC. This datasheet is quite good as it includes schematics and the chip is relatively straight forward to use.
This is a 48V, 350W 3phase brushless motor wired in a delta configuration. There are 51 windings. Around the outside of those windings are a heap of small magnets glued to the inside of the wheel. There are 3 heavy duty wires for the 3 phases but this motor also has 5 fine wires for position feedback. 2 (red and black) are 5V power for 3 hall effect sensors. the other 3 match the color of the 3 phases and are the hall effect sensor outputs.
This motor has the halleffect sensors placed 120 degrees apart but you should check your motor as there are other configurations. These feed back signals are used like a commutator in a brushed DC motor but can also be used as an encoder to monitor speed and direction of the motor.
After some careful testing I found that my motors hall effect sensors required 5V and had open collector outputs. If you do not have specifications for your motor then be careful when testing these. As the sensors are embedded in the windings it is very difficult to replace them. The open collector outputs are ideal as the control IC in the datasheet is best run at higher voltages.
These motors are ideal for large robot projects, they are cheap, have a good power / weight ratio and if you know what your doing you should be able to get very precise speed / position control. I got these brand new for aproximately $100 USD each.
This motor with the aluminium wheel and tyre (420mm OD) weighs 7.5Kg. A standard 350W brushed motor with a gearbox and an equivalent wheel (including bearings) will weigh a lot more than that.