I’m a student and working on a project that involves designing a ‘DC-DC converter of an electric bicycle with regenerative braking system’. I am using MOSFET half-bridge topology as the DC motor drive. When I tested the circuit, motoring works but regeneration didn’t. My question is must the DC motor reverse its direction for regeneration to occur?
The motor does not reverse its direction when you go into regeneration but what does reverse is the direction of current flow. Thus you need a second bridge in regeneration to conduct current in the opposite direction to that when you are motoring. You could also consider using an H-bridge topology. Remember also that when you are regenerating the motor(generator) voltage is higher than that of the battery (which is what forces the current back into the battery) so your voltage regulator will need to sense whether it needs to provide motoring energy (Battery volts higher than motor and forward bridge conducting) or absorb regenerated energy (Motor volts higher than battery and reverse bridge conducting).
Normally, an half-bridge topology is enough for both motoring and regeneration. H-bridge is necessary if you want to operate in both directions. For your application (bicycle), I guess only one direction of rotation is enough.
In order to regenerate energy, the average voltage supplied by your converter must be lower than the emf of the motor. In these conditions, the current flow will be reversed, and the motor will act as a generator.
You do not need to have a motor voltage higher than the battery voltage for regeneration. The regenerative DC to DC converter is there to supply the right voltage to the motor. If the supplied voltage is higher than the emf, you get power into the motor. If the supplied voltage is lower than the emf, you get power from the motor.
Remember that the emf is proportional to the speed. If you are running in one direction, the emf will always be positive. When the full battery voltage is applied to the motor, you are running at full speed. (Do not forget to put your helmet ;-)