A bidirectional motors has its brushes aligned central to the turns of the armature. A unidirectional motor has its brushgear or commutator offset to favour one direction of rotation, to minimise sparking and so prolong life of brushes and commutator. In the unidirectional BBC, power flow in single direction whereas in BBBC power flow in both the direction.

In DC motor, the magnetic neutral axis and geometric neutral axis are not remaining same due to armature reaction when current flow from armature, hence brushes need to move forward or backward, depending on direction of motor for bidirectional DC motors, to avoid sparking, and is eliminated by providing inter poles between the main poles.

AC induction or Synchronous motors could be made bidirectional, simply by changing the sequence of the phase as it changing the direction of rotating field …..

Dr Rana Hamza Shakil thank you for your contribution to the discussion

I agree with Bhupendra Desai in the unidirectional BBC, power flow in single direction whereas in BBBC power flow in both the direction. PID controller is designed to obtain the fine output response with less percentage overshoot. Unidirectional BBC has a wide range of voltage variation and bidirectional BBC has high energy density. A unidirectional speed controller is a device that helps control the speed and direction of a motor in one direction only. It receives a signal and uses this signal to change the voltage or current to the motor, which in turn controls the speed of the motor. Unidirectional devices are the semiconductor devices that allow current to flow only in one direction whereas bidirectional devices allow the flow of current in both the directions. The direction of a relationship can be either bidirectional or unidirectional. A bidirectional relationship has both an owning side and an inverse side. A unidirectional relationship has only an owning side. As unidirectional ac voltage controllers use only one thyristor, it is economical as compared to bidirectional. But it has more harmonic content and also it introduces a dc component. As unidirectional ac voltage controllers use only one thyristor, it is economical as compared to bidirectional. But it has more harmonic content and also it introduces a dc component. Prefer bidirectional associations: Unidirectional associations are more difficult to query. In a large application, almost all associations must be navigable in both directions in queries. A bidirectional motors has its brushes aligned central to the turns of the armature. A unidirectional motor has its brushgear or commutator offset to favour one direction of rotation, to minimise sparking and so prolong life of brushes and commutator. A unidirectional speed controller is a device that helps control the speed and direction of a motor in one direction only. The unidirectional DC–DC converter realizes voltage matching and power decoupling between FC and DC bus [8]. The bidirectional DC–DC converter realizes voltage matching between SC and DC bus, and bidirectional flow of instantaneous energy.