In one of the previous posts we discussed the basic working concept of BLDC motors and learned how a Hall sensor is used for activating the motor's electromagnet through an external attached electronic circuit for sustaining a continuous rotating motion of the rotor.
The method of implementing a fixed stator electromagnet and a rotating free magnetic rotor ensures enhanced efficiency to BLDC motors compared to the traditional brushed motors which have exactly the opposite topology and therefore require brushes for the motor operations. The use of brushes makes the procedures relatively inefficient in terms of long life, consumption and size.
Although, BLDC types may be the most efficient motor concept, it has one significant drawback that it requires an external electronic circuit for operating it. However, with the advent of modern ICs and sensitive Hall sensors this issue now seems to be quite trivial when compared with the high degree of efficiency involved with this concept.
In the present article we are discussing a simple and basic control circuit for a four magnet, single hall sensor type BLDC motor. The motor operation may be understood by referring to the following motor mechanism diagram:
The image above shows a basic BLDC motor arrangement having two sets of permanent magnets across the periphery of an external rotor and two sets of central electromagnet (A,B,C,D) as the stator.
In order to initiate and sustain a rotational torque either A, B or C, D electromagnets must be in an activated state (never together) depending upon the positions of the north/South poles of the rotor magnet with respect to the activated electromagnets.
To be precise, let's assume the position shown in the above scenario with A and B in a switched ON state such that side A is energized with South pole while side B energized with North Pole.
This would mean that the side A would be exerting a pulling effect over its left blue North pole and a repelling effect on its right side south pole of the stator, similarly the side B would be pulling the lower red south pole and repelling the upper north pole of the rotor....the entire process could be then assumed to be exerting an impressive clockwise motion over the rotor mechanism.
Let's also assume that in the above situation the Hall sensor is in a deactivated state since it may be a "south pole activated" Hall sensor device.
The above effect would try to align and force the rotor such that the south locks on face to face with side B while the north pole with side A, however before this situation is able to transpire the Hall sensor is brought in a close proximity to the shifting upper south pole of the rotor, and when this just transits across the Hall sensor it is forced to switch ON, sending a positive signal to the connected control circuit which instantly responds and switches OFF electromagnets A/B, and switches ON electromagnets C/D, making sure that the clockwise moment of the rotor is yet again enforced maintaining a consistent rotational torque on the rotor.
The above explained switching of the electromagnets in response to the Hall sensor triggering signal can be very simply implemented using the following straightforward BLDC control circuit idea.
The circuit does not need much of an explanation since its too basic, during the switch ON situations of the Hall sensor, the BC547 and the coupled TIP122 is correspondingly switched ON which in turn turns ON the corresponding sets of electromagnets attached across their collector and positive, during the switch OFF periods of the Hall sensor, the BC547/TIP122 pair is switched OFF, but the extreme left TIP122 transistor is switched ON activating the opposite sets of electromagnet.
The situation is toggled alternately, continuously as long as power remains applied keeping the BLDC rotating with the required torques and momentum.