This page serves to be a brief introduction to the differences between a brushless DC motor and a brushed DC motor.
A brushless DC motor is a synchronous motor which uses an electronic controller (ESC) to switch DC currents to the motor windings producing magnetic fields which will effectively rotate in space with a permanent rotor magnet that follows it. Simply put, an analogy would be like hanging a carrot in front a Hamster as it runs through the wheel. It will never catch the carrot, but it attracts the Hamster to keep running after it. Similarly, the magnetic field generated by the Stator causes the Rotor to follow the magnetic field by attractive forces. How this is done will be detailed later in how Phases are controlled.
When we apply a current through a coil, it generates a magnetic field. So if we apply the appropriate current, the coil will generate a magnetic field that will attract the rotors permanent magnet. Now if we activate each coil one after another the rotor will keep rotating because of the force interaction between permanent and the electromagnet. In theory, this can be done using 3 coils, but in order to increase the effectiveness of the motor, two opposite coils are winded together in a way that will generate opposite poles to the rotor poles, getting double the attractive force.
As previously mentioned, the coils generate a magnetic field that the rotor interacts with. However, this is only considering the attraction of the rotor. To further increase the speed and efficiency of the motor, BLDC motors utilize coils to both attract and repel the rotor in the desired direction.
If we take a look at the current waveform we can notice that in each interval there is one phase with positive current, one phase with negative current and the third phase is turned off. This gives the idea that we can connect the free end points of each of the three phases together and so we can share the current between them or use a single current to energize the two phases at the same time.
Here’s an example. If we pull up phase A High, or connect it to the positive DC voltage, with some kind of switch, for example a MOSFET, and on the other side, connect the phase B to ground, then the current will flow from VCC, through phase A, the neutral point and phase B, to ground. So, with just a single current flow we generated the four different poles which cause the rotor to move.
All of this sounds super complicated to do. And that’s because it is. But an Electronic speed controller does all the phase changes for us so that operating a BLDC motor isn’t actually that difficult. An ESC enables the BLDC motor to be used in the same way that a motor driver would operate a brushed DC motor. To activate the correct phases is done using a Hall-Effect sensor embedded in the stator that tells us the position of the rotor.
As the rotors permanent magnets rotate the Hall-effect sensors sense the magnetic field and generate a logic “high” for one magnetic pole or logic “low” for the opposite pole. According to this information the ESC knows when to activate the next commutation sequence or interval.
A brushed DC motor is an internally commutated electronic motor designed to be run from a direct current power source, utilizing an electric brush for contact. Similar to the brushless DC motor, brushed DC motors also utilize wound coils to create a magnetic field which are free to rotate to drive a shaft.