The Difference Between Brushed Motors and Brushless Motors

DC motors are divided into brushed motors and brushless motors, where the "brush" refers to carbon brushes. So, what do carbon brushes look like?



Why do DC motors need carbon brushes? What's the difference between having carbon brushes and not having them? Let's delve into it further!



Principle of Brushed DC Motors

As shown in Figure 1, this is a schematic diagram of a brushed DC motor structure. There are two fixed opposite polarity magnets, with a coil placed in between. The ends of the coil are connected to two semi-circular copper rings respectively. The ends of the copper rings are in contact with fixed carbon brushes, and then the carbon brushes are connected to the DC power supply at both ends.



Fig  1

After connecting the power supply, the current flows as indicated by the arrows in Figure 1. According to the left-hand rule, the yellow coil experiences an upward electromagnetic force, while the blue coil experiences a downward electromagnetic force. The motor rotor begins to rotate clockwise. After rotating 90 degrees, as shown in Figure 2:



Fig  2

At this point, the carbon brushes are precisely positioned in the gap between the two copper rings, and there is no current flowing through the entire coil circuit. However, due to inertia, the rotor continues to rotate. When the rotor rotates to the position described above, the coil current is as shown in Figure 3.



Fig  3

According to the left-hand rule, the blue coil experiences an upward electromagnetic force, while the yellow coil experiences a downward electromagnetic force. The motor rotor continues to rotate clockwise. After rotating 90 degrees, as shown in Figure 4:



Fig  4

At this point, the carbon brushes are also positioned in the gap between the two copper rings, and there is no current flowing through the entire coil circuit. However, due to inertia, the rotor continues to rotate. This process is then repeated in a cycle.



Principle of Brushless DC Motors

As shown in Figure 5, this is a schematic diagram of a brushless DC motor structure. It consists of a stator and a rotor. The rotor has a pair of magnetic poles, while the stator is wound with many sets of coils, with six sets of coils shown in the diagram.



Fig  5

When current is applied to stator coils 2 and 5, magnetic fields are generated by coils 2 and 5. The stator acts as a bar magnet, with 2 as the S (south) pole and 5 as the N (north) pole. Due to the attraction between like poles, the rotor's N pole rotates to the position of coil 2, and the rotor's S pole rotates to the position of coil 5, as shown in Figure 6.



Fig  6

Next, when the current to stator coils 2 and 5 is turned off and current is applied to stator coils 3 and 6, magnetic fields are generated by coils 3 and 6. The stator acts as a bar magnet, with 3 as the S pole and 6 as the N pole. Due to the attraction between like poles, the rotor's N pole rotates to the position of coil 3, and the rotor's S pole rotates to the position of coil 6, as shown in Figure 7.



Fig  7

Similarly, when the current to stator coils 3 and 6 is turned off and current is applied to stator coils 4 and 1, magnetic fields are generated by coils 4 and 1. The stator acts as a bar magnet, with 4 as the S pole and 1 as the N pole. Due to the attraction between like poles, the rotor's N pole rotates to the position of coil 4, and the rotor's S pole rotates to the position of coil 1.

At this point, the motor has rotated half a turn... The principle for the second half of rotation is the same as the first, and it is not repeated here. We can simply understand the brushless DC motor as if there is a carrot in front of a donkey, and the donkey will keep moving towards the carrot. How can accurate current be applied to different coils at different times? This requires a current reversal circuit... which is not detailed here.

Comparison of Advantages and Disadvantages

Brushed DC motors: Quick startup, prompt braking, smooth speed control, simple control, simple structure, and inexpensive. The key point is inexpensive! Inexpensive! Inexpensive! Additionally, it has a large starting current and produces high torque at low speeds, capable of carrying heavy loads.

However, due to the friction between carbon brushes and the commutator, brushed DC motors are prone to sparking, heating, noise generation, electromagnetic interference to the external environment, low efficiency, and short lifespan. Because carbon brushes are consumable items, they are prone to failure and require replacement after a period of use.

Brushless DC motors: Since brushless DC motors eliminate the need for carbon brushes, they produce less noise, require no maintenance, have low failure rates, long lifespans, and stable operation time and voltage. They also have minimal interference with radio equipment.