Detailed explanation of the principle of brushed DC motor

Why do power tools (such as hand drills, angle grinders, etc.) generally use brushed motors instead of brushless motors? If you want to understand it, it really cannot be explained in just one or two sentences.

DC motors are divided into brushed motors and brushless motors. The "brush" here refers to carbon brushes. What do carbon brushes look like?

Why do DC motors need carbon brushes? What is the difference between having carbon brushes and not having carbon brushes? Let's read on!

Principle of brushed DC motor

As shown in Figure 1, this is a DC brushed motor structural model diagram. There are two fixed opposite magnets, with a coil placed in the middle. The two ends of the coil are connected to two semicircular copper rings, and the two ends of the copper rings are in contact with the fixed carbon brushes, and then the two ends of the carbon brushes are connected to a DC power supply.

▲ Figure 1

After connecting the power supply , the current is as shown by the arrow in Figure 1. According to the left-hand rule, the yellow coil is subjected to a vertical upward electromagnetic force; the blue coil is subjected to a vertical downward electromagnetic force. The motor rotor begins to rotate clockwise, and after rotating 90 degrees, it is as shown in Figure 2:

▲ Figure 2

At this time, the carbon brush is just in the gap between the two copper rings, and there is no current in the entire coil loop. However, under the action of inertia, the rotor continues to rotate.

▲ Figure 3

When the rotor rotates to the above position under the action of inertia, the coil current is shown in Figure 3. According to the left-hand rule, the blue coil is subjected to a vertical upward electromagnetic force; the yellow coil is subjected to a vertical downward electromagnetic force. The motor rotor continues to rotate clockwise. After rotating 90 degrees, the carbon brush is just in the gap between the two copper rings, and there is no current in the entire coil loop. However, under the action of inertia, the rotor continues to rotate. Then repeat the above steps and keep circulating.

Brushless DC Motor

As shown in Figure 5, this is a DC brushless motor structural model diagram. It consists of two parts: the stator and the rotor. The rotor has a pair of magnetic poles; there are many groups of coils wound on the stator, and there are 6 groups of coils in the figure.

▲ Figure 5

When we pass current through stator coils 2 and 5, coils 2 and 5 will generate magnetic fields. The stator is equivalent to a bar magnet, with 2 being the S (south) pole and 5 being the N (north) pole. Since like magnetic poles attract each other, the rotor N pole will rotate to the position of coil 2, and the rotor S pole will rotate to the position of coil 5, Figure 6.

▲ Figure 6

Then we remove the current from stator coils 2 and 5, and then pass current into stator coils 3 and 6. At this time, coils 3 and 6 will generate a magnetic field, and the stator is equivalent to a bar magnet, where 3 is the S (south) pole and 6 is the N (north) pole. Since like magnetic poles attract each other, the rotor N pole will rotate to the position of coil 3, and the rotor S pole will rotate to the position of coil 6, Figure 7.

▲ Figure 7

Similarly, remove the current from stator coils 3 and 6, and then pass current into stator coils 4 and 1. At this time, coils 4 and 1 will generate a magnetic field, and the stator is equivalent to a bar magnet, where 4 is the S (south) pole and 1 is the N (north) pole. Since opposite magnetic poles attract each other, the rotor N pole will rotate to the position of coil 4, and the rotor S pole will rotate to the position of coil 1.

So far, the motor has rotated half a circle... The second half of the circle is the same as the previous principle, so I won't go into details here. We can simply think of the brushless DC motor as fishing a carrot in front of a donkey, so that the donkey will keep moving towards the carrot.

So how can we supply accurate current to different coils at different times? This requires a current commutation circuit...I won't introduce it in detail here.

Comparison of the advantages and disadvantages of the two

Brushed DC motor: fast starting, timely braking, smooth speed regulation, simple control, simple structure, and cheap price. The key point is cheap price! Cheap price! Cheap price! Moreover, it has a large starting current, a large torque (rotational force) at low speed, and can carry a heavy load.

However, due to the friction between the carbon brush and the commutator, the DC brush motor is prone to sparks, heat, noise, electromagnetic interference to the external environment, low efficiency and short life. Because the carbon brush is a consumable part, it is easy to malfunction and needs to be replaced after a period of use.

Brushless DC motor: Since brushless DC motors do not require carbon brushes, they have low noise, no maintenance, low failure rate, long service life, stable running time and voltage, and less interference to radio equipment. But it is expensive! Expensive! Expensive!

Power tools are very commonly used tools in life. There are many brands and fierce competition. People are very sensitive to prices. Moreover, power tools need to carry heavy loads and must have a large starting torque, such as hand drills and impact drills. Otherwise, when drilling , the motor may easily stop running because the drill bit is stuck.

Just imagine, brushed DC motors are cheap, have large starting torque, and can carry heavy loads; brushless motors have low failure rates and long life, but they are expensive and their starting torque is far inferior to that of brushed motors. If you were asked to choose, which one would you choose? I think the answer is self-evident.