Why do power tools (such as hand drills, angle grinders, etc.) generally use brushed motors instead of brushless motors? To understand, this is really unclear in a sentence or two.

DC motors are divided into brushed motors and brushless motors. The “brush” mentioned here refers to carbon brushes. What does the carbon brush look like?

Why do DC motors need carbon brushes? What is the difference between carbon brushes and no carbon brushes? Let’s look down!

Principle of brushed DC motor

As shown in Figure 1, this is a structural model diagram of a DC brush motor. Two fixed magnets of the opposite sex, a coil is placed in the middle, both ends of the coil are connected to two semi-circular copper rings, both ends of the copper rings are in contact with the fixed carbon brush, and then DC is connected to both ends of the carbon brush. power supply.

▲ Figure 1

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

▲ Figure 2

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

▲ Figure 3

When the rotor turns 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 vertically upward electromagnetic force; the yellow coil is subjected to a vertically downward electromagnetic force. The rotor of the motor continues to rotate clockwise, and after rotating 90 degrees, at this time, the carbon brush is just in the gap between the two copper rings, and the entire coil circuit has no current. But under the action of inertia, the rotor continues to rotate. Then repeat the above steps, and the cycle continues.

DC brushless motor

As shown in Figure 5, this is a structural model diagram of a brushless DC motor. It consists of a stator and a rotor, in which the rotor has a pair of magnetic poles; there are many sets of coils wound on the stator, and there are 6 sets of coils in the picture.

▲ Figure 5

When we pass the current to the stator coils 2 and 5, the coils 2 and 5 will generate a magnetic field. The stator is equivalent to a bar magnet, where 2 is the S (South) pole and 5 is the N (North) pole. Since the magnetic poles of the same sex attract each other, the N pole of the rotor will rotate to the position of coil 2, and the S pole of the rotor will rotate to the position of coil 5, as shown in Figure 6.

▲ Figure 6

Then we remove the current of the stator coils 2 and 5, and then pass the current to the stator coils 3 and 6. At this time, the 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 the magnetic poles of the same sex attract each other, the N pole of the rotor will rotate to the position of coil 3, and the S pole of the rotor will rotate to the position of coil 6, as shown in Figure 7.

▲ Figure 7

In the same way, the current of the stator coils 3 and 6 is removed, and the current is passed to the stator coils 4 and 1. At this time, the 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 the opposite magnetic poles attract each other, the N pole of the rotor will rotate to the position of coil 4, and the S pole of the rotor will rotate to the position of coil 1.

So far, the motor has rotated half a circle…. The second half circle is the same as the previous principle, so I won’t repeat it here. We can simply understand the brushless DC motor as fishing a carrot in front of a donkey, so that the donkey will always move towards the carrot.

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

Comparison of advantages and disadvantages of the two

DC brush motor: fast start, timely braking, stable speed regulation, simple control, simple structure and low price. The point is that it is cheap! Cheap! Cheap! And it has a large starting current, large torque (rotation force) at low speed, and can carry a heavy load.

However, due to the friction between the carbon brush and the commutator segment, the DC brush motor is prone to sparks, heat, noise, electromagnetic interference to the external environment, and low efficiency and short life. Because carbon brushes are consumables, they are prone to failure and need to be replaced after a period of time.

Brushless DC motor: Because the brushless DC motor eliminates the need for carbon brushes, it has low noise, no maintenance, low failure rate, long service life, stable running time and voltage, and less interference with radio equipment. But it’s expensive! Expensive! Expensive!

Power tools are very commonly used tools in life. There are many brands and fierce competition. Everyone is very price-sensitive. And power tools need to carry a heavy load and must have a large starting torque, such as hand drills and impact drills. Otherwise, when drilling, the motor can easily fail to run because the drill bit is stuck.

Just imagine, the brushed DC motor has low price, large starting torque, and can carry heavy loads; although the brushless motor has a low failure rate and a long life, it is expensive, and the starting torque is far less than that of a brushed motor. If you were given a choice, how would you choose, I think the answer is self-evident.

Reviewing Editor: Tang Zihong

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