Why do power tools (such as hand drills, angle grinders, etc.) generally use brushed motors instead of brushless motors? If you want to understand, this is really not clear in one or two sentences.
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 having carbon brushes and not having carbon brushes? Let's move on!
DC brush motor principle
As shown in Figure 1, this is a structural model diagram of a DC brushed motor. Two fixed heterosexual magnets, a coil is placed in the middle, and the two ends of the coil are respectively connected to two semicircular copper rings. The two ends of the copper ring are in contact with the fixed carbon brushes, and then the two ends of the carbon brushes are respectively connected to DC power supply.
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 motor rotor starts to rotate clockwise, after rotating 90 degrees, as shown in 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. However, under the action of inertia, the rotor still continues to rotate.
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 motor rotor continues to rotate clockwise, after rotating 90 degrees, as shown in Figure 4:
At this time, the carbon brush is just in the gap between the two copper rings, and the entire coil loop has no current. However, under the action of inertia, the rotor still 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 DC brushless motor. It is composed of a stator and a rotor, in which there is a pair of magnetic poles on the rotor; there are many sets of coils wound on the stator, and there are 6 sets of coils in the figure.
When we pass current to the stator coils 2 and 5, the coils 2 and 5 will generate a magnetic field, and the stator is equivalent to a bar magnet, where 2 is the S (south) pole and 5 is the N (north) pole. Because 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.
Then we remove the stator coil 2,5 current, and then pass the current to the stator coil 3,6. At this time, the coils 3 and 6 will generate a magnetic field, and the stator is equivalent to a bar magnet, in which 3 is the S (south) pole and 6 is the N (north) pole. Because 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.
In the same way, the current of the stator coils 3 and 6 is removed, and then the stator coils 4 and 1 are fed with current. 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 opposite magnetic poles attract each other, the rotor N pole will rotate to the coil 4 position, and the rotor S pole will rotate to the coil 1 position.
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 feed accurate currents to different coils at different times? This requires a current commutation circuit... I won't go into details here.
Comparison of advantages and disadvantages
DC brushed motor: quick start, timely braking, smooth speed regulation, simple control, simple structure, and cheap price. The point is cheap! cheap price! cheap price! Moreover, it has a large starting current, a large torque (rotational force) at low speeds, and can carry a heavy load.
However, due to the friction between the carbon brush and the commutator, the brushed DC motor is prone to sparks, heat, noise, electromagnetic interference to the external environment, and has 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.
DC brushless motor: Since the DC brushless motor eliminates the carbon brush, it has low noise, no maintenance, low failure rate, long service life, and the running time and voltage are relatively stable, and the interference to radio equipment is small. But it's expensive! Expensive! Expensive!
Electric tools are very commonly used tools in daily life. There are many types of brands and fierce competition. Everyone is very sensitive to price. Moreover, electric tools need to carry a heavy load and must have a large starting torque, such as electric hand drills and impact drills. Otherwise, when drilling, the motor is easily unable to run because the drill bit is stuck.
Just imagine, a brushed DC motor is cheap, has a large starting torque, and can carry heavy loads; although a brushless motor has a low failure rate and a long life, it is expensive and its starting torque is far inferior to a brushed motor. If you could choose, how would you choose? I think the answer is self-evident.
