# Why are brushed motors commonly used in power tools, but brushless motors cannot be used?

2023-10-18Archive

Why do power tools (such as electric drills, angle grinders, etc.) typically use brushed motors rather than brushless motors? If you want to understand, it's really unclear in one or two sentences.

DC motors are divided into collector and brushless. The "brush" referred to herein refers to carbon brushes. What does a carbon brush look like?

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

Principle of DC commutator motor

As shown in Figure 1, this is a block diagram of a DC brushed motor model. Two fixed opposite sex magnets, coil is located in middle, and two ends of coil are respectively connected with two semi-circular copper rings. The two ends of copper ring are in contact with fixed carbon brushes, and then two ends of carbon brushes are respectively connected to DC power supply.

▲ Picture 1

After power supply is connected, current is indicated by arrow in Figure 1. According to left-hand rule, yellow coil is subjected to an electromagnetic force directed vertically upwards, and blue coil is acted upon by an electromagnetic force directed vertically downwards. The motor rotor starts to rotate clockwise after turning 90 degrees, as shown in Figure 2:

▲ Figure 2

At this time, carbon brush is just in gap between two copper rings, and entire circuit of coil is de-energized. However, under influence of inertia, rotor still continues to rotate.

▲ Figure 3

When rotor rotates to above position due to inertia, coil current is shown in Figure 3. According to left hand rule, blue coil is subjected to an electromagnetic force directed vertically upwards, yellow coil is acted upon an electromagnetic force directed vertically downwards. The motor rotor continues to rotate clockwise after turning 90 degrees, as shown in Fig. 4:

▲ Figure 4

At this time, carbon brush is just in gap between two copper rings, and entire coil circuit is de-energized. However, under influence of inertia, rotor still continues to rotate. Then repeat above steps and cycle will continue.

Brushless DC Motor

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

▲ Figure 5

When we pass current to stator coils 2 and 5, coils 2 and 5 will generate a magnetic field and stator is equivalent to a bar magnet where 2 is south pole and 5 is north pole. (North pole. Since magnetic poles of same floor are attracted to each other, N pole of rotor will turn to coil position 2, and S pole of rotor will turn to coil position 5, as shown in Figure 6.

▲ Figure 6

Then we remove current from stator coils 2 and 5, and then we pass current to stator coils 3 and 6. At this time, coils 3 and 6 will generate a magnetic field, and stator is equivalent to a bar magnet, in which 3 is south (south) pole, and 6 is north (north) pole. As magnetic poles of same floor are attracted to each other, N pole of rotor will turn to coil position 3 and S pole of rotor will turn to coil position 6, as shown in Figure 7.

▲ Figure 7

Similarly, remove current from stator coils 3 and 6, and then apply current to stator coils 4 and 1. At this time, coils 4 and 1 will generate a magnetic field, and stator is equivalent to a bar magnet, where 4 is south (south) pole , and 1 is north (north) pole. As opposite magnetic poles are attracted to each other, N pole of rotor will turn to coil position 4 and S pole of rotor will turn to coil position 1.

So far, motor has turned half a circle... second half circle is same as previous principle, so I won't repeat it here. We can simply understand a brushless DC motor as catching carrots before donkey, so donkey will always move towards carrot.

So how can we supply exact current to different coils at different times? To do this, you need a current switching circuit ... I will not go into details here.

DC commutator motor: quick start, timely braking, stepless speed control, simple operation, simple structure and low price. The bottom line is cheap! low price! low price! At same time, it has a large starting current, large torque (rotation force) at low speeds, and can carry a large load.

However, due to friction between carbon brush and commutator plate, commutator DC motor is prone to sparking, heating, noise, electromagnetic interference in environment, low efficiency and short life. Because carbon brushes are consumables, they can fail and need to be replaced after a while.

Brushless DC Motor: Since brushless DC motor does not have a carbon brush, it has low noise, maintenance-free, low failure rate, long service life, and running time and voltage are relatively stable, and radio equipment interference minimal. small. But it's expensive! Expensive! Expensive!

Power tool functions

Power tools are very often used in everyday life. There are many types of brands and competition is fierce. Everyone is very price sensitive. In addition, power tools must be able to withstand a large load and have a large starting torque, such as electric hand drills and impact drills. Otherwise, when drilling, engine simply will not be able to work due to jamming of drill.

Just imagine, a brushed DC motor is cheap, has a large starting torque, and can handle heavy loads; although brushless motor has a low failure rate and long life, it is expensive and its starting torque is far inferior to brushed motor. If you could choose which one would you choose? I think answer is obvious.