Comparing Brushed and Brushless Motors: Working Principles, Performance, and Speed Control

Brushless motor: A brushless DC motor consists of a motor body and a driver. It is a typical product of mechatronics integration. Since the brushless DC motor operates in a self-controlled manner, it will not add starting windings on the rotor like a synchronous motor starting with heavy load under frequency conversion, nor will it produce vibration and loss of steps when the load changes suddenly. The permanent magnet of small and medium capacity brushless DC motors now mostly uses high magnetic grade rare earth neodymium iron boron (Nd-Fe-B) materials. Therefore, the volume of rare earth permanent magnet brushless motors is reduced by one frame compared to the same capacity three-phase asynchronous motors.

Brushed motor: A brushed motor is a type of rotary motor that converts electrical energy into mechanical energy (motor) or converts mechanical energy into electrical energy (generator) that contains a brush device. Unlike brushless motors, the brush device is used to introduce or extract voltage and current. The brushed motor is the basis of all motors, with features such as fast starting, timely braking, smooth speed regulation within a large range, and relatively simple control circuits.

I. Differences in working principles between brushed and brushless motors

1. Working principle of brushed motor

The brushed motor is the earliest type of motor that people have come into contact with. In high school physics classes, it is often used as a model to demonstrate the motor. The main structure of a brushed motor includes the stator, rotor, and brushes. By rotating the magnetic field, torque is obtained to output kinetic energy. The brushes and commutators are in continuous contact and friction, playing a role in conducting and commutating during rotation.

The brushed motor uses mechanical commutation, where the magnetic poles are stationary, and the coils rotate. When the motor is working, the coils and commutator rotate, while the magnetic steel and carbon brushes do not rotate. The alternating change of the current direction in the coils is completed by the commutator and brushes in the motor.

In the brushed motor, this process involves arranging the two power input terminals of each group of coils in sequence to form a circular cylindrical object, which is integrated with the motor shaft. The power source passes through two small columns made of carbon elements (carbon brushes) at two specific fixed positions, pressing on the two points of the circular cylindrical power input ring above the coils to electrify one group of coils.

As the motor rotates, electricity is supplied to different coils or different poles of the same coil at different times, causing the N-S poles of the coil's magnetic field to have an appropriate angle difference with the N-S poles of the nearby permanent magnet steel stator. The magnetic fields attract and repel each other, producing force and driving the motor to rotate. The carbon electrode slides on the line connection head of the coil, like a brush rubbing on the surface of an object, hence the name "brush."

This mutual sliding causes wear on the carbon brushes, resulting in losses that require periodic replacement of the carbon brushes. The intermittent on-off of the carbon brush and the line connection head also causes electrical sparks, which can interfere with electronic devices.

2. Working principle of brushless motor

In a brushless motor, the commutation work is completed by the control circuit in the controller (usually a Hall element + controller, or more advanced technology such as a magnetic encoder).

The brushless motor uses electronic commutation, where the coils are stationary, and the magnetic poles rotate. The brushless motor uses an electronic device to sense the position of the permanent magnet through the Hall element and uses an electronic circuit to switch the direction of the current in the coil in a timely manner, ensuring that the correct direction of magnetic force is produced to drive the motor. This eliminates the disadvantages of brushed motors.

These circuits are the motor controllers. The controller of the brushless motor can also achieve functions that brushed motors cannot, such as adjusting the switching angle of the power supply, braking the motor, reversing the motor, locking the motor, and using the braking signal to stop supplying power to the motor. The electronic alarm lock of electric bicycles now fully utilizes these functions.

The brushless DC motor consists of a motor body and a driver, which is a typical product of mechatronics integration. Since the brushless DC motor operates in a self-controlled manner, it will not add starting windings on the rotor like a synchronous motor starting with heavy load under frequency conversion, nor will it produce vibration and loss of steps when the load changes suddenly.

II. Differences in performance between brushed and brushless motors

1. Brushed motor structure is simple, with a long development history and mature technology

The first practical motor produced when the motor was invented in the 19th century was the brushless form, i.e., the induction asynchronous motor. This type of motor has been widely used since the advent of alternating current. However, asynchronous motors have many insurmountable defects, causing the development of motor technology to be slow. Especially, direct current brushless motors have not been able to be put into commercial operation for a long time, and it was not until recent years that they have gradually been put into commercial operation. In essence, they still belong to the category of alternating current motors.

The brushless motor was not invented until a few years after the motor was invented. However, people soon invented the brushed DC motor. Since the brushed DC motor has a simple structure, easy processing and manufacturing, convenient maintenance, and easy control, it has been widely used since its inception.

2. Brushed DC motor has fast response speed and large starting torque

The brushed DC motor has a fast response speed at startup, a large starting torque, stable speed, and almost no vibration from zero to maximum speed. It can drive larger loads at startup. The starting resistance (inductive reactance) of the brushless motor is large, so the power factor is small, and the starting torque is relatively small. There will be a humming sound at startup, accompanied by strong vibrations. The load that can be driven at startup is small.

3. Brushed motor runs stably and has good starting and braking effects

The brushed motor adjusts the voltage to control the speed, so the startup and braking are stable, and the operation is stable when running at a constant speed. The brushless motor usually uses digital frequency conversion control, first converting alternating current to direct current, then direct current to alternating current, and controlling the speed through frequency changes. Therefore, the brushless motor runs unsteadily at startup and braking, with large vibrations, and only runs stably at constant speed.

4. Brushed motor has high control accuracy

The brushed motor is usually used with a reducer and encoder to increase the output power of the motor and improve the control accuracy. The control accuracy can reach 0.01 millimeters, allowing the moving parts to almost stop at any desired location. All precision machine tools use DC motor control accuracy. The brushless motor is unstable at startup and braking, so the moving parts will stop at different locations each time, and a locator or limit switch is required to stop at the desired location.

5. The cost of using brushed motors is low, and maintenance is convenient

Since the brushed motor has a simple structure, low production cost, many manufacturers, and relatively mature technology, it is widely used, such as in factories, machine tools, and precision instruments. If the motor fails, only the carbon brush needs to be replaced, which only costs a few cents and is very cheap. The brushless motor technology is not mature, the price is high, and the application range is limited, mainly used in constant speed devices, such as air conditioning and refrigerators. If the brushless motor is damaged, it can only be replaced.

6. No brushes and low interference

Since brushless motors eliminate brushes, the most direct change is the elimination of electrical sparks produced during the operation of brushed motors, greatly reducing the interference of electrical sparks on remote control wireless devices.

7. Low noise and smooth operation

Without brushes, the frictional force during the operation of a brushless motor is greatly reduced, making the operation smoother and quieter, which is a great support for the stability of model operation.

8. Long life and low maintenance cost

Without brushes, the wear of the brushless motor is mainly on the bearings. From a mechanical point of view, the brushless motor is almost a maintenance-free motor. Only necessary dust removal maintenance is required occasionally.

III. Differences in speed control between brushed and brushless motors

In fact, the control of both types of motors is voltage control, but because brushless DC motors use electronic commutation, digital control is required to achieve this, while brushed DC motors use brush commutation, and traditional analog circuits can be used to control them, which is simpler.

1. The speed control process of a brushed motor is to adjust the voltage of the motor power supply. After adjustment, the voltage and current pass through the rectifier and brushes and are converted into the magnetic field strength of the electrodes. This process is called voltage regulation.
2. The speed control process of a brushless motor is to keep the voltage of the motor power supply constant and change the control signal of the electronic commutator. The microprocessor then changes the switching speed of the large-power MOS transistor to achieve speed control. This process is called frequency regulation.

In conclusion, brushed and brushless motors have their unique advantages and disadvantages in terms of structure, working principle, performance, and speed control. Brushed motors have a simple structure, low cost, and high control accuracy, making them suitable for applications that require precise positioning and speed control. However, they have the disadvantages of wear and tear, electrical interference, and high maintenance costs. Brushless motors, on the other hand, have the advantages of long life, low noise, and low maintenance costs. They also offer better speed control and efficiency. However, they have a more complex structure and higher cost compared to brushed motors. Therefore, the choice between brushed and brushless motors depends on the specific application requirements and cost considerations.