The most powerful popular science of brushless DC motor (collector's edition)

Brushless DC motor (BLDC), also known as electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor that uses a direct current (DC) power supply. A brushless DC motor is essentially a permanent magnet synchronous motor with position feedback that uses a DC power input and converts it into a three-phase AC power supply using an inverter.

There are various types of motors, and the brushless DC motor is the most ideal speed-regulating motor today. It combines the advantages of DC motors and AC motors, with the advantages of DC motors’ good adjustment performance and AC motors’ simple structure, no commutation sparks, reliable operation, and easy maintenance. Therefore, it is very popular in the market and widely used in automobiles, home appliances, industrial equipment and other fields.

01 Development History of Brushless DC Motor

The brushless DC motor is not the earliest product, but developed on the basis of the brushed motor. Its structure is more complex than that of the brushed motor. The brushless DC motor consists of a motor body and a driver. Different from the brushed DC motor, the brushless DC motor does not use a mechanical brush device, but a square wave self-controlled permanent magnet synchronous motor, and replaces the carbon brush commutator with a Hall sensor, and uses neodymium iron boron as the permanent magnet material of the rotor.

However, when electric motors were invented in the last century, the practical motors produced were brushless.

1740s: The invention of the electric motor begins

Early models of electric motors first appeared in the 1740s through the work of Scottish Benedictine monk and scientist Andrew Gordon. Other scientists, such as Michael Faraday and Joseph Henry, continued to develop early motors, experimenting with electromagnetic fields and discovering how to convert electrical energy into mechanical energy.

1832: Invention of the first commutator DC motor

In 1832, British physicist William Sturgeon invented the first DC motor that could provide enough power to drive machinery, but its application was severely limited due to its low power output.

1834: The first true electric motor is built

Following in Sturgeon's footsteps, Thomas Davenport of Vermont, USA, made history by inventing the first proper battery-powered electric motor in 1834. This was the first electric motor with enough power to perform tasks, and his invention was used to power a small printing press. In 1837, Thomas Davenport and his wife Emily Davenport received the first patent for a DC motor.

But their motor design still suffered from the same power and efficiency issues as William Sturgeon's design, and unfortunately, due to the high costs involved in battery power, Thomas went bankrupt and the machine was never used commercially.

1886: Practical DC motor invented

The first practical DC motor that could run at a constant speed under variable weight was invented in 1886 by Frank Julian Sprague, and it was this motor that provided the catalyst for the widespread use of electric motors in industrial applications.

It is worth mentioning that the practical motor adopts a brushless form, that is, an AC squirrel cage asynchronous motor, which not only eliminates sparks and voltage losses at both ends of the winding, but can also deliver power at a constant speed. However, asynchronous motors have many insurmountable defects, which has led to slow development of motor technology.

Soon after the brushless motor was invented, people invented the DC brush motor. The DC brush motor became the mainstream as soon as it came out because of its simple structure, easy production and processing, convenient maintenance, and easy control.

1887: The AC induction motor is patented

In 1887, Nikola Tesla invented the AC induction motor and successfully patented it a year later. It was not suitable for road vehicles, but was later modified by Westinghouse engineers. In 1892, the first practical induction motor was designed, followed by a rotating bar-wound rotor, making the motor suitable for automotive applications.

1891: Development of the three-phase motor

General Electric began developing the three-phase induction motor in 1891. To exploit the wound rotor design, GE and Westinghouse signed a cross-licensing agreement in 1896.

1955: The era of brushless DC motors begins

In 1955, D. Harrison et al. in the United States first applied for a patent to replace the mechanical brushes of brushed DC motors with transistor commutation circuits, which officially marked the birth of modern brushless DC motors. However, there was no motor rotor position detection device at that time, and the motor had no starting capability.

1962: Invention of the first brushless DC (BLDC) motor

Thanks to advances in solid-state technology in the early 1960s, in 1962, TG Wilson and PH Trickey invented the first brushless DC (BLDC) motor, which they called a "DC motor with solid-state commutation." The key element of the brushless motor is that it does not require a physical commutator, making it the most popular choice for computer disk drives, robots, and aircraft.

They used Hall elements to detect the rotor position and control the winding current commutation, making the brushless DC motor practical, but due to the limitation of transistor capacity, the motor power was relatively small.

1970s to present: Rapid development of brushless DC motor applications

Since the 1970s, with the emergence of new power semiconductor devices (such as GTR, MOSFET, IGBT, IPM), the rapid development of computer control technology (single-chip microcomputer, DSP, new control theory), and the advent of high-performance rare earth permanent magnet materials (such as samarium cobalt and neodymium iron boron), brushless DC motors have developed rapidly and their capacity has continued to increase.

Later, with the launch of the Mac Classic brushless DC motor and its driver in 1978, and the research and development of square wave brushless motors and sine wave brushless DC motors in the 1980s, brushless motors truly began to enter the practical stage and developed rapidly.

02 Basic knowledge of BLDC motor

(1) Structure of brushless DC motor

The brushless DC motor mainly consists of a rotor made of permanent magnetic material, a stator with coil windings and a position sensor (optional).

stator

The stator structure of a BLDC motor is similar to that of an induction motor. It consists of stacked steel laminations with axial slots cut for winding. The windings in a BLDC are slightly different from those of a conventional induction motor.

BLDC Motor Stator

Typically, most BLDC motors consist of three stator windings connected in a star or “Y” configuration (without a neutral point). Additionally, based on the coil interconnection, the stator windings are further divided into trapezoidal and sinusoidal motors.

BLDC Motor Back EMF

In a trapezoidal motor, both the drive current and the back EMF are trapezoidal in shape (sinusoidal in case of a sinusoidal motor). Typically, motors rated for 48 V (or less) are used in automotive and robotics (hybrid cars and robot arms).

Rotor

The rotor portion of a BLDC motor consists of permanent magnets, usually rare earth alloy magnets such as neodymium (Nd), samarium cobalt (SmCo), and neodymium iron boron (NdFeB).

Depending on the application, the number of poles can vary between 2 and 8, with North (N) and South (S) poles placed alternately. The following figure shows three different arrangements of the magnetic poles.

(a): Magnets are placed on the outer circumference of the rotor.

(b): It is called electromagnetic embedded rotor, in which rectangular permanent magnets are embedded in the iron core of the rotor.

(c): Insert the magnet into the rotor core.

BLDC Motor Rotor

Position sensor (Hall sensor)

Since there are no brushes in a BLDC motor, commutation is controlled electronically. In order for the motor to rotate, the stator windings must be energized sequentially, and the position of the rotor (i.e., the north and south poles of the rotor) must be known in order to accurately energize a specific set of stator windings.

A position sensor, usually a Hall sensor (which works on the Hall effect principle), is used to detect the position of the rotor and convert it into an electrical signal. Most BLDC motors use three Hall sensors that are embedded in the stator to detect the position of the rotor.

The output of the Hall sensor will be high or low, depending on whether the north pole of the rotor is near the south pole or the north pole. By combining the results of the three sensors, the exact order of energization can be determined.

(2) Working principle of brushless DC motor

Click on the video to understand the working principle of BLDC in seconds

As the name implies, brushless DC motors do not use brushes. Instead of utilizing a commutator to regulate the current inside the coils, brushless DC motors use an electronic commutator to deliver the current, which creates an alternating current signal that causes the motor to drive.

The working principle of brushless DC motor is similar to that of brushed DC motor. The Lorentz force law states that as long as a current-carrying conductor is placed in a magnetic field, it will be subjected to a force. Due to the reaction force, the magnet will be subjected to equal and opposite forces. When current passes through the coil, a magnetic field is generated. This magnetic field is driven by the magnetic poles of the stator. Like polarities repel each other and opposite polarities attract each other. If the direction of the current in the coil is continuously changed, the magnetic poles of the magnetic field induced by the rotor will continue to change, and the rotor will continue to rotate under the action of the magnetic field.