Brushless motors — design, operation and advantages

Find out why you should choose a brushless motor.

Brushless motors — design and operation

In order to understand the phenomenon of brushless motors, which are rapidly entering the power tool market, it is necessary to understand the principle of operation of the formerly applied commutator motors and learn about the main differences between these two types of motors.

The basic task of an electric motor is to convert electrical energy into mechanical energy, ie, to set the shaft of the driven device in motion. In a conventional motor, the commutator of a rotating rotor receives an electric charge from graphite brushes, which conduct electricity through direct contact with the rotating part. The resulting magnetic field generated by the commutator causes rotational movement by means of special magnets.

The design and therefore the operating principle of a brushless motor is slightly different from that of a commutator motor. The commutator and brushes in this device are replaced by coils that are wound on a stationary core, the so-called stator (in a brush motor the coils are placed on a moving rotor). It is these coils that, when current flows through them, create the magnetic field that puts the rotor into motion. A very common and basic solution is to use three windings. However, due to the torque ripples occurring, manufacturers also offer motors equipped with sets from one to even eight pairs of poles.

Due to the lack of direct contact between the rotating part and the fixed coils, there is no need to use graphite brushes. The use of permanent magnets in the brushless motor design allows for higher torque and improved efficiency. This is all thanks to the high energy density of these magnets.

Why it is worth choosing brushless motors

The design of a brushless motor offers a number of advantages. The most important differences in favour of brushless motors include:

• Quieter operation and fewer failures. During operation, the brushes rub against the rotating parts, causing noise. Brushes are the most common wear-and-tear component in commutator motors, which significantly reduces their service life.
• Improved work safety. During operation, brushless motors do not cause sparks which are dangerous for the user. This advantage allows the motor to be used in explosive and flammable environments.
• Higher efficiency of the device. Elimination of mechanical resistance enables higher rotor speeds and improved torque.
• Improved battery life. A brushless motor draws less current than its traditional counterpart. This fact is extremely important for rechargeable devices, which can run much longer on a single charge.
• Better control precision. BLDC motors enable high precision torque control, which helps reduce inertia and power consumption.
• Significantly reduced or zero maintenance. If used properly, the motor should operate without failure or the need for external intervention in the structure. This is because the absence of brushes means that there are no longer any critical parts inside the machine that wear out during normal operation. The benefit of this fact is that the motor can be installed in a place that is difficult to access.

Applications where a brushless motor is unbeatable

Brushless motors are ideal for computer equipment that requires safe, uninterrupted operation for long periods. Examples include hard disk drives and computer fans, which are designed to continuously reduce component temperatures during computer use.

As mentioned above, thanks to their reduced power consumption and extremely compact dimensions, brushless motors are indispensable in professional cordless devices such as drill-drivers, drills and angle grinders. Battery-powered operation is more economical and longer. A very popular industry today that relies heavily on brushless motors is the manufacture of electric and hybrid vehicles.

The use of brushless technology in the construction of washing machines and air conditioners is equally important. Spark-free operation is essential due to the high electrocution risk. In addition, washing machine manufacturers offer a longer warranty on products equipped with brushless motors due to the more stable operation of the drum, which allows for fewer breakdowns. Another benefit of purchasing such a washing machine is the reduction of running costs.

Types of brushless motors — BLDC and PMSM

The most commonly used brushless motors are BLDC motors (brushless DC motors), ie, a solution employing a trapezoidal EMF (electromotive force) waveform, and PMSMs (permanent magnet synchronous motors) with a sinusoidal EMF waveform. A distinction is also made between DC and AC drives.

Brushless motor control

The use of windings on the stator makes it possible to control the motor by varying the currents applied to the windings and to manipulate the torque value on the fly. As already mentioned above, a distinction is made between BLDC and PMSM motors. The motor type marking indicates how the device is controlled. Trapezoidal control allows for regulation when no control of the speed or the torque of the motor is required. Such motors are equipped with Hall sensors, which determine the position of the rotor. This solution is less complicated, but also less accurate. The main disadvantage of this design is the problematic adjustment at low speeds.

In PMSM motors, ie, those with a sinusoidal EMF waveform, thanks to the application of special encoders, undesirable commutation effects occurring in BLDC motors are eliminated, thanks to which easy control is possible even at low rotational speeds. In addition, such a solution minimises the risk of torque ripple.

The third and most advanced type of brushless motor control is vector control. This solution consists in using a special transformation block upstream the PI (proportional-integral) controller. This results in improved control accuracy compared to the previously mentioned types, while eliminating ripples and offering better control at low speeds.

Summary

The use of a brushless motor is undoubtedly a solution that is worth the attention of both the manufacturer and the consumer. The former gets rid of the risk of frequent breakdowns. Products gain in quality, while the number of necessary assembly components is reduced. The buyer, on the other hand, receives a product that will serve them for years and will not require any special interference in its operation. Despite a certain price difference, the peace of mind and security are bought for a lifetime. It is therefore worth considering the offer of devices equipped with brushless motors.

Image credit: ©stock.adobe.com/au/Mikhail Nesytykh

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Originally published here.