What is a brushless DC motor? What is the difference between brushless motor and brushed motor?

A brushless DC (BLDC) motor refers to a type of motor that lacks metal components (brushes) that contact the rotor to supply direct current to the coils. This article describes the mechanism, configuration, method of controlling rotation of brushless DC motors, and main applications for the motors, contrasting them with those of brushed motors.

A brushless DC motor avoids the need for a commutator and brushes by having its permanent magnets in the rotor. The rotor’s rotation is sustained by detecting the position of its magnetic poles and adjusting the electric current flow through the coils accordingly. For this reason, a brushless DC motor requires a driver (an electronic drive circuit). Meanwhile, the rotor shaft position is detected using a hall effect sensor or other magnetic sensors (although motors capable of working without such a sensor also exist).

Hall effect sensors use the Hall effect to determine magnetic field strength. By converting this information into an electrical signal, the driver can detect the position of the permanent magnet (the orientation of its north and south poles) and switches the electric current flow through its coils in a way that keeps the motor shaft rotating.

Brushed DC motor (left) and brushless DC motor (inner-rotor type: right)

A brushless DC motor has a permanent magnet in the rotor and wound coils in the stator, whereas a brushed DC motor is the opposite, with wound coils in the rotor and a permanent magnet in the stator. In addition, while brushes and a commutator are essential for the rotation of a brushed DC motor, a brushless DC motor requires a drive circuit instead.

Brushless DC motor control

A three-phase brushless DC motor with an outer rotor, for instance, comprises three stator coils connected to six switches. The magnetic polarity (N or S) of the coils is determined by the on/off status of these switches, causing the rotor to rotate. In other words, the current flow is controlled to alternate the stator magnetic polarities, resulting in attractive and repulsive magnetic forces that drive the rotor to high speed.

The components of a control circuit for a brushless DC motor may include a magnetic polarity sensor to detect motor speed, a circuit to compare the actual motor speed with the reference speed, a circuit to calculate the required drive voltage, and a drive circuit to deliver this voltage.

Excitation sequence of 3-phase brushless DC motor

Brushed DC motor control

A brushed DC motor's rotation is governed by the interaction of brushes and a commutator. The stator houses the field magnet, and electrical current courses through the rotor coil. The brushes and commutator facilitate current flow from brush to commutator, inducing a magnetic force in the coil according to Fleming's left-hand rule, propelling rotor rotation. However, nearing 90°, the magnetic force shifts inward, threatening to halt rotation.

To counter this, the current is cut just before reaching 90°, allowing inertia to carry the rotor past that point. Subsequently, the current resumes in the opposite direction, propelling the rotor from 90° to the opposing 270°. The alternating current direction, reversing every 180° due to rotor rotation, sustains the motion.

Motor torque and speed are regulated by adjusting current in the coil, without altering its geometry, winding count, or the field magnet's flux density. Higher current results in increased torque and faster rotation.

Brushed DC motor configuration

The physical contact between the brushes and commutator in a brushed DC motor means that these parts wear out over time. Additionally, the associated risks of electrical noise, sparking, and acoustic noise make this contact problematic at high speeds. Although brushed DC motors are cost-effective upfront, the requirement for maintenance, such as routine inspections or part replacements, has the potential to elevate their long-term costs.

In contrast, brushless DC motors do not experience the wear of the brushes and commutator or the electrical noise caused by brush contact. They are easier to control compared to brushed DC motors, utilizing a technique called sinewave drive to provide quiet operation with minimal vibration and electrical noise. This is why brushless DC motors are often selected for applications that demand long life and high efficiency.

Brushless DC motors are broadly divided into those with an outer rotor and an inner rotor.

Outer rotor motor

Permanent magnets are located on the rotor which rotates around the outside of the stator coil windings, providing stability of rotation.

Inner rotor motor

Permanent magnets are on the rotor inside, with the stator coil windings outside. This configuration allows for precise control, similar to a brushed DC motor.

Brushless DC motors: Outer-rotor motor (left) and inner-rotor motor (right)

Brushless DC motors that use permanent magnets in the rotor are called permanent magnet brushless DC motors (PM motors) or permanent-magnet synchronous motors. They can be further divided into the following types based on how the permanent magnet is fitted into the rotor.

The embedded magnets of IPM motors make them mechanically safer than SPM motors and capable of faster speeds.

SPM and IPM for inner-rotor brushless DC motors

Due to their characteristics of small size, high output, low noise and vibration, and long life, brushless DC motors find applications in a wide range of uses.

At ASPINA, we specialize in brushless DC motors among various types of DC motors. In addition to developing standalone brushless DC motors, we provide system products that incorporate drive and control systems along with mechanical design. Our comprehensive support extends from prototyping to commercial production and after-sales service.

We understand the diverse needs of different industries, applications, and customer products, as well as specific manufacturing arrangements. That is why we offer customization solutions. Our solutions are tailored to meet the specific functions and performance requirements of each project.

We are here to support not only customers with clear requirements or specifications but also those who facing challenges during the early stages of product development.

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