Brushless motors are synchronous electric motors that have a magnetically (AC induction) or electronically (DC) controlled commutation system instead of a brush-based mechanical commutation system.

Brushed motors are used in many applications, but have an important disadvantage in that the brushes wear out. Brushed motors also limit the speed of the motor, and the sparking from making and breaking connections creates electrical noise. In addition, the position of the electromagnet on the armature makes the motor difficult to cool. Brushless motors overcome these limitations because there are no brushes to wear out and there is no sparking. In addition, the electromagnets on the stator are easier to cool. Brushless motors are also more efficient because of their non-contact system control and because they do not contain brushes that add to friction losses.

Table 1 - Comparison chart for brushed vs. brushless motors.

How do Brushless Motors Work?

Brushless motors incorporate the use of rotating magnets that spin around a permanent coil of wires. By applying current through the wires, a magnetic field is generated that forces the magnets (which are attached to rotors) to rotate a shaft. Brushless motors use either alternating current (AC) or direct current alternated between positive and negative via a controller.

Selecting Brushless Motors

The most important factors industrial buyers need to consider when selecting brushless motors are the type of motor, the performance requirements of the application, and motor construction.

Motor Type

Brushless motors can be powered by either direct current (DC) or alternating current (AC).

AC induction motors are motors that run using AC. Most AC motors operate by simply feeding from an AC power source, making them more effective for applications that require more constant speed and torque. Some AC motors have variable-frequency controllers in order to incorporate multiple speeds.

DC brushless motors function basically the same as AC motors, but instead apply alternating (positive and negative) DC via a controller. Since DC brushless motors require the use of a control and timing sensors, they are generally better suited for applications requiring variable speeds or torques.

Servomotors are a specific class of DC brushless or AC induction motors which include an additional sensor for positioning. These sensors provide very precise rotation and speed control, allowing a controller to specify, for example, the number of turns a motor will make after it is shut off.

Performance Requirements

When selecting any type of motor, it is necessary to find a motor whose performance specifications meet the application requirements. The most important performance specifications for brushless motors are shaft speed, torque, and the design voltage.

Shaft speed is the speed at which the output shaft of a motor spins. Shaft speed specs generally refer to the no-load speed, which is the maximum speed the motor can reach when no torque is applied.

Torque is the rotational force or load that a motor generates. Torque specs generally refer to the stall torque and the continuous torque. Stall torque is the torque at which the shaft speed is zero, or the motor stalls. Continuous torque is the maximum τ at normal running conditions.

Design voltage is the rated voltage at which a motor is designed to operate. This specification must be matched with the available system voltage in order to be able to effectively power the motor.

Motor Construction

Motor construction is an important specification to consider when specifying brushless motors, as different motor constructions are designed for different applications.

AC Motor Construction

Three common types of AC construction are induction, synchronous, and universal.

Induction motors derive their name from the fact that current is induced into the rotor windings without any physical connection with the stator windings (which are directly connected to an AC power supply). Induction motors areadaptable to many different environments and capable of providing considerable power as well as variable speed control. Typically there is "slip" or loss of exact speed tracking with induction motors.

Synchronous motors operate at constant speed up to full load. The rotor speed is equal to the speed of the rotating magnetic field of the stator; there is no slip. Reluctance and permanent magnet are the twomajor types of synchronous motors. A synchronous motor is often used where the exact speed of a motor must be maintained.

Universal motors can operate at approximately the same speed and output on either DC or single-phase AC power. Universal motors arealso known as AC/DC motors.

DC Motor Construction

DC motors can be constructed in various ways. These include permanent magnet, shunt wound, series wound, compound wound, disc armature, and coreless/slotless.

Permanent magnet motors have a magnet permanently embedded into the assembly and no wound field. They offer constant speed with varying load (zero slip) and excellent starting torque. Compared to wound types, permanent magnet construction provides higher efficiency but less speed regulation.

Shunt wound motors exhibit minimum speed variation through load range and can be configured for constant horsepower over an adjustable speed range. They are used for applications where precise control of speed and torque are required.

Series wound motors exhibit high starting torques for permanently attached loads which are required to prevent damage from high speed conditions. These motors develop a large torque and can be operated at low speeds. They are best suited for heavy industrial applications that require heavy loads to move slowly and lighter loads to move quickly.

Compound wound motors are designed with both series and shunt winding for constant-speed applications requiring higher torque. They are often used where the primary load requirement is a heavy starting torque, and adjustable speed is not required. Applications include elevators, hoists, and industrial shop equipment.

Disc armature motors, also called "pancake" or "printed armature" motors, use flat rotors driven by an axially-aligned magnetic field. Their thin construction allows for low inertia, resulting in high acceleration. These motors are good for applications requiring a quick startup and shutdown while bearing a constant load, such as in an electric vehicle.

Coreless and slotless motors incorporate cylindrical winding that is physically outside of a set of permanent magnets. Because the winding is laminated and excludes an iron cage, these designs have much lower inertia. They boast high acceleration, high efficiency, excellent speed control, and little to no vibration. They are commonly used as servomotors for process control applications.

Image credit:

Aerotech, Inc.