Last revised: October 24, 2024
Reviewed by: Scott Orlosky, consulting engineer

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 eventually 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 can make 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 the rotor) to rotate a shaft. Brushless motors use either alternating current (AC) or direct current switched between polarity of the rotating magnetic field.

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 polarity 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, constant speed under a varying load.

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 specifications generally refer to the maximum speed that the motor can run continuously without overheating, under a no-load condition.

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 torque 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 are adaptable 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 two major 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 are also 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.

Brushless Motors FAQs

How does the construction of a brushless motor impact its performance and efficiency?

The construction of a brushless motor significantly impacts its performance and efficiency in several ways:

Noise and Vibration: Brushless motors produce less noise and vibration compared to brushed motors. This is because they lack brushes and commutators, which are common sources of noise and vibration in brushed motors. The absence of these components allows for smoother operation and can enhance the overall performance of the motor by reducing mechanical wear and tear.

Speed and Longevity: Brushless motors can operate at high speeds, up to 100,000 RPM, and have a longer operating life. This is due to the method of commutation, which is achieved without mechanical brushes. Instead, brushless motors use magnetic Hall sensors or sensorless drives, limiting contact to the ball bearings. This construction reduces wear and extends the motor's lifespan, making it suitable for applications requiring high speed and reliability.

Energy Efficiency: Brushless motors are known for their energy efficiency. The construction allows for quiet operation and higher RPMs, which can contribute to energy savings in various applications. This efficiency is partly due to the reduced friction and heat generation from the absence of brushes.

Size and Torque: The size of the motor is a crucial factor in its performance. While reducing the number of windings can increase speed, it may also reduce torque. Therefore, the construction must balance these factors to ensure the motor meets the required performance specifications without compromising on size or torque.

How do magnetic Hall sensors work in brushless motors?

Magnetic Hall sensors play a crucial role in the operation of brushless motors by providing precise rotor position feedback, which is essential for the electronic commutation process. Here's how they work:

Rotor Position Detection: Hall sensors are used to detect the exact position of the rotor in a brushless motor. They produce a low or high signal based on the poles (north or south) of the permanent magnet (PM) of the rotor. This allows the motor controller to determine the rotor's position accurately.

Commutation Control: In most brushless DC (BLDC) motors, three Hall sensors are embedded in the stator. These sensors provide signals that indicate the passage of the rotor's magnetic poles. The combination of these signals helps determine the exact sequence of commutation, which is crucial for energizing the correct stator windings in sequence to maintain motor rotation.

Signal Processing: The Hall sensors output a digital high or low signal for 180 electrical degrees of rotation, and a low level for the other 180 degrees. The three sensors are typically offset by 60 electrical degrees, aligning with the electromagnetic circuits of the motor. This alignment ensures that the motor drive voltages are applied correctly to maintain smooth operation.

Types of Hall Sensors: Hall effect sensors can be digital or analog, with digital sensors further divided into unipolar and bipolar types. Unipolar sensors switch on or off based on the presence of a magnetic pole, while bipolar sensors switch states when in proximity to opposite magnetic poles.

How does the electronic commutation process work in brushless motors?

The electronic commutation process in brushless motors is a critical aspect of their operation, enabling efficient and precise control of the motor.

Rotor position detection and signal processing are as they were described in the preceding paragraph.

Commutation Sequence: Based on the signals from the Hall sensors, the motor controller determines the correct sequence for energizing the stator windings. This sequence is crucial for maintaining the rotation of the motor. The controller switches the current through the windings in a specific order to create a rotating magnetic field that interacts with the rotor's magnetic field, causing it to turn.

Power Electronics: The actual switching of the current is managed by a power electronic system, which changes the polarities of the windings based on the rotor's position. This electronic commutation replaces the mechanical commutation found in brushed motors, eliminating the need for brushes and commutators and reducing wear and tear.

This electronic commutation process allows brushless motors to achieve high efficiency, reliability, and performance, making them suitable for a wide range of applications.

What are advantages of electronic commutation over mechanical commutation?

Electronic commutation offers several advantages over mechanical commutation, particularly in the context of brushless motors. Here are some key benefits:

Reduced Wear and Tear

In brushless motors, electronic commutation eliminates the need for brushes and commutators, which are subject to mechanical wear. This results in a longer lifespan for the motor, as the only significant wear component is the ball bearings.

Higher Efficiency and Performance

Brushless motors can achieve higher speeds (up to 100,000 RPM) and maintain high performance over a longer period due to the reduction of friction and heat generation associated with brushes.

The electronic commutation process allows for precise control of the motor, enhancing its efficiency and performance.

Noise and Vibration Reduction

The absence of brushes and commutators in brushless motors leads to less noise and vibration, contributing to smoother operation and improved performance.

Energy Efficiency

Brushless motors are known for their energy efficiency, partly due to the reduced friction and heat generation as well as the switching noise and lac of sparking from the absence of brushes. This makes them suitable for applications where energy savings are important.

Reliability

Electronic commutation provides more reliable operation as it is less prone to the mechanical failures that can occur with brushes and commutators. This reliability is crucial for applications requiring consistent performance.

What are the types of Hall effect sensors used in brushless motors?

In brushless motors, Hall effect sensors are used for detecting the rotor's position and ensuring proper commutation. Here are the types of Hall effect sensors used in these motors:

Digital Hall Effect Sensors

Unipolar Sensors: These sensors are activated by either the north or south pole of a magnet and switch to the OFF state when the magnetic field diminishes or is removed. They are suitable for applications where a simple ON/OFF signal is sufficient.

Bipolar Sensors: These sensors switch ON when in proximity to one magnetic pole and switch OFF when near the opposite pole. In the absence of a magnetic field, they remain in their current state (ON or OFF). Bipolar sensors are useful for applications requiring detection of both magnetic poles.

Analog Hall Effect Sensors

These sensors provide a continuous output signal proportional to the magnetic field strength. They are used in applications where precise measurement of the magnetic field is necessary, allowing for more detailed feedback on the rotor's position.

Selection Criteria for Hall Effect Sensors

Sensitivity: The sensitivity of a Hall effect sensor is determined by its placement relative to the magnet, the air gap, and the magnet's strength. High-sensitivity sensors have low magnetic operating points (BOP) and release points (BRP), which enable the use of smaller magnets and contribute to a more compact motor design. This can lead to more efficient motor performance.

These types of Hall effect sensors are integral to the efficient operation of brushless motors, providing the necessary feedback for precise control and commutation.

How does sensorless control work in brushless motors?

Sensorless control in brushless motors is a technique that eliminates the need for physical sensors, such as Hall effect sensors, to determine the rotor position. Instead, it relies on the back electromotive force (BEMF) generated in the motor's stator windings. Here's how it works:

Back EMF Detection

As the rotor of a brushless DC motor spins, it induces a voltage in the stator windings known as back EMF. This voltage is proportional to the rotor's speed and position.

Position Estimation

Sensorless control algorithms use the BEMF to estimate the rotor's position. This estimation is used to determine the correct timing for commutation, which involves switching the current in the stator windings to maintain motor rotation.

Commutation Control

The motor controller uses the estimated rotor position from the BEMF to control the commutation sequence. This ensures that the stator windings are energized in the correct order to produce a rotating magnetic field that interacts with the rotor's magnetic field.

Advantages

Sensorless control reduces system cost and complexity by eliminating the need for additional sensors and wiring. It also increases reliability, as there are fewer components that can fail.

Challenges

One challenge of sensorless control is accurately detecting BEMF at low speeds, where the signal can be weak. Some systems use techniques like microstepping to overcome this limitation until the motor reaches a speed where BEMF detection is reliable.

Sensorless control is particularly suitable for applications where cost is critical, and where the motor operates at moderate to high speeds. Note that the BEMF is an estimate based on the motor characteristics. If the motor fails and needs replacement, the replacement motor may behave differently than the one that it replaced.

Brushless Motors Media Gallery

References

Electronics360—Controlling a three-phase BLDC motor with Hall sensors

GlobalSpec— Innovative FOC Motor Control: Improve Time To Market By Eliminating Software Development

Image credit:

Aerotech, Inc.