There are different types of Brushless DC motors. These are classified based on design, type of parts, power signal, and other criteria. The different types of the device provide different benefits and are, therefore, used in a range of applications. Here, I will be taking a detailed look at the brushless DC motor types in use today. I will also be exploring the advantages and drawbacks of each type based on parameters such as torque, speed, and efficiency.
Common Types of Brushless DC Motors
Different types of BLDC motors are utilized in different environments such as industrial machines, electric and electronic appliances, vehicles, and robotic devices. The types of brushless DC motors can be classified as below.
Based on Design
- In-runner brushless DC motor (also called inner rotor)
- Out-runner brushless DC motor (also called outer rotor)
Based on Use of Sensor
- Sensored brushless DC motor
- Sensorless brushless DC motor
Based on Number of Poles
- Single Pole brushless DC motor
- Multi pole brushless DC motor
Based on Type of Power Signal
- Sinusoidal brushless DC motor
- Trapezoidal brushless DC motor
The types of BLDC motors are explained next.
In-Runner Brushless DC Motor
This type of BLDC motor has the rotating part (rotor) inside an assembly of electromagnet coils (stator). This brushless DC motor construction allows for heat dissipation through conduction, since the stator coils are mounted on the motor’s casing. An in-runner brushless DC motor attains peak speeds easily and is best for applications that require higher RPM characteristics. These motors often do not use many poles in the rotor. As a result, their performance reduces at lower speeds.
Out-runner Brushless DC Motor
This out-runner BLDC motor is basically the opposite of the in-runner type. It’s also called outer rotor BL motor and uses rotating outer case around a stationary inner piece. Out-runner BLDC motors generally use a higher number of permanent magnet poles on the rotor. That translates to more torque and smoother operation. The main disadvantage of the out-runner brushless DC motor lies in its slow speed. These types of motors are, therefore, better suited for low-speed, high-torque applications.
Sensored Brushless DC Motor
A sensored BLDC motor is one that relies on sensors to provide rotor position data. These types of BL motors offer reliable performance at lower speeds. At lower rpm, the sensors provide accurate data to enable a smooth rotation. The main drawback of sensored motors shows up at higher speeds when the sensor feedback becomes unreliable. Harsh conditions such as dusty or high-heat environments also affect the sensors and, therefore, motor operation. These motors fit low rpm applications best.
Sensorless Brushless DC Motor
This type of motor does not use sensors. Instead, the controller relies on the back electromotive force generated in the stator coils to calculate the rotor position. These types of brushless DC motors offer the best performance at high speeds. You can also use them in tough environments, seeing that they do not use sensors. Their shortcoming becomes evident at low speeds when the back EMF is too low to be read by the controller, or when starting from a stationary situation. These motor types suit high-speed, low-cost applications and harsh conditions.
Single Pole Brushless DC Motor
A single pole BLDC motor uses a rotor that consists of a single pair of poles, North and South. This type of brushless DC motor design has its advantages and disadvantages. Starting with the benefits, the motor can attain very high speeds of rotation. On the downside, the performance of single pole motors drops significantly at lower rpms, affecting rotational stability and efficiency. They are, therefore, best used in high speed applications.
Multi Pole Brushless DC Motor
Multi pole motors use several poles on the rotor, mostly up to eight. As mentioned earlier, these are placed so that opposite poles face each other. More poles provide a smoother rotation, but at the expense of speed. As a result, these types of BL motors do not achieve high rpm levels and are mostly used in low-speed applications that require high torque levels.
Other Types of Brushless DC Motors
It’s a common practice to group BLDC motors by their power and back EMF voltage signals. The motors in this category are discussed below.
- Trapezoidal Brushless DC Motor- a trapezoidal BLDC motor uses a power signal that forms a trapezoid shape. This is a simple commutation method when compared to the sine wave type. The method involves powering one pair of terminals at a time, while the third terminal remains disconnected. A drawback with the signal, despite its simplicity, is the torque ripple effect that it causes. This makes the motor operation less smooth, especially at lower rpm. At higher RPM, these motors perform better than the sinusoidal types and fit applications that mostly use high speeds.
- Sinusoidal Brushless DC Motor – these brushless DC motor types use a duty cycle signal that forms a sinusoidal shape. In this commutation method the controller tries to power all stator windings and using a current that follows a smooth sinusoidal curve. The result is a cancellation of the torque ripple effect common in trapezoidal motors. A sinusoidal BLDC motor offers a smooth performs, but mostly at lower speeds. At high speeds, the torque reduces. You will, therefore, find them more suited for low-speed situations.
Brushless DC Motor vs. Brushed DC Motor
A brushless motor differs from the brushed type in several ways including construction, method of operation, and components, among others. These differences, in turn, affect the performance and durability of the motors in different ways. To help you grasp the differences quickly, I divided the comparisons based on the mentioned features and other parameters.
- Construction– A brushless motor uses electromagnets for the stator and permanent magnets for the rotor. A brushed motor has the opposite of that, with rotating coils making the rotor and a permanent magnet the stator.
- Commutation Method– A brushed motor uses brushes and a mechanical commutator. The commutator for brushless DC motor is electronic, and dependent on solid state electronic components.
- Speed– brushless motors attain higher levels of speed (up to 100 000 rpm) than the brushed types, which usually rotate at 20 000 rpm highest
- Torque Variation– Brushless motors maintain torque at different speed levels. Brushed motors experience a slight loss of torque at higher speeds.
- Control- BLDC motors rely on an electronic controller to operate. Brushed motors do not need it, unless when operating on variable speed.
- Noise Level– Brushed motors produce more noise than BL motors, mostly due to the mechanical action of the brushes against the commutator. The brushless DC motor noise is always very low since it uses fewer mechanical parts.
- Maintenance Requirements– Compared to the brushed types, brushless DC motors are low-maintenance devices, seeing that they do not use high-wear parts such as brushes and commutator.
- Efficiency– The brushless DC motor efficiency is usually way higher than that if brushed motors, up to 85%-90%. This results from the lack of brushes, low energy losses, and low heat levels when working. Brushed motors normally attain 75%-80% efficiency.
- Sensors– most brushless motors use sensors to sense the rotor position, with a few operating sensorless. A brushed motor does not need any sensor.
- Power Consumption– brushless motors operate on a duty cycle signal, which reduces their power consumption considerably. Brushed motors, on the other hand, maintain a constant flow of current and are unsuitable in applications that require preserving power or where overheating is an issue.
- Cost- the cost of building a brushless DC motor is generally high cost when compared to the conventional brushed types. That mostly results from the inclusion of expensive electronic controller circuits and, at times, sensors.
- Lifespan– brushless motors are zero-maintenance devices that last a long time. Brushed motors, on the other hand, contain parts that require regular maintenance and that wear out too quickly.
To summarize the comparison between brushless and brushed types of DC motors, it’s evident that brushless motors offer more advantages. In applications where brushed motors are preferred, the advantages include low cost and ability to operate in rugged conditions.
Different types of brushless DC motors offer different benefits when used in specific applications. When choosing the type for your project, it helps to understand their construction, advantages, and disadvantages. The most common of these BLDC motors are described in this chapter. In the next part of the guide, I you will learn about the different uses that brushless motors are out to.