The brushless DC motor is a hi-tech device that uses complex electronics to operate. This section will provide you with a deeper understanding of the motor. Among the areas I will be touching are; the brushless DC motor working principle, construction, and components. You will also be learning about the benefits offered by the motor over the others. First, brushless DC motor basics such as meaning and history.
What is a Brushless DC Motor?
The brushless DC motor is a type of direct current (DC) motor that does not use brushes. In a conventional brushed motor, a brush and commutator assembly conveys current to the coil windings. A brushless motor lacks these parts. Instead, it employs semiconductor switches to serve the same purpose.
Because it use electronic switches, a brushless motor is also called an electronically commutated motor (ECM)- or simply EC motor. Other names that the motor goes by are BLDC motor and BL motor. BL stands for brushless, and denotes the device’s construction and operation.
Brushless motors came around many years ago, in the early 1960s, following advancements in the field of solid state electronics. Today, the brushless DC motor use is rapidly on the uptake and it use cuts across different electrical machines and systems.
You will learn about the applications of brushless DC motors in a later chapter. For now, something about its design or, in other words, the BLDC motor construction. This will provide you with an idea of how the motor differs from those that use brushes.
Brushless DC Motor Construction
The brushless DC motor design is almost similar to that of the brushed type, save for a few components. It contains both rotor and stator, for example, which primarily consist of permanent magnets and electromagnets respectively. Additional parts that are specific to BL motors include sensors and the electronic controller. In a nutshell, the main brushless DC motor components are:
- Position Sensors
- Electronic Controller
The parts, their images, and functions are discussed in more detail below.
The stator is the part that produces a shifting magnetic force to cause the rotor to spin. It’s usually either inside and surrounded by the rotor or outside and enclosing the rotor. The brushless DC motor stator typically consists of laminated steel stampings. These are normally stacked together to form a magnetic core. A coil of wire is then wound around the core (as shown in the image above) and connected to the controller.
The steel pieces of a brushless DC motor stator are usually either slotted or slotless. Slotless cores produce high speed motors due to the low inductance. However, the design increases the cost to manufacture the motor since it requires a higher number coil turns.
The stator is also normally made up of 3 different windings connected using either a star or delta pattern. Due to the coil arrangement, a star pattern produces a higher torque level at lower speeds. A delta pattern, in contrast, produces a low level of torque at low motor rpm.
The coils of a brushless motor are further classified as trapezoidal and sinusoidal types based on the type of operating signal or back EMF voltage. A trapezoidal brushless DC motor stator has the drive current and back electromotive force (EMF) assume trapezoid shapes. The Sinusoidal motor, on the other hand, has the signal forming a sine wave.
The rotor in a BLDC motor is a permanent magnet. It provides the magnetic field that reacts with that of the stator and enables a rotational motion. Different materials are used for this part. The most common are rare earth materials such as Ferrite, Neodymium, alloy of Neodymium, Boron, and Samarium Cobalt.
Just like the stator, the rotor of a BL motor may be on the outside (in an in-runner motor) or inside (in an out-runner motor). Each configuration has its up and downsides, including the ability to produce compact motors, rotor momentum, and more.
The number of poles in a brushless DC motor rotor mostlly varies from 2 to 8. These are normally arranged sequentially, with opposite poles bordering one another. The placement of the magnets also differs based on application. They may be placed on the rotor outer surface, embedded, or inserted into the core.
It’s good to mention how the number of magnets or poles on the rotor affects the working characteristics of brushless DC motor assemblies. Having more poles increases the motor’s torque. However, it decreases the speed and smoothness of operation. The magnet material also has an impact on the motor, with high ferrite materials increasing torque.
As earlier indicated, a BLDC motor is electronically commutated. For that to happen, the motor’s controller must know the position of the rotor at any time. Doing so allows it to switch power to the right electromagnet and at the right time. Most BL motors, therefore, incorporate position sensors. A brushless DC motor sensor is usually the Hall Effect type, a resolver, or optical encoder.
Each type of sensor offers benefits and drawbacks when used in specific situation. An optical sensor gets affected by dirt easily, for instance, and does not fit applications that are too dusty. Resolvers are, on the other hand, robust and great for harsh environments. Hall sensors provide for low-cost brushless DC motor components, but are affected by high heat and vibrations.
A majority of BL motors use Hall sensors. These work by generating voltage from a magnetic field. The voltage is then used to determine the field’s intensity. A Hall sensor’s output is ether high or low depending on the type of pole (North or South) that has passed near it.
A sensored motor relies on sensor data to calculate the position of the magnets on the rotor as well as the pole orientation. That way, its controller automatically commutates the stator coils. The brushless DC motor hall sensor is usually placed in the stator coils, although sensor positions vary by type of motor.
This is the brushless DC motor commutator. Its function is to switch on the motor’s stator windings in turns, thereby allowing the permanent magnet rotor to spin. The controller also regulates various aspects of the motor operation, such as torque and speed.
There are different types of controllers used with BLDC motors. These are generally classified as sensored and sensorless controllers. A sensored controller operates a motor that uses sensors, a sensorless controller the type motors that uses no sensors.
A brushless DC motor controller circuit can be a microcontroller or digital signal processor. It can also be a dedicated IC that integrates all the control features. It’s one of the most important parts, though, when it comes to the operation of brushless motors.
Minor components of a BLDC motor, but which are important parts of the construction, include the following.
- Bearing– this part supports the shaft at either end of the motor assembly and allows its smooth rotation
- Shaft– a metal rod that holds the rotor and rests on the motor’s housing. It projects on one end of the motor to transmit motion
- Casing– this is the housing that protects the parts of the motor assembly. Usually made from a lightweight material such as aluminum
How a Brushless DC Motor Works
The brushless DC motor working principle is more complex than that of a brushed motor and uses more electronic components. To help you understand its operation, I will use the parts that you have just learned about such as stator, coil windings, and rotor assembly. Here is the working of a Brushless DC motor explained.
- A BLDC motor primarily consists of a rotor (rotating part) and stator (stationary part). The stator is usually a series of electromagnets made of magnetic cores around which are wound wire coils. The rotor on the other hand, is composed of several permanent magnets arranged end to end, with opposite poles facing each other.
- When current flows through the coil of a brushless motor, a magnetic field is generated. The magnetic field reacts with that of the rotor’s permanent magnet next to it, producing either an attraction or repulsive force. In a typical brushless DC motor operation, two coils are energized at the same time. One coil attracts a part of the rotor, while the other repulses it. This causes the rotor to move.
- For continuous motion, the next coil must energize, while the previous coil loses magnetism. The magnetic field then literary rotates around the stator, while the rotor practically chases it. This causes a continuous revolution of the rotating part of the motor.
To ensure smooth rotation and continuity, the magnetic forces must occur on the right coil and at the right time. This is made possible by using an electronic circuit, or what’s called the brushless DC motor controller circuit.
But the circuit must sense the position of the rotor so it can calculate the switching pattern. For that, most BL motors use sensors, which can be magnetic or optical. Most BL motors use Hall sensors, as mentioned earlier. When the rotor moves, the sensor generates a voltage. The magnitude of the voltage depends on whether the nearest pole is North or South.
There are also BLDC motors that do not use sensors to detect rotor position. Instead, they depend on the back EMF or back electromotive force to do that. This is the voltage produced in the stator coils when the motor is in operation.
Brushless DC Motor Speed Control
One of the most important aspects of a BLDC motor is its speed control. This feature is normally implemented by the use of high-tech systems such as field-oriented controls, vector systems, and others. A pulse-width modulation (PWM) signal is generally used to change the power in BL motors. Basically, this system varies the voltage and duty cycle durations to change the rpm.
Brushless DC Motor Torque Control
The characteristics of a brushless DC motor torque are closely related to the current flowing in the stator windings and back EMF. By manipulations these and other parameters, the motors controller is able to control the torque characteristics. BL motors come with the rated torque specifications. In some situations such as drive applications, a gear set is often employed to increase the torque level.
Brushless DC Motor Advantages and Disadvantages
Brushless DC motors have their up and downsides, especially when used in certain applications. These should inform your decision when choosing between a BLDC and brushed motor. The advantages and disadvantages of BL motors are described below, starting with the advantages.
Brushless DC Motor Advantages
The benefits off a BLDC motor include:
- Efficiency- with a BLDC motor, you get more torque per watt ratio. Reasons for the high efficiency include the fewer moving parts, pulsed power control, and reduced power losses since it uses no sliding contacts.
- High Torque to Weight Ratio– when designing a brushless DC motor, its working principle allows engineers miniaturize parts without a significant reduction of torque. These motors are commonly used in small electric products where brushed motors would be too big.
- Low Noise- because they do not use brushes and sliding contacts, BLDC motors do not produce a lot of noise. That explains one of the reasons for their use in computer parts such as fans and hard disks, among other low-noise applications.
- Longer Lifespan- with fewer wear parts, brushless motors last a long time. Most BL motors provide more than 10 000 hours of service life, which is one of the longest lifespan for an electric motor.
- Safe Operation– brushless DC motors do not produce sparks when in operation, which makes them suit environments where sparks could ignite vapor and start a fire. These include hospitals and factories. In brushed motors, the mechanical switching of voltage that happens between the brushes and commutator and leads to arcing.
- Effective Cooling- the brushless DC construction involves placing coil windings on the outer casing of the motor assembly. This helps with cooling through conduction. BL motors also use pulsed power and do not produce a lot of heat.
- Sealed Construction- with the low heat generated by BL motors and heat dissipation through conduction, brushless motors usually do not need to be actively cooled by air. This allows for closed assemblies, shutting off dust and other elements that would cause damage.
- Reduced Electromagnetic Interference– BLDC motors do not cause electromagnetic interference (EMI). The commutation is electronic, which allows for a smooth change of current direction.
- Reduced Centrifugal Force on Rotor- the brushless DC motor rotor is normally a permanent magnet or magnets. That means reduced centrifugal force when the motor is working, and a better dynamic response.
- Reduced Maintenance Costs- brushless motors are easier and less costly to maintain. There are neither brush wear to worry about no commutator erosion to take care of. Only the bearing is bound to wear out, which is usually after many hours of service.
Brushless DC Motor Disadvantages
A BLDC motor also has its downsides, which makes it unsuitable for specific uses. The disadvantages include:
- High Initial Cost- the brushless DC motor price is one of the highest, and can go up to several hundred dollars for the large types of the motor. Reasons for the high cost include the electronic controllers used with them and the inclusion of sensors in the assembly.
- Delicate Electronics and Sensors- BL motors rely on electronic controllers and (often) sensors to operate. These parts limit the use of the motors in harsh environments such as dusty conditions.
- Need for Gear Reduction- the high RPM of BLDC motors makes them unsuitable for low-speed use such as drive applications. When used in such situations, a gear reduction system is normally requires, which increases costs.
The brushless DC motor suits many uses. As we have seen the benefits it offers far outweigh the downsides. Being knowledgeable with its operation and construction is, therefore, essential. You are now conversant with the brushless DC motor meaning, working principle, and how it differs from the regular brushed type. In the next part of this guide, I will expand on the information by discussing the types of BLDC motors in use today, including their design, working, and other aspects.