BLDC Motor
What is BLDC Motor
A brushless direct current motor (BLDC) is a small but powerful type of electric motor that uses direct current as its power source. Because of their size and efficiency, BLDCs are increasing in their popularity and the number of applications they're used for. A motor converts supplied electrical energy into mechanical energy. Various types of motors are in common use. Among these, brushless DC motors (BLDC) feature high efficiency and excellent controllability, and are widely used in many applications. The BLDC motor has power-saving advantages relative to other motor types.
Lifetime
The lifetime of brushless motors is much longer than that of brushed motors because the brush and commutator don't rub against each other and get worn down (like they do in brushed motors). In fact, brushed DC motors last about 1000 - 3000 service hours, whereas BLDC motors last about 20,000 service hours.
Output Speed and Precision
Brushless motors also have a higher output speed, they can typically go about six times faster than brushed motors.
The speed of BLDC motors is also thought to be more consistent. This is because there could easily be discrepancies between the speed of the rotor, and the speed of the revolving magnetic field of the stator in an AC motor. As we mentioned earlier in this article, this phenomenon is called “slip”, and it's directly related to power losses from heat, which also occur in brushed DC motors. Slip does not occur in BLDC motors, which eliminates discrepancies in speed that would be caused by it.
Lower Operational Cost and Less Maintenance
Brushed motors have a lower capital cost than brushless motors because brushless motors require a controller to supply a “voltage sequence between the three phases”. However, brushed motors do wear down more quickly; sometimes the brushes must be replaced, and this brings up maintenance costs. If you consider BLDC motors more of an investment, they can be less expensive in the long term because they require practically zero maintenance, and last much longer.
Efficiency
Brushless motors are more efficient than brushed motors because, again, the lack of brushes means that no brushes will be constantly in contact with the commutator (like they are in brushed motors). This lack of contact, means there's also a lack of friction and heat created in brushless motors. Less energy lost in the form of heat, makes a brushless motor more efficient. In fact, BLDC motors typically have an efficiency of 85-90%, while brushed motors are usually only 75-80%. This means that for applications like power tools, for example, this improved efficiency extends battery life (meaning more can be done between charges).
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Brushless DC Motors Do Not Have Brushes
Brushed DC motors combine a rotating armature with stationary magnets (the stator), using brushes and commutator to supply current to the armature coil.
Brushless DC motors, in contrast, have a rotor, stator, rotation sensor, and control circuit. In place of the brushes and commutator, the current through the stator coil is controlled electronically by semiconductor switching.
Brushless DC motors can be divided into those with an outer rotor and those with an inner rotor.
Outer-rotor motors
In these motors, a rotor containing permanent magnets rotates around the central stator. This configuration eliminates the need for the magnets to be kept small and facilitates equipment downsizing as it allows the outer rotor to be used as a fan driveshaft, for example.
Inner-rotor motors
These motors have the opposite configuration to that of traditional brushed DC motors, with a cylindrical rotor containing magnets rotating inside coils that are arranged concentrically. This enables the rotor to have a small moment of inertia and facilitates miniaturization of the motor.
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.
Components Used in Brushless Dc Motors
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.
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Brushless DC motor |
Brushed DC motor |
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Rotor |
Permanent magnet |
Wound laminated core |
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Stator |
Wound laminated core |
Permanent magnet |
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Control |
Magnetic sensor and drive circuit |
Brushes and commutator |
Motor controls for brushless and brushed DC motors
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.
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.
Durability And Noise Issues
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.
Applications for 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.
Without the carbon brushes and copper commutator, the only wear items in a BLDC motor are the bearings. This makes the BLDC motor far superior when long operating life is required.
Because the windings are stationary and in thermal contact with the outer case, heat is dissipated more easily, allowing for higher power operation in the same package size. Also, with no brushes and commutator, the overall length is shorter.
Without the mechanical contact of the brushes, higher speeds and peak loads are practical.
Carbon brushes are a design feature from early electric motors. They are still used today as an inexpensive alternative to brushless motors.
In brushed motors, a carbon brush serves as the means to conduct electricity to the rotor. The disadvantage is that friction between the stationary brushes and moving rotor can cause wear and loss of power.
Brushless motors replace the less-durable brush design with an electronic controller. The controller controls which coils in the stator are active, making for an ever-changing magnetic pull.
BLDC motors boast a higher speed range and better torque to speed ratio. They also deliver a higher level of torque relative to their size making them ideal in applications such as power tools.
As their name implies, brushless DC motors do not use brushes. With brushed motors, the brushes deliver current through the commutator into the coils on the rotor. So how does a brushless motor pass current to the rotor coils? It doesn't-because the coils are not located on the rotor. Instead, the rotor is a permanent magnet; the coils do not rotate, but are instead fixed in place on the stator. Because the coils do not move, there is no need for brushes and a commutator.
With the brushed motor, rotation is achieved by controlling the magnetic fields generated by the coils on the rotor, while the magnetic field generated by the stationary magnets remains fixed. To change the rotation speed, you change the voltage for the coils. With a BLDC motor, it is the permanent magnet that rotates; rotation is achieved by changing the direction of the magnetic fields generated by the surrounding stationary coils. To control the rotation, you adjust the magnitude and direction of the current into these coils.
BLCD Control Methods
With rotational information provided by dedicated sensors or back EMF, BLDC control can be implemented by one of three methods: trapezoidal, sinusoidal, and field-oriented control (FOC).
Trapezoidal control is the simplest method for powering a BLDC, energizing each phase in sequence. Coils are energized in either a high or low state or can be left floating. While broadly applicable, this is often not as effective as using more advanced techniques and can produce audible noise.
Sinusoidal control energizes each BLDC coil using variable duty-cycle PWM techniques to simulate analog outputs. This allows for a much smoother transition between states, using a lookup table to determine the correct signal. Coils are often energized in a saddle pattern, rather than a pure sinusoidal output.
Field-oriented control (FOC) works similarly to variable-output sinusoidal control, but also takes the motor's changing winding currents into account when calculating voltage inputs. FOC can produce constant torque and speeds with low acoustic noise and is the most efficient way to drive a BLDC motor.
Key Considerations When Choosing a Brushless Motor
When it comes to choosing a brushless motor, there are a few key considerations that you should keep in mind, such as:
Speed and Torque: One of the most important considerations when choosing a brushless motor is its speed and torque capabilities. It is important to select a motor with enough power to complete the desired task while not overloading it.
Size: Another key factor to consider is the size of the motor, which will determine the space requirements of your application. Smaller, lighter motors are typically more efficient but may have a different torque or power output than larger motors.
Cost: As with any purchase, cost is an important factor in selecting a brushless motor. When comparing prices, consider factors such as efficiency and durability to determine which motor is the best value for your application.
Control System: Depending on the application, you may need a specific control system to operate the motor. Either analog or digital systems can control brushless motors, so make sure to select one that is compatible with your specific needs.
Environment: Consider the environment in which your motor will be operating. Different motors are designed to work in different environmental conditions, so select one that suits your application's environment. This includes factors such as temperature, humidity, and dust levels.
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