By Jason Kelly
Brushless direct current electric motors, or BLDC motors, are electronically commutated motors powered by a dc electric source via an external motor controller. Unlike their brushed relatives, BLDC motors rely on an external controller to achieve commutation, which is the process of switching current in the motor phases to generate motion. Brushed motors have physical brushes to achieve this process twice per rotation, while BLDC motors do not, and due to the nature of their design, they can have any number of pole pairs for commutation. In this blog post we will review BLDC motor basics, look at common methods of commutating BLDC motors, and introduce a new solution for gathering position feedback.
The most common configuration of BLDC motors is 3-phase. The number of phases match the number of windings on the stator, while the rotor poles can be any number of pairs depending on the application. Because the rotor of a BLDC motor is influenced by the revolving stator poles, the stator pole position must be tracked to effectively drive the 3 motor phases. Therefore, a motor controller is used to generate a 6-step commutation pattern on the 3 motor phases. These 6 steps, or commutation phases, move an electromagnetic field which causes the permanent magnets of the rotor to move the motor shaft.
For the controller to commutate the motor effectively, it must always have accurate information on the position of the rotor. Hall effect sensors have been the popular choice for commutation feedback since the inception of the brushless motor. In a typical scenario, 3 sensors are required for 3-phase control. The Hall effect sensors are embedded into the stator of the motor to detect rotor position, which is used to switch the transistors in the 3-phase bridge to drive the motor. The three sensor outputs are commonly noted as U, V, and W channels. Unfortunately, there are some drawbacks to this method of position feedback. While the BOM cost of the Hall effect sensors is low, the cost of integrating these sensors into the BLDC can double the total cost of the motor. Additionally, the controller only gets a partial picture of the motor's position from the Hall effect sensors, which can cause problems in systems where precise position feedback is required to operate properly.
In today's world, systems that require BLDC motors need far more precision in position measurement than ever before. To accomplish this, incremental encoders can be paired to the BLDC motor in addition to Hall effect sensors. This presents a system that provides improved position feedback, but now requires the motor manufacturer to add both Hall sensors in the motor, along with an incremental encoder after assembly. A better option skips the Hall effect sensors altogether and replaces the incremental encoder with a commutation encoder. These commutation encoders, such as CUI Devices' AMT31 series or AMT33 series , have incremental outputs for precise position tracking, along with commutation outputs that match the motor's specific pole configuration. CUI Devices' commutation encoders, being digital, allow for these parameters, including pole count, resolution, and direction, to be programmed. This provides the engineer flexibility during prototyping and testing as well as a reduced encoder SKU count across multiple designs. For more information about commutation versus incremental encoders and hall sensors, read our technical paper.
When current is applied to a motor it spins, and conversely when you spin a motor, it generates current. If you were to spin a BLDC motor, you would see outputs on the 3 phases similar to the image below. To properly align a commutation encoder or even Hall effect sensors to a BLDC motor, the resulting commutation waveform should be aligned to the back EMF. Traditionally, this results in an iterative process requiring a second motor to drive the first, and an oscilloscope to observe the waveforms. This can be time consuming and add significant costs during the manufacturing process.
With an AMT capacitive encoder, the alignment process is nearly instant and only requires a power supply. Once the encoder is mounted, the user needs only to apply power to the two phases that correspond to the desired starting position of the AMT encoder and send the alignment command. In doing so, the user has essentially set the starting position of the encoder's commutation waveform and the motor's back EMF waveform.
In addition to the ease of alignment, the AMT encoder's commutation signals are much more precisely aligned to the motor poles. Aligning a commutation encoder to a motor just sets the start position (i.e. where the commutation waveform begins). If done properly, the commutation waveform should perfectly match the motor's back EMF waveform. However, this is not always achievable. A typical alignment with hall sensors or an optical encoder is on the order of ±1 electrical degrees. AMT encoders, on the other hand, can achieve much greater precision, typically within ±0.1 electrical degrees. The AMT encoder's waveform begins when U and W are both high (third state in the above waveform); consult your motor manufacturer for the appropriate back EMF diagram to determine which phases should be energized during alignment.
Along with the programmable pole count and resolution features, the AMT series offers a direction setting for commutation applications – a unique option not provided by most other commutation encoder manufacturers. Put simply, the direction tells you which way the encoder's shaft should rotate for the commutation signals to advance. Typically, commutation encoders are placed on the back shaft of the motor. In this scenario, the commutation signals advance through their states when the motor is turning counter-clockwise (as viewed from the back of the motor). However, if you put the encoder on the front shaft, you have essentially flipped the encoder upside down and now when you rotate the motor counter-clockwise (viewed from the back), the encoder's shaft is actually rotating clockwise (viewed from top down on the encoder). This means the motor's poles are rotating the opposite direction as the encoder's poles, as shown in the image below. Other technologies that do not include this programable option require the physical swapping of the encoder disk or the U, V, W channels to accomplish the same task. For applications utilizing multiple BLDC motors with varying directional requirements, this programmable feature can be particularly useful.
BLDC motors continue to grow in use and can excel in many applications when afforded a tight control loop and high accuracy position sensing feedback. Hall effect sensors have been the go-to solution for many years due to their low BOM cost, but they often fall short in providing a complete picture of a motor's position unless paired with an incremental encoder. However, CUI Devices' AMT commutation encoders provide an all-in-one solution that eliminates the need for Hall effect sensors and incremental encoders altogether. CUI Devices' AMT31 or AMT33 commutation encoders are the most versatile options on the market due to their flexible programmability and simple installation. A basic understanding of commutation encoder principles as outlined in this blog can make them a compelling option for your next BLDC motor project.
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