Maximising the effectiveness of BLDC motor commutation
01 June 2018
Brushless DC (BLDC) motors have seen strong uptake over the course of the last two decades, supplanting traditional brushed motors. They are capable of delivering major advantages over their brushed counterparts - including significantly higher operational efficiency levels and longer working lifespans (as they are not as prone to mechanical wear and tear).
Domestic appliances, automobiles, items of consumer electronics, industrial automation equipment and medical instrumentation are just some of the many areas where they are now commonplace. In contrast to brushed versions, brushless BLDC motors need external controllers in order to take care of commutation activities. The following article will investigate the numerous methods by which this can be accomplished.
In simple terms, commutation is the process of switching the current within the motor phases in order for motion to result. For brushed motors the physical brushes are used for this purpose (twice in each rotation). BLDC motors do not have this mechanism however, so an alternative mechanism is required. The construction of BLDC motors means that they can (in theory at least) have any number of pole pairs for commutation.
Although BLDC and brushed motors share the same fundamental principles of operation, the additional external control circuitry has to be factored in. To some degree the more sophisticated control and feedback circuitry associated with BLDCs was responsible for holding back their adoption in certain applications. More stringent international environmental guidelines are now leading to greater proliferation though. Engineering innovations in relation to motor drive technology, as well as access to better permanent magnets have further supported this migration to BLDCs in recent years. With more and more opportunities opening up for BLDC motors, an increasing number of engineers are having to consider how they will implement this technology into their system designs.
BLDC control circuitry adds a completely new dimension to the motor system. Let’s now look at each of the key aspects of this. As most of us will already be aware, BLDC motors are offered in either 1, 2 or 3-phase configurations, but it is generally 3-phase versions that are used in professional designs. How many phases there are reflects the number of windings on the stator, while the rotor poles can be any number of pairs - depending on the application expectations.
Since the rotor of a BLDC motor is influenced by the revolving stator poles, it is necessary to know the stator pole position at any given time. This is where the motor controller comes in. It creates a 6-step commutation pattern for the 3 motor phases (as shown in Figure 1). These steps (which are referred to as commutation phases), move an electromagnetic field which in turn causes the permanent magnets of the rotor to keep the motor shaft in motion. Via a high frequency pulse-width modulated (PWM) signal the motor controller alters the average voltage that is applied to the motor, and thus determines what speed the motor will run at.
If the operational efficiency benefits of BLDC motors are to be fully realised in this arrangement then a tight control loop between the motor and its accompanying controller will be needed. The acquisition of accurate feedback data on an ongoing basis is critical here - allowing fully effective control of the motor to be maintained. This feedback data can be achieved by either sensor-based or sensor-less techniques.
Techniques for acquiring feedback data regarding positioning
It has been commonplace for many years to employ magnetic sensors (utilising the Hall Effect) to deliver commutation feedback in BLDC motor systems. For 3-phase control, only three sensors are required, and with a very low per-unit cost involved generally, they are easily the most economical option to achieve commutation from a purely bill-of-materials perspective. These sensors are incorporated into the stator so that the rotor’s positon can be correctly derived. From this data, switching the transistors in the 3-phase bridge can be scheduled so as to drive the motor with optimal efficiency.
There is a limitation that starts to come into play though. These magnetic sensors can only provide angular position within each electrical cycle and, as the number of pole pairs is ramped up, the ratio of electrical cycles per mechanical revolution will increase. In response to this the addition of an incremental rotary encoder to the BLDC motor can prove to be invaluable. The tasks can then be demarcated as follows - the mechanical encoder taking care of precisely tracking position, rotation, speed and direction, while the magnetic sensors are used for motor commutation.
Optical encoders are another, slightly more elegant, option. Here light is generated by LEDs and passed through a disk with notches at specific intervals to generate output patterns. Although this technique has clear benefits over mechanical encoders, there are still some compromises that will need to be made as a result of taking this approach.
Since BLDC motor commutation relies on accurate rotor and stator positioning data, attention must be paid to ensuring that the commutation encoder’s U, V and W channels are correctly aligned with the BLDC motor’s phases. For optical encoders that have fixed patterns on their optical disks and magnetic sensors that need to be placed manually, alignment of a BLDC motor can be a time-consuming and laborious process that requires specialist equipment plus extensive training to undertake. In addition, use of optical encoders can prove to be quite restrictive, as they are dependent upon fixed patterns on their optical disks. This means that the motor pole count, quadrature resolution and motor shaft size all need to be known in order to specify a suitable encoder. There is also the potential for the optical disks to break because of exposure to heavy vibration.
An attractive alternative to optical commutation encoders, are ones that are capacitive based. Here, rather than transmitting light generated by LEDs, an electrical field is transmitted. A PCB rotor containing a sinusoidal-patterned metal trace replaces the optical disk and is used to modulate the electrical field. The receiving end of the modulated signal is then passed back to the transmitter where it is compared against the original signal.
The AMT31 series of commutation encoder devices from CUI Inc. provide both the incremental outputs (A/B/Z) and the commutation outputs (U/V/W) necessary for BLDC motor control. Each incorporates a capacitive ASIC, plus an on-board microcontroller. Through these it can generate outputs digitally - which means the user can set the zero position for the encoder with far greater convenience than the methods that are mandated by the other encoder types already outlined. It dispenses with the need for backdriving the motor, examining the output signals using an oscilloscope, or similarly onerous procedures. Most importantly it saves a considerable amount of time.
By employing capacitive technology, the quadrature resolution and commutation outputs can be adjusted dynamically, accessing a list of different quadrature resolutions to find the optimal one for the motor system. As well as speeding the development/deployment process, it is highly advantageous during prototyping, when different options may need to be quickly evaluated. As each capacitive encoder unit can support multiple resolutions and pole pair counts, there are clear logistical/supply chain benefits to be considered.
By furnishing BLDC motors with a tight control loop the full operational benefits of these devices can be realised. Advanced encoder technology, such as that described in this article, is showing itself to be pivotal in maximising the effectiveness of BLDCs. By improving the accuracy less power is wasted and end user control is enhanced.
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