What you’ll learn:
- Startup techniques employed to overcome the problem of determining magnet position in BLDC designs.
- Cost-saving and power-efficiency incentives using sensorless motor drivers.
- Motor-driver solutions that leverage a field-oriented-control (FOC) system.
Today, the market trend is to use brushless dc (BLDC) motors for high-power-density motor design. To drive the BLDC motor properly, the precise position of the magnet that’s being sensed must be known. This typically requires an angle sensor, such as a magnetic Hall sensor or encoder.
Compared to a dc motor, the BLDC solution adds cost and complexity to the controller design. To optimize cost, some BLDC applications may be able to use a controller without a motor angle sensor. This article describes multiple advanced, sensorless startup techniques to overcome the challenge of determining the magnet position, which is needed for proper BLDC motor startup.
Traditional Sensorless Startup Method
Traditional sensorless drivers use the “dc alignment startup” method, where the controller forces a dc current in the three-phase winding so that the rotor magnet gravitates to the known position. This solution forces the alignment of the current so that the rotor aligns properly with the magnet. The method is shown in Figure 1, with alignment to Phase U. After rotor alignment completes, the motor can begin to rotate from that position.
The dc alignment method is simple, easy, and can be applied to many types of BLDC motors. However, when the motor or connected load has high inertia, it increases the time to settle to the alignment position. As a result, overall startup time tends to be longer than sensored-BLDC drivers.
Also, depending on the parked position of the rotor, the rotor may move backward to align with the stator. That small amount of backward spinning causes an undesirable effect for some applications, such as pumps.
When using this traditional dc alignment method, motor startup—from speed zero to robust, consistent high-inertial spinning—may take more than five seconds.
Conventional IPD Startup Method: Inductance Saturation
Initial-position-detection (IPD) methods are available that leverage inductance saturation to enable sensorless detection of the initial rotor position without moving the shaft (Fig. 2). Inductance-saturation techniques leverage the state of inductance in the stator winding.
If the total flux generated by the winding exceeds a certain point, the inductance will become saturated. The stator receives the overall flux generated by the winding and by magnetic flux. The magnetic flux component varies depending on the rotor position, so the inductance-saturation current changes with the magnetic flux:
- When the position of the magnet is aligned to the stator, the inductance saturates easily.
- When the magnet is offset 90 degrees from the stator, the saturation effect from the magnet is zero.
Compared to motor startup that applies the traditional dc alignment method, IPD using inductance saturation is significantly faster: Traditional dc alignment could take more than five seconds and may include reverse rotation, while IPD enables detection of the initial rotor position within, typically, less than 1 ms—without any reverse rotation.
IPD using inductance saturation provides initial detection accuracy of 60 degrees. However, such performance comes with an unwanted noise component: Typically, six test current pulses must be injected into the motor, which produces an unwanted click sound six times.
Advanced IPD Startup Method: Saliency with Inductance Saturation
In a permanent-magnet synchronous motor (PMSM), the inductance at the motor terminal varies according to the rotor position. This phenomenon, called “saliency,” is intentionally designed into some PMSMs.
For example, depending on application needs, some PMSMs may use an interior PMSM (IPMSM) to enhance torque characteristics, while others may use a surface PMSM (SPMSM), which has comparatively less saliency. The saliency behavior characterized over the angle —i.e., the inductance difference depending on the rotor position—is depicted in Figure 3.
Observing inductance can make it possible to detect the initial position of a rotor. Because saliency exists regardless of the flux direction from the magnet, saliency characteristics between 0 and 180 degrees are identical to those between 180 and 360 degrees. Thus, use of saliency requires another method, such as the inductance-saturation measurement, to identify the north pole or south pole:
- When the magnet (d-axis or inverse d-axis) is aligned to a phase, inductance will be at the maximum.
- When the magnet is offset by 90 degrees (q-axis or inverse q-axis), the inductance will be at the minimum.
One type of IPD technology developed by Allegro MicroSystems leverages the inductance-saturation and saliency effects and employs a dedicated analog detection circuit within the motor-controller driver. Saliency measurement is achieved using a high-precision comparator, enabling detection of saliency characteristics typically within six PWM periods. After the saliency measurement is complete, a test current pulse is applied to the winding to check the inductance saturation, which is used to identify the magnetic pole.
When utilizing this method, alignment can be ignored, detection of the initial position within less than 1 ms is possible without moving the shaft, and it provides additional flexibility: If the target motor has low saliency, the initial position can be detected using inductance saturation only.
Compared to conventional IPD methods that rely only on inductance saturation, the Allegro IPD method uses a two-stage IPD method, so it reduces the required test pulse currents from six to two—thus reducing the number of click sounds. Also, leveraging the saliency effect improves initial detection accuracy from 60 to 30 degrees compared to the conventional IPD method. The Allegro IPD method offers faster, more accurate, quieter initial detection capability, leading to quicker, reliable, non-reversing startup. An evaluation GUI is available to validate the startup performance.
Conclusion and Recommendations
The table presents a comparative summary of the BLDC motor startup methods discussed in this article. Due to the speed, accuracy, non-reverse, and noise features of the Allegro IPD method, it’s well-suited for fan blowers for both general and automotive purposes.
The available sensorless motor controllers that use the Allegro IPD method include A89301 and A89306 for the general blower fan market, and A89307 for automotive fan blowers. They drive a motor using a FOC system. Driving parameters—including the Allegro IPD method—can be chosen by an I2C communication interface and saved into internal non-volatile memory (NVM), and the motor can spin with preprogrammed parameters.