Using the PIC18F2431 for Sensorless BLDC Motor Control
Description
AN970
Using the PIC18F2431 for Sensorless BLDC Motor Control
Author: Padmaraja Yedamale Microchip Technology Inc.
With a sensorless method of commutation, the Hall sensors, the sensor magnets, the sensor wires and the PC board can be eliminated.
This simplifies the motor construction and reduces cost.
Inherently, systems with fewer components are more reliable.
Applications like the compressor control in a refrigerator or in HVAC systems, which generates heat and an elevated temperature, may accelerate Hall sensor failures.
Using sensorless control improves the reliability of total system.
There are two disadvantages to sensorless control: • The motor must be moving at a minimum rate to generate sufficient BEMF to be sensed.
• Abrupt changes to the motor load can cause the BEMF drive loop to go out of lock.
As the BEMF signal is proportional to the speed of rotation at standstill and at low speeds, the amplitude of the BEMF poses a challenge to determine the zero crossover point.
In order to overcome this problem, the motor is normally run in open loop during starting and is accelerated to a speed where the BEMF has sufficient amplitude to detect and then synchronize with the zero crossing point.
Running the motor in open loop makes it difficult to know the rotor position, when starting from standstill.
So, the rotor can rotate in either direction when energized.
Applications which require very low motor speed may be forced to use sensors for control.
There are many applications in appliances, automotive, and industrial applications where very low speed is not an issue, and the motor needs variable speeds of about 25% of the rated speed.
Sensorless control is ideal for such applications.
Another requirement is that the sensorless control needs extra resources on the control circuit side.
Depending upon the method used, it may require a fast A/D converter on the microcontroller, or comparator and filter circuits.
INTRODUCTION
Brushless Direct Current (BLDC) motors h...
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