A novel method for measuring rotational speed of BLDC motors using voltage feedback

Applications of BLDC (Brushless Direct Current) motors are increasing each day. Air conditioners, electric pumps, fans, printers, robots, electric bikes, doors, windows, sun roofs, seats, mixers, food processors, blenders, vacuum cleaners, toothbrushes, razors, coffee grinders, etc. BLDC motors are most commonly used in easy to drive, high speed and long life applications. They have become widespread and are available in all shapes and sizes from large-scale industrial models to small motors for light applications (such as 12 V BLDC motors). In some cases it is needed to have a feedback of the motor's rotational speed in order to control it. Common speed meters use hall-effect sensors to measure the exact speed of the motor; and some kinds of BLDC motors include these sensors inside to give to the controller board. Nevertheless in some places it is hard or even impossible to locate these sensors around motor. Here we present a simple method for measuring the speed of a BLDC motor using simple electronic elements and a low cost microcontroller unit for measuring and transferring data. The main idea is to measure the frequency of the voltage between two phases of the motor; as we know the rotational speed of a BLDC motor is proportional to the phase signal frequency generated by the driver.