Brushless sensorless DC motors have been used extensively to power devices such as blowers. In order to start a brushless sensorless DC motor, a calibrated sequence of commutation pulses must be applied to phase coils in the motor. These commutation pulses increase at a frequency rate that closely matches the rate at which a rotor of the motor accelerates to start and maintain a rotation of the rotor.
Brushless sensorless DC motors generally utilize back electromotive force (EMF) signals generated in the phase coils to determine the rotor position. Each of the back EMF signals has an amplitude that varies linearly with motor speed and has a zero value when a rotor is not rotating or is rotating at low speeds. Thus, motor control circuits have difficulty in detecting the rotor position when the rotor is not spinning or is spinning at low speeds. Thus, techniques for starting brushless sensorless DC motors have required that the start-up sequence of commutation pulses be “tuned” to a motor/load combination.
The tuning/calibration process is a time-consuming process that requires an engineer to characterize motor start-up parameters, such as the duration and amplitude of each of the commutation pulses, that correlate to the acceleration characteristics of the motor/load combination. However, if the motor torque, load, or applied voltage at a customer site varies substantially from those used during the tuning/calibration process, the preset start-up parameters (such as a pre-set commutation pulse duration or amplitude), may not properly start the motor at the customer site.
Thus, it would be desirable to have an adaptive method for starting a brushless sensorless DC motor that can sense a position and a speed of the motor during start-up when a rotor is not rotating or rotating at low speeds, and then adjust the duration and amplitude of commutation pulses to repeatedly start up the motor under varying operating conditions.