Apex Precision Power has developed a MCU program with a soft start algorithm for driving motors with the Apex Precision Power SA303 and SA306. Download it free from the SA303 or SA306 product page at www.Cirrus.com.

By employing a microcontroller and monitoring the instantaneous currents in all three phases， a closed-loop algorithm can be developed for start-up purposes which would hold the peak current near 15 amperes without actually exceeding it. A small amount of headroom makes good sense so programming for a 12 ampere motor current would be best. The advantage of this approach is that it optimizes the run up and keeps the current and the acceleration as high as possible. In this approach the duty cycle would be modulated， based upon the current sensed in the three phases， as depicted in Figure 2. Further discussion on the use of microcontrollers to drive brushless motors is available in References 3， 4， and 5.