I am trying to achieve speed control of a sensorless Brushless dc motor (Back Emf method). Initially its speed is zero so Back emf is zero, so there will be no feedback, in that case what do i do? In my current implementation I can only hear a high pitch noise but no rotation. please help.Circuit Diagram of my ESC I Hope this helps. As for the code, Im not sure. Could anybody help me out?

  • 2
    \$\begingroup\$ I think this question is fine. It asks "how do I sequence/start up the driving of a brushless motor when using back EMF rather than separate sensing." It's quite possible the answer can also be found in various textbooks, but I think it's a fine question to ask here. My guess is that those who put it on hold actually don't understand what the problem is in the first place? The one thing missing from the question is a description/diagram of the current implementation. \$\endgroup\$
    – Jon Watte
    Aug 18, 2013 at 22:37
  • \$\begingroup\$ There is an application note from Microchip which explains this process pretty well. Sadly, their site seems messed up right now, I think it is AN901 \$\endgroup\$
    – Matt
    Aug 25, 2013 at 4:05
  • \$\begingroup\$ See the basics of transistors first, how to drive them properly. T7-T9 are not driven properly at all, you drive the base with 3volts and collector is with 12volts, will never work. \$\endgroup\$
    – user68731
    Feb 26, 2015 at 23:28

1 Answer 1


you could use open-loop to start up, so you would linearly increase the pwm duty-cycle and commutation frequency until you reach some speed at which you can get meaningful signals... or just use 2 pots and see what works.

you also need to do something about driving the high side transistors... their source is floating so you have to drive the gate higher than that value. you'd need some sort of bootstrap for n-channels or like a push-pull type thing if you use p's.

also i would recommend testing it on a resistor bridge before connecting it to a motor, just my $.02.


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