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Permalink Reply by Andrew Dunlop on May 26, 2012 at 11:46pm

If only it was that simple.  Unfortunately your circuit will not be able to drive a brushless motor.  The term "brushless DC motor" is a bit of a misnomer - these devices are in fact synchronous 3-phase AC motors with permanent magnet excitation.  There's no DC involved at all.

ESCs provide a 3-phase drive from DC supply, and perform synchronisation and speed control.  They are quite complex devices and relatively inexpensive for all that they do.

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 12:56am

Thanks!

Upon closer examination of these Motors you are indeed correct. I completely missed the 3 'hot' connectors.

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Permalink Reply by Jake Stew on May 27, 2012 at 12:16am

ESCs are cheap, try ebay. It's the last thing I would worry about trying to cobble together.
There's a lot more to an ESC than your schematic. The PWM control signal is not the same as the one that's fed into the motor. You've got 3 motor wires and you need to feed PWM power into each with the right timing and switching to get the motor to run.
It's not that it would be difficult to make a decent ESC, but you simply can't make one for even close to as cheap as you can buy them, unless you want to make 10k+ of them and happen to own a factory in China.

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 1:01am

Oh I'm not looking to cheap out, I just thought I understood the concept and so I tried to get creative. I really wanted to understand what is all going on behind the scenes.

So pretty much the ESC is acting like an inverter then?

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Permalink Reply by Andrew Dunlop on May 27, 2012 at 2:37am

Yes, although you won't hear speed controllers referred to as inverters.  Although a BLDC motor will follow a 3-phase AC signal at low rpm (open loop), once a load is applied and/or the speed increases, maintaining synchronisation requires a knowledge of rotor position.  Back EMF is the most commonly used technique to estimate rotor position, probably because it requires minimal additional hardware.

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 11:11am

Thanks! I think I'm getting closer to having a better understanding, I've read several articles on the internet but they just didn't click.

I know Back EMF varies with the load on the Motor; so for a constant load (e.g. Propeller) ... Let's say we're moving at a constant RPM (e.g. 3000 RPM) ... The Back EMF should be constant as long as the 'average' Voltage being applied to the Motor is constant right?

I'm just not seeing why Rotor position is important to know here. I guess my question is ... Why is this is closed loop control system and not an open loop control system.

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 3:34pm

Wow, you guys explain better than the articles I've been searching. Thank you!

Lastly, how exactly is the Back EMF being measured? All 3 leads are used to 'Pulse' the phases of the Motor ... Wouldn't the ESC require some feedback connection to know the Rotor's position?

All the ESCs I have seen have 3 terminals (one for each phase) ... How do I hook it up so that the Back EMF is monitored?

Or is all this done automatically?

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 4:50pm

Alrighty, that does make sense.

Now, I'm still a little confused on how the ESC 'controls' the Motor speed. I've done some reading and to help you better understand my question I made a sketch in Visio.

I would like to know  ... When the Phases operate as they do below, is the Motor in Full Throttle (100%)?

How then does the diagram change for say 75% Throttle? I'm assuming that the ESC itself is always powered, and that for 75% duty cycle the ESC just uses PWM to power the individual phases for the appropriate time?

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Permalink Reply by Sunmi Akintewe on May 27, 2012 at 4:56pm

So for 75% Throttle ... When each phase is supposed to be ON ... Does the ESC just cut down that ON time to 75% of its regular time via PWM?

I've included a diagram (just for Phase A for simplicity) to explain what I mean.

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