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Basic Electronics » Building a motor driver/ESC for AC motor.

May 05, 2010
by met_fredrik
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As the title says I want to build an ESC for a big ac brushless motor.

But I don't even know where to start! I need the max load current to be about 200 amps. Would it be hard for a beginner to build something like that? And what components would I need?

The motor I'm talking about is this one: [Turnigy Brushless Outrunner](http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5142&Product_Name=Turnigy_80-100-B_130Kv_Brushless_Outrunner_(eq:_70-55)

Thanks!

May 05, 2010
by Ralphxyz
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I believe that is a DC motor not AC.

What exactly is a ESC?

Ralph

May 05, 2010
by met_fredrik
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Hmm, the only reason I assumed it was an AC motor is because it is 3-phased. Electronic speed controller. I checked it up now, it is 3 -phase AC.

May 06, 2010
by JKITSON
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I viewed the info on the motor and it is a DC brushless outrunner motor used primarily in the model airplane hobby..

Depending on what power you want and with proper cooling you can use from 12vdc into the 60 vdc range. All ESC's (electronic speed controls) have the max voltage and current ratings published for them...

Good luck Jim

May 06, 2010
by met_fredrik
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Are you sure it is DC powered? Yes, I have used it for an airplane. They also sell a 200 amps ESC for it, but the ESC is of poor quality hence I wanted to build my own.

Any tips for the components etc?

Thankyou!:)

May 07, 2010
by mrobbins
(NerdKits Staff)

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Hi all,

When it's a brushless motor, it's hard to really call it "AC" or "DC" -- each phase will certainly be powered by a changing voltage. DC really refers to the fact that the original power source will be a DC source (like a battery), but there's a controller between the DC and what the motor coils see. Anyway, I don't mean to get stuck on this terminology, as it doesn't really add anything to the discussion.

The big picture is that you'll use power MOSFETs as switches for each coil -- probably requiring three half-bridges (or 3 high side MOSFETs and 3 low side MOSFETs) in total. You will then need to switch them at the correct times based on either sensor feedback, or sensorless feedback from the motor's own back-EMF. That's not a trivial project! Let me try to point you to some resources:

This article has some basic overview of brushless motor control.

For more details about implementation, Atmel has published a series of application notes about motor control. Some of these directly address microcontroller-based control of a BLDC motor, such as:

Atmel provides sample source code for each of these -- although I'm not sure it will be too useful on its own.

In any case, your 200A motor requirement is quite large, and that means that your circuit is more sensitive to power dissipation issues, switching time, etc. It also means that breadboard-based design is probably not feasible (too much resistance). If you have the proper diagnostic equipment like an oscilloscope, are willing to make PCBs, and want to do some math, this could be a neat project!

Mike

May 11, 2010
by met_fredrik
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Thanks for a very in depth answer Mike! After some reading I've decided to make a micro version of the esc for a 0.27A(noload) 3-phase brushless motor.

If I understand the basics correct I need something like this:

3 power mosfets and 3 smaller mosfets for giving the current to the motor. I can just use the MCU directly to the smaller mosfets right? And then using the back-emf as a sensor for when to send a pulse.

What type of component should I use to register the back-emf? I can't connect it directly to the adc input, it would kill the MCU instantly!

I don't have very much experience from the subject but help is greatly appreciated!

Thanks!

May 27, 2010
by met_fredrik
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I've gotten hold of some power mosfets. Datasheet: FS3206 mosfet.

They are more than powerful enough but if I want to make a starter regulator for about 0.5 amps, would they still work? Or would the resistance be to high?

June 02, 2010
by mrobbins
(NerdKits Staff)

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Hi met_fredrik,

That looks like a hefty power MOSFET, with a saturation I_D of over 100 amps at a 5V gate drive.

I am not entirely sure where your 3 power MOSFETS + 3 smaller MOSFETs idea is coming from. Can you post a quick sketch perhaps?

The link above "Sensorless control ..." shows using three half-bridges. That means that each of the 3 terminals of your motor can be connected either to the positive voltage supply or to ground. In practice, at any given time, you basically have one terminal connected to the positive supply, one terminal connected to ground, and the third is "floating" but is used as your back-EMF input. After going through a R-C lowpass filter, that voltage is read and watched to signal rotation (so the chip can know when to start triggering the next phase).

Anyway, it's all a bit complicated. The "Sensorless control..." PDF I linked to above is really quite good, although it's terse and doesn't explain where everything comes from. I wish that I could point you to something more thorough on this topic!

Mike

July 09, 2010
by Ralphxyz
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met_fredrik, did you make up a esc?

Want to share any insights?

Ralph

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