over current protection for bldc motors

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so far I have come across designs that uses simple shunt resistor and measures the voltage across it....this is quite bad for high current like in the order of 50A....It would be nice to be able to actually get some values as measured readings through the microcontrollers adc to report back as how much current is being consumed rather than just work only in the high current range and work as a protection trip.

I am thinking of using a current transformer instead of a shunt resistor...has anyone used any? any recommendations?

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actually cant use a current transformer...since it is DC! lol...hmh..

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hhm... I just saw ACS714 it provides a current path direct through its die! Up to 30A. How can it with those thin SO8 pins???

Then there is the MLX91205..which uses a current path on the pcb track...

I wish ACS714 did up to 50A...

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hhm....this is nuts ordering for small quantities and paying 30+$ for shipping from digi-key!

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There is ACS714's big brother, ACS756, and it goes up to 100A.

http://www.allegromicro.com/en/P...

-Pantelis

Professor of Applied Murphology, University of W.T.F.Justhappened.

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I just wondered what the accuracy would be in measuring low current level eg in 5-10A range...using the ACS715 units...but it seems it should work fine...as per datasheets...40mV/A.

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So, what are your requirements actually? 5-10A, 50A, something else?

-Pantelis

Professor of Applied Murphology, University of W.T.F.Justhappened.

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now the question I have for you guys is... should I put the ACS756 on the positive rail or the ground rail of the supply?

I am leaning more towards positive rail...

I would be happy with a 50A max requirement for now :) I have just ordered the part from farnel..

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Ok, because you mentioned ACS715 and I got a bit confused. Sorry, can't help with the high-side or low-side dilemma, I don't know much about motor current sensing and the application notes that I found with a (quick) look don't mention anything. :(

-Pantelis

Professor of Applied Murphology, University of W.T.F.Justhappened.

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Concensus is to use high side where possible....

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luvocean1 wrote:
now the question I have for you guys is... should I put the ACS756 on the positive rail or the ground rail of the supply?

It doesn't matter. The part that the current passes by is just a copper bar (~130uR, according to the datasheet). It senses the current via a hall effect sensor, that is galvanicaly isolated from the bar.

Felipe Maimon

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yes I was asking the question from a design perspective :) I chose now positive rail. The advantage being the ADC measurements all stay referenced from ground. That way I dont need to do differential measurements from each phases etc...

Think I just finished all the pcb work...now will order soon and in the mean time coding begins!

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For such currents, I saw using linear hall effect sensors that have something else integrated inside. Placement is more or less critical, but calibration can be done easily.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Years ago on my electric car, I used the DC motor power cable resistance as the shunt. I used a high current low resistance shunt to calibrate the measurements, then removed it. This eliminated the shunt loss.

It all starts with a mental vision.

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Well I have finished my design and have sent it off to seeedstudio.com. Writing code is almost there... I am just translating my previously written code for atmega164 to atxmega32A4. So its not that hard. Shall be waiting for a board to arrive now...will keep you guys posted!

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ok I just realised there is a problem...the ACS756 device outputs signed values centered at 2.5V (assuming 5V supply) in other workds 0Amp current flow will give 2.5V. From 0->50Amp positive will give 2.5V->5V output.

Now I am using the xmega32A4....I need to first offset the voltage to 0. Hmm.. what is the quick way of doing this?

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Why not do it in the math? You will lose 1 bit spanning from 2.5V to 5V, however 9 should work for your application. Just subtract 511 or what ever the ADC offset count is.

It all starts with a mental vision.

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Hhm...it seems you can drive the ACS756 with 3.3V supply...which means its Voutput will be sitting around 3.3/2 = 1.65V ish when there is no current. Essentially also means the Voutput max is 3.3V. This is according to the datasheet. But the datasheet advises me to contact Allegro if I am using 3.3V as the prefered supply voltage...apparently the sensitivity will have to be scaled as well. I am still waiting to hear a confirmation from Allegro.

But if this is the case then I might just have to simply scale the 3.3V max Vout to 2.7V ADC input of the atxmega32A4. Then my range of current values will be from 2.7/2 = 1.35 to 2.7V...not a big margin but should work. Hhm... yes and then I will have to use math to subtract the voltage offset of 1.35

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As it can sense current in both directions, connect the wires in the opposite order. Then you'll get a 0-2.5V output, with 0 V being the maximum current.

Felipe Maimon