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Are AC and DC parameters related linearly? 
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#1
Dec712, 12:37 AM

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I've a small wind turbine that is rated at about 24V. The turbine produces AC voltage but it's got an internal recifier converting this to DC. I need to create an embedded system that measures the output of this turbine displaying AC voltage, current and power output.
I did some research online and have come to the conclusion that I might need specific programmable IC's (Microchip for example has one). However, they need external current transformers to tap into the main output of the turbine. Also, they need to be programmed separately (apart from the microcontroller that I'll be using). At least, that is how I understood. I was wondering, since the turbine has an internal rectifier(unfiltered) converting the output to DC, can I not simply measure the DC parameters? Voltage is easy (an ADC should do). Current? Current sensors are easy to find. So, power = voltage * current. But, this is under the assumption that AC parameters (voltage and current) are linearly related to DC paremeters after passing through a bridge rectifier. Is this correct? 


#2
Dec712, 01:23 AM

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Welcome to PF;
If it is really just a rectifier  then it won't be producing DC. Hook it up to an oscilloscope and see. Also see: http://en.wikipedia.org/wiki/Rectifier But certainly  P=VI in DC and P=V_{rms}I_{rms} for AC If the output is really DC you will have lost the frequency and phase information though. If you don't need that, then why do you need to work in AC at all? 


#3
Dec712, 04:42 AM

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So, if I use a capacitor to filter the output, then measure voltage and current, can I get an approximation of the AC voltage and current being produced by the turbine? 


#4
Dec712, 05:29 AM

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Are AC and DC parameters related linearly?
Do I understand you that you must somehow get the rms values of the voltage and current from the generator before it is rectified? The DC values are related to the rms values  but the relationship depends on the physical electronics in the rectifier circuit  eg. it has resistances in it which will reduce the DC voltage. Presumably the voltage also varies with the speed of rotation? So I guess I don't understand which AC values you need. 


#5
Dec712, 05:46 AM

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I hope I'm clear with my problem. 


#6
Dec712, 06:56 AM

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You can find a relation between the RMS voltage before ACDC conversion to the DC voltage after conversion by studying the electronics of the ACDC converter circuit. The details depend on the actual circuit being used. 


#7
Dec712, 09:12 AM

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I think I've caused unnecessary confusion. What I meant was, I'll use two multimeters, one to measure the AC voltage right before rectification and the second multimeter to measure DC voltage after rectification. I'll vary the RPM of the turbine to generate different voltages. I'll use this data to plot AC voltage versus DC voltage. This will give me a mathematical relationship between the two which I can use in my microcontroller code to display AC voltage even though it measures only DC voltage. I hope this makes sense. :)



#8
Dec712, 10:51 PM

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Fair enough... a moving coil voltmeter displays rms volts on AC iirc.
Digital voltmeters may do something special. Since you plan to measure everything, what was it you wanted to know? 


#9
Dec812, 07:47 AM

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#10
Dec812, 11:00 PM

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Oh right... empirically determining a relationship is usually best. The relationship need not be strictly linear though, so you'll want enough data to draw in a curve. OTOH I wouldn't expect a complcated relationship either.



#11
Dec1012, 11:37 AM

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Is this alternator a single phase device? Afaik, they are usually three phase with full wave rectificaton for each winding (as in motor car alternators). This tends to give a pretty steady raw DC output voltage when they all add up. It would be worth while checking this out before you get started.



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