Are AC and DC parameters related linearly?

diff_edge

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?

Simon Bridge

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_rmsI_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?

diff_edge

Thanks, this forum seems to be a wonderful place.
Yes, my mistake. However, using a large value capacitor should fix this right?

I do not need frequency or phase information. Also, I do not need instantaneous voltage or current. As you said, V_rms and I_rms is all I need. I need this because my system has to show AC output (voltage, current and power).

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?

Simon Bridge

But why?

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.

diff_edge

Exactly.
This is what I'm looking for. I want to know if I can find a relationship b/w AC voltage and DC voltage and similarly for current too. Definitely the numbers might not be the same, I can plot the AC voltage versus DC voltage to find a relation. I can then use this relation in my code to "correct" the measured DC voltage to find the AC voltage.

Just V_rms and I_rms before rectification.

I hope I'm clear with my problem.

Simon Bridge

You will have to open it up and see what components are being used.If you compute the AC V_rms and then plot it against the DC voltage, you will just get the relationship you used to calculate it in the first place though.Does not make sense!

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

diff_edge

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. :)

Simon Bridge

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?

diff_edge

Ah, I just wanted to know if this plan of mine was not flawed in some way. Thanks a lot for your help. I'll go ahead with this now.

Simon Bridge

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.

sophiecentaur

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.