This page explains why you need "...the phase angles to find the (real) power.":
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In your case, the current waveform is not sinusoidal, so the notion of phase angle becomes more complicated.
A 3-phase wattmeter would give the results you want, but as a practical matter, you should probably use an oscilloscope with trace math to carry out the measurement of your 3-phase power. See:
While what you say is true those instruments tend to be expensive. For most any purpose any "TrueRMS" class handheld DMM or small benchtop TrueRMS DMM will do OP's task quite nicely. OP may have to read the datasheet to verify that it includes DC in the "TrueRMS" calculation. Well designed RMS to DC converter ICs have been around for over 20 years.
I just poxy hate it when OP does not state the problem clearly, completely and correctly the first time. You are stuck with using a digital Scope that does math. You will have to "simultaneously" digitize both voltage and current. Then do the calculations.
ok phil, so you are telling me that I can not put a shunt resistor in line with one of my fan windings and simply measure the voltage waveform across that shunt, and convert that waveform to an RMS voltage and then divide that by the shunt resistance in order to get my RMS current? And once I have both the RMS voltage and current, that I can't multiply those values together in order to see how much power the winding is consuming?
I'm not just directing this at Phil, if anyone thinks this would not work please let me know, I don't see what is wrong with this approach.
ok... after reading some of the documents you guys have pointed me to, what I'm gathering is that if I do Vrms*Irms I will just get the apperant power, and in order to convert that to the real power I need to multiply by the cosine of my phase shift, but that is only true for sinusoids, since I have a messy pulse I think my situation has become a lot more complicated.... I'm dealing with a pretty small fan and feeding the windings from a microcontroller sending out pulses to each winding, like a pwm to control the speed.... these watt meters look like they are for big AC type circuits, are there watt meters for low voltage non-sinusoidal applications?
Then read this page for some more of the same explanation:
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followed by this page to explain how to actually measure the power delivered to your 3 windings, except that you will probably need to use a scope with trace math, rather than a wattmeter. The scope will have to be able to multiply the instantaneous voltage and current and integrate (average) that product. You can use the two-wattmeter method to get the
3-phase power delivered to the motor windings, with the scope taking the place of the wattmeters.:
Those links were really helpful, thanks for sending them... from what I'm gathering according to the yokogawa links what I'm trying to do, find the power going to the windings, can be done by "Active power is calculated by averaging the products of the instantaneous voltages and currents" and "Ideal active power is expressed as the product of the instantaneous voltages and currents averaged over one period of voltage or current. " but then it also says "We know that the active power from the voltage and current of the distorted wave is the sum of the active powers obtained from the products of the voltages, currents, and power factors of the same harmonic component (frequency)"
so would you say I can just multipy the instantaneous values of V and I to get the real power? I don't even need RMS anything? Weird... I thought the point of RMS is cause you can't do that.......
Why not buy (or rent) one of the Tektronix TPS2000 series digital 'scopes (or equivalent)? For about about $3500 you can get a digital scope, voltage probes, current probes, ISOLATED inputs, power calculation software, download ability, portability, and more! And it's a nice scope for general purpose too! It will open up measurement and design opportunities, that lie outside the realm of your existing equipment. There are all kinds of what appear as "el-cheapo" solutions out there, but after you've blown a few of them up, electrocuted yourself, wasted a lot of time, why not go for the gusto, and get something decent the first time! When I first got seriously into building electronic stuff about 20 years ago, I bought a Tektronix 'scope for about 1600 dollars. It's still running today, and has paid itself off, over and over. Your other choice is either to hand build something (that exists as off-the-shelf), or cobble some existing devices together to do a half baked job. What's you time worth? The saddest thing to see is soemone building something that exists "off-the-shelf", and end up wasting far more time, energy, and effort than the price of the purchased device.
What they're getting at here is that you could do a Fourier analysis of your waveforms and calculate the power due to each harmonic and add them all up by taking the square root of the sum of the squares of the individual powers. But this adds the complication of doing a Fourier analysis. It would be easier to do the instantaneous product of the current and voltage waveforms.
One way to look at the calculation of the RMS value of a voltage waveform v(t), is that you are calculating the square root of the average of v(t)*v(t), where v(t) is the instantaneous voltage as a function of time. For power, you calculate the average (no square root needed) of v(t)*i(t).
For the RMS calculation, you multiply the voltage waveform times itself; for power you multiply the voltage waveform times the current waveform. The calculations are quite similar.
So, for your power measurement, use the two wattmeter method on the Yokogawa page, but with the scope with trace math substituted for a wattmeter. Make two measurements with your scope and add the results, paying attention to a possible negative sign for either of the measurements.
Yes... but they have to be the V and I taken _at the same instnat in time__, you multiply them and that gives you the instantaneous power at that time....then repeat for many periods.. take all the power results and average them and you will have the true average power...
but why do you want to measure the true average power of a small fan motor?
lets get to the meat of the real problem you are trying to solve..
well basically Im tryign to characterize a small fan, and I want to know how much power this little blower is consuming... the blower comes with a little controller that sends pulses out to each winding in order to keep the fan spinning. That is the only power the fan gets, the controller gets power from somewhere else but I'm just interted in the fan... so I have set out to measure the power each winding consumes, and that's how've I've ended up with all these issues.
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