can someone recommend a really TRUE RMS meter?

"panfilero"

** The formula for Volt Amperes or VA.

.... I don't see why I would need the phase angles to find the power.

** Fraid you do and there will not be a simple number either.

...... Phil

I'm travelling at the moment, but I'll see when I get back to the lab. I bought it in 1991-ish. It's a thing of great beauty in its way.

Cheers,

Phil Hobbs

Video converters run well into the tens of MHz. The Analog one I'm using is 14bits at 75MHz (though it's double correlated).

--
Keith

** Not off eBay you didn't.

This is a HP 3400A

This is a HP400A

WTF is Hobbs on about ???

...... Phil

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.

There is a significant and important difference between VA and watts for motors. Study up.

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.

much thanks. J

In principle that will work, but if the waveform is complicated, so will be the calculations.

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?

appreciate the help

I gave you the answer in an earlier post.

Go read this page:

Then read this page for some more of the same explanation:

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.:

.

There are cheap in-line meters intended for use by radio-controlled vehicle hobbyists made by companies like Medusa Research.

"Phantom"

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.......

thanks

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..

Mark

Thanks Mark,

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.

I don't want to criticize you at all, but as you do not seem to understand the difference between real and apparent power you are Way way out of your depth wanting to worry about the power used by a small fan.

John G.

I didn't say I bought it off ebay. I bought it for work, new.

Cheers,

Phil Hobbs

So, can't you measure _that_?

Good Luck! Rich