Measuring power consumption of spiky load

[Grunff]

Hi all,

This would probably fit better in another NG, but I know quite a few people on here who will be able to provide some good ideas, so here goes.

I have an experiment where a voltage of ~250 VDC is applied across a load, resulting in a current consumption of around 2A. The experiment runs for around 5 minutes.

The nature of the load is such that the current consumption is very spiky - it may average to around 2A, but you get very short, sharp spikes, probably of 10-15A.

I need to be able to measure, with 1% or better accuracy, the total energy dissipated in the load over the duration of the experimental run.

I don't need instantaneous V or I values - all I need to know is the total energy consumption over the run.

The only sensible way I can think of doing this is to use a data logger, and sample V and I at a high rate thoughout the experimental run, then calculate average power over the run, then multiply by run time.

However, this seems like a very round-about way of achieving my goal. Anyone have any thoughts on other ways of doing this? I must stress that it is the electrical energy input to the system that I'm interested in, so measuring temperature rise of the load etc. is out of the question.

TIA

[John]

you need a digital data logger connected to a PC. eg Maplin sell multimeters with RS232 output which give a serial data o/p. so if the load has no phase problems you just need to measure the current. I don't have detailed knowledge of the specs.

serial software interface program like ProCom or VisualBasic Studio is also needed to sample the readings in the PC. Or GPS navigation software for NMEA might do the same sampling job at a pinch.

[Grunff]

If I'm going to go down the data logger route, I will probably go for one of these:

I used one before in a previous project, and found it to be a very nice piece of kit.

But that wasn't my question really - I can see how to get the measurement I want by sampling V and I with a data logger - but I can't help wondering if there isn't a cleverer, simpler way, given that all I want is total energy.

Another way of asking the question - I can see how to integrate the data electronically, after collecting a huge number of data points. But I'm wondering if there isn't another way of doing the integration. If that makes sense.

[John]

Right, ideally you need to multiply I and V and integrate the product. Easy if you had an 1960's analogue computer, but these are all in museums now. The only analogue route I can think of is to use an analogue multiplier (search in Google) to get the product of V and I. Then construct an integrator using an OP amp.

still need to sample the integrator's output, so the integration may as well be done in the PC rather than in an IC..

[Lee]

I know you can't measure the temp rise of the load, but is there any reason why you couldn't measure the temp rise of an in series resistor bolted to a known thermal mass?

Been a while since I did physics ;) But shouldn't there be a definite relationship between total real power and the temp rise, even for spikey loads?

Lee

[James Salisbury]

Use some voltage to frequency converter chips and count the clicks?

[andrewpreece]

I may remember incorrectly, but old rms power meters used to work on the temperature rise reated by the power passing through, didn't they? The time constant of such a meter may be long enough to integrate the spikes out. However, the output will be analogue which isn't convenient for data analysis if the rmspower varies significantly on a timescale longer than the thermal time constant of the meter. You could run theoutput to a pen plotter ( if available ) and integrate visually the area nder the curve. All very laborious of course. Perhaps apparatus that is essentially a calorimeter would work ( i.e. pick off some of the load power with a series resistor which sits inside a calorimeter: wait five minutes and measure the temperature rise ).

Andy.

[mark]

If you want super accurate, you'll need to sample the instantaneous voltage and current rapidly and record the results for the length of your experiment.

From that you can work out the power consumption by computing the instantaneous power, and then integrating that over the period of the experiment.

For less accuracy, you could:

1. Assume a constant mains supply voltage & frequency
2. Using a True RMS meter with peak avg/min/max record facility, measure the current.
3. Compute the avg/min/max power from the meter readings and assumed supply voltage.

For example, a Fluke 87 meter would do this quite happily.

Cheers, Mark.

[Grunff]

It's a regulated DC power supply.

It's DC.

But the rest is spot on ;-)

[Grunff]

I think this is very cunning (also suggested by Lee) - it may well be the way to do it. The only thing that concerns me is what kind of big power resistor to use and guarantee that it will remain sufficiently ohmic.

[Andy Dingley]

There are still plenty in the Burr-Brown or Analogue Devices catalogues. This is a common enough task and an analogue multiplier is still an appropriate solution to it.

Given how cheap accurate and stable op-amps have been for years, it's not that hard to build your own anyway.

[Grunff]

But (and please correct me if I'm wrong) I still need to sample the output of the multiplier at a reasonably high rate, then integrate the data over the time of the run, right?

So all the multiplier means is that instead of logging V and I, we log the power? That means I still have to get a reasonably fast data logger. If I'm going to do that anyway, then I'd be inclined to skip the multiplier, grab the raw numbers, and do the calculation afterwards - or have I missed something?

[Andy Dingley]

Depends how analogue retro you want to be. You _could_ integrate it in an analogue stylee, before sampling. Although I wouldn't. I have done this, but not for about 15 years. A/D and data capture is just faster these days.

The reason for using an analogue multiplier is when the power factor (i.e. the ratio between the measured values) is all over the place. This may need to be "sampled" at a high rate, so that appropriate pairs of values are multiplied together.

Your "spiky" load is probably not that spiky, compared to this requirement. I'd expect you could easily sample and mulitply digitally, with a speed well in excess of the experimental variation (You'll need to actually work this out anyway, to do your error analysis).

The other option is of course a pair of synchronised analogue sample-and-holds (dead easy), then digitising their values at leisure.

[Tony Williams]

[snip]

Do you have an idea of the timing, of the shortest duration of a current spike?

[snip]

I'm not sure that Average*Time will get the total energy. Don't you have Integrate the instantaneous power over duration of the run time?

Take V+I readings at fixed time intervals (significantly shorter than the shortest spike), multiply V*I, and accumulate.

The multiply and accumulate could either be done in real time, or the raw V+I data log could be processed off-line, say with something like MathCad.

[andrewpreece]

A 25W wirewound would be good to better than 1% probably - the blocky gold finish type with solder terminals, RWR### I think they might be called. I have no idea of what type of power you are trying to measure. Anyway, you need only sample a fraction of it, then multiply up the result to get the power consumed by the load. Might be better to do it over a longer period than 5 minutes though so temperatures in a calorimeter have a chance to equalise. The Fluke 87 idea sounds like a lot less hassle, is that the one that measures 'true rms'? I'd check that it has a sampling time fast enough to record your glitches though.

Andy.

[mark]

OK. Depends on how good the regulation is as to whether this assumption holds. Does it sag a bit?

The current isn't static is it? It's a spikey load. There will still be a min/max/avg figure even for DC.

thanks :) Mark.

[Grunff]

It doesn't under normal load, but I'm sure that under the high peaks it does, yes.

Right, OK - I thought you'd misread as AC. I see where you're coming from now.

[Huge]

I thought hedgehogs ran on worms anyway, not 'leccy.

Sorry.

[Grunff]

Yeah, I know the ones - I have a bunch of them somewhere.

I think I can set up a reasonably accurate calorimeter, and a few calibration runs should allow me to compensate for its characteristics quite nicely.

I will look into this further, thanks.

[Grunff]

Ah, ok, gotcha.

I think based on all the good advice so far, I'm going to go for a two pronged approach. I'll set up a calorimiter, and use the resistor method discussed elsewhere in the thread, and I'll also get a data logger (as fast as I can afford) and do the measurement that way as well. Should be interesting to compare the results.