Power Meters Logging

[Alex_Suhisky]

When you are dealing with a few hundreds units it becomes way too expensive.
So i am thinking about using SCT 013-30 sensor plus rectifier bridge loaded with resistor and paralleled capacitor. That will give me a DC voltage proportional to the apparent power.

Makes sense?

[RTSLVU]

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@Alex_Suhisky wrote:

When you are dealing with a few hundreds units it becomes way too expensive.
So i am thinking about using SCT 013-30 sensor plus rectifier bridge loaded with resistor and paralleled capacitor. That will give me a DC voltage proportional to the apparent power.

Makes sense?

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Sure, but you pay for real power not apparent power.

Also the capacitor will throw off your measurements even further. (I remember trying that a long time ago)

One thing that you can do is if you can sample the voltage and current simultaneously at a high enough rate (like in the Khz) you can just do the math on each sample and be reasonably accurate.

[Alex_Suhisky]

That is true, however first of all there is a "Basic Averaged DC-RMS.vi " in Waveform Measurements and second of all there is a specifics of monitoring freezers. Your goal is not to know the true power, but to be able to see how two stages of freezers are working. This allows you to see a lot of problems even long before they actually happen. And for THAT reason, I would think the apparent power works even better.

So if that idea works, the solution becomes really inexpensive, I recon 🙂
I just computed that the 1 uF capacitor plus 120 Ohms resistor in addition to the sensor make total around $10  😉 That's what I call inexpensive. 😁
What do you think?

[RTSLVU]

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@Alex_Suhisky wrote:

Tthere is a specifics of monitoring freezers. Your goal is not to know the true power, but to be able to see how two stages of freezers are working. This allows you to see a lot of problems even long before they actually happen. And for THAT reason, I would think the apparent power works even better.

Interesting concept, I can't say I know enough about refrigeration to know how well that would work. 

[Alex_Suhisky]

Well, in short, there is a first stage which is basically just a fan, that circulates an air inside the unit, and the second stage is an actual compressor. So, by just seeing how two of those stages are working you can have a very good idea about the freezer health. I mean in addition to the temperature monitoring of course 😁

[RTSLVU]

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@Alex_Suhisky wrote:

Well, in short, there is a first stage which is basically just a fan, that circulates an air inside the unit, and the second stage is an actual compressor. So, by just seeing how two of those stages are working you can have a very good idea about the freezer health. I mean in addition to the temperature monitoring of course 😁

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So if you are just looking at trends over time and sudden changes in data then a simple VA measurement is probably good enough. 

[DavidBoyd]

In general, I'm with @RTSLVU on this - if you want "real" (pun fully intended) measurements, your cheapest solution per channel is probably something like an Accuenergy Acuvim (panel meter plus a CT will be a few hundred USD).  Add an RS-485 interface to a PC (LabVIEW driver code is available... I wrote a full driver for its default ModBus serial interface) and you have what you need to poll lots of devices.

I stopped to have a look at Accuenergy's website (it's been a few years), and I now see that they've added an AcuRev series, which can support perhaps 9 or 18 single phase channels.  You still need CTs, but that would undoubtedly be more cost-efficient over multiple Acuvim-L's.  (I didn't seek pricing.)  ModBus serial is still their default interface, so I'd expect the LabVIEW driver on hand would be trivial to adapt.

The SCT 013 style CT used by hobbyists is maybe 10 USD in unit quantities, but then you'll need to gimmick up your capacitor/resistor thingy, and now we need to discuss how you'll get those analog signals into your PC.  Are you discounting that cost because you already have some USB DAQ on hand?

Do you have any calibration/traceability requirements, or is this all just to give you a go/no-go of "yup, the compressor kicked on again"?

Best of luck!

Dave

[Alex_Suhisky]

Hey Dave. I agree, I assume too much, basing on my own experience. I assume that the power monitoring is needed for this particular purpose and I assume that the DAQ is already there, since temperature monitoring for freezers always comes first and only then - other optional thingies. And normally it is a thermocouple, since it is a universal and most feasible, because of several reasons, way etc... 
Accuenergy Acuvim, in addition to parts, will require an enormous amount of labor per unit and space for installing those meters and then - wiring them to the DAQ, so it would cost thousands, not hundreds as I see it.
Yes, you are right, most of people want to see their freezers' stages at work, how often, in what combinations they kick in, how long they stay on etc. It gives you a very good idea about the health of the unit. In combination with temperature data, you can also see how efficient (or inefficient) each stage is and the freezer as a whole and what particularly problems it has.
So, I assume that there is actually rare need in monitoring true power consumption of freezers for commercial purpose, however I might be wrong.

If you are interested, for freezers' monitoring I use 8018 module from Super Logics as a DAQ (for thermocouples) and Moxa's Nports to talk to them over the net. It is quite cost-effective solution for my conditions and it works pretty well.
So, yes, monitoring true power consumption for commercial purpose is a very different task which would require much more sophistication and greenies. No doubt.  🙂

[DavidBoyd]

Alex,

Thanks for clearing some of that up - as always, it's difficult to offer useful suggestions when you've only got a hazy view of the problem space.

Totally separate thought, if you're willing to go the consumer electronics route - the folks over at YoSmart (yosmart.com) have some dandy remote-control AC switching devices which also accumulate power consumption statistics, at about 35 USD in unit quantities (plus you need one "hub", for potentially hundreds of nodes, which I think is also about $35).  No LabVIEW involved. Their entire web-based data collection/control infrastructure is all ready to go.  I've recently started using their stuff (not for work) and am VERY impressed with their engineering execution.

This AC monitoring assumes your refrigerators are single-phase and probably limited to 15-20A of draw.  You may want to contact their applications engineers if your needs exceed their offerings, but you envision a substantial installation.

I'll add, that they also have temp/RH wireless devices that use the same infrastructure, and I know they support commercial refrigeration surveillance.

Oh, and I know sorta "know a guy" within their tech organization. Happy to refer you!  (Are you US domestic? I didn't scope your profile.)

Dave

[Alex_Suhisky]

Thank you, David. Interesting products at yosmart.com. There are lots of similar things at aliexpress.com for much better price 🙂
Problem is that I cannot use nothing wireless though... Government stuff , you know... 
Yes, I am located in MD and I am dealing mostly with freezers, not refrigerators.