Measure on a chiller

Hi, I'm a final year student in civil engineering. 

I need to measure the real and reactive power of a chiller in a company. There are three phases (3 wires) and currents up to 200A. 
So I won't use the EmonTX (limited to 125A I think) but an Arduino, voltage and CT sensors.

As I didn't know anything about Arduino, I intended to use the code EmonLib.cpp. And that's the point of my question. Do you think it's a good way to proceed ? Can I use the code in its raw form? 

In addition, I don't understand what are these calibrations, VCAL, etc, and how to determine it. Is it specific to EmonTX? 

Thank you and sorry for the poor English. I tried to do it understandable

Robert Wall's picture

Re: Measure on a chiller

There is nothing to stop you using an emonTx - the input transducer (c.t. in this case) is the controlling factor. With a 200 A c.t. and appropriate burden, either the emonTx or the Arduino will do exactly the same job.

As you need real power across three phases, you have a problem. Normally, I would recommend the emonTx kit, as all you have to do is assemble it. But to measure real power you need the three phase voltages, and that's not part of the kit. However, it will be relatively easy to copy the voltage input two more times on stripboard and add two more voltage inputs.

If you don't want to do that, (and especially if the firm is paying), then I recommend you use three emonTx kits - one for each phase. You can stack them and use one as the master to control the other two. This post tells all. That method is also more likely to give you more accurate results because you can sample the waveforms often and measure continuously, whereas if you are measuring 6 values (3 x voltage, 3 x current) in one processor, either you measure the wave relatively infrequently or you must sample each phase in turn.

For c.t's, your best choice is a Magnelab split core 600 A  SCT-1250-000. You need the 600 A version because they are designed for 0.333 V output and you want 1 V, so you will have a burden resistor 3 times larger and you'll need a c.t. 3 times more powerful (VA rating) to drive it. The ratio is 1:7500 so 200 A primary current will give 26.66 mA secondary current in the burden resistor, so 39 Ω looks to be right. (Magnelab have confirmed it's OK to do this.)

The calibration theory - how the constants are derived - is in Building Blocks here.

[Added] You might have a problem with sensing the voltage though. You will need either the neutral (i.e 4 wires which you say you do not have) or you must create an artificial star point in order to measure the phase voltages. I think you must talk to the electrician who maintains the electrical systems at the firm.

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