Archived Forum

Which meter and which CTs?

You can look at the report on the YHDC ct in Building Blocks - that tells how I tested the CT up to 250 A using an ordinary moving iron 5 A meter. OK, I didn't have to buy the variac or the transformer - I had those already - and you don't either: if you have a multimeter that can read alternating current, you select a load whose running current is close to but less than that maximum, and use that. If you don't have or cannot borrow a suitable multimeter, you will have to rely on the tolerances of the CT and the various other factors that influence calibration, and then over time adjust the calibration by small amounts until it agrees with your energy supplier's kWh meter.

A couple of years back, I posted a Digital Meter Emulator sketch which runs on an emonTx V2 or similar platform.  This sketch generates a visible pulse whenever 1 Wh of energy has been recorded by its CT.

Calibration is adjusted by altering the value of powerCal.  When the CT is clipped next to the supply meter, and both visible indicators are pulsing at the same rate, the calibration is correct.

This same powerCal value can be used with any of the sketches that I've posted.  Recorded values of power/energy should therefore be fairly accurate.

According to my Summary Page:

DigitalMeterEmulator.  This sketch uses V and I sensors to determine real power, with the on-board LED being set to behave just as the LED on a digital supply meter.  Originally posted on 26/10/12 at http://openenergymonitor.org/emon/node/857#comment-6867, this tool is available at
http://openenergymonitor.org/emon/sites/default/files/DigitalMeterEmulat...

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I'm looking for a way to calibrate my meter's CTs. I need to pass through a constant known current of 10A in order to read their values and record this.

If you switch on a kettle or similar, the supply meter will pulse at a fairly quick rate.  If you count the rate of these pulses, that will tell you how much power is flowing.  If your meter's CT is clipped next to the supply meter, then the same amount of power will be flowing through both of them.   The current won't be exactly 10 Amps, but it will be known reasonably accurately by counting the pulse rate. 

Thanks for the response. 

How stable is the current using the variac / transformer method. Surely the heating of the load affects it?

In the report's test rig picture, whats the function of both resistors. I'm assuming one of them is a variable resistor which is used to alter the current magnitude. 

Secondly why do you go through a secondary transformer, why not just go from the autotransformer directly to the circuit?

One resistor is to sense the current, (for feeding into an oscilloscope etc), the other is the load. Both are shown as fixed resistors, which they are. What's the point of having a variable resistor in there? - there's absolutely no need. The step-down transformer makes it quite safe to work on live as it's only 6.5 V maximum, therefore the power requirements are minimal and although the heat sink that the load is on gets quite warm, the dissipation is only in the tens of watts range. The current is stable enough to take readings - provided you don't try to do it when the mains voltage is bouncing around too much. Because the dissipation in the resistors is kept down, self heating isn't a major problem.

Ok got it, thanks. Didnt quite look properly at the 0.33 resistor.

Still though, why not just down bring the voltage to 6.5 with the variac. Why are you going through an additional transformer?

With a single variac and perhaps a variable resistor you would be able to get a custom current while minimizing power loss. 

I suppose I could use this method with a 7.2V and 0.72 ohm load. But 72W power dissipation in the resistor, is this really negligible?

"Still though, why not just down bring the voltage to 6.5 with the variac." You need to take a step back and think the problem through in total. Two questions for you: How do you accurately adjust the voltage, and is the low voltage side then isolated from the mains?

I never said the dissipation in the resistor is negligible - if it can get a large multi-finned heat sink warm it clearly is not negligible.

"How do you accurately adjust the voltage, and is the low voltage side then isolated from the mains"

I'm not too sure on your questions. Surely adjusting the variac until you reach your desired voltage (measured) is enough. Accuracy as measured by the voltmeter.

As far as isolation is concerned, the variac is still a transformer, we only changing the secondary coil.

I'm just trying to find out the purpose of the second transformer. 

If I create a setup such as yours - besides for cost implications - do I need that transformer