simple solution measuring mV

[CAI_Support]

Ross,
 
You are great resources for all those different sensors!
 
78M6631 IC maybe reasonable, but the demo board is $350 each.
By going through its datasheet, it does not seem to be able to measure the power flow direction.
It is easier to measure DC power flow direction, but I checked with a few sensor manufactures, none of them have a chip to detect AC power flow direction.

 

[Efried]

Ross,
 
You are great resources for all those different sensors!
 
78M6631 IC maybe reasonable, but the demo board is $350 each.
By going through its datasheet, it does not seem to be able to measure the power flow direction.
It is easier to measure DC power flow direction, but I checked with a few sensor manufactures, none of them have a chip to detect AC power flow direction.

 
power flow direction is measurable via shunt- some of the beforementioned energy metering chips have input channels for both transducer and shunt.

 

[Efried]

Efried, on 15 Sept 2014 - 03:29, said:

But the voltage signal should also be equal to all metered devices in the area and only 120° staggered on all three phases, so may be a way for savings on the input side to multiple single chip electricity meters at one location.



Please be very careful making such dangerously WRONG assumptions.

You CANNOT and MUST NOT ASSUME that the voltage on one phase is representative of the voltage on the other phases.


 
...or assume the voltages are 120 degrees apart but the phase relationship assumption will usually get you within about 2-3% accuracy. With 3 wire systems analysis, the voltages associated (delta system) with the same phase line currents are not available from measurements and this is always the case (2-3%) without some fancy trig calculations to emulate them.

My silly question as non electrician: I assume the voltage over time depends on the influence of all connected devices on that phase also depending on the impedance of the grid not on the single device we are measuring. I think the grid voltage should be a hard driver, well may be not during zero transition, but here the power error should be low then... Is this correct?

 

[CAI_Support]

AC power flow direction and amount is not simple measurement via shunt.
I asked Allegro Microsystems about AC power flow direction. they told me they can not detect it by using their shunt products.
I think that measurement must combine AC power phase.  With two phase power or three phase power, that is not a easy job.

 

[rossw]

My silly question as non electrician: I assume the voltage over time depends on the influence of all connected devices on that phase also depending on the impedance of the grid not on the single device we are measuring. I think the grid voltage should be a hard driver, well may be not during zero transition, but here the power error should be low then... Is this correct?

 
In an ideal world, yes - the three phases would be identical voltage and waveform, and exactly 120 degrees apart.
 
In the real world, there are distribution considerations. Loads may not be uniformly balanced, switch gear and lines may not be identical and faults actually happen.
Particularly with 3-phase power and imbalanced loads, you may see significant differences in voltage phase-to-phase.
Absolutely during fault conditions you will see MAJOR differences in phase voltages. A phase that has blown a fuse for example, will be (to some extent) "synthesised" by loads from the other phases. You may see for example, 230V, 215V and 80V (phase-to-neutral).
It is common to see 5% or more voltage difference between phases. This can make for quite inaccurate power calculations if you happen to be measuring the wrong phase!

 

[CAI_Support]

Ross is absolutely correct.  Consider three phase AC power has capacitor load on one phase, coil load on another, and triac load on the 3rd phase.

 

[Efried]

 
In an ideal world, yes - the three phases would be identical voltage and waveform, and exactly 120 degrees apart.
 
In the real world, there are distribution considerations. Loads may not be uniformly balanced, switch gear and lines may not be identical and faults actually happen.
Particularly with 3-phase power and imbalanced loads, you may see significant differences in voltage phase-to-phase.
Absolutely during fault conditions you will see MAJOR differences in phase voltages. A phase that has blown a fuse for example, will be (to some extent) "synthesised" by loads from the other phases. You may see for example, 230V, 215V and 80V (phase-to-neutral).
It is common to see 5% or more voltage difference between phases. This can make for quite inaccurate power calculations if you happen to be measuring the wrong phase!

Thanks rossw that 5% guess is perfect for my application. I was referring to effects on one phase though.
As with regards to your fear CAI_Support think about the reaction and not only the cause. On the other side of the disturbing you do have a powerful counterpart - the grid. Theory says that generated currents may alter voltage over the networks impedance. So if you have a disturbing charger, a good private network and are located near a transforming station, not a lot will happen voltage wise. But I admit the grids voltage itself may have flaws. For my application I may live with the limitations of greatly simplifying the measuring architecture, for metering you may not. I guess that the WC may be very useful in home automation once we may use an AC current sensor.
Here are spec. from a Chinese supplier, I guess they will have 9% wattage accuracy not 3%:
* Operating loading:3680W, 230Vac
* Lowest measurable current: 0.005A
* Power Resolution: 0.5W
Accuracy:
* Voltage: +/-3%
* Current: +/- 3% +/- 0.01A
* Wattage: (>=1W) +/- 3% +/- 2W
 
and here is one solution with ZigBee having an accuray of .5%..
http://www.aliexpress.com/item/CC2530-Zigbee-Smart-electricity-metering-socket/1950853699.html

 

[Efried]

For WC8 this is easy to do. For WC32 it is harder, because wc32 uses resistor networks, and its traces are tiny small.

 
 
Do you think about a WC version having jumpers, selecting voltage level for ADC Input?
This would make the device much more versatile!
 
thanks

 

[CAI_Support]

With IO board, it can adjust the gain on the amplifier thus change the range of the measurement.