Power monitoring 240VAC devices

Question

I'd like to monitor the power usage of a number of single phase mains devices (0-10A). The system is to be added to an industrial automation system (on DIN rail), and should be able to talk to a microcontroller (analogue or digitally).

It can be installed inline or around the cable (ie clamped). What are the advantages of each?

I'd like to know about the method used for current/power measurement, the accuracy (for a 0-10A range), a suitable off the shelf system/parts, and a rough cost.

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To clarify:

I'm interested in using an off-the-shelf system if at all possible. If there's an existing solution that suits my needs, I'd love to hear about it (and answers to the questions above - how it works, accuracy, cost). There's no sense redesigning the wheel here, it's not an academic exercise.

Answer 1

off-the-shelf devices

Many off-the-shelf power monitoring devices do most of what you want to do. I wouldn't be surprised if several companies sell industrial versions on DIN rail mounts -- please tell us if you find any. If you can't find any, perhaps you could use commercial versions:

• http://openenergymonitor.org/emon/ (thank you, TiOLUWA)
• http://www.opto22.com/lp/optoemu.aspx
• http://www.killawattplus.com
• http://themodlet.com/
• http://en.wikipedia.org/wiki/Kill_A_Watt
• http://en.wikipedia.org/wiki/Home_energy_monitor
• http://en.wikipedia.org/wiki/Wattmeter#Electronic_wattmeter
• http://en.wikipedia.org/wiki/Automatic_Meter_Reading

They use a variety of current sense techniques and communication protocols.

unnecessarily cranky response to other "answers":

A typical power monitoring device measures the instantaneous current and voltage many times per second. From those numbers, it calculates the instantaneous real power, and from the instantaneous real power it can calculate the average real power RealP over one or more full power cycles.

Also from that data the device can calculate the power factor Pf over one or more full power cycles with

Pf := RealP / (Vrms * Irms) -- by definition

Some textbooks imply that the power factor is the cosine of something, perhaps something related to zero-crossings. That's doing it wrong. It may happen to give the currect result for perfectly linear loads with ideal sine-wave power, but not for nonlinear loads, or linear loads with our less-than-ideal not-exactly-sine-wave power. Instead, good power monitoring devices use the above equation, which gives the correct power factor even with nonlinear loads or non-sine-wave power or both.

Some textbooks imply that one can calculate the real power by multiplying the power factor times something. That's doing it wrong. It is an unnecessarily round-about method of getting the real power, since the device must have already calculated the real power in order to get the power factor.

Answer 2

For something fast and simple, you can try the open energy platform.

It's an Arduino based power monitoring system, complete with GLCD display if you will. It comes with open source arduino sketch for relatively accurate energy meansurement, implementing the above formulars, and the site details the design and the energy measurement method used as well.

the use a simple AC-AC adaptor to set down the voltage, and a Current Transformer for the current that snaps onto the mains line.

the design is very simple, and can be easily modified to send the calculated Vrms and Irms values to another systems via any communication medium like RS232, I2C, etc.

You could also use AFEs (Analog Front End) chips from Microchip, Maxim or Cyrus Logic, however these will require alot more programming expertise.

Answer 3

For power measurments : You will need to Claculate VRMS, IRMS ,Power factor & Power

Power (real power ) =VRMS * IRMS * PF

Lets start with current :

There's 3 methods for current sensing "Shunt resistance , Rogowski coil & hall effect " to know which one to choose see this question Shunt Vs Rogowski Also for shunt resistance connection & circuitry check this Shunt structure ,any way whether you used shunt or Rogowski , you will take the output of this sensor into an ADC , the ADC will give you samples , in your MCU you will make an equation that transform the ADC reading into a real signal , i.e if the reading was X=0.5 then it will match a Y= 7 Ampere at the real signal Then for finding the IRMS , you will collect those reading in a buffer say buffer length [64] sample , after the buffer is full sum (sample * sample) "Square & sum them" , then divide them by their number (64) , later take the root mean square So IRMS= Sqrt(Sum(Sample*Sample)/No of samples) The more samples you take the more accurate result you will obtain, Also this topic will explain another method for it "Ac measurements "

Now lets move to voltage: For voltage measuring , you will need a simple voltage divider & then take the ouput signal to a ADC , do the same calculations you as you did in IRMS to find the VRMS So VRMS= Sqrt(Sum(Sample*Sample)/No of samples)

After you done now you have VRMS & IRMS .

SO we need to find P(average power = real power ) P= VRMS * IRMS * PF

, so we need the power factor

For power factor you will have to detect the zero crossing of current or voltage , then after you detect it , use a timer in the MCU to count how many "usec" between the voltage & current zero crossing , Now you have the difference between them , use math & you will have PF

So , finally you can calculate Power ,

For power :- You will accumulate them & divide by number to get KWH & so on .

It happened I've been building a system like this & asked a lot here in stack , so this what I got till now .

The reason why I didn't mention Hall effect sensor that it is expensive .

A simple algorithm to find VRMS or IRMS :- 1-X = function(ADC_Reading) ; // Equation to take back the reading to original 2-Sum+=X*X; 3-Is this sample number (64,128...) if yes go to 4, if not go to 1 4-Y=sum/(64,128,...) 5-IRMS or VRMS= sqrt(Y) 6-Reset All & repeat

Further step in your system , you will have to make a calibration because you want less error in reading & so on to achieve a better accuracy , it happened I asked for this on stack to Calibration

Answer 4

There are many energy measurement solutions.

10A @ 240V is about 24 Ohms. You can get you a precision 0.1Ohm resistor(or lower) and simply use an ADC from a uC to measure the voltage across the resistor.

The current through the resistor is then simply I = 10 V where V is the voltage across the resistor which is in the range about 1Vrms.

Because P_tot = P_S + P_L where P_S = power dissipated in sense resistor and P_L is power in load(what your trying to find), and

P_tot = 240*I = 240*10*V = 2400*V

P_L = 2400*V - P_S = 2400*V - 10*V^2

So simply knowing V is enough to compute the power in the load.

If your power source is not constant you can use another ADC to monitor the voltage across the sense and load and use that instead of 240.

This is the easiest and cheapest way in general(it can scale much better since you can probably use one cheap PIC and a few sensing resistors).

Anyways, I can't do all your homework for you ;)