Power Measurement Revisited

Spot Measurement

I finally got around to tinker with things again and opted to digress from the zigbee standalone mote-like development to revisit the power measurement.

I have three types of sensors to measure currents lying around and decided that given I already wrote the C++ classes and modbus integration for total home energy consumption, I could easily create a spot measurement modbus slave device to monitor specific loads.  Note that all my arduinos+zigbee are modbus slave devices and only the mote-like devices shall use a different protocol.


The sensors in questions include a SeeedStudio Non-Invasive AC Current Sensor,  a general purpose 0-20A for experimentation CR Magnetics  (CR8410-1000) Hall effect bases chips from Allegro ACS712ELCTR-20A-T







With known loads, I compared the output from each of the sensors as illustrated below.

Allegro hall effect chip.

What I liked about the hall effect chip was a dc offset of 2.5V was already in place to feed into the A/D of the Arduino and fit in the 0-5v range requirement. The CT from CR magnetic was clean and was sensitive down to the half-amp range which I wanted for spot measurements. The non-invasive one was bulky and noisier on the low end.

I did not spend much effort on the linearity of the sensors.  Like the total home energy solution completed a while back, I generated a scatter plot for know currents vs my calculated currents then fit a line of the type mx+b to use to linearize the calculations in the desired operating regions.

 Phase Shift

Now when it comes to delays introduced by the CT relative to the voltage, it can introduce errors in non-unity power factor loads. I put both signals on the scope and examined the phase offset introduced. My scope software can calculate the phase difference between two signals. I was not getting too much phase delay to warrant compensation for my home application.

 Housing the Components

A plastic electrical box was used to host the arduino (fio) and xbee + power supply + voltage measurement transformer. Not the prettiest, but it serves the purpose.

The same host software was used for total home energy monitoring to poll, display, and trend the information from my modbus aware device. The chart below is for the power consumption of my PC and other stuff plugged into the power bar.