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Measurement of electrical energy consumption today

2014-11-27 05:20  
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Electromechanical energy counters were popular for more than 50 years, but today you can find them only in older houses or in low budget appliances because today electronic energy counters taking their place. And there are meny reasons to do so. For instance measurement accuracy, flexibility, multi-rate support, memory, statistics, remote reading and control, automatic register and so on. All these features can be buil in to one small chip which will serve with high reliability and low price.

Lets see how energy (active power) consumption is measured. Active power which is required in form of alternate current (AC) can be calculated as average power over one period:

power1.PNG

 

Normally power comes in sinusoidal form, then

sinus.PNG

 

So we get

power2.PNG

 

where cos??a€“ power coefficient which depends on load properties.

Power consumption over time t can be integrated as follows:

power3.PNG

 

One of first IC power measurement solutions were AD7750 which is converting current and voltage to digital and calculated instant active power constantly.

AD7750.PNG

 

Device has two inputs. Differential voltage value between V1 and V1- is proportional to current value. It is usually read from measurement transformer. Input voltage is amplified with programmable gain amplifier and then goes to sigma-delta second order modulator with 900kHz sampling frequency. Then signal is passed through HF filter and then delayed to compensate phase shift in filter.

In second input V2 and V2- differential voltage is proportional to load voltage. This voltage is amplified by 2 and modulated with second sigma-delta modulator. Then instant current and voltage values are digitally multiplied to get instant active power. Then results are passed through LF filter to get current active power. Current power value is converted to frequency and output to F1 and F2 one normal and other inverted phase. Frequency is proportional to average active power. Having this data there are many ways to display and register energy. One of the ways is to use stepper motor, which turns depending on frequency. This chip outputs 275ms wide signals in frequency range from 0 to 1.8Hz. Chip also detects reverse energy flow. Then REVP changes it’s state. FOUT pin gives conversion with higher rates and is used for calibration purposes.

AD7750 gives high precision a€“ about 98% in dynamic range 500:1 and 96% in rage 1000:1. Analog Devices produces more IC for energy measurements. AD7756 can be used for single phase, AD7754 triple phase. Data can be read through SPI interface by microcontroller or from internal ADC. Hobbyist may use these chips for home automation projects.


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