Hi Brendon,
640 sample pairs per cycle is approximately 1/2 degree of an AC cycle. Phase shift for the VTs and CTs that are affordable and typically used have phase shift in the 2 degree range. Moreover, the phase shift of the CT alone varies by up to several times that depending on current at the low end. So the effect of any phase shift of the voltage is secondary to the phase shift of the VT and CT. This is not true of slower devices, but true with IotaWatt’s high sample rate.
More importantly, when collecting sample pairs at high speed, IotaWatt uses the current sample directly, but averages the bracketing voltage samples to fully compensate for that timing phase shift.
Lastly, after the sample pairs are collected, the relative phase lead between the VT and CT is calculated and the sample pairs are skewed and interpolated to compensate for that difference.
It does. IotaWatt measures up to 15 channels, and it can only measure one thing at a time. So it samples the various channels round-robin style and maintains a time weighted average for each channel. The basic pulse is:
Sampling is initiated about a ms before zero crossing.
Upon zero crossing sample accumulation begins
After two more crossings (one full cycle) accumulation stops.
The samples are processed to develop voltage and power and saved.
At this point, no more sampling can be done until the next zero crossing, so control is returned to the main loop and services that handle web requests, SD logging, time-synchronization, and logging to internet services like Emoncms are dispatched.
As the next zero crossing approaches, we go back to the sample code and start over.
So it takes 1.5 AC cycles to measure a channel. At 60 cycles, that amounts to 40 channels sampled per second. Realistically, it achieves about 38, which means each channel in a fully loaded IotaWatt gets sampled 2.5 times/second or about every 400ms.
If you have an appliance that uses power in bursts shorter than 400ms, maybe. In a more practical vein, most power limiting schemes use PWM and limit total power by clipping each AC cycle using a semiconductor. Light dimmers work this way. The amount of clip doesn’t vary much from cycle to cycle. With it’s high sample rate, IotaWatt does a great job of sampling these clipped cycles.
Let me know how that works out.
And can you provide more information about the “simultaneous sampling, 16 bit, 20Mhz SPI ADC” that you reference. Specifically, a link to the datasheet and the cost.