Question: At what rate does the POPS (Per Outlet Power Sensing) Cabinet PDU collect the energy readings?
Engineer Answer: During a given AC cycle for a particular outlet, our chip samples 120 times per cycle. At 60 Hz, this is a sampling rate of 7.2 KHz. At 50 Hz, it is 6 KHz. These rates allow our 10-bit ADC (analog-to-digital converter) to perform 4X oversampling (effective 11-bit ADC) of the Nyquist rate for the 14th harmonic of 50 or 60 Hz, effectively capturing complex waveforms.
At 60 Hz, each outlet is measured 2.5 times per second. At 50 Hz it is 2.083 times per second. For current, voltage, active power, apparent power, power factor, and crest factor, the reported value is a low-pass-filtered average of the previous four measurements (1.6 seconds at 60 Hz, 1.92 seconds at 50 Hz). For energy, the actual energy consumption during the measured cycle is accumulated for the exact number of cycles until that same outlet is measured again.
The time reference is the AC cycle itself. Every five seconds we re-adjust our sampling clock to compensate for temperature changes and to keep our sampling clock to within +/- 240 nanoseconds of the actual AC cycle time.
We calibrate each measurement circuit, use zero-phase CTs, use differential ADC inputs for noise-immunity, and are scaled for a current crest factor of 2.5 at 16 Amps to avoid clipping/railing of the digitized waveform with high-crest-factor devices.
Follow up Question: Here, if I have to measure the energy consumed by my machine during the time interval T1 - T2, I will have to first read the PDU energy readings at time T1 and later at time T2. The difference between the two readings will give me the amount of energy consumed during the time interval. Is this correct ?
Follow up Engineer Answer: You are correct. To determine consumed energy for a given period of time, you must take a start reading and subtract it from the end reading. We do not provide storage for interval-based energy readings due to NVRAM constraints. However, we do offer our Sentry Power Manager (SPM) appliance that is specifically intended to continuously poll several thousand CDUs and store the accumulated data for various reports.
Follow up Question 2: At what rate is the energy reading updated? Is it the same like power - 1.6 per second?
Follow up Answer 2: The energy reading is updated 2.5 times per second for 60Hz and 2.083 times per second for 50Hz. As mentioned previously, a microcontroller exists for each 8 outlets. Starting with a zero crossing for the first outlet, all pertinent information for that outlet is sampled 120 times through one full cycle. For the next two cycles, the information for the first outlet is processed. At the zero crossing for the start of the fourth cycle we capture all information for the second outlet, then process that information for the next two cycles. This continues for a full 24 cycles, 3 cycles per outlet over an 8 outlet span, and repeats continuously. For each outlet, the information captured in one cycle is considered valid for the next 23 cycles. At 60Hz, 24 cycles occurs 2.5 times per second. At 50Hz, 24 cycles occurs 2.083 times per second.
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