380V 50Hz 3 Phase Power monitoring

I am looking to set up IotaWatt for a commercial Chiller System in Thailand. 3 Phase Power here is 380V and 50Hz. For my reference voltage transformer, will I need to buy transformers that step down from 380VAC (phase to phase) to 9VAC, or do I just connect the step-down from neutral to each phase (230VAC) to 9VAC?

Are there any other issues I need to be aware of when using 380VAC 3 Phase 50Hz power?

You can use derived reference with one 230V phase to neutral or direct reference with three 230V phase to neutral.

When measuring a 380V three-wire delta load such as a motor, you would use a CT on all three wires, or you can use a CT on just two wires with derived 380V reference.

Be advised that I no longer have 230V transformers.

OK that is helpful. Thank you. Accuracy is very important here so I have my best fluke DVM to do the calibration. When I calibrate, I would measure the Phase to Neutral voltage with my Fluke and then adjust the calibration to match that voltage (approx. 230VAC)?

The transformer can be 230VAC primary and 9-15V on the secondary? How many VA are required?

Also, if I have 12 x 3 phase measurements to make, do I need VT on each IotaWatt, or can that frequency/voltage data be shared to the other units? For accuracy, I don’t want to use any derived data points.

Finally, with ACV calibrated to within 0.05% and CT of 0.1 or 0.2 class, what is the accuracy of the final kW measurement?

Understand that IoTaWatt is not a meter, it is a monitor. The standards, calibration and certifications for meters are not applicable to IoTaWatt. It is not for trade. Most users report Wh totals over periods of days to months in agreement with their utility meter in the 0.5% to 1% range, but that is anecdotal. If you require measurement for billing or public safety, I would say that IoTaWatt is not appropriate and you should get a meter certified to the appropriate use case.

To obtain the best results, a voltage reference transformer that is supported in the configuration tables should be used. While RMS voltage can be field calibrated, phase angle cannot. The VTs in the table have measured phase angles.

The IoTaWAtt uses 0.5% resistors, so accuracy beyond that should not be expected.

The VT outputs can be split and shared between multiple IoTaWatt, but there are some restrictions:

  • Polarity must be strictly controlled. In a setup where multiple VTs are used for direct reference, reversing polarity to one of the units can damage all of the units.

  • All of the units that share a VT must have their own independent USB power supply.

VTs should not exceed 15V RMS no load. They only draw about 1.2 mA (13K Ohm). Minimizing phase shift is important. In my experience shift is inversely proportional to weight, so larger heavier transformers usually have the lowest phase shift and work best.

Is the phase shift why you wouldn’t want to use the VT to also power the Iotawatt? (It could be rectified and regulated but would draw about 100-140mA). I’ve thought it would be nice to only need one power supply for the Iotawatt.

Have you measured some of your sample VTs to see what the phase shift would be under that load? Could it be calibrated for?


No, that’s not the reason.