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I've got a device (microcontroller-based energy monitor, https://shelly.cloud/knowledge-base/devices/shelly-em/) which I'm using to measure energy consumption of 240V split-phase devices (motors). Since the device is powered from one of the phases, it believes the voltage used by the monitored device is 120V, when it's actually 240V.

The device can't be powered by using both phases, it requires a neutral, but it can handle supply voltages of 240V. I'd like to use a small step-up transformer to provide it a 240V supply, but most of the devices I can find are designed for much higher current loads and are thus both larger and more expensive than needed here.

How would I go about choosing a proper, plain, transformer to use in this application?

Kevin P. Fleming
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    How does the device know the difference between a neutral or the other phase? – Kartman Feb 21 '22 at 12:17
  • Why just do not keep in mind that the measured value is 0.5 of real? And maximum voltage is 230V, do not exceed it. – user263983 Feb 21 '22 at 12:49
  • Or is the same version that is sold in EU, where RMS is 230 V and peak 325 V? Which would make 110 2-phase 180 degree phase angle to be within limits? – Ralph Feb 21 '22 at 15:37
  • @Kartman: That's a good question; I'm assuming that it wouldn't be safe to connect to 240V across a split-phase system, but I don't have enough knowledge to know why. I've contacted the manufacturer for guidance and they may be able to answer that question. – Kevin P. Fleming Feb 22 '22 at 13:12
  • @user263983: It's integrated into Home Assistant along with many other energy-monitoring devices, so the data needs to be consistent with them. – Kevin P. Fleming Feb 22 '22 at 13:13
  • @Ralph: There is only one version of the device, manufactured in the EU and sold there too. Possibly the answer to your question is the same as the one Kartman asked. – Kevin P. Fleming Feb 22 '22 at 13:13
  • As long as the circuit has adequate clearance from N to earth then there should be no difference. I doubt the device has an earth, so you’re home free. – Kartman Feb 22 '22 at 13:15
  • Kartman is correct. @user263983 mentioned the voltage levels and not exceeding max, that's what I was really referring to. If I understand correctly he was considering if 230 V is the max peak voltage, which would make sense in 110 V systems. But if the Shelly is the same as in EU, then the maximum peak voltage limit cannot be only 230 V, it's actually maximum RMS voltage. Makes all the sense too, usually consumer electronics are specified in RMS voltage. – Ralph Feb 22 '22 at 16:11
  • @ralph No, you did not understand correctly. For AC RMS value used. Guide for device shows maximum RMS 230VAC, european standard. In NA residential electricity is 240VAC, RMS too. – user263983 Feb 22 '22 at 20:13
  • @KevinP.Fleming, you ever get this working? Looking to make a similar setup. – Andrew Sep 28 '22 at 15:27
  • Sorry, I did not. I switched to a different Shelly device (the 3EM) which can monitor more power phases. The data is fed into Home Assistant where I use a 'template sensor' to add the readings together to get the proper values. – Kevin P. Fleming Sep 29 '22 at 16:22

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So, simple; go and buy a step-up transformer and make sure that the rating of the transformer is enough, or the output power you want is less than the rating of your transformer.

ocrdu
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Akhil Basoya
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  • It appears that the only low-cost way to do that is to buy a 'travel' step-up transformer; searching Digi-Key for bare transformers the least expensive I can find is more than US$50, because it is rated for 60VA. – Kevin P. Fleming Feb 22 '22 at 13:14
  • @kevinp.fleming any transformer with selectable 120/240V can be used. You can connect it as autotransformer. If galvanic insulation is not required. – user263983 Feb 22 '22 at 13:59