AC input voltage limits


I’m hoping to install an Iotawatt at a friends off-grid cabin. He generates power from the creek. I’m not exactly sure on the details, but I think the pump is spinning an inductive motor which generates 3-phase 600 ish volts. That goes from the power house up to the cabin, to a transformer that creates standard 240 split phase. It should be a pretty standard install, one voltage reference on one of the legs, one CT clamp on each phase, plus one clamp on the dump loads.

The interesting bit is the variable voltage due to fluctuations of the water flow. There is a load controller that is basically a PID that turns on and off baseboard heaters in an attempt to keep the voltage steady at 120. If a breaker goes or some heavy loads are turned on suddenly there can be quite a fluctuation in the voltage while load controller is compensating. Extreme situation could be between 80 VAC and 160 VAC (so peak voltage could be upwards of 200?)

If I’m understanding the design correctly, the upper limit on the voltage input is clamped at 3.3v to avoid damaging the MCU, so at some point, the calculated voltage will top out. So even if the cabin is at 160 VAC, the Iotawatt might be reading it as 140.

What is the calculated max input with the 12K/1K divider?

If I needed to increase the range, is it as simple as replacing R32 with a higher value resistor and calibrating the VT?


Rube Goldberg meets Thomas Edison :roll_eyes:

A little off point, but this system as described doesn’t make sense to me. Producing three phase power and then converting to split-phase isn’t really possible as I understand it, unless he is only using two of the three phases. You can put a transformer primary across two of the three-phases and then have a center tapped secondary to get split-phase. Another approach is to gound one of the phases and carry it as a neutral with the remaining two phases. With that you don’t really get true split-phase, but more like a 120/208 scenario.

The other thing that has me befuddled is that for the dynamo to produce voltage that varies with flow, that would indicate it spins faster which would change the frequency as well as the voltage. That’s not a problem for IoTaWatt, but could be for other things.

As far as IoTaWatt is concerned, the 230V reference transformer should work. If you don’t have the standard Euro two-pin plug, you can use a “universal” power strip. Rigging that up should be child’s play for your friend and it would be good to 250V or so. I wouldn’t recommend trying to use the 120V transformer at 200V.

Any of the USB power supplies, Euro or US, are good from 90V to 270V, so not a problem there.

You would get some interesting data from such a system, watching the voltage and frequency move up and down.

For my nickel, I’d approach the regulation problem at the source by adding a flywheel and some type of mechanical governor that regulated the water flow. A split-phase generator would also help. Depending on the power needed, one could easily be salvaged from an old genset. Most of those get discarded when the motor fails and the dynamo is still in good shape.

Thanks for the reply!

It is a pretty interesting setup, quite awesome really, they’re generating upwards of 5 kilowatts. I think I befuddled that description with some poor assumptions on my part. There is some kind of mechanical flow control so the pump (coupled to the inductive-motor-generator) spins at a constant rate. I think it may be a spear-point valve. The settling pond at the intake helps reduce debris and keep a steady flow moving through the system. I assumed it was changing water flow that caused the fluctuations in voltage, but it’s more from the changing domestic loads.

I’m not entirely clear on the theory here, so please bear with me, and interject if I’m muddying the waters further. The motor is wired in C-2C: one winding has no caps, one winding has one cap, and the last winding has two caps. The 600 volts that goes up to the cabin is off of the winding with 1 cap. The cap selection sets the frequency. What arrives at the cabin is 600 VAC single phase, which then goes to a 120/240 center tapped transformer. I suppose it’s similar to the standard North American delivery.

That makes good sense to use the 230V Euro transformer. I guess I was worried that for the majority of its time, measuring around 120V, it wouldn’t be as accurate (further from its nominal 230V). I started looking at industrial voltage transducers but those start around $200! And I think they’re essentially the same thing: transformer, voltage divider, op-amp for bias.

Sampling will be very interesting. I’m considering having the Iotawatt sample every second, then writing to a local RPI with EmonCMS every 10 or 20s. They have satellite internet so it’s possible to update to, but not more than every minute I would think.

Thanks again for the amazing work on the Iotawatt! I’m looking forward to using it. The web interface looks great.