How about an alternative plan: steal some power off the current sense transformers? If you don't need to use the radio often, you can design your measurement equipment to have low operating current and provide just a bit of charge current to the LiPo.
What i neglected to say earlier is that the PSU does not have to supply the entire load all of the time (the battery is there to help). In some cases the motor is DOL, sometimes fed from a VFD and then there will be an undervoltage cut-off as well as a filter to stop the worst noise from getting through.
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At this time i am not too worried about the happiness or otherwise, of the VFD. The motors are relatively big (at least several kVA up to 100s of kVA). The VFD will hardly notice a couple of VA whatever the PF.
It is certainly possible to do this, but there are a number of issues you'll need to look at.
Undervoltage cutoff doesn't mean anything in the context of a VFD. If the VFD is operating at all, it is spitting out square waves* of the DC link voltage (approx. 600V). To get a lower
equivalent voltage, the pulses just get narrower.
If we look at the data sheet for the 3W open frame module, the recommended deployment for EMI compatibility (Figure 2) has a 0.1uF X cap and 470pF Y caps. When subject to (round numbers) a 10 kV / us output from the VFD, the X cap would try to pass 1000A. The common mode choke will of course restrict this, but you could be looking at some ugly current stresses.
These current spikes could be measured by the VFD and make it a) do slightly strange control stuff or b) trip into fault protection.
There will also be high frequency current flow through the Y caps. The common mode choke will try to stop the current, but that means it will have a lot of flux going on (and
at the switching frequency of the drive, not just 50Hz) and might get very hot.
In addition to X and Y cap issues, VFDs with longer cables are known for having reflected voltage wave issues. That is to say, the VFD generates sharp square-wave pulses which go down the cables like a transmission line. When they reach the terminals of the motor, there is an impedance mismatch and some of that wave reflects back. Depending on where you measurement box is mounted, it could be subjected to a 600V pulse (the DC link voltage of the VFD) followed by a step up to a higher level (maybe 1.5X or even 2X if the motor is very small). These voltage levels will want to rush into your device and may blow up your DC link. You can add a filter inductor to take the edges of, but then you have to check for possible resonances (note VFDs operate over a wide variety of switching frequencies from 400 Hz to 8 kHz or more).
Measuring the voltage of a VFD output? Impossible - no. Difficult-ish, yes. Not a trivial task by any means but for that i have the help of a platoon of PhD's. That part will be done.
Ah, that will help. Also, have you considered the possibility of a) getting information out of the VFD via Modbus or similar or b) measuring the line side of the VFD?
The Mornsun modules are real interesting. Actually, just the thing i was looking for. Now to find a distributor... Any info on that?
Mornsun offer some kind of sample request on their website. And they have
a list of European distributors. I haven't used them before, but their datasheets look like they give a damn.
Once this is done, there is a line of 6 kV motors waiting in line for similar add-ons. Now that will be a challenge. Maybe i need to reinstate my IEEE membership to get access to some relevant papers...
!!!
That will be interesting. Please be aware that once you get into medium voltage (>= 1kV AC), a lot of things change: insulation doesn't insulate (creepage, ageing, partial discharge), applicable standards change, electrical safety rules change, over voltage classes become a thing...
* Yes, OK, there are multilevel drives out there which spit out more interesting waveforms than straight-up square waves. But they are unusual in Low Voltage (LV) (< 1kV AC) settings. We may see more 3 level drives in the LV market soon because the cost of semiconductors is dropping and 3 level drive allows for improved efficiency (reduced iron losses in motor).