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I have a what I assume is a very unusual question, and an unusual situation. I do appologize if it has been explored before, but I could not find anything about it when searching. I also appologize for any nomenclature errors, since I am not an electrical engineer by trade.

The situation is as follows:

I currently live in a country with 400 V, three-phase power (three 230 V phases). At my house I have a 1 kW, three-phase electric motor running a pump in my well which provides fresh water for the house.

The motor can be wired as either 400 V star or 230 V delta connection according to the plate on the housing.

Now the region I’m in suffers from intermittent power outages in winter, lasting anywhere from a couple of minutes to a few days. I figured I would try to run the water pump on backup power, and have devised the following setup:

12 V battery bank -> 230 V pure sinus inverter -> VFD (to create a three-phase supply) -> motor. From what I gather based on my research the VFD will produce a 230 V, three-phase supply and not a 400 V one, correct? As such, the motor would theoretically need to be wired in ”delta” in order to function at peak efficiency.

However, since this is an emergency solution, and not something to be used often or for long periods of time, it makes little sense to change the wiring terminal on the motor (from the current star to delta) whenever running on battery.

So in short the question is: Would the motor run at all when fed with 230 V, three-phase power while wired in ”star”, and if so, will it damage it in some way? I am aware that it would at the very least produce significantly less torque, which I think should be okay in this situation, but it would be difficult to explain to the missus why we need a new motor for the pump…

I have also thought about getting a single-phase motor to replace it, but I figured it would be better to balance the load over the phases.

If anyone has some other solutions to the problem that would of course be welcome as well!

EDIT: The solution would be automatically disconnected from the mains through a series of relays while the power was out, which would then by extension also allow the batteries to be connected. Similarily the emergency power would be automatically cut when main power returned.

Found the solution below; easiest would be to get a single-phase motor.

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    \$\begingroup\$ There exist single-phase VFDs with 380 V 3 ph output. Pure sinus inverter is not required because VFD rectifies AC input voltage. \$\endgroup\$
    – Vladimir
    Commented Aug 9, 2022 at 22:25

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No, the 400V wired motor will not run properly on 220 VAC from a VFD, although you could possibly adjust the output to half speed which would supply 25 or 30 Hz. You could get the same torque for the pump, but I don't know if it will pump water under those conditions.

Another option would be a set of switches or relays to dynamically rewire the motor in delta for emergency conditions.

A third option would be to use two 12V batteries and two 220V inverters. You would need to hack the inverters to access the 320 VDC bus link directly, and connect that in series to get 640 VDC for a 440 VAC VFD. But you need to be very careful about isolating the two inverters, and some VFDs will not work on DC through the AC input terminals, so you may need to access the DC link bus, and provide proper protection with DC rated switches and fuses. I have actually done this:

https://www.youtube.com/watch?v=18pCH_qxZ4k

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  • \$\begingroup\$ Brilliant. I think the most resonable thing to do is to actually replace the motor with a single-phase variant, and just run that off of a beefy inverter. The thought of building two external wiring-boxes (one delta and one star) and switch between them did occur to me, and the thought was to use a series of relays to disconnect everything from the mains while power was out. But the work is likely not worth it over just replacing the unit. Thank you for the reply! \$\endgroup\$
    – Viktor
    Commented Aug 10, 2022 at 17:45
  • \$\begingroup\$ @Viktor Actually,you could just buy an ordinary off-the-shelf manual star-delta switch for much less than a new motor. \$\endgroup\$
    – TooTea
    Commented Aug 10, 2022 at 18:01

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