What is the cheapest way to run a 1 kW, 3-phase motor for about 20 minutes on a 12 VDC battery?
Will it be a 2 kW inverter to power a VFD or is there something else? And will a 100 Ah battery do, and what type of VFD?
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\$\begingroup\$ How big of battery are you planning on? Your 100A battery will probably not cut it. What is the 100A rating? \$\endgroup\$– GilNov 22, 2022 at 4:53
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\$\begingroup\$ 1Kw @ 12V = 83 AMPS! The 12 volt draw will be even higher after inverter losses. Good luck! \$\endgroup\$– DoxyLoverNov 22, 2022 at 5:43
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\$\begingroup\$ is your battery big enough for 100 amps? \$\endgroup\$– user253751Nov 22, 2022 at 9:38
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Assuming about 80% overall efficiency, a 1kW motor at full load will draw about 100A from the 12V battery. That is a 1C load, and for a lead-acid battery the Peukert effect will make effective capacity of about 50 A-h. Running it for 20 minutes will take it to about 30% SOC, which is OK for a deep discharge type battery.
It may be difficult to find an inverter with 380 VAC output, whereas those for 220 VAC are commonly available. So you may need two batteries and two 220 VAC inverters. However, you can't simply wire the AC outputs in series, unless the phases are synchronized. So what you can do (and I have done it), is to access the internal DC bus links, which are about 320 VDC, and wire them in series for 640 VDC. Then apply this DC power to the inverter's DC bus link. Make sure the batteries and the inverter enclosures are isolated, as they will have a very high (LETHAL) DC voltage on them.
My crude video shows this concept:
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\$\begingroup\$ surely wiring inverters in a way that gives double voltage is liable to just blow them up \$\endgroup\$ Nov 22, 2022 at 9:38
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\$\begingroup\$ I did connect the DC outputs together with a resistor (12 ohm 5W IIRC) to limit surge current, and I connected them after they were powered up by using a switch. The only current surge will be from the charged capacitors in the inverters, into the uncharged capacitors in the VFD. Each inverter still has its usual 320 VDC and only the VFD will see 640V. \$\endgroup\$ Nov 22, 2022 at 9:57
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\$\begingroup\$ so I think you meant to apply to the VFD's DC link. This is liable to blow up a VFD designed for half that voltage. Of course modifying two inverters and a VFD voids the warranty on all 3. \$\endgroup\$ Nov 22, 2022 at 10:00
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\$\begingroup\$ It is an old 480V 5 kW (8 HP) VFD that I bought for about $60, and the inverters were only about $30 each. Used 480V VFDs can be purchased pretty cheap on eBay, as most hobbyists and DIY freaks don't have 480V available. These VFDs are not really designed for single phase, as the capacitors are insufficient, but using DC from an inverter, using a high frequency switching supply, means the capacitors can be smaller \$\endgroup\$ Nov 22, 2022 at 10:08