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Can I use a VFD to drive a 3 phase motor if I drive the VFD with 48V from some series car batteries? The VFD has a single phase input and 3 phase output. Is it just a rectifier in there that charges up large DC bus caps? If it is, why can I not just pour charge in with DC?

I want to make an electric lawnmower and I got these parts from the dumpster at work (the motor and the VFD).

Thanks!

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    \$\begingroup\$ Find the manual and it just might tell you. \$\endgroup\$ – Sean Boddy Apr 4 '15 at 21:03
  • \$\begingroup\$ Already did...no mention of that \$\endgroup\$ – testname123 Apr 4 '15 at 21:31
  • \$\begingroup\$ Then don't do it. Or do. Off label use of electronics is generally dangerous. If it even works you'll wear out the input stage prematurely. And you fished it out of a dumpster? \$\endgroup\$ – Sean Boddy Apr 4 '15 at 21:35
  • \$\begingroup\$ A lawnmower is hardly a good target for a VFD \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 19 '18 at 20:15
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If it is any type of industrial VFD, it is probably looking for either 240 VAC or 480 VAC. Low horsepower VFD's can almost always run off of single phase, but you still need to get the internal DC Buss up to about 340VDC or 650VDC, so 48V won't do it.

Inexpensive fan/pump type VFD's generally don't directly expose the DC Buss connections, but they are easily available. Even if applying the proper DC voltage there, you will have to come up with a precharge circuit to bring the buss capacitors up slowly.

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  • \$\begingroup\$ How about if I run a UPC with several 12V in parallel and then take that output to the VFD? It's a small one that runs on 120Vac single phase \$\endgroup\$ – testname123 Apr 6 '15 at 16:51
  • \$\begingroup\$ That might work, but now you are starting to look at severe conversion losses. Honestly, you would be better off just using the batteries and a simple PWM generator to power a DC motor. A 90 VDC motor would only run at half speed, but much simpler to implement, and more efficient. \$\endgroup\$ – R Drast Apr 6 '15 at 17:10
  • \$\begingroup\$ I agree...I think I need at least 2 hp for a riding lawnmower, but those aren't easy to find \$\endgroup\$ – testname123 Apr 6 '15 at 17:23
  • \$\begingroup\$ 2HP would be light for a push mower. For a rider, you seriously need a minimum of 7.5 HP. \$\endgroup\$ – R Drast Apr 6 '15 at 17:35
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As said, the VFD will look for a higher voltage than you have, and your 2HP is a lot more power than you might think it is so it will consume a battery faster than you think. Let's look at an example, assuming you have a 230V VFD.

  • To run a 230V 3 phase AC motor rated 2HP, you first will need 30 12 volt batteries in series.

    2HP 230V 3 phase will be roughly 6.8A Full Load.

    Assume 1/2 hour of mowing time, on the surface that looks like you only need 3.4 amp-hours of battery power.

But it doesn't really work that way, because to keep the motor at full speed, you must give it full voltage. Yet as soon as you start to discharge, the voltage is dropping as the batteries drain. Without going too deep into it, you run into a chase scenario wherein you have to keep adding cells in parallel to increase storage capacity, but that adds weight, which drains them faster, which requires more batteries.

So you might be able to START it with 30 x 5AH 12V motorcycle batteries that weight 2lbs each, but it will likely only run at full speed for 2 minutes. If you go with a 55AH battery so you can run for 20 minutes, each battery now weights 40lbs, so you have 1200lbs of batteries on your mover!!! Now because your riding mower has to carry the batteries too, the 2HP motor can't move them and you and the mower, so you have to go to a 20HP motor, and the process starts over. Next thing you know, you are attaching a mower blade to your Prius!

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  • \$\begingroup\$ "on the surface that looks like you only need 3.4 amp-hours... as soon as you start to discharge, the voltage is dropping" - another problem is that a typical 5Ah lead acid battery only has about half its rated capacity at 1.4C, so you really need about 10Ah (and then you are only discharging at 0.7C, so voltage drop isn't as bad). So 120lbs of battery might be enough... \$\endgroup\$ – Bruce Abbott Sep 29 '16 at 1:23

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