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View Full Version : Somewhat OT. running a three phase motor from DC?



fyffe555
12-04-2006, 02:59 PM
Have a challenge. need is to run a large (7-15hp rated) three phase motor from batteries with speed control and forward and reverse. Trying to fit some readily available motors into a boat. Number of batteries isn't a problem. I know some vfd's convert single phase to dc then produce three phase and timing.. power electrics isn't my forte......

Geof
12-04-2006, 04:01 PM
There is a commercially available, expensive way to do it;

http://www.philtek.com/products.html

I briefly looked into making my own battery powered three phase inverter to run a CNC machine; that was how I found the link above. I think it would have been possible but when I started looking at cost and safety I came to the conclusion that it was better to run a genset. Not exactly on option for you. :)

JRaef
12-04-2006, 04:54 PM
Many (if not most) VFDs allow you to input DC directly into them, as long as it meets the DC bus voltage requirements. Just ask your favorite VFD manufacturer. The reason why some do not is only because of where they tap off power to run the electronics. Most take their power from the DC bus, so they will be happy with a DC input. A few however have a separate AC power supply for the control power, so if you supply DC, it bypasses that power supply.

You can also do a search for AC vehicle drives or small "traction drives", which also take DC and put out AC, then you don't end up wasting $$ on the rectifier section of a VFD.

One thing to be careful of however, you should not use the dynamic braking feature of a VFD when being supplied by batteries. The regenerated voltage may come back onto the batteries too fast and damage them.

fyffe555
12-05-2006, 11:43 AM
thanks guys, so its possible, even reasonable. Just need to figure a way to do it without paying more than my house is worth......

Andrew

Geof
12-05-2006, 12:18 PM
thanks guys, so its possible, even reasonable. Just need to figure a way to do it without paying more than my house is worth......

Andrew

All you need to do is find someone who fried the rectifier side and maybe the capacitors on a big VFD. You do not need these parts. Your batteries take the place of the capacitors so all you need is to know the upper and lower DC bus voltage and put in a correct number of 12 volt deep cycle batteries in serial.

You are going to be putting many batteries in serial to get maybe well over a hundred volts which can be very dangerous when you are dealing with DC from a source that can supply extremely high current so you need fuses between the batteries that are large enough to carry your working current but which will blow and separate the batteries if you get a short circuit.

So now you have a "DC bus" which is at the operating voltage for the VFD. Some VFDs draw all their requirements both control and power from this bus, some use a single phase of the input to drive the power supply for the control. If the one you have scrounged uses the bus voltage for everything you are (more or less) up and running once you have the DC voltage source in the form of batteries.

If your VFD needs single phase 120 volt AC for the control power supply you just need to go and get a small inverter that will take a 12 volt DC input and give you 120 volts AC out at maybe 500 watts. You power this from one of your 12 volt batteries. This is a real hokey way to do it because this slightly discharges this battery below all the others so the batteries get unbalanced...but it works in the short term.

And I suppose I should put the disclaimer that if you choose to do something like this it is your decision and I make no claim as to the reliability of my ideas.

fyffe555
12-05-2006, 12:44 PM
All you need to do is find someone who fried the rectifier side and maybe the capacitors on a big VFD.

That had crossed my mind, sounds like a job for ebay...

I'm familar with the issues of large battery bank management having been perpherally involved with some large UPS for computer facility projects. In fact that's where the battery banks would come from. That would include the mid bank fusing as you suggested and charge and monitoring systems.

A VFD will run both clockwise and anticlockwise?

thanks

Andrew

Geof
12-05-2006, 01:26 PM
...A VFD will run both clockwise and anticlockwise?

thanks

Andrew

Even if it doesn't with 3 phase you just have a switch to flip two lines for reverse.

fyffe555
12-05-2006, 01:33 PM
/slaps head/ duh, thanks......

Geof
12-05-2006, 01:39 PM
/slaps head/ duh, thanks......

Never mind 'slaps head'... first time I was told how to reverse a 3 phase I think it was phrased as 'switch two sets of wires' or at least that is what I heard. So I switched two sets, it kept running the same way and I was really pissed.

NC Cams
12-05-2006, 10:42 PM
Facetious he mused:

You'se guys need to look at some 3 phase AC Bridgeport mill schematics - they show how to wire the contactor so that a knife switch could flop the phases needed to reverse motor rotation.

Everytime I forget how, I pull up the PDF and its head slap time again - DUH, DOH, SLAP.....

Thanks, I"m feeling much better now.....