Voltage Conversion

You only have 2 choices:

1) An inverter, and a VFD would not work, the output is 3 phase. You need a plain inverter anyway since you don't need the converter section.

2) A motor / generator set where you drive a DC motor that is coupled to an AC alternator. If it is stand-alone (i.e. not grid connected), it will need to be a synchronous alternator.

Why I mentioned using the VFD is because I have been dealing with a local company that does, in fact, uses VFD's for this purpose. Only two of the three output phases are used, and the voltage between the two used phases is 208 (which is OK for most 240VAC applications). The D.C. input voltage can be connected direct to the VFD's DC buss, if desired, bipassing its input diodes. This will boost the VFD's efficiency because it lowers heat losses.

The only way to do this in a stable & efficient way is through a motor - generator combination.

The power you need is also to high for cheap simple conversion systems.

But why are you going with this stupid North American system to create a 120/240V supply?

Just do it as we do it in Europe: Star-Delta and balance your output over the three fazes. (if possible) Your efficency will rize enormously.

If you don't need perceft sinusoid can make an static converter using one transformer and making electronic switching. Is an inverter as used in boating and camping, but designed for 750 VDC.

Thanks for your input! A perfect sinusoid is not required.

Are you suggesting an autotransformer (1 primary and 2 secondaries) similar to the pole type transformers? So I pulse the primary at 60Hz using a uC and some power semiconductors....and then collect the stepped down voltage on the two secondary windings? I need to design the turns ratios to get 120VAC on the secondaries (outside to center tap=COMMON). Then I can get 240VAC between the two outside taps (L1 to L2).

Any thoughts on where I can get such a transformer?

A VFD is definitely NOT an option, as you say you have 750VDC.

A DC to AC inverter might work. but first tell us where do you get your 750VDC from? sounds like a an unusual power source.

Wangito.

Allow me to differ with you somewhat.

I have a Yaskawa GPD 515/G5 technical manual showing their 600V VFD is made to handle input voltges from 445V to 733V (Vrms?). My 750VDC shouldn't present a problem.

The drive produces outputs from 0 to 575VAC. I would lock the voltage at 120VAC and the frequency to 60Hz. But this is still expensive--hence my original post.

The source of the 750VDC? Sorry, can't tell you! (Don't you just hate that?)

Reverse feed a DC eletric arc welder. Your voltage will need to be droped via step down transformer to the highest rating of the welder.

It may help to use a voltage devider to lower the voltage to around 480 volts. Then go through a inverter. All inverters have a rough sine wave but the better ones can run anything. Once you convert to ac then transformer to any voltage you like. It will be a expensive electrical conversion so you may want to drive a H.V. Dc Motor or possibly a series of Regular voltage dc motors coupled to generators. Generator loads would have to be well ballanced to use more then one. Seems to me that if you want to convert 750 vdc to standard household current then you are getting the power very cheap! Remember, you need only 60 Hz.