On Fri, 19 Jul 2002, Richards, Justin P wrote:

>Yes, you are correct.  I had not considered that current was going to
>flow in the opposite direction.  The drawing I made sort of obscured
>this.  I can think of a way to eliminate this but then the uneven demand
>placed on the batteries would be increased and as suggested present more
>problems than it has solved.
>
>I assume with the PWM H bridge design that as the Voltage is to peak
>that PWM frequency peaks then slows down again.  Am I visualizing this
>correctly.  I will spend some time with this idea.

Actually the best way to do this is to build a large buck regulator
followed by a H bridge that only switches polarity. The buck regulator fed
with Vpeak will guarantee minimum losses where it matters (at and near the
peak of the sine). You can use any SMPSU chip to drive the buck regulator,
its output voltage will have to span 0V and be modulated with a rectified
sine wave waveform (which could come from a PIC PWM output or from
somewhere else). The H bridge simply inverts polarity every half cycle and
is un-critical (relatively speaking).

I think that you really should start thinking about a hipot insulation
transformer on the output. A 5kVA unit will not be much larger than 2
stacked beer sixpacks (universal engineering measure) and you can probably
buy one surplus for under 200$. This will save you lots of trouble and
maybe your life while you work on the project.

I am wondering if there is a way to use a 3-phase 5kVA wye/delta
transformer as is and drive its (wye) primary with the mosfets. The delta
would be disconnected at one point and the wye is perfect for direct
steering with PWM (two out of three phases, with the delta output's two
phases connected in parallel, the third unconnected). This needs some more
thinking imho. Maybe the best way is to drive all three phases and use as
is. I think that 3 phase wye output transformers are available for lower
voltage (like 48V). It could be used as is as uptransformer.

Peter

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