sort of

You take the 240Vac and rectify it to give you a 336Vdc. Then with a H-bridge and suitable current-control (or just go for open-loop voltage control) you PWM the relevant FET's to produce a sinewave fundement which you require of 216Vac

you wount get 240Vac out due to volt drops, modulation depth.... so keep that in mind

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Here's another thought. Get a 230V 3-phase VFD control for fractional HP and use just one phase plus a ground. I don't know if that is legal in the UK. In the US, our mains have a white wire ("common") that is grouded at the service entrance. We can't use the ground (green), but we can use the white wire in the configuration described. One can get FHP VFD's that run on 230V for about $100USD. I checked the UK eBay and didn't see any listed. Maybe they are called something else in the UK.
John

 

Is there a way of getting 240vac constant output because the pump stops if the voltage drops below 230vac

 

Of course you can get variable frequency oscillators but he wanted to vary the frequency at set time periods which is possible using a VCO and another oscillator but it's probably more convenient to use a PIC.

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would these be suitable to make the h-bridge with http://www.rapidonline.com/netalogue/specs/47-0320.pdf
If so what voltage do i need to apply to the gate to activate it?

 

Those MOSFETs are perfect.

Now the only problem is delivering 20V to the gates on top of the 325V.

I would use pulse transformers to do this as they can do this and isolate the high voltage side from the low voltage side.

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short of boosting the DC-link.. no
either use a transformer on the input to raise the link a bit, or add a boost stage on the DC-link.

@ that FET... 400V blocking should be ok UNLESS you have a really poor layout and lots-o-stray inductance creaps into your power layout, then that 80V headroom is going to vanish!
There is already going to be a voltage overshoot at turn-off (even with the optimum layout you could do... cause it won't be the ideal) so just be careful. I would be tempted to go for 500V FET's but that is just me


depending on yr switching frequency a pulse-transformer (with simple rectifier & cap) will be good, otherwise a bit more in-depth drive will be needed (ie a DCDC with opt-driver)

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I thought it was +/- 20v? or have i read the data sheet wrong


How would i do that?

 

They do a 600v verious aswell suppose this would be safer

 

yer take the 600V one (as long as the current is still OK).
NOTE FET's have a nasty (ie to calculate with) SOA, so do your homework before you commit. But that is just me I have managed to kill a 1200V diode (in a basic chopper configuration) from a 350V link because I was driving the IGBT too hard (trying to turn it on in 500ns with 600A flowing in the main coil... really should of been happy with 900ns) and there were a bit to much stray inductance around

at the post above
Yes a FET is driven with +/-20V at its gate... W.R.T its source. Have a look at the two FET's which are on the positive rail (ie their DRAIN's attached to the 325V bus). Assuming that the FET is on, the source and drain can be assumed to be shorted (yes in practice Ron and current means this isn't the case... but for illistration...). So if we now see that the source potential is the same as the drain potential (which is 325V), if we want to keep this FET conducting we need the Gate potential to be 20V above the source potential (ie 345V) IF we were to reference w.r.t. the low-side of the DC-link.

That is what isolated gate-drive cct are needed. Sure measure the gate potential w.r.t. to the low-side of the dc-link and you will see it at 345V. But using a transformer to provide the power (and the signal in some cases) means that you develope the 20V across that trasformer BUT it can "float" with the FET as it switches, everybody wins

Basically get a small pulse transformer which has enough blocking potential to cope with yr link (and then some...) and on the "floating" side rig up a small recitifer using Shokky diodes and a small cap (plus a gate terminating resistor + series gate resistor). On the primary side of said transformer some FET's that you then pass a square-wave two when you want to fire the FET

I have a cct somewhere, will find. Personally I prefer putting down a small 1W Newport DCDC with a FET opto-drive, alot simpler

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Now im really confused (A diagram would help )

What are Shokky diodes? I cant find an referrence to them?

 

*schottky*

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From what ive read on pulse transformers they amplify the wave input, If the wave input is 5v from the oscillator how does it produce the 345v that is needed to trigger the FET? Is it the winding configuration or does it need to be conected to the 325v source?

 

no and yes...
Well yes a transformer does "transform" the voltage/current to a higher/lower level, but that isn't what said transformer is used for in this case. It is needed to provide a floating/independent reference point (as well as isolation). A simple 1:4 pulse transformer is all that is needed (even a 1:2 depending on FET and XFMR drive ).

Yes the GATE of the upper FET's will be at a potential of 345V w.r.t the negative side of the link (when they are ON), but that doesn't mean you are going to be supplying 345V to said gate.


So take figure1. This is a crude setup of what you want. The FET drives are almost what you want (don't go exact, there are bit missing... personally I prefer DCDC+Opto over XFMR but thats just me)

Anyway the potential of the Gate of FET driven by G1 will be at 345V w.r.t. to 0V1, however... with respect to its source it will only be 20V.
That is what the XFMR in the gate-drive is doing, it is allowing a local supply (between gates & sources) to float to whatever potential it has to BUT still providing a potential across the secondary of the XFMR (and thus the FET's gate&source of 20V (if designed such).

Try not to think of voltage w.r.t. to a common voltage, think of them w.r.t. whatever at the time of query, then you can see THAT as long as a FET's gate potential (w.r.t to its source) is ... +20V then the FET will turn on. The source may be at a completly different potential w.r.t. another voltage reference point, but w.r.t. the gate is all the FET cares about.


Also I have put in an isolated gate-drive for the lower two FET's as well. Some may argue it is a waste since 0V1 and 0V2 could be the same potential so why bother with the isolation and just simplify the design ... HOWEVER I will say this now that 0V1 is gonna be noisy and you really don't want that noisy polluting yr control rails, so don't pollute it

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I understand now

Thanks for all your help, Ill do a schematic and post it up