You correct Tracker - the AUTOSTART feature on this inverter is not very useful at all, becauase it requires at least a 60W load to cause the inverter to actually start and stay on.
However - by the end of this little blog you'll see it's not really neccessary to use it anyway!!
So the 3kVA transformer arrived actually while i was away from home doing the rest of the off grid install at west wyalong.
What a beauty it was!! I can't say i've ever seen such a large toriodal transformer !!
The primary windings are actually 2x separate 30V 50A windings which can be connected in parallel to double the current rating, the secondary is a single 240V output.
So anyway i set about carefully connecting it up to the MOSFET banks (heatsinks).
To increase that chances of the NOT blowing anything up i had 4x 7Ah small SLA batteries connected
by multimeter set on the 10A current range and a couple of very thin strand of wire to serve as a fuse
These precautions were justified as i flicked the switch to "ON" and the inverter soft started, the transformer growled, the multimeter beeped as it showed me over 10A flowing from the battery and i saw the strands of wire move as they heated up to a dull red colour
Quickly switching off to preventing disaster i sat back in the chair as the dryness in the back of my throat subsided.
Playing it safe i reconnected the small 160VA toroidal transformer and powered up to make sure very thing was still OK - and it was.
Then i reconected the 3kVA toroidal transformer but with only one of the 30V windings in use.
This time the result was a little less severe with only 8A flowing, this was clue as to the nature of the problem...
After a fair bit of head scratching, meaurement with the CRO and experimentation the picture started to become clearer as to what was going on:
Transformers can be modeled by the PI equivalent circuit:http://en.wikipedia.org/wiki/File:Transformer_equivalent_circuit.svg
For a toroidal transformer the magnetising inductance (Xm) and core loss resistance (Rc) are very large because the magnetic circuit is so tightly made and couples the windings very well, which means the no load current and losses are small.
The tight magnetic coupling also means that the leakage inductance on both the primary (Xp) and secondary side (Xs) are very small - at full load where less of the applied voltage is stolen by these parastic components the result is less losses and higher efficiency.
Transformers that are less than ideal compromise these characteristics.
And of this should help improve the inverter's efficiency and reduce the wasted power, but it didn't, so why not??
Directly across the output of the transformer secondary exsists a 4.7uF polyester capacitor.
As it turns out the original transformers are made deliberately lossy with a disproportionately large amount of leakage inductance, as this forms a filter with the 4.7uF capacitor on the output of the transformer to help filter, block and smooth the switching waveform generated by the inverter into a nice clean sinewave.
Refered to the primary side by multiplying by the (turns ratio)^2 this capacitor appears as approximately 300uF. And due to the ideal nature of the toroidal transfomer with very little leakage inductance, this quite large apparent capacitance was virtually short circuiting the MOSFETS as they switched at each transition from on to off.
Hence the reason for the high current draw.
Removing this capacitor and powering up saw the no load DC input current drop to about 0.25A, which was great except now the PWM switching transitions and hash from the MOSFET's appeared at the output
Problem indentified - but not not solved
Obviously some low pass fitering is required some distill the fundemental sine wave from the square switching waveformhttp://en.wikipedia.org/wiki/Fourier_series
Absolutely beautiful mathematics discovered over 200 years ago, with totally practical applications that make all inverters - stand alone and grid tie possible.
Some considerable trial and error and measurements resulted in the following changes:
1. The 4.7uF capacitor on the secondary side was reduced to 0.47uF - This Capacitor MUST
be an X2 class self healing MPP (metalised polyproplene) capacitor ie one rated for contineous safe opperation at mains voltages
2. Some inductance must be added to the primary side of the transformer to help block and filter the switching waveform.
3. A final LC filter was added on the secondary high voltage side to wipe off any traces of the switching noise to make the sinewave as pure as possible.