Various aspects of home brew inverters

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Joined: 07/11/2007
Location: Australia
Posts: 1904
Posted: 09:46am 08 Jul 2018      

  Warpspeed said   Everyones transformer will be different, different toroid size, different number of secondary turns, even the batch of steel in the core can make a slight difference.

The capacitor Mark was using originally was unmarked, but turned out to be 7uF and that gave a resonant frequency of 65Hz. It was a bit too big.
Then we tried 2uF, 4uF and 6uF and the third attempt was right on the money at 76Hz.

So its not much use copying what worked for someone else, you are going to have to make the measurement yourself and find out what capacitance your own transformer requires to hit the magic 75Hz number (within one or two Hz).

I did some experimenting with this today.
The toroid came from a 2.5Kw Latronics LF inverter, I re used the original 230V winding.

All was hooked up as suggested but I was handicapped by my audio oscillator which had a rather course frequency adjustment. It could be set to 50 or 75Hz with a bit of knob fiddling.
I tried that little kit digital oscilloscope first, too small screen to notice amplitude differences accurately.
Then I connected my analogue CRO, this was better but it is still hard to see just exactly where the peak occurred.

So a different approach was used. I have a multimeter which can measure AC RMS values. Turning the oscillator amplitude down so it remained below 2.000V at peak resonance it was very easy to see just where this peak was. A few millivolts difference where shown when one Hz above or below resonance. Its not a sharp peak which is why I found it hard to see on my old CRO.
It corresponded with what I saw on the CRO but was much more precise.

The frequency was monitored on another multimeter, there is no scaling on my home built audio oscillator.

Interesting results with this toroid, a 1uF cap resonated well below the optimal 75Hz.
2 x 0.47uF in parallel gets closer.
0.47 & 0.33uF in parallel was just about spot on.

As Tony mentioned, different toroids behave quite different. It would pay to have a selection of mains rated caps on hand for this test.

BTW, it appears no fancy oscilloscope is required as the RMS multimeter seems to do a better job. Perhaps just monitoring the frequency on the little kit oscilloscope to see if all is running sweetly.