Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 10:04am 04 Jul 2018 |
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).
For a battery powered off grid inverter the capacitor is always going to be located across the secondary, because the large turns ratio (9:1 ?)would need a capacitor 81 times as large on the primary to do the same job.
A grid tie inverter is different, because it works from a much higher dc input voltage, so the transformer ratio is going to be much closer to 1:1 in many cases, and the capacitor can just as easily go across the primary, which is where you often find it in many grid tie inverters.
Just make sure you use a suitably rated capacitor. You can test with any old junk at low voltage to find the optimum final capacitance value. But make sure what finally goes in is going to be up to the job.
Cheers, Tony.