Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 10:39am 08 Jul 2018 |
Klaus,
The peak can definitely be difficult to see very accurately, but its there somewhere.
One thing to be rather careful of is that you are not tuning the transformer to a harmonic of 75Hz.
With the oscilloscope peaking at a harmonic of 75Hz (say at 150Hz or 225Hz) may result in lower peak, with an odd lumpy looking waveform.
With a multimeter you don't get the same visual clues of waveform distortion that there will be with an oscilloscope.
The signal generator may be tuned to 75Hz o/k, but there might be a visible peak at one of the signal generator harmonics which might lead you astray, and its fairly easy to get caught that way. That 1uF does sound rather small which is a bit of a concern.
One way to be more certain is to tune the signal generator over a much wider frequency range, especially much higher, to make sure you settle on the largest real peak there is. Then use the multimeter for finer final measurement.
Mark,
A saturating choke (or having no choke at all) means that the bridge will be switching straight into a highly capacitive load which can produce some very high peak currents in the mosfets.
Definitely not recommended.
Anything that make the inverter operate in a higher stress mode can certainly make it more vulnerable when other external load extremes are also applied to it.
That double Aerosharp choke with the twelve turns will work fine without a ferrite choke. Adding an additional ferrite choke may help very slightly with idling current, but that will saturate beyond a medium load, leaving the big iron choke to do the job which it will do with ease.
By the way, I think your iron choke has about 3 metres of that 25mm squared wire on it. You might correct me on that.
I remember you clearly marked off the length, but not sure what that measurement was.
Anyhow 3 metres works out to 0.0172ohms x 3 metres divided by 25mm squared = .00206 ohms.
At 100 amps that will drop 0.206 volts, less than half a percent of 48v.
If you tried really hard you might get 50mm squared worth of stranded copper wire onto that choke, and gain a quarter of a percent. Its just not worth worrying about.
The transformer primary on the other hand, really needs all of that that 70mm squared because the wire length in the primary is much greater than in the choke.
Cheers, Tony.