I want to build a three-phase boost converter a la Flux and for the current levels I anticipate, the inductors are important components.
I initially considered air core types since physical size was not an issue, but Flux convinced me that ferrite cores were a better way to go. I'm still concerned about about core losses and saturation issues at high current levels.
Unfortunately, I didn't have any suitable cores for low frequencies (plenty for high freqs) so was looking to purchase. Then, a stroke of luck came my way. A friend just happened to have six new, identical flyback transformers for CRT monitors and he let me have them at the right price - free.
These use a double C type core that measure 67 x 40 x 15 mm with a 44 x 15 mm winding window when assembled. The winding legs are round and 15 mm in diameter. These use two phenolic spacers between the halves to set the gaps. They measure only about 0.2 mm thick. I'm assuming that the two gaps add in effectiveness but this may be a wrong assumption. In any case, it seems a pretty small gap compared to what Flux was talking about. I don't have a clue as to what the ferrite mix is.
Since I now have six core sets and need three inductors, I decided to stack two cores for greater power handling. Since these will have to handle the full machine output current (up to 150 Amps battery current), I needed to use the maximum wire size consistent with needed inductance. I wound a test coil of 16 turns of four-in-hand #14 square wire which is equivalent to about #7 round. I know Litz wire would be better but this is what I have. This pretty much filled the winding window and was a bear to wind.
I measured the inductance by using a known 5% capacitor in series and finding the series resonant frequency with a signal generator and scope. Based on the capacitor Xc, this computed to 262 uH at about 100 kHz, which is the lowest my signal generator will go. This inductance is in the ballpark for a reasonable boost ratio. Using the known current voltage drop method, I measured the DCR (Direct Current Resistance) as 3.98 milliohms.
Now, to what I DON'T know. To get a handle on core losses, one needs to know the ESR (Equivalent Series Resistance). Without a network analyzer, I don't know how to measure this. Of course, even if I knew, there's not much I can do about it.
The other issue is saturation current. This is something I could adjust by changing the gaps with the compromise of decreased inductance and boost ratio with larger gaps. It seems that one needs to find the knee point where inductance rapidly decreases with increasing current. Since these will be operating with two widely different frequency components, the PWM and the alternator line frequency, it seems a sticky problem. Anyone with ideas?
