high current inductors from copper tubing?

[PB2]

Originally Posted by star882

20' of tubing wound into an 8" diameter coil yields just over 9 turns. At a spacing of 1/8" between turns, it yields about 18.6uH. Looks like it would be just within reach for a 50kHz switching frequency. Air is virtually lossless as a core material and resistance losses should be small. There might be a more optimal geometry but I'm not sure how to figure it out.

I wonder what's the highest inductance that can be obtained with 20' of tubing with only an air core...

Seems that a spreadsheet where you keep the length constant would help. Or you could take a shot at the Calculus to find a maximum H for minimum length. Someone must have done it, here you go in .pdf:
http://www.phy.auckland.ac.nz/Staff/...;20Problem.pdf

An accurate calculator suggested in the above reference:
http://hamwaves.com/antennas/inductance.html

I believe that more layers will help as a result of mutual coupling, say 3 layers of 3 turns as compared to your 9 turn example.

[PB2]

Star, I noticed that you have several posts about EVs. My nephew is interested in building an EV; he has started, but seems that the cost for batteries, controller and charger is quite high. He's thinking about building them or abandoning the project.

I noticed a DIY controller at the link you provided and I forwarded it to him.

Please let us know how the charger works out if you end up building it.

Pete B.

[star882]

I'm only doing the "design side" of the project for now. I'll actually build it when I need a large power supply that does not need isolation.

[Andrew Eckhardt]

For maximum inductance you'd be trying to approximate a Brooks coil.

I think by the time you figure out your total power switch and rectifier loss, and proper thermal management there, you'll be looking at the slower or at least mid-range side of switching speed. At minimum cost you will probably find gapped ferrite or iron powder most attractive.

[star882]

How much does the core material cost in small quantities?

I did some research on the Brooks coil, and one of the problems is the poor heat dissipation, which the tubing solves by cooling the coils from the inside. I have not found a calculator that can calculate the maximum inductance from a wire of a given length and size. That would be of great help for determining the most optimum geometry.

With lower voltages and/or higher currents, air core inductors can in fact be very practical as the inductance needed would be smaller.

For the EV charger, maybe some surplus microwave oven transformers can be used as inductors. The switching frequency would need to be reduced, perhaps to the point where cheap standard recovery rectifiers and SCRs can be used. The main disadvantage would be the weight. However, a charger with a power circuit consisting of one large SCR, one large rectifier, and a few microwave oven transformers acting as inductors would be very easy for the DIYer to build.

[PB2]

Some surplus parts, that might be of use:
Semiconductors, SCR, IGBT, etc

[woody]

Star, look at DIY Audio & Video - FAQs, Tutorials, and Calculators for Speaker Boxes, Crossovers, Filters, Wiring and more

on the left side of the page is a section for calculators.

When you get to low resistance coils below .5ohm or so you may want to
do your own math. I don't think this calc displayes results below .1 ohm
or maby that was .01 ohm?

Play with the coil legnth and diameter to find optium size.

[PB2]

Seems a Brooks coil is the most efficient geometry (as Andrew mentioned above) for a length of wire. I believe that if you provided say a 1/16" gap between layers and windings for air flow that you'd end up with good enough cooling just with passive cooling. 6 turns X 6 layers is 36 turns with a box of about 1.5" and a 3" core; this calculator predicts about 125 uH:
Ness Engineering Tech Data - Brooks Coil Formulas

[star882]

Maybe the idea is more practical than I first thought. Perhaps it can even be used in a high current boost converter to step up, for instance, 288v from the battery pack to 400v for operating a motor inverter as well as step it back down during regen. (400v is about the peak value of 277v AC.)

[star882]

Paul and Sabrina's Cheap 3 Phase Inverter (AC Controller) with Field Oriented Control - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
That project aims to allow common 3 phase industrial motors to be used for homemade EVs. One problem is that most industrial motors operate at 208-230v minimum (for rated power), which translates into 300-325v plus overhead of DC. Not exactly practical for a battery pack. But to step up a 180v battery to 400v with a maximum load current of 200A (translates into about 100HP) at an operating frequency of 15kHz only requires about 16.5uH. That is practical for a copper tubing inductor cooled with oil. One major problem is that the metal of the car would act as a shorted turn. However, hypermilers often add "Kammbacks" to their cars. If the Kammback is made from a nonconductive material, the inductor can be installed inside it in such a way that the metal would be far away enough to not cause problems. I'm not sure about heat dissipation of 1/4" tubing when conducting 200A, but I would not expect it to be a problem as the load would usually be much less, maybe as low as 15A when cruising.