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Induction Heater - Series Resonant - uC/PLL Tuning [ReactorForge]

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Josh Campbell

Sat Aug 11 2012, 08:33PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Jrz126 wrote ...

Looks great.

Where'd you get the ferrite cores for the coupling transformer? What material is it?

The grey ones are iron powder toroids material type 3 with a frequency range of 50 to 500KHz distributed by Amidon.
T225-3 Iron Powder Toroid

The yellow ones are also iron powder toroids material type 26 for use as high temperature transformer cores. They are similar in quality to Micrometal's type 26 cores but I'm unsure of the manufacturer. These are nice to have around as they take a lot to saturate at sub 100KHz Fo ranges.
AT225-26B High Temperature Iron Powder Ferrite Core

The coupling transformer configuration isn't final but it is working very well at high power levels. For example running near 10KVA (240VAC in limited to 40A) the toroid core never passed 115 degrees F on long runs. I have an assortment of power conversion rated powdered iron toroid cores from Micrometals that I need to experiment with. I may end up using a type 52 for the final stage gate transformer (that's actually what's on it now) and for the coupling/matching transformer as they seem to perform much better than the type 26 material up to 250KHz and have lower losses than the 26 below 100KHz (Their design tool is pretty useful.)

I have a BOM of all the parts and suppliers that I'll list once I finalize it all.

teravolt

Mon Aug 13 2012, 03:59AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

do your tank caps get hot and are you still just using the one

Josh Campbell

Mon Aug 13 2012, 02:21PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Right now I'm using both of them since I have a a smaller H work coil attached. With both of them attached and running from 240VAC mains limited to 50-60A they run only slightly warm, showing around 115-130Â° F on the IR thermometer. The front cap does get a bit more warm than the back one, the next tank I build I'll put them equidistant from the work coil rather than front to back like they are now. I'll post some vids or pics of a long run and show the temperatures of the working components.

teravolt

Mon Aug 13 2012, 04:52PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you switch the caps will the front one still get hot? what do you think the circulating current is?

Josh Campbell

Mon Aug 13 2012, 07:52PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

If the caps are arranged to be an equal distance from the work coil then they will heat evenly since the current will not favor one or the other as is the case with one being closer. (The cap closest to the work coil will be carrying the most current.)

When running at 10KVA my 300A clamp on meter pegs out hard when attached just behind the work coil. I'll do some tests with the variac at reduced power levels and plot a graph of tank current as supply voltage increases.

I have some hall effect current sensors on the way that I will be using to determine mains draw current and maybe for fun, tank current at high power levels.

Dr. Dark Current

Mon Aug 13 2012, 09:05PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You can measure the current by putting a small cap in parallel to the tank caps and measure the current through it (if the cap is 1000 times smaller than the tank one, you will get 1000 times lower current).

Josh Campbell

Tue Aug 14 2012, 02:14PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Dr. Dark Current wrote ...

You can measure the current by putting a small cap in parallel to the tank caps and measure the current through it (if the cap is 1000 times smaller than the tank one, you will get 1000 times lower current).

I'll try that, although I may have to wait until the caps are rearranged to be an equal distance from the coil or take one out since the placement of the small cap relative to the tank cap(s) would likely make a difference in current reading. I should also be able to work it out mathematically since I know the input V & I along with the peak tank voltage from the scope readings. Hall effect sensors will be here today too!

I'm working on finishing the control board to get everything off of the breadboard, once that is finished I'll have more time to look at interesting things like tank current.

klugesmith

Tue Aug 14 2012, 03:15PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

For tank current measurement, a permanently installed Rogowski coil might be well suited.

Its sensitivity (in units of nH) can be calculated from geometry. An integrator to give you voltage proportional to current could be passive and trivial. Especially in your narrow frequency range of interest.

If you're interested, I could give you one of my PCB Rogowski coils as described here:

Josh Campbell

Tue Aug 14 2012, 03:47PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

klugesmith wrote ...

For tank current measurement, a permanently installed Rogowski coil might be well suited.

Its sensitivity (in units of nH) can be calculated from geometry. An integrator to give you voltage proportional to current could be passive and trivial. Especially in your narrow frequency range of interest.

If you're interested, I could give you one of my PCB Rogowski coils as described here:

A standard closed loop CT would obviously not work due to inductive heating of the core, my clamp on was quite hot after just clamping it on and removing it in under a second or two. But an open loop Rogowski coil would work great, good idea! And since the Rog coil has no iron core to saturate, it's response is very fast. I used something similar to measure magnetic fields in another project. The pcb is a very cool idea but I think i would rather have the coil on a flexible substrate so I could wrap it around the 1/2" tank bus tube. I wrapped the wire on a 1/4" flexible pvc tube in the past.

Gabriel35

Tue Sept 04 2012, 12:02PM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

How are you measuring those waveforms on your scope? Using Differential probes right?

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