RE: drsstc

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Wagner" <jwagner-at-cc.hut.fi> 

Hi,

just thought I'd comment as well...

On Wed, 20 Oct 2004, Tesla list wrote:
>Original poster: "Rajesh Seenivasan" <rajeshkvs-at-hotmail-dot-com> I am trying 
>to design tesla coil and induction heater at home.
>I am trying to use that in my design. I have a doubt in my design. I 
>designed a CW H-Bridge inverter using IGBT bricks. Now, I want to connect 
>a inductance (tesla or induction heating transformer) and a capacitance 
>(capacitor bank) across the output of two legs of H Bridge inverter. L and 
>C are in series. So, the resonant freq is given by,
>f = 1/( 2*pi*sqrt(LC) )

I've an old Excel design sheet that could give you some ideas on some of 
the calculations, even though the sheet is for a slighly different subject: 
http://www.hut.fi/~jwagner/ele-tmp/LLC-resonant-welder-designsheet.xls
(but I haven't checked yet if it's up to date, and there _might_ be some 
errors there...)

For induction heating you might want to use a parallel-resonant LC circuit 
to get the current (and thus magnetic field) increased, instead of the 
voltage (electric field). Inductive heating works with induction, i.e. 
magnetic field.

Here's a paper from IEEE that has some calculations on power and current, 
that you could also use as a starting point:

   http://www.hut.fi/~jwagner/electr/tmp/01296120.pdf

>Now, my doubt is: Can I run this setup at exactly resonant frequency ?

Yes.

How well it runs, or when it explodes, depends on the quality factor of 
your series resonant circuit and how much the IGBT modules can handle. The 
circuit has enough resistive and radiative losses that you don't actually 
get zero impedance. Of course even this impedance can still be too low for 
the IGBTs to survive long runs...

What you can do though instead of boring maths ;-) is to just plug in the 
LC circuit and a load an run at the resonant freq, but start with a really 
small input voltage to your IGBT system. Slowly increase the voltage, and 
monitor the input current. When the current seems "hmm, this might be too 
much", then _don't_ increase the input voltage further, okay? ;-))

>I ask this question because i studied that at resonant frequency, the 
>series LC circuit's impedence becomes ZERO. So, if L and C are
>connected across the outputs of two legs of HBridge, will it short both 
>the output together because of Zero impedence ?
>In other words, how do I calculate the current through the series LC 
>circuit at resonant frequency ?

Can't remember right now how one would calculate the max tank current in 
the steady state, but it was via the Q (quality factor, Q=2pi*f*L/R) and 
then input power considerations. Something like "P=Q^2/2R=UI" or there you 
go ;) Maybe Google will help, "lcr tank current", "rlc tank current", ..., 
etc. Or that electronics-basics book that I don't have at hand right now. ;o)

>Another doubt:
>In one of the message, Mr.Vladimiro said:
>"The family (Semikron) 5 is suited to work in square wave >20kHz and in 
>resonant mode >100kHz". Lets consider the resonant frequency is 60kHz. Can 
>I run the setup at a frequency more that resonant frequency, say 75kHz to 
>make sure that current drawn is not huge (say, 30A) ?

Yes, though you could also go _lower_ in freq. The circuit will look a bit 
more capacitive than resistive then (vs inductive if you go higher freq), 
but, who cares ;-)) Power draw will drop, in either scenario.

By the way, if the IGBT module doesn't already contain internal 
fast&soft-recovery freewheeling diodes then you'll have to add some 
external ones.

cheers,
  - Jan

-- 
****************************************************
  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
  http://www.hut.fi/~jwagner/ - jwagner-at-cc.hut.fi