using 100 capacitors in parallel for high current...

[Glyph]

Hi everyone,

Ok, before you start laughing at the idea just hear me out:

I'm trying to build an induction heater and i need a powerful capacitor capable of handling 100-300 amps RMS current at about ~ 120Khz.

The capacitor is being used in the resonant tank circuit of the induction heater.

The size of the capacitor should be around ~1uF

After looking around the only capacitors i could find that fit the bill are manufactured by Celem, Vishay and General electric. Unfortuantely none of them are willing to sell individual units to a hobbyist like myself.

So i'm thinking of using in parallel about 100 smaller 0.01uF film capacitors each rated at 2 amps RMS for a total of about 200 amps capability and 1uF total capacitance.

(I can almost hear you guys snickering...)

In terms of layout i'm going to get two copper plates and drill 100 holes through both and thread the axial capacitor leads through both and solder them down. The configuration is sandwhich style with a plate, capacitor, and another plate. The plates are each about 1/4" thick and while they won't handle 200amps without heating up I intend to keep them cool with heatsinking and active fan cooling. Hopefully the heat from the capacitors themselves will also conduct to the plate where it will be dissipated.

The capacitors i intend to use are 15mm in diameter so giving each 5mm room and arranging them in square fashion the assembly should be 20cm x 20cm x 4cm.

I'll also blow air in between the capacitors to try and keep them cool directly.

Now after you guys stop laughing i'd like to hear your opinions if i'm on the right track. I understand that there will be differences of ESR and ESL between each capacitor that will imbalance the load across them. Is this going to lead to a catastrophic failure where one capacitor blows causing the next one to blow and so on? Is the whole concept worth trying or am i completely off my rocker and should try instead to set up a dummy corporation and get the real capacitors from Celem, Vishay or general electric? Is there an alternative means for meeting the requirement of a 200 amp RMS 1uF capacitor?


Thanks for your assistance.

 

[audioguru]

Hee, hee, haw, haw, hee, hee.
What are you going to drive it with? A powerful radio transmitter re-tuned to 120kHz??

 

[Glyph]

A full-bridge driving a LCLR circuit which the capacitor will be a part of.

 

[rjvh]

you mentioned already that some companys make your disered capacitor why not put the effort to get it there

i know for sure that they will sell it to a company, so why not aproache a company and ask them to buy it for you with your money

easyer quicker and probably better and maybe cheaper

good luck

Robert-Jan

 

[Glyph]

Can you suggest any?

 

[rjvh]

try your local swich panel maker that makes capacitor banks for powerfactor control they have acsess to that kind of stuff

Robert-Jan

 

[Glyph]

i guess i'll follow that lead.

Btw, Is the original concept of parallel capacitors completely unworkable?

 

[rjvh]

in my opinion not (as far i learned on school) but why reinvent if you know that it is already out there?

Robert-Jan

 

[Glyph]

Whats already out there is kinda hard to access.

I tried getting a company, actually a university, to get it for me.

We got as far as getting a quote and shipping details, but the companies demand we use estabalished and highly traceable bank transfers for payment. Certified cheques, money orders and credit cards were unacceptable. The university refused to go further because in doing such a transfer they would have record it as part of their budget and they weren't willing to help me in that way, even if i was paying for it.

Now that i think about it, i'll probably go ahead and set up that dummy corporation and use it as front to purchase this capacitor. I know some lawyers that would help me and i think i can this going.

On a technical note, just for the sake of interest, what pitfalls would i encounter with the parallel capacitor setup?

 

[rjvh]

you go already to great lenkt to get this capacitor i have to say

try your swich panel maker the capacitor what you want is not that unusual in power factor corections

on the technical side i don't see that much problems if you work secure and neat

don't get shocked is the advice

and make your conections good and protect them for people/animals of touching it (housings)

corosion protection (depend on the enviorment you wan't to use it in) is a factor to consider

Robert-Jan

 

[RODALCO]

Powerfactor caps are rated for 50 or 60 Hz and may be including 3rd and 5th harmonics, not for 120 kHz. rjvh

 

[Boncuk]

~F capacitor

WIMA Germany has developed super caps with 400 and 500F, a 600F type is under development. They can deliver high current pulses without any question. The only problem: The charging voltage must not be higher than 2.5V.

Check out https://www.wima.de

and search for super cap.

No idea about the price.

Boncuk

 

[Boncuk]

~F capacitor

Sorry, I was in error. WIMA's home page only offers German language.

Here are some downloaded data sheets, supercap type R and supercap type C.

There is a 2.700F type C. It can deliver pulses up to 1.000A. According to WIMA's info they can be wired in series to achieve the required voltage, but must be well isolated (minimum distance on a board!)

Again regards

Hope the info helps.

Boncuk

 

[Glyph]

Nice looking caps.

Although their capacitance is too high for my application by a factor about a hundred million.

I need a small value cap (1uF) that can handle huge currents. this is because the cap is used in a resonant tank circuit who frequency depends on the size of the capacitor and inductor. too high a capacitance, and the frequency becomes way too low for my application of induction heating.

nice looking caps though.

 

[Rolf]

What am I missing?

Nice looking caps.

Although their capacitance is too high for my application by a factor about a hundred million.

I need a small value cap (1uF) that can handle huge currents. this is because the cap is used in a resonant tank circuit who frequency depends on the size of the capacitor and inductor. too high a capacitance, and the frequency becomes way too low for my application of induction heating.

nice looking caps though.

I have newer seen a capacitor rating for Amps! Is this something that you invented? I Goggled "capacitance reactance formula" to refresh my education on the subject. And I still don't understand what you are talking about.
Question, where is the object you are going to heat being placed? For instance when heating plastic sheeting the plates on either side are part of the capacitor and the plastic becomes its dielectric.

 

[Glyph]

I have newer seen a capacitor rating for Amps! Is this something that you invented? I Goggled "capacitance reactance formula" to refresh my education on the subject. And I still don't understand what you are talking about.
Question, where is the object you are going to heat being placed? For instance when heating plastic sheeting the plates on either side are part of the capacitor and the plastic becomes its dielectric.

YES!!! i also invented the internet, and my greatest invention of all: water.

Maybe you should google "capacitor ripple current"

oh wait, i'll do it for you since you clearly don't understand what i'm talking about: http://www.bhc.co.uk/electric03.htm

how about induction heating? maybe you should spend 20 seconds in google and look for how that works. oh even better, i'll spend those 20 seconds for you: http://www.ameritherm.com/aboutinduction.php

... actually only took me 12 seconds.

What you're talking about is dielectric heating, a known, but lesser used, heating technique. Information can be found here: **broken link removed**

I'm sorry if my imprecise terminology of saying "amps" rather than "ripple current" threw you off. Although ripple current is usually almost always given in amps.

Maybe i'll also invent the words "impolite" and "condescending" to characterize individuals such as yourself.

 

[Roff]

I have newer seen a capacitor rating for Amps! Is this something that you invented? I Goggled "capacitance reactance formula" to refresh my education on the subject. And I still don't understand what you are talking about.
Question, where is the object you are going to heat being placed? For instance when heating plastic sheeting the plates on either side are part of the capacitor and the plastic becomes its dielectric.

The problem is with ESR. For example, 300 amps through 1 milliohm will dissipate 90 watts.

Glyph, you have probably already considered voltage, but just in case you haven't:
1uF has a reactance of 1.3 ohms at 120kHz. At 300 amps RMS, that's 390V RMS, or 560V peak. Keep that in mind when you're choosing capacitors.

 

[Glyph]

yeah good point.

I calculated the ESR of my "big dumb bank" to be about 0.4-0.6 milliohms according to the manufacturer specifications for the individual units. I think the heating is going to be an issue so thats why i wanted to put fan cooled heat sinks on the bus-plate and blow air across the capacitors. Don't know if that will be enough but i won't know until i try.

The voltage rating for the capacitors is 3kV. should be enough to handle the working load and (hopefully) handle any unexpected spikes before the overvoltage protection kicks in.

heating i think will be my biggest problem. i'm trying to get those specially built capacitors because they got water cooling tubes and other goodies built in to handle it. Although if i can't get it then i'll put on some safety googles, earplugs, and hide behind a wall when i power up my 'big dumb bank'.

 

[Rolf]

Microwave oven.....

Some microwave ovens have a 1uf capacitors rated at 2000v in their klystrons cavity circuit. If that would work then they should not be to hard to get hold of.

 

[Roff]

Have you thought about immersing them in transformer oil? I found this interesting thread on the topic when searching for "dielectric oil".