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tank capacitor contruction for induction heating

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Hernan

Fri Sept 04 2009, 12:31PM

Registered Member #1614 Joined: Wed Jul 30 2008, 03:08PM
Location: Argentina
Posts: 52

Hi guys, I decided to start this thread because I want to make a tank capacitor for my own LC series induction heater.
I Â´ve got 54 vishay sprague 715P capacitors rated at 0.0033uF 600Vdc/500Vac so the total tank would be 0.1782 uF with a work coil of 4.6uHy the resonant freq. is about 175Khz.
fortunately I could find the datasheet on web . the derating curves says that I could put a maximun of 400Vrms aprox at the resonant freq. so the maximun currrent for each cap would be:

with C= 0.0033uf and Freq 175Khz

Imax = Vmax/ Xc= Vmax . 2 . PI . Freq. C = 1.45 Amps rms each cap

1.45 . 54 = 78.3 amps for total
Are the above calculations correct?

my question is how can I solder them for proper current sharing between them and the possible setup for water cooling

I 'll estimate your comments

regards
HernÃ¡n
]sprague_715p.pdf[/file]

Gabriel35

Fri Sept 04 2009, 05:11PM

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

I think that if they will be immersed in water, a "S" or "U" setup will work nice, with acceptable space occupation.

Sulaiman

Fri Sept 04 2009, 08:51PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

General principles;
- Bus conductors should be as short and wide as practical to minimize inductance
- Each capacitor should 'see' a similar path to the load,
e.g.1 Good

e.g.2 Bad

In the second diagram the left-most capacitor will be stressed more than the right-most one, whereas in the first diagram all capacitors will suffer the same !

Proud Mary

Fri Sept 04 2009, 09:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Wouldn't it be less effort, and cheaper, to simply buy a capacitor of that size and rating?

uzzors2k

Fri Sept 04 2009, 09:41PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Not if you can get a deal on smaller capacitors in bulk, which is easy. A single capacitor suitable for induction heating, or DRSSTCs for that matter, are hard to find and cost dearly. I squeezed a quote out of Celem for a conduction cooled power capacitor, which I've attached. It cost as much as all of my projects over the last year combined. The 100 mini capacitors I used in my induction heater have worked alright, and only cost about 20 dollars with shipping.

]qut20007.pdf[/file]

Hernan

Fri Sept 04 2009, 10:43PM

Registered Member #1614 Joined: Wed Jul 30 2008, 03:08PM
Location: Argentina
Posts: 52

thanks all for the replies
I thought in two cooper discs and the caps between them but the radius must be 90mm to contain the 54 caps. but is a problem to find the discs.

I like the first scheme proposed by Sulaiman because is easy to build with cooper strips.

I agree with Eirik the cost of mine was u$s 15 for the 54 caps compared with u$s300 from one conduction cooled capacitor

But in future I plan to buy some of them

regards,
HernÃ¡n

GeordieBoy

Sun Sept 06 2009, 05:05PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I'd mount all of the PP/foil capacitors in parallel between two solid aluminium blocks. You could take the leads through the blocks and secure on the outside with mounting hardware. Seal the holes and fill the gap between the blocks with your circulating water.

If you keep the lead lengths as short as possible you will minimise the lead resitance and get what little conduction cooling you can through the capacitor leads. The problem is that capacitor dielectric (plastic) and typical capacitor packaging (also plastic) really don't allow the heat to get out, so most cooling will be through the leads.

That's why dedicated IH caps use special design to support conduction cooling.

-Richie,

Hernan

Mon Sept 07 2009, 03:11AM

Registered Member #1614 Joined: Wed Jul 30 2008, 03:08PM
Location: Argentina
Posts: 52

Richie, your idea to remove the heat of the dielectric of the caps is very good so I 'll need to use distilled water between the blocks ?

I thought to use a container with silicone fluid and submerge the caps on it. then put a cooper tubing with circulating water to cool the silicone like a heat exchanger but I don't know if this setup will remove the heat properly.

regards,

HernÃ¡n

Proud Mary

Mon Sept 07 2009, 03:21AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Vegetable dielectric oils such as ordinary kitchen sunflower seed oil, or rape seed oil, will do fine. (Google "vegetable dielectric oil" if in doubt!)

You could keep this cool with a Peltier device if you have the amperage to spare!

Finn Hammer

Mon Sept 07 2009, 05:03PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

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