SSTC Primary Heating / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

SSTC Primary Heating

1 2 next

Move Thread

LAN_403

Sigurthr

Wed Dec 05 2012, 01:40PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Hey everyone!

I just recently modified my large 1.5KVA SSTC to allow for interruption/modulation and also doubled the filter capacitance on the dc bus. In doing so I took the primary off for the first time since initial construction and was surprised to see that the primary (600V 200degC rated insulated 10ga made of twisted 22ga conductor) got hot enough to warp a 0.5mm polypropylene insulator I use to electrically and mechanically insulate the primary from the secondary. I always run the coil with forced air cooling over the primary because it feels warm (~120F) to the touch after a minute or so of low power (500VA) operation. The fets run cool with tiny heatsinks when under 600VA, but the primary always heats up over 100VA or so. At full power I can only run it for 30sec before the primary gets too hot to touch.

The DC resistance of the primary is quite small, my DMM says less than 100mOhms, and since it is a halfbridge driven coil there should be no DC component flowing through the primary. Inductors aren't supposed to waste energy as heat I thought. Likewise, this isn't a DRSSTC with hundreds of amps being pumped through the primary due to a resonant capacitor... so what the heck is causing so much primary heating? With the variac cranked the coil draws about 13Arms of current at 120Vac input, and there is only 1880uF of filter capacitance, so the halfbridge is not seeing the full ~180V peak, there is definitely a large amount of ripple.

Any suggestions for ways to reduce thermal power loss?

dex

Wed Dec 05 2012, 02:17PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

the simplest solution is to wind new primary with thicker wire or litz wire.
1.5 kw of high frequency continuous power isn't joke.things get hot .

Steve Conner

Wed Dec 05 2012, 02:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

At Tesla coil frequencies, the skin depth in copper is very small. The AC resistance of a thick conductor is much higher than the DC resistance, so it gets a lot hotter than you'd expect.

That's one reason why copper pipe is popular as a primary conductor. The copper in the middle hardly passes any current, so you can save money by leaving it out.

Skin effect also happens in stranded conductors, unless the strands are insulated from each other as in Litz wire.

Sigurthr

Thu Dec 06 2012, 08:34AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Wow, so it's just pure AC resistive heating. I was expecting that I had missed something more reactive at play. Good to know then, thanks guys!

Litz wire it is then, since there's no way to preserve the tight coupling with stiff uninsulated tubing (not to mention an exposed shock hazard). Anyone know of a good place to order Litz wire online? Ill start searching in the meantime.

Dr. Dark Current

Sun Dec 09 2012, 09:00PM

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

It may also be eddy current heating, especially in the outer turns, where the magnetic fields of adjacent turns don't cancel out.

Polonium210

Sun Dec 16 2012, 07:01PM

Registered Member #3450 Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61

13Amps @ 120VAC???
Is your primary vertical or flat spiral? You may have a coupling issue not allowing enough energy to be efficiently transferred to the secondary meaning all that energy is being dissipated as heat.
Also adding primary turns will decrease current draw.
You could also try a primary made from #8 stranded audio power wire.

Sigurthr

Tue Dec 18 2012, 07:53AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

It is a vertical primary, with very tight coupling; modeled at almost 0.54k .

Secondary turns are already kind of high, over 1500t of 30awg on a 4" PVC coil form.

*accidentally wrote primary turns

Uspring

Tue Dec 18 2012, 04:20PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Primary turns are already kind of high, over 1500t of 30awg on a 4" PVC coil form.

You probably meant your secondary. If that has a too large inductance you might have an impedance mismatch between your TC and the arc load. That will limit the power, that is transferred to the secondary and consequently heat up the primary. Whether you have a mismatch depends mostly on your primary-secondary inductance ratio. It is hard to tell without knowing your coil specs.

Polonium210

Wed Dec 19 2012, 01:39AM

Registered Member #3450 Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61

Sigurthr wrote ...

Primary turns are already kind of high, over 1500t of 30awg on a 4" PVC coil form.

Sorry, I meant adding primary turns.

Sigurthr

Wed Dec 19 2012, 07:44AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Oops, haha! Yes I meant secondary turns. I'll fix the post, heh.

Pri: 10t of insulated 10ga on secondary coil base.
Sec: ~1530t of 30ga heavy poly enamel on 4" PVC form (actually 4.3" OD)
Toroid: 12" dia with 3" dia ring

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob