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How to calculate max power for a given secondary coil?

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FreakyG

Fri Jun 12 2015, 04:44PM

Registered Member #1734 Joined: Thu Oct 02 2008, 04:07PM
Location: Heemstede, Netherlands
Posts: 8

Hi all,

I have a secondary coil which I wound some while back and would like to use in my first DRSSTC project, I thus am trying to work backward as normal but seem to be struggling with finding the info I need.

What I am trying to do it calculate the max possible power rating for my coil given that I know the conductor diameter and insulation thickness of the wire, the number of turns and the physical parameters of the former thus I have the fres of the coil, DC reactance etc. (all with various toploads)

Now I need to design a primary circuit with coupling etc. and cannot for the life of me figure out how to go about deciding on the design parameters.

I would seriously appreciate any assistance you can give in the form of documentation, calculations, software, equations, etc.

Thanks in advance
Grant

Blackcurrant

Fri Jun 12 2015, 08:33PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

If you could guess at the maximum safe wire temperature you could simple say max power = I^2.R
I'm sure if you look up radio ham coils they probably know some good figures for this. A coil that's running with on time then longer off time will be able to take more. You could use the average power.

I think you need to figure out how much current your switches can take before designing the primary tank circuit
look at some other ready built ones to get an idea

Hazmatt_(The Underdog)

Fri Jun 12 2015, 10:11PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Okay, with TC's you might think its DC R, IT's not.

You will want to calculate the skin depth and the effective AC resistance of the wire. It will come out to something like 1.4 x DC R.

Then because this is an AC system, the current will be the RMS current, squared x AC R, for one frequency. AC R will change over the sweep of frequencies.

How do I find I peak or I rms of my TC?

Maximum current that is cycling through the system will be at the base lead. Insert a CT (10:1) or some such, around the base lead. Rectify the current and filter it. If you construct a circuit in a controlled environment that can help you determine the actual scaling factor, you can then get the measured current to calibrate to a correction factor, and thus build a calibrated base current monitor.

Taking it a step further, A shunt resistor can both take off voltage and current. Those two values sent to a logarithmic amplifier circuit (or build a VSWR meter) can square the inputs, V x I and give you the true RMS power at the base of the coil.

I was going to do this but I'm not there yet, too tired after work.

But directional bridges are pretty easy to build too, and you could go that route as well. Directional coupler to an AD8307 can be calibrated to give you a wideband RF power measurement easily.

Sulaiman

Fri Jun 12 2015, 10:33PM

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

my 2c worth ... very difficult.

. as above 'skin effect' and more significantly 'proximity effect' cause the current to mainly flow in parts of the surface of the wire,
I think that a factor of 1.4 is too low, more like double that.

. the current waveform is sinusoidal, increasing in amplitude with each cycle, in bursts, then a cooling period.
and since heating is proportional to Isquared the later part of the waveform envelope has much greater heating power than the beginning
so far all calculable if the waveform and ac resistance are known

. how long (how many ac cycles) will the drive be on ?
. how quickly will the current 'ring up'?
. how long will the current be near constant?
etc.

the only predictable 'calculation' of heating is to copy an existing design

the good news;
I have seen reports of all kinds of failure of a secondary, I can't remember one due to I^2.R heating
(others have more experience)

loneoceans

Fri Jun 12 2015, 10:53PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Agree with Sulaiman, it's very difficult to calculate the 'max power', given that is a very loose constraint. What do you exactly mean by max power?

As mentioned, I^2Z heating even with skin effect taken into account will not at all give you a good approximation of the losses. Current flow and distribution will also be affected significantly by many other factors, such as geometry of the primary and secondary coil, circuit configurations, coupling, etc (what Sulaiman talked about with proximity effects). Usually some geometries can lead to severe heating on the bottom of the secondary coil much more so than what you would expect from simply the current flowing etc, and so on. So even if you know the power dissipated in the secondary, heating is not even and its likely that the bottom of the coil will see significantly more heating than the rest of the coil.

In practice for normal DRSSTCs though, heating of the secondary coil is really usually not a problem. Since this is your first DRSSTC, I suspect that the secondary coil is very unlikely to be the bottleneck in your system, given that most DRSSTCs are actually very low duty cycle, and usually people have more problems with their transistors instead. When you start building more really powerful ~CW-type DRSSTCs running hundreds or thousands of RF cycles, then thermal problems and heating start to become much more relevant.

Do you know JavaTC? It's a great starting place to design the coil and it gives you a good coupling value recommendation based on your secondary coil geometry.

FreakyG

Sat Jun 13 2015, 02:24PM

Registered Member #1734 Joined: Thu Oct 02 2008, 04:07PM
Location: Heemstede, Netherlands
Posts: 8

Thank you all for your replies!

I agree that my question regarding 'max power' was very probably way too loose, I guess what I am wanting to do is rather attempt to formulate parameters which I can use to design my tank circuit (choice of IGBT's etc) that would give me optimal streamer output without 'overloading / overheating' my secondary coil. Assuming that I am going to invest an enormous amount of time (and cold hard cash) on this most pleasurable endeavour I would hate to waste time and resources building a driver that was way too powerful or indeed was so underpowered that it resulted in disappointing performance.

I am by no means an academic in any sense of the word so please forgive any bad terminology or any undue oversimplifications ...

It just seemed to me that for a technology that is now over 100 years old there 'should' be well documented methods of characterising any given coil such that an 'optimal' driving circuit can be designed around some form of minimum and maximum figures.

Ok, so it seems that I am going to have to do a whole lot more reading and learning to do to gain a better understanding.

(any recommended sources? best resources for learning about high frequency low duty cycle bridges?)

Loneoceans, yes I know JavaTC extremely well as I built a hugely successful SG coil back in SA which consistently gave 7 foot streamers. The only parameters I cannot get it to give me obviously are all centred around what power input is required for given streamer length lol

OK so just for my edification ... what would your 'best guess' suggestions be for a driving circuit for an exceedingly well constructed secondary using heavy insulated 32AWG wire, 3" diam 15.7" height at 1840 turns with an fres of 295Khz bare and 152Khz with topload?

Loneoceans, I love your new dual bridge inverter for your QCW design, could this be an option for me for this coil?

Hazmatt_(The Underdog)

Sat Jun 13 2015, 03:12PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If you want to know more about coils themselves, pick up a couple of books, "Radio Engineers Handbook" By Terman, and "The Theory and Design of Inductance Coils" By Welsby.

Uspring

Sat Jun 13 2015, 05:42PM

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

An estimate of the power dissipated in the secondary can be got from the Q of the secondary, which JavaTC calculates. There are 2 Qs for the secondary, one, which is determined from the winding resistance (javaTC calculates that) and another one, which is determined by the arc load. The fraction of total power going into the winding is roughly:

Qloadedbyarc/Qjavatc

Typical vallues of Qjavatc might be around 200 and for Qloadedbyarc about 5. A 2kW coil would dissipate about 50W in the secondary. These values are very much ballpark. Your secondary will also be limited by flashovers.

It just seemed to me that for a technology that is now over 100 years old there 'should' be well documented methods of characterising any given coil such that an 'optimal' driving circuit can be designed around some form of minimum and maximum figures.

The missing part is a realistic arc model. While it is in principle understood how arcs work, a quantitative description is a mess of plasma physics, ionisation crross sections, mean free paths, thermodynamics, heat conduction, gas physics etc.
Another approach is to model arcs on a semiempirical basis. This has been done, but it suffers from a scarcity of actual measurements of arc voltages and currents of TCs to fit these models to.

Hazmatt_(The Underdog)

Sun Jun 14 2015, 05:40PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

if you want to see how the arc is loading the coil, you would need to use a VSWR measurement, which would give you an average of how the arc loads the secondary.

I would estimate that the power would be reflective up to 50% of the input (to the coil, maybe 40% of actual input power) because I don't think it would be drained by more then half the input power during an arc cycle. But that theory will have to be tested in the future when I have some time.

OR, you guys can build a dual directional coupler yourself and make the measurement, they're fairly simple to build, I'm just not sure what power level to build one to. It's hard to tell the actual power delivered to the coil, probably at least a 1kw bridge for safety margin.

This one is a 20dB coupler for typical measurements. A TC coupler would probably need to be a 40 dB coupler.

Uspring

Mon Jun 15 2015, 09:46AM

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

With an excuse to the OP, since this is getting off topic:
For an arc characterisation you'd need a measurement of the arc voltage and current. The main difficulty is, that the arc current would have to be measured between the toroid and breakout point. The only reliable way to do that I know of is to put a battery powered DSO there.
Top load voltage can be indirectly inferred by a measurement of the current between the secondary top and toroid.

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