FAQ - Ballasting a power transformer.

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Richard Hull
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FAQ - Ballasting a power transformer.

Post by Richard Hull » Sat Apr 26, 2014 8:34 pm

In a homemade fusor's linear, low frequency, mains supply, various transformers can be used.

The term ballasting refers to installing a current limiting device (usually a resistance) in either the primary or secondary of a transfomrer that is expected to receive sudden heavy loads that might damage the transformer, itself, by exceeding its ratings or upset the electrical mains and their circuit protection (fuses or breakers).

The fusor, being a vacuum device, can see current surges of tremendous range. This is due to the fact that as the voltage in the rarified gas environment in raised, the high voltage circuit's impedance goes from virtually infinite to a dead short as the glow is struck or, if advanced too fast, proceeds to a townsend or even arc discharge. For the amateur to maintain some degree of safe contol over the circuit, some form of ballasting is needed.

The internally shunted trasformer...............

The self limiting or magnetically shunted transformer is the choice for the demo fusor power supply. These high voltage transformers can not be short circuited to the point of blowing any fuse or straining the capacity of a variac or wall outlet. They are effectively, impedance limited and in most all versions are limited to 20ma (oil burner type), 30-60 ma, (neon sign type) or even 5-10ma, (odd impulse type oil testers and very small electronic transformers). With the foregoing understood, there is no reason at all to current limit or ballast such transformers.

The nature of these transformers is to lower their secondary voltage as the load increases. (see FAQ on these type transformers.) This is not good for those doing fusion as a good source of steady high voltage is needed right up to the limit of the transformer's rated voltage and current.

The large power transformer............

The non-shunted, power transformer is needed to do fusion when constructing your own fusion capable high voltage supply. Such transformers will destroy themselves and or blow fuses, pop circuit breakers and otherwise tax the circuit they are in if they are not ballasted against massive current transients common to the operation of a fusor doing fusion. These are often high voltage transformers found in radio transmitters, microwave ovens, x-ray machines and power distribution systems, (pole transformers, potential transformers, etc).

For continous, massive, rapidly changing loads in the kilowatt range, it is best to ballast the trasformer in the primary circuit of the transformer. This can be done with large resistive loads, such as electrical heaters in series with the primary or a magnetic choke or variable inductance. No fusor has massively changing loads in the kilowatt range. Thus, never ballast a fusor's transformer in its primary circuit.

It is far better to ballast a fusor in its high voltage secondary circuit. using a resistor of between 10,000 and 60,000 ohms. The wattage of this resistor needs to match the power expended or expected in the running fusor on a 1/10 basis. 600 watts of power in the fusor needs a 60 watt ballast resistor.

Let us say you have a 60kv supply but plan on using only 50kv and have a run current of 15ma. That is 700 watts so a 70 watt 60,000 ohm resistor will be needed. A sudden transient of 50ma will drop 3,000 volts across the resistor which is 150 watts! For a short period, the resistor can hand this, allowing the operator to adjust for the surge using a variac or similar primary circuit control device. By placing the ballast resistor under oil, it will be kept cool. In the case of an old xray supply or distribution transfomrer, it might be placed in the oil tank with the transformer.

The actual value in ohms of the resistor is not critical. It is a strain relief for the delicate secondary in your precious HV transformer. 100k ohms is rather high and extreme and 20k ohms is a bit on the low side. For a 40kv supply I would chose 50-60k ohms for at 10ma fusion current it would only drop about 400-600 volts out of the 50kv applied in normal running. If you tried to draw an ampere in an excursion eventthe voltage would drop to below extinction turning the fusor off or throwing it into relaxation oscillation mode until you back off the variac or controller.

Such power resistors were once quite common, but are a rather rare item today. Hamfests, techfests and old electronics surplus stores are about the only inexpensive source for these large resistors today. Check those sources out.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Ballasting a power transformer.

Post by Dennis P Brown » Sun Apr 27, 2014 10:21 am

So, one could use Nichrome wire as the resistance source? If so, then an aquarium heater might serve as a resistor? These devices come in many different wattage ranges of 50, 75, 100, 125, 150, 200 watts and higher; disassembling one of these units is trivial: they have a ceramic core wrapped with nichrome wire that would appear to be what one would need. Just remove the core from the glass housing, disconnect the electronics and switch. The wrapped core would be available. Then submerge this core/wire assembly in oil. Considering the voltage (even in oil) one might buy the larger 300 watt unit and remove all the wire from the core (wrapped too close together), cut the wire to the length required to achieve the desire wattage, then re wrap the core with greater spacing between the wire loop on this ceramic core.

These are easily obtained at local pet stores or on line and are rather cheap. Hamfests aren't always available when one needs one nor does every area have one. Fish keeping is a very popular hobby. Even wally-Mart carries these devices

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Re: FAQ - Ballasting a power transformer.

Post by Chris Bradley » Sun Apr 27, 2014 11:15 am

It should also be explained that a resistive ballast must be capable of bearing the full supply voltage. Imagine that you have a 10kohm resistor in series with your fusor. The fusor is 'unlit' and high impedance. You apply 10kV. That 10kV is now across the fusor and there is virtually no voltage across the resistor, because the fusor is the highest resistance in the circuit and little or no current is flowing.

In a flash (quite literally!) that fusor could light up a plasma and now may be the lowest resistance in the circuit. At least, at the moment the plasma enters a discharge mode its resistance will drop to its lowest point. Now it is the resistor bearing up the voltage across the circuit, that is 10kV.

Momentarily, the resistor will see 1 amp at 10kV. It will be a fleeting moment of all the capacitive and inductive energy within the circuit discharging, limited only by the inductance of the circuit (hence, one should aim to ballast the secondary circuit, in series with the fusor). The resistor may really see 10kW of near instantaneous power for a fleeting microsecond before the circuit energy dissipates, but the power tolerance of the resistor is a matter of integrated 'energy', not simply power, so it should be fine (so long as it can tolerate the lower sustained power).

However, what is not fine is if the resistor is a physically short, unencapsulated type wherein that instantaneous voltage across it, 10kV worth, can cause an arc from one lead to the other. You may find resistors rated to 10kV that are only a couple of cm long. These are not suitable. The resistor body may be rated for 10kV but there will likely be breakdown between the leads of the resistor. Such ratings presume the user understands this and encapsulates the resistor, or puts it under oil. I would generally aim to ensure the leads of an unpotted ballast resistor (or any component, come to that) are spaced apart by at least 5cm for each 15kV. This is a minimum, and you may still experience significant corona at these separations, but it should be sufficient to prevent arcing if it is caused to act momentarily as a circuit protection.

The ballast resistor also serves another curious purpose in regulating the circuit. One might imagine that once a fusor discharge sparks up that the ballast resistor will 'cut in' and promptly kill the newly formed plasma. This is not what happens. Because the resistor is passive it will instantaneously share out the power going into the circuit, and in doing so act as a current limiter both as protection for the circuit, but also to control and regulate the current passing through the fusor. This is a very complex function that is unlikely to be well controlled by an operator, because the circuit behaves as it does and the impedance of the fusor is complex and reactive, but it is possible to slowly increment the voltage across a circuit composed of the fusor and a ballast and cause the fusor to very gradually enter discharge modes which are not 'glow-discharge', an then on into the regime just at the edge of glow discharge. In such circumstances the voltage across the resistor will remain low. But the voltage rating of the resistor should not be chosen on the basis of the current-limiting function (where it sees a low potential across it), it needs to be rated for the circuit protection function (the full voltage).

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Re: FAQ - Ballasting a power transformer.

Post by Chris Bradley » Sun Apr 27, 2014 11:43 am

Dennis P Brown wrote:So, one could use Nichrome wire as the resistance source? If so, then an aquarium heater might serve as a resistor? These devices come in many different wattage ranges of 50, 75, 100, 125, 150, 200 watts and higher
These are generally low resistances. In general, you may 'size' your ballast resistor according to the 'circuit limiting' function I describe above. Let's say we have Richard's example of a 700W 50kV supply for a theoretical 14mA. If we chose the highest resistance value in the suggested range you have there, 50W, and I'll presume that is a 110V rating, that's a resistance of about 250ohm. Now, at 50kV, a 250ohm resistor would only be able to regulate the current to below 200A. This is somewhat more than the 14mA capability of the supply! No good!

It is therefore the function of the ballast resistor to avoid a current drain, such as 200A, being attempted. Instead, we size the ballast so that if 14mA is being drawn at the full power of the circuit the resistor will have the due power rating. Let us aim for a 100W resistor power rating. P=I^2.R, so we have an R requirement of 100/(0.014)^2 = 500kohm.

Now in practice we do not want to have such a high resistance because it sucks up some of the applied voltage. 500kohm at 0.14mA is 7kV out of the 50kV applied. Although this would be a 'bullet-proof' ballast resistor for the circuit, so long as you are content with a 7kV drop in your circuit, it is largely unnecessary due to the transient nature of the high current modes. So let's look at limiting the ballast to drawing off only 1.4kV, which is more minimal out of 50kV. Then we have a resistance of 50kohm.

That 50kohm resistor could, in theory, allow a pulse of 50kV^2/50kohm = 50kW through. However, because the resistor will cause the circuit to be 'pulled down' due to the power limitation of the supply, the maximum current seen would be P=I^2.R, so I= 120mA. The resistor would then absorb the 700W of the supply until the plasma settled back down to a stable discharge.

So in the various states of function attached to a 700W supply, this 50kohm resistor may face an instantaneous 700W (though, possibly higher if there is capacitance in the circuit), or a continuous 10W (I^2.R at the maximum rating). Sizing at 10W only would risk burning it out when it needs to fulfil the circuit protection function, whereas sizing at 700W would be 'wasted' as it will spend most of its life in the current limiting function. One then makes a pragmatic decision about what power rating is needed, versus availability and cost.

I hope this describes the sort of iteration one might want to do when sizing up a suitable ballast resistor value and power & voltage ratings.

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Re: FAQ - Ballasting a power transformer.

Post by Dennis P Brown » Sun Apr 27, 2014 12:09 pm

Excellent post Chris and discussion of the problem and issues on resistance ballasting a HV transformer; I was mislead by the post about using a heater element - obviously, those are not acceptable.

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Re: FAQ - Ballasting a power transformer.

Post by Richard Hull » Sun Apr 27, 2014 6:10 pm

Nichrome of the type in heaters is a PRIMARY ballast and should never be used as such unless you are drawing kilowatts rather continuously.

High ohm resistors of the type suitable for secondary ballasting in a fusor are normally about 1.25 inches in diamter, 6 to 12-inches long and fully porcelin potted turn to turn over its entire length. Only old time electronics buffs, radio amateurs and hamfesters know what one of these looks like. They all contain thousands of turns of wire the width of a human hair and are of a special alloy of nichrome.

Those with fusor experience will know that 10k-100k ohms is the range you will need with 20-60k being the most often used. A variac and a trained user will never burn one of these resistors out. I have accidentally melted two grids in fusor IV during nasty excursions since 2004, but have never replaced that original 63 kilohm 50 watt wirewound that I tanked within the x-rAY transformer case back in 2002.

Most such resistors, if found for sale today in a catalog, may run nearly $100.00! At a hamfest I find them for $1.00 to a maximum of $5.00. They are leviathans of another age when wasted power was never and issue. Yet they are our best friend in controlling those transients so common to our work.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Ballasting a power transformer.

Post by Dave Xanatos » Wed Apr 30, 2014 8:30 pm

I have three Clarostat 100W vitreous enameled resistors available to me, 40k, 50k and 60k, they're about 6.5" long by 3/4" diameter - would these work as transformer ballasts or are they physically still too small?

Thanks!
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Re: FAQ - Ballasting a power transformer.

Post by Richard Hull » Thu May 01, 2014 7:46 am

Yes, they are fine and are the type I use on fusor IV.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ - Ballasting a power transformer.

Post by Johannes westman » Mon May 26, 2014 6:45 am

Just a question about this. When I do the calculations using an ohm's law calculator the calculations come out in the megaohm range for resistors - where am I going wrong with this? I'm calculating for about 10-15 mA, and the calculator gives me somewhere between about 2.7-4.0 megaOhm.

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Re: FAQ - Ballasting a power transformer.

Post by Chris Bradley » Mon May 26, 2014 8:02 am

I explained it in my post. Did you read it, or is there something that needs further explanation? No practical resistor value is a 'bullet proof' ballast. You have to understand what the ballast resistor does, and then decide on your own compromise.

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