FAQ - Ballasting a power transformer.
Posted: Sat Apr 26, 2014 4:34 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
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