BALLAST

Ballasting is a term that you will no doubt run into quite frequently in amateur HV work. The Ballast is something placed in a circuit to limit the current draw of the device being energized. When, for example, the output of a transformer is run in a dead short to ground or across a small air gap, the current will tend to rise until either the transformer overheats and fails or the overcurrent protection device (fuse, breaker) is triggered to shut down the supply circuit.

By Ohm’s Law (I=V/R), we see that, with voltage constant, resistance placed in the circuit will limit the current and indeed, a resistor may be used for this purpose. The resistor will dissipate the excess current as heat so it will need to have the appropriate power rating and will probably require heat sinking and active and aggressive cooling to function properly. The inverse relationship tells us that the effective current decreases as the resistance increases. Heating elements from electric clothes dryers, ovens, and water heaters, among other things, have been used as resistive ballast.

It is often preferable to take advantage of a property of inductors called ‘Reactance’ symbolized by a capital X. Inductive reactance is the ‘apparent’ or ‘equivalent’ series resistance of an inductor. For Inductors: X_L= ωL , where ‘ω’ is equal to 2 times π times the frequency (f) of the current (or 6.28f ) and L is the measured inductance in Henries. The value of X is reported in Ohms and its value may then be substituted into the Ohm’s Law equation above to determine the maximum current the inductor will allow to pass. For example, an inductor is measured with an LC meter to find that it has an inductance of 0.038 Henry (or: 38 millihenries). By the formula above, the inductive reactance is found to be: 14.33 Ohms in a circuit with a 60 HZ frequency. For the mathematically challenged, a useful JAVASCRIPT Reactance calculator can be found on Bill Bowden’s Website (along with many other useful features). Plugging this value into the Ohm’s equation, we find that in a constant 120V circuit, the current will be limited to a maximum of 8.37 Amperes. NOTE: that LC meters are not intended for iron frame inductors. Their small output voltage is not sufficient to magnetize the frame of the inductor and the reported value will be low. To determine the inductance of iron frame inductors, a test voltage is applied to the coil, using a Variac with an ammeter in series with the circuit and the value of the current at the known voltage is used to calculate the inductance via the formula: L=V/ωI. The referenced website also has a calculator for Ohm’s Law – Thanks Bill !!!!

The inductor, basically an electromagnet, will ‘hold up’ the excess current in its magnetic field rather than simply running it out to heat. The convenience of not having to bother with forced cooling of a resistor usually makes this the preferred method of current limitation. This is not to say that the inductor may not warm up, but this can usually be remedied with paralleled inductors of large enough mass that the cores don't saturate and get hot. If cooling were to become necessary, this may be accomplished by simply training a fan on it. Some have tried submerging the inductor in transformer oil, but that is not necessarily as helpful for heat sinking low voltage apparatus as it is for dielectric insulation of high voltages, unless the oil is also circulated and cooled.

Inductors may be built or salvaged from other electronic equipment. They may be built from a simple coil of wire on a spool (air core) or iron may be inserted into the center of the coil, or the core of a transformer (especially a BIG one) may be rewound with THHN to give an inductor (single winding transformer).

Inductors may also be created by putting the leads of a transformer winding in series within a circuit (the chosen winding should be rated for voltage and current of the circuit in which it will be used). The magnetically coupled winding may be left open or its leads may be shorted together. If left open, the inductance will normally be Very Large. Shorting the same winding will result in lowering the inductance, usually by several orders of magnitude. Variable inductance ballast may be produced from a simple arc welder by placing the power cord in series with your circuit, shorting the output cables together, and moving the shunts in and out of the core with the current control. Slide chokes and autotransformers (either Variac or multi-tap types)often make good adjustable ballast inductors. A very interesting device, the Saturable Core Reactor is a special class of adjustable inductor that is covered separately.

To get precise control within a specific current range, it may be advantageous to use multiple inductors in series, parallel or a combination of both.  As with resistors, the inductance (and by extension, reactance) of inductors in series is additive.  When in parallel, inductance of the total array is the reciprocal of the sum of the reciprocals of the individual inductances, or for ‘n’ number of inductors: 1/L_total = 1/L₁ + 1/L₂  . . . . . .  + 1/L_n.   On our power supply control deck, we used a series of 2 paralleled pairs of inductors to achieve our goal.  Here is the schematic and associated calculations.

Here is a picture of a 240 lb. dry transformer I am using to ballast a 10kVA electrical distribution tranformer (pole pig). It is holding up nearly 150 Amps in its magnetic field without ever even getting warm!