Note: this is the text of an article that originally appeared in Signs of the Times magazine, January, 2003.

Transformer Loading by Secondary Voltage Measurement

By Telford Dorr

When a neon transformer is loaded with the proper amount of correctly processed neon tubing, and the wire length between the transformer and tubing is kept short, the transformer will operate within the manufacturer’s intended ratings. This will lead to the maximum possible operating lifetime for the transformer.

Why is this? When a transformer is loaded with too much tubing, it operates at a higher secondary voltage than is intended. Along with this, high frequency oscillations can occur (especially with neon-filled tubes), further stressing the transformer with high voltage spikes, leading to secondary winding breakdown. Operating with too little tubing causes excessive winding current, leading to overheating and winding failure.

The problem becomes how to determine whether the loading is correct or not. When a transformer is correctly loaded, it will operate with the correct secondary current, at the correct secondary voltage. It will draw its rated power from the line and operate at its rated power factor. While we may initially estimate the correct tube load using standard transformer loading charts, we should always double-check the actual loading by measuring as many of these parameters as possible.

Some measurements are more sensitive to loading errors than others are. For instance, the classical way of checking loading is by measuring the operating secondary current and comparing it to the short circuit secondary current. Typically, a transformer rated at 30 ma secondary short circuit current operates at 24 ma when properly loaded. This represents a 6 ma current change out of 30 ma, or 20%.

We may also determine proper secondary loading by measuring the loaded secondary voltage and comparing it to the open circuit unloaded secondary voltage. When properly loaded, most US made transformers operate at about one half of their open circuit voltage. This represents a change of 50%. As you can see, checking loading by measuring the secondary voltage is more sensitive to change than by measuring secondary current, and you don’t have to break the secondary circuit to make the measurement.

I first learned of this method of loading determination from a good friend in the Netherlands, Dirk Boonstra, who is a strong advocate of the method. Neon transformers seem to vary a bit more in their design operating parameters in Europe than they do here, and not all are designed to operate at the 50% secondary voltage point. The ratio of the operating secondary voltage to the open circuit secondary voltage is referred to in Europe as the “G” factor. It is the same numerically as the power factor - the cosine of the angle between the total power (volt-amps) and the ‘real’ power (watts) - of the transformer. This is determined by modeling the neon transformer as an “ideal” transformer with inductance in series with each secondary output terminal, and calculating the voltage drops (and their phase angles) across the neon load and the internal inductance. This discussion of power factor applies only to regular magnetic neon transformers of low power factor, or to the uncorrected power factor of high power factor types.

For example, if the rated power factor of the transformer is 0.50, we would expect the properly loaded secondary voltage to be approximately 0.50 times the open circuit voltage. Unfortunately, in the US, transformer rating plates typically don’t list the power factor, so we have to use whatever information the manufacturer provides in their literature. For example, Allanson specifically states that they recommend loading to 50% of the rated secondary voltage. The general rule of thumb is to load to approximately 50% of the rated secondary voltage.

What equipment do you need to perform this measurement? You need a good DVM and a matching high voltage probe. Ideally, the DVM should be a “true RMS” type, although a regular one will work by applying a correction factor. [See sidebar: “True RMS”] The high voltage probe typically reduces the measured voltage by a factor of 1000 before sending it to the DVM. For example, a voltage of 7500 volts AC applied to the probe will display on the meter as 7.5 volts AC.

One point to be aware of: some high voltage probes are sensitive to the high voltage electrical field around a transformer, and the reading may vary depending on how the probe is held. I try to hold the probe such that it points directly at the transformer (as opposed to sideways, parallel to the HV wiring). Watch out for this effect.

The voltage measurement is always made relative to ground, specifically the transformer case that you have bonded to a good electrical ground as required by code. The voltage you expect to read is a function of the internal transformer construction (usually specified on the rating plate). Specifically, you need to know if the transformer secondary is mid-point grounded, grounded on one end, or ungrounded. Transformers that have fully isolated secondary windings (e.g. Magnetek isolated types, and small core and coil units) cannot use a regular DVM and HV probe to check their loading - you need a special high voltage voltmeter for safety reasons. Note that this method is fully usable with NEC 600-23(b) (or UL 2161) transformers, as the current draw by the DVM is negligible and should not cause tripping. This method is not reliable with solid state transformers unless you have special probes and meters that can handle their operating frequency.

As an example, let us assume we have a sign which contains five lengths of 15 mm blue-filled tubing, each tube being 10 feet long, connected to a 12,000 volt 30 ma transformer which has a mid-point grounded secondary winding. Each set of electrodes adds about 1 foot to the electrical length of the tube. Thus, we have a total of 55 electrical feet of 15 mm tubing connected to this transformer, which the standard transformer loading charts indicate is the proper load. We can check this by measuring the voltage at each secondary transformer hub. We should read approximately 3,000 volts AC (½ of ½ of 12,000 Vac) at each hub. This means the tubing is operating at 6,000 Vac end-to-end, or at one half of the rated open circuit secondary voltage. Failure to operate at this voltage indicates that there is most likely something wrong with the tubes – incorrect processing, wrong fill pressure, etc.

Does this mean you should abandon the time-honored method of loading by secondary current in favor of secondary voltage? No, you should not. You should do both! Why? Because it’s not an ideal world out there, and there are other factors which can (and do) influence loading other than tube length. The most problematic of these is secondary wiring capacitance. By measuring both current and voltage, and comparing them to the expected values, you can get a better picture of the true nature of the load seen by the transformer. For example, excessive secondary wiring capacitance tends to cancel the current limiting inductance of the transformer, causing excessive current to flow. If you just used the secondary current as your sole determination of loading, you might conclude that, because the current is too high, the transformer is too big, and install a smaller one to compensate. This would most likely lead to operating the new transformer at too high a secondary voltage, leading to secondary breakdown. By loading by voltage measurement and then checking the operating secondary current (at the transformer), you could deduce that the transformer is the correct size but that there is too much wiring capacitance. You cannot see this using only one measurement method.

The bottom line is that when a transformer is correctly loaded, all measured parameters should be correct at the same time. Having one parameter correct at the expense of another ignores an underlying problem of some kind, which will most likely lead to future component failure and a warranty service call.

Sidebar: True RMS, and other AC voltage types

When we measure AC voltage, we need to specify what type of voltage it is. This is because with alternating current, the actual voltage does not have a fixed value, but varies with time. We have created several named voltage types: peak, average, and RMS, each which describe a specific characteristic of the actual AC voltage waveform.

The most well known of these is RMS (or root-mean-square) voltage. This is the numerical value of voltage that will cause the same amount of power to be dissipated in a resistive load that the equivalent value of DC voltage would. The RMS voltage of a sine wave is 0.707 times the peak voltage (the voltage of the highest point of the waveform). The average voltage value of a sine wave, on the other hand, is 0.637 times the peak voltage. This is the value a mechanical voltmeter or regular DVM actually responds to, although it typically is calibrated to indicate the RMS value. This is fine when measuring a sine wave, because the correction factor for the meter is a part of its calibration. On the other hand, for a waveform like a square wave, the RMS, average, and peak voltages all have the same value. For this type of waveform, you have to apply a correction factor to the meter reading, thus compensating for the internal meter correction, to find the actual RMS voltage.

Why should you use a true RMS type of DVM instead of a regular one? This is because a true RMS meter has special circuitry that responds to the true RMS voltage, and thus displays the correct voltage regardless of waveshape. No correction factor need be applied. As the voltage across an operating neon tube is more of a square wave than a sine wave, a non-RMS meter will read incorrectly as the internal correction factor is wrong for this waveshape. If we multiply the indicated voltage by 0.9 (or 0.637 / 0.707), we can correct the reading. See your DVM manual for more details on this.