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Testing and Using Power Transformers

Tips including a good procedure for drying a high-voltage power transformer

So you have an old tube-type transmitter as a backup. It ran perfectly when you last turned it on two years ago to substitute for your then-ailing solid-state main transmitter. Now you need it again.

Filaments come up, high voltage comes up for a few seconds, and then BANG! Off it goes.

Ouch, what happened? Answer: There was an arc-over in the high-voltage power transformer.

Gosh, it worked fine last time! What transpired in those two years?

Answer: Moisture crept into the transformer windings, just waiting for high voltage to be applied so it could create an arc-over path.

To prevent this scenario, run the transmitter for a few hours every six months to heat the components and drive out moisture. You’ll smell it too … something like an old electric heater being started for the first time in the fall. Also best to check the inside of the transmitter for mouse houses or snakes before energizing with power. You’ll often be amazed at what you find. There might even have been a roof leak that dropped water inside the transmitter.

Now you go looking for a replacement high-voltage transformer to fix the old transmitter. Your engineer friend 60 miles away says he has the exact replacement from the same model transmitter that he “parted out” a couple years ago. Good news.

You get the transformer, but you are smart enough not to install it right away. Instead, you warm it up to drive out moisture.

How do you do that? Well, you can put it in an oven and heat it to 150 degrees for a few days. Your wife might not appreciate that, especially if the transformer weighs in at a hefty 400 pounds. You can blow hot air on it with an electric heater, but there is a better way.

Fig. 1: Heating transformer coils with a battery charger and variable AC power supply.
Photos by Mark Persons

Fig. 2: Testing high voltage using a variable AC power supply on a primary winding. A good procedure for drying a high-voltage power transformer, even if it has not doused been in water, is to put a traditional non-regulated car battery charger on the primary winding (Fig. 1).

Yes, this is direct current, but the transformer is not being used as an AC transformer in this case. Hopefully the current will be in the range of what the charger can deliver, probably 10 amperes or less. Older chargers have a 6-volt output, which will reduce the current. I use a Sencore brand PR57 AC Powerite variable AC transformer to set the battery charger’s input voltage to get the output current I want. Let the charger’s current warm/heat the transformer for a day to drive moisture out. Make sure the current you use is less than the maximum allowed amperes for that winding. If this is a three-phase power transformer, do the same for all three poles of the transformer.

Now hook the transformer primary to a variable AC transformer, as mentioned above (Fig. 2). For safety, use a three-conductor cable with the ground connected to the transformer’s core. Slowly run the up the voltage while measuring the secondary voltage with a high-voltage AC meter. Likely your variable AC supply will only deliver 0–130 VAC. Assuming the transformer has been built for a 240 VAC input, the next step is to connect the primary to 240 VAC for a full-voltage test.

Perform this drying procedure with every high-voltage transformer, modulation transformer, filament transformer or power supply choke that has been sitting in storage for any length of time. It is common for moisture to be “drawn up” from a concrete floor into a transformer. Components like this should be stored sitting on wood.

Sometimes a transformer winding will short to its iron core. Use an ohmmeter to verify it hasn’t. A Sencore LC53 Z-Meter can apply 600 volts between a winding and the core. That is better than a regular ohmmeter that puts a mere 10 volts or less across its leads. The PR57 Powerite has a leakage test built-in that will do the same test, but at 120 VAC.

Let’s say you are installing an old transmitter into service after it has been stored.

Check the transmitter with a fine-toothed comb to locate and clean previous arc-over spots. You will want to clean dirt, dust and grime off high-voltage components, especially the insulators. Not a bad idea to heat transformers for a few days. Look for and replace any burned or obviously heated components. The idea is to restore the transmitter to its former glory. Check for and replace any failed light bulbs. This will help in troubleshooting when things do not go well.

If you are changing frequency on a transmitter, start by testing on its original frequency and power level into a dummy load. Make sure it is running properly before attempting a frequency conversion. This will eliminate difficult questions when it doesn’t operate correctly at the new frequency. While you are at it, check for manufacturer’s bulletins on updates to the design.

Fig. 3: When working on equipment, use a grounding rod to ensure there’s no high voltage present. If the transmitter was built before 1980, look to see if oil-filled transformers and/or oil-filled capacitors have a “Non-PCB” label. Best not to put high voltage on components filled with PCB (polychlorinated biphenyl) oil. Most of those components were replaced 25 or more years ago when this substance became a nationally recognized health hazard. Studies show a link between PCB exposure and cancer. There are companies that specialize in disposal of PCB filled components.

On another matter, every transmitter should have at least one high-voltage shorting rod (Fig. 3). If a transmitter doesn’t have one, buy or build one. Mine has a 12-inch-by-1/2-inch diameter PVC handle with a 6-inch-by-1/4-inch diameter metal rod at the end. A heavy wire connects the metal rod to chassis ground in the transmitter. Always, always, always use one before sticking your hands on high-voltage components that you “assume” have no voltage at the moment. A loud BANG from a shorting stick on high voltage will tell you that your assumption was wrong.

Be safe! There is a shortage of broadcast engineers and we can’t afford to lose any more. It makes perfect sense.

Mark Persons, CPBE, is a longtime contributor. His website is