Converting DC Voltages

Dear Control Engineering: At the ABB Automation and Power World, there was a lot of discussion of dc power distribution. How can that be practical if transformers can’t be used to change voltages?

If you’re old enough, you may remember the days when having a power supply capable of delivering dc power at various voltages meant having a transformer to convert mains ac to the desired voltage and then rectifying it. Multiple voltages meant multiple taps and maybe a rheostat. Something as simple as a radio normally depended on a (heavy and expensive) transformer to boost voltage for tubes, or reduce it for transistors.

If you’re really old, you may remember car radios that had tubes. (I do.) Since 12 Vdc was not high enough voltage to make a vacuum tube function, a mechanical device called a vibrator created crude ac or chopped dc mechanically. This could be fed into a transformer and stepped up to a useful tube voltage and then converted back into dc.

Changing ac to dc, or dc from one voltage to another simply wasn’t practical without some sort of rotary converter. (If you ever had to work on a three-bearing MG set, you can be happy these ultimately disappeared.) This was one of the reasons that Edison lost the battle with Tesla. If Edison wanted 100 Vdc for residential us, he had to generate it at that voltage which proved to be impractical for many reasons. This is a major reason why we have ac distribution today.

In my travels I once saw an interesting version of a mechanical VFD. A natural gas engine was coupled to a dc generator. Power from the generator was fed to a motor/alternator set. Since the dc could be varied, it allowed the alternator to rotate at any speed and therefore generate ac at any frequency. This was used in a motor repair shop, so it was only used for testing purposes. I can’t imagine what the overall efficiency was, but I doubt it was very high. But I digress.

Anyhow, the current technology that has become ubiquitous is switched-mode power supplies. While you can go into greater depth at your leisure, the basic idea is that semiconductors can turn on and off very quickly. This action cleverly combined with other components, including inductors, capacitors, and transformers, can convert just about anything to anything, and it is highly scalable.

The charger I have for my iPhone is a prime example. Barely more than 1 cubic inch, this device can convert 120 Vac mains power to 5 Vdc used by USB circuits. The first time I saw one of those, I didn’t think it was possible. “Where the heck did they put the transformer?” I asked. The answer is simple: there isn’t one. The power converter for your laptop can probably work at any voltage around the world if you have the right plug. No problem. Switched-mode power supplies can be very efficient, reliable, small, and relatively inexpensive. They are common now and will be even more so.

For industrial use, dc distribution can be very practical. If your plant has a dc bus, anything that uses a motor can run off a VFD powered from the bus. Other devices can use converters or switched-mode power supplies as needed. There are all sorts of possibilities.

Peter Welander, pwelander@cfemedia.com

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