Why There are no dc Transformers

Q:
from your understanding or research of electromagnetic induction, can u explain why transformers can be used in AC power systems and not DC power systems for power distrubution
- Anonymous
A:
That’s a really good question.

For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it’s connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won’t drive any electrons around. A CHANGING magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday’s law of induction. You get a changing field from an AC current, since the current which makes the field is changing.

Mike W.

(published on 10/22/2007)

Follow-Up #1: transformers

Q:
YOU EXPLAINED THAT TRANSFORMERS CAN’T WORK ON DC BUT I WOULD LIKE TO KNOW THAT WHAT ID THE EXACT EFFECT OF A TIME VARYING FIELD AND ONE MORE THING IN HVDC TRANSMISSION TRANSFORMERS ARE BEING USED, WHAT IS THAT?
- MAYANK (age 22)
BANGLORE, KARNATAKA, INDIA
A:
The effect of time-varying magnet fields is to produce an electromotive force (EMF) driving charged particles around. The effect is described exactly by Maxwell's equations. The current in the primary of the transformer produces a magnetic field. When the current changes, so does the field. That makes an EMF which drives current in the secondary.

I'm guessing that in the application you describe some dc current is chopped into an ac current (maybe by switching some contacts back and forth) and the ac current is then run through a transformer. It could then be converted back to dc by a rectifier.

Mike W.

(published on 10/22/2007)

Follow-Up #2: DC transformer

Q:
can I use a 10:1 120vac - 12vac transformer that I have and back feed it to make it a step up but use it in a dc circuit and transform 28vdc into 280vdc?? My question is can I feed an ac transformer with dc? Will it still work? And if it does can I wire it in reverse??
- David moran (age 28)
A:

No, it will not and this has a historical importance.

The reason is that transformers work via induction of electrical forces by changes in magnetic fields, so the constat fields produced by dc currents won't work at all. Here's a more detailed description.

In the simplest case, the transformers consist of a metal rectanglular core, around two sides of which two separate wires are wound (see the image from Wikipedia). It works like this: to red side, you apply 400VAC, which generated a magnetic field (B) along the axis. Because iron is ferromagnetic, virtually all B flux will be transferred along it to the through the secondary windings. Both AC and DC will generate the magnetic field, so no problem till this point. However, because B is proportional to the current (I) ,AC will induce an oscillating B but DC a steady B. Harvesting the current is based on Faraday's law, which says changes in B will induce currents. Therefore if you apply DC, no current. However, you still will be consuming energy. What makes the transformation is the difference in the number of turns on both sides, so it would be OK if you supplied 28VAC to get 280VAC.

There are other ways today to modulate DC voltage, such as step up converters (or alternator + transformer + rectifier), however compared to an iron core and bunch of wires, they are far more complex. This is the reason why AC investors won the competition against DC in early 20th century. To reduce energy losses, you need to step up and down and it is far easier to do this with AC.

Tunc

(published on 04/03/2015)

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