No matter if permenant magnets, or electromagnetic field is used to generate electricty, there is no way to generate DC directly (except batteries). So, why do we say there cannot be DC transformers? Does it mean we cannot supply a transformer with DC ?
9
-
3\$\begingroup\$ Have you thought how a transformer works? \$\endgroup\$– Solar MikeCommented Dec 29, 2018 at 20:24
-
3\$\begingroup\$ Because DC current is constant, flux does not change. AC current changes, flux changes. So transformer is possible. A DC generator can produce DC directly. \$\endgroup\$– StainlessSteelRatCommented Dec 29, 2018 at 20:24
-
\$\begingroup\$ Your premise is wrong. There are DC transformers. Just not at room temperature. \$\endgroup\$– Spehro 'speff' PefhanyCommented Dec 29, 2018 at 20:26
-
\$\begingroup\$ Going along with @SpehroPefhany comment, DC transformers are much too expensive for off-the-shelf use. Before this century is over the power utility companies may have some areas being fed by DC power, but that is just idle speculation on my part. \$\endgroup\$– user105652Commented Dec 29, 2018 at 20:34
-
\$\begingroup\$ Your premise is wrong; while not usually practical, 'homopolar generators' are true DC-output gemerators. \$\endgroup\$– Whit3rdCommented Dec 29, 2018 at 21:02
|
Show 4 more comments
3 Answers
\$\begingroup\$
\$\endgroup\$
2
PV cells generate DC directly by changing photons into electrons, forcing current flow. Also a DC generator does produce DC, using a commutator.
If you are asking such a question about why DC transformers do not exist, you should Google or watch some informative videos on transformers to learn how they work (ignore HVDC transformers which are not conventional transformers but more solid state electronics).
Because DC current is constant and doesn't change, the flux created by it's magnetic field is also constant and doesn't change, so no change in flux is present to induce an emf to the secondary winding by mutual inductance. On the other hand, AC magnetic flux does change with a given frequency, hence it induces an emf across secondary.
You CAN supply a transformer with DC but it will not do anything useful and just overheat the windings due to the low DC resistance as they rely on reactance to limit current flow. This is why isolation transformers can sometimes be used to block DC noise in signals.
-
\$\begingroup\$ I see thank you so much, and sorry I forgot about PV cells, I thought about machinery first. Then, if I supply a conductor with AC, and put it in a permenant magnet (not moving it though), would it still count as a change of flux due to AC? (sorry if this question sounds stupid) \$\endgroup\$– VyunCommented Dec 29, 2018 at 21:53
-
\$\begingroup\$ Sorry, I don't really understand your question. A change of flux with respect to which field? \$\endgroup\$– David777Commented Dec 30, 2018 at 0:48
\$\begingroup\$
\$\endgroup\$
2
Just as DC can be used to drive motors (via commutators), generators can provide DC outputs via the same mechanism. As long as you grant that "DC" means of constant polarity without requiring constant voltage.
As a general rule, any motor which is DC without the intervention of electronics (such as BLDC motors) can be used as a DC generator.
answered Dec 29, 2018 at 20:44
-
\$\begingroup\$ I know this, but can't we supply the generator (without commutators or not) with DC (in such case, is it the exciter that we feed first) ? Cause, without a commutator, if I am not wrong, what we produce is AC, not DC, due to the periodic polarity change. \$\endgroup\$– VyunCommented Dec 29, 2018 at 21:45
-
\$\begingroup\$ @Vyun - See Leandro Alsina's answer. And normally exciters are fed from DC. And yes, without commutators, a generator will produce AC. So I don't understand your question. \$\endgroup\$ Commented Dec 29, 2018 at 22:11
\$\begingroup\$
\$\endgroup\$
6
Transformers rely on magnetic field inductance, this phenomenon only occur in a VARIABLE magnetic field, such is a sine function. When you drive an inductor with DC current, there is a static magnetic field (like a fixed magnet), but this field cannont be induced on a secondary inductor, because it is fixed in time, there is no variance in time.
-
\$\begingroup\$ But, isn't the rotation of a conductor with a current flow in a magnetic field what creates the induced emf? \$\endgroup\$– VyunCommented Dec 29, 2018 at 21:47
-
\$\begingroup\$ i think you are talking about an alternator, this is other machine \$\endgroup\$ Commented Dec 29, 2018 at 21:51
-
\$\begingroup\$ I mean, unless we keep polarity constant of an alternator, there is no way to supply a generator with DC. \$\endgroup\$– VyunCommented Dec 29, 2018 at 21:54
-
\$\begingroup\$ If you consider a DC current flowing through a conductor, it produces a stationary magnetic field with a north and south pole. If this conductor rotates around another conductor, yes now the magnetic field is rotating. So as opposite poles move closer the induced emf increases and decreases as they move further away. This is where the sine wave's shape come from, the angular position of one conductor's magnetic poles with relation to the others. \$\endgroup\$– David777Commented Dec 29, 2018 at 21:58
-
\$\begingroup\$ @David: No poles are created in the magnetic field around a current carrying conductor. Try drawing a diagram and see if you can find where the poles would be created. \$\endgroup\$ Commented Dec 29, 2018 at 23:01