Is DC-Generator Commutator a Rectifier?

This is really just playing with words.

If you didn't have the (split ring) commutator, and had slip-rings connected to the windings, you'd have AC (which you could convert to (lumpy) DC using some other form of 'rectifier' - e.g. a diode bridge).

In this context, the commutator 'rectifies' the AC to provide (lumpy) DC.

a silicon bridge rectifier will have a dual diode forward drop of about 1.4 volts for the two diodes. So if you are dealing with a 5 VAC = very lossy. Slip rings have zero forward drop, so can be better at low voltage. They do not really rectify, they switch the voltages so it looks like rectification. Multi-phase commutating rectifiers will not work, but silicon diode multiphase rectifiers will work and that is what they do in alternators.

You can also create commutating devices with switched FETs, that have very low voltage drop. All they have is the drop due to current through their forward resistance, which can be .01 ohms or so (depends on the size of them)

Man, you are hard! Sometimes (at least in my case) I don't know enough about something to ask a question in the right way, so it comes out as a stupid question. You don't learn anything if you are afraid to ask stupid questions; besides, there's usually someone sitting on the sideline saying, "Durn, I never thought about that before."

Kyoto,

You are so right.

"Durn, I never thought about that before."

As a maintenance electrician, amongst other things, I had always just accepted whatever I was confronted with, never thinking back beyond the transformers or batteries. A.C. was A.C. and D.C. was D.C.

But one day in a class for a bakery maintenance crew where they were trying to bring everybody up to the same page, the issue of rotary generation came up. I was surprised to realize that I had never thought very deeply about it just assuming there were generators that turned out D.C. and others that turned out A.C.

It was then that I realized that the sine waves, single phase or three phase, were the natural product of rotary generation.

I know this will make me look stupid because I had been an electrician, self taught, for many years. There just had never been a reason, before that class, then to simply accept what was given in the wires in front of me.

So yes; you are right, sometimes there are things we never thought about, or had to think about before.

Our friend calling somebody stupid for raising the question needs to think about who is stupid. He could have been talking about me.

j.

In the world of rotary DC generators, or "dynamos" as they used to be called, "commutate" and "rectify" mean the same thing.

In the case of DC electric motors, the "commutator" doesn't rectify anything, but is merely a switch that energizes the appropriate coils on a wound armature. It is called a commutator because of its resemblance to the commutator of a dynamo.

Mechanical commutation isn't always associated with a rotary machine. In very early automobile radios, the high voltages required for the vacuum tube circuits of the day were usually provided by a vibrator (interrupter) and a step-up transformer, followed by a vacuum-tube rectifier, often a gas-filled type like an 0Z4. However, some of these radios used what was called a "synchronous" vibrator, which provided interrupted DC for the low-voltage primary of the transformer, and which also commutated, or rectified the high-voltage output of the transformer, thereby eliminating the need for a rectifier tube.

Although many of the functions of mechanical switching devices are now handled electronically, mechanical switches will only be supplanted when something equally robust can be obtained as inexpensively.

SRY frenz.....i know its a stupid question to ask........but this was one asked by ur prof ..................anyway thanks all for reply

S'ok, gopi_cr4, it ain't that dumb a question (if it is, blame the prof!) - but it's just a way of looking at it. If you've got to write an essay (or even a few paragraphs) for an answer, I hope this has given you some pointers.

BTW - ignore the idiot with his head up his a**.

gopi, rather ignore this a**h*** who has his head inside his a**. Given the technical qualification and knowledge you seem to possess this is the dumbest question I would consider. A rectifier is for converting AC into DC which is generated by an alternator, whereas a DC generator+commutator generate only DC where is the question of rectification? You are not feeding AC to the commutator to convert AC into DC. Oh! why I am answering the stupid question???

Ganapa,

If the issue raised by the question is who is stupid???

Consider the rotary generator output before it is switched by the commutator.

Is the output a sine wave?

Can a rotary device put out anything but a sine wave?

If in fact the output after the commutator is a bumpy d.c. is the commutator not in fact a mechanical rectifier?

Is there no conceptual correlation between the old style d.c. dynamo of Edison and its commutators and, say, the modern automobile ALTERNATOR, and its solid state rectifiers?

j.

I think you're a stuck-up Jack-A...who thinks he's got all the answers!!!

Even the way you explained the mechanical rectification is bleak compared to some of the others.

Furthermore, even if you're a pro in electronics this attitude of yours does not have a call for on this blog. Therefore, our attitude is down right inane and I doubt it if you even know the difference between your left & right. Frankly, I cannot comprehend why did you join this blog with an attitude like yours?

You'd better establish your own blog and set your own standards there, as much as you like, but NOT here, OK?

So, stop acting like a jerk!

The commutators act as a synchronous rectifier in generator mode and a synchronous switch in motor mode. This is similar to what is being done on a modern brushless DC motor. In the modern version, various sensors detect the position of the rotor and activate the corresponding transistors to have the correct flow of current. There are various refinements added but the basis is the same as what has been developed using mechanical means over a hundred of year ago. They were very clever back then.

Here is a link to a New York Times story about the recent shutting down of the Edison DC generating plant on Manhattan's Lower East Side.

http://cityroom.blogs.nytimes.com/2007/11/14/off-goes-the-power-current-started-by-thomas-edison/

I am searching for technical data on that plant if it exists on the web.

But, how do you suppose that plant generated and distributed huge amounts of direct current, which I myself have used for battery charging in lower Manhattan, for all those years. Rectifiers?

Want to bet it was switching commutators.

Not a stupid question at all.

j.

In the mid-60s, Con-Ed began installing 150 and 250KW rectifiers in large buildings and the street, respectively due to burnouts and maintenance headaches. So not commutating recitifiers; but rectifiers none the less. The few DC customers now have rectifiers on THEIR side of the meter.

Yes Kyoto,

And if you read down into the article whose URL I posted you find that all these years later there are customers with large installations that still require D.C., such as elevator motors where it is cheaper to install the rectifiers to replace the D.C. from the now closed Edison plant.

Another customer that in fact uses massive amounts of D.C. happens to be the New York subway system, parts of it over a hundred years old, whose third rail distributes D.C. from massive rectifying sub-stations, those stations distributed every couple of miles or so, because of resistive losses of D.C. over distance, throughout the system.

j.

I did read your article and am agreeing with what you said. I was just saying that I do not believe those DC customers have commutating rectifiers as in DC generators or MG sets or amplidynes to provide their power. Most likely selenium or tube type rectification. I would be interested in knowing what they did have; just for grins. The last customer that did receive DC from Con-Ed was a residence (or residential structure); the one the wire was cut by the old worker who was in the supervisor's words, "Closest to Edison", back in April 2007. Everyone else now get AC from them and convert it on their premises.

thankx again for reply