Hi,

I have two questions:

1) why DC is used for excitation? why not AC?

2) How to find pole specification (how may poles) by physical examination of cylindrical pole rotor?

regards
anka rao

 

Responding to Ankarao's Sep 29, questions:

1) Why DC? Why Not AC?
First consider two coupled wound rotor motors. The stator of the first is powered from a fixed frequency source. The second's stator is connected to the slip rings of the first, while its own slip rings are connected to a variable resistor. This arrangement, called concatenation, results in a machine having variable shaft speed.

An alternator's voltage is different if the rotor is supplied with an AC voltage! What follows is a very simplified discussion. The output voltage of an alternator, having its rotor connected to an AC supply, would yield a wave shape similar to AM or Amplitude Modulation. That is because the generated air-gap flux wave has a two varying components...
one related to the rotor speed, the other rotor supply frequency.

2) Number of Poles by Observation.
Unlike a squirrel-cage rotor an alternator with a 2-pole nonsalient (cylindrical-rotor) will have 2 sets of slots in the rotor. They are not
necessarily distributed around the rotor circumference. Instead they are diametrically opposed! A 4-pole rotor will have 4 groups.

If the slots are evenly distributed around the circumference, then the end-turn construction will yield the number of poles... if the end-bells are removed!

I'm sorry if my description sounds convoluted, but it's like describing a ratchet... without using you're hands!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

1) Try searching http://www.wikipedia.com for alternator or synchronous generator. You should also try http://www.canteach.candu.org; there is some really great information on this site!

This is a good question, and Mr. Corso's answer seems to suggest that it might be possible to use AC for excitation, though the results would not be what was desired (if the answer is understood, and that's not certain, either.?.?.?).

In general, the principle says that as long as there is relative motion between a magnetic field and a conductor that a voltage will be generated in the conductor. The only magnetic fields this author has ever seen used in power generation used DC as the excitation for the rotating magnetic field(s), be they salient pole or non-salient pole rotors.

2) One can easily determine the number of poles of a synchronous machine from the rated speed (in RPM) and frequency of the machine from the following formula: p = (102 * F)/n, where "p" is the number of poles, "f" is the frequency of the machine, and "n" is the speed, in RPM--regardless of the type of rotor (salient or non-salient).

This author has only seen one modern synchronous generator (built in the 1980s) with a salient pole rotor (it is rated at about 10 MW).

markvguy

 

Markvguy, you took the second easiest solution, but not the easiest, to determine the number of poles. Unfortunately, your method was not what Ankarao requested. He asked, "How is it done by physical observation?"

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Mr corso and MVguy,

Thank you very much for ur explanation.

1) I under stood after referring electromagnatism basics, DC is given as filed for generating a steady magnetic field.

2) I could found the two number poles on generator rotor. the winding pattern is different at the poles. It is cylindrical pole rotor and the slots (for winding) are uniform except at the pole area.

regards
ankarao

 

As you described, Mr. Corso, it is very difficult to determine the number of poles of a cylindrical (non-salient) pole rotor by looking at it without the end-bells removed--which usually requires a good deal of effort and planning.

Perhaps the originator of this thread was not asking about how to determine the number of poles by visually looking at the rotor, but rather by some other method--such as using an ohm-meter.... We'll probably never know. (Free advice is a funny thing...)

In either case, we are both also probably presuming "large" electrical alternators, and the author could be describing a very small alternator (not likely, but possible). It's hard to imagine a condition where one would be looking at or holding a rotor without having some means of knowing something about the alternator it is associated with--paperwork, the nameplate of the machine, etc.--unless one were in a shop where a number of large electrical rotors were being serviced and one was just observing and didn't have access to the paperwork associated with them...

Again, we'll probably never know, eh?

markvguy