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Re: excited rotor field power in generator-alternator

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>| >| snipped-for-privacy@ipal.net wrote: >| >|> When a generator design uses an electromagnetic rotor field (e.g. an >|> alternator), what is the typical ratio of power needed to energize >|> that field compared to the power the generator can produce with that >|> field? Based on the designs of generators I have seen from some >|> manufacturer web sites, it seems the ratio quite low. But what I am >|> interested in is what kind of scientific basis there is in this. >|> >|> In the designs I see, the power for these fields are transferred from >|> the regulator governing the field strength to the exciter stator >|> windings, to the exciter rotor windings, through a rectifier bank, >|> and then to the main rotor windings as a DC current. But why does >|> this have to be DC? Is it more efficient to be DC? Since I see no >|> AC designs, there must be some reason DC is better there. >|> >|> -- >|> ----------------------------------------------------------------------------- >|> | Phil Howard KA9WGN |
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| >|> | (first name) at ipal.net |
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| >|> ----------------------------------------------------------------------------- >| >| From my own experience, a 2000KW 3phase alternator that I worked with had >| a regulator that consumed 32KW. That was old technology, (mag-amps), and >| the regulator cabinet was water cooled. Still, it was pretty efficient for the >| times. >| >| ARM > >62.5 to 1 is pretty good. It's got to be even higher than that when you >consider the waste in the regulator which must be high if it had to be >water cooled. > >What if the regulator controlled a small generator on the end of the shaft >which was then used as the direct exciter power source. The first generator >would not be directly voltage sensed, but instead, would vary in voltage >to power the exciter of the 2nd generator, where the voltage sensing for >the regulator would be present. Assuming only 20 kW was needed for the 2nd >generator exciter, the 1st generator would probably only need to be rated >to 24 kW or so. The exciter of the 1st generator would only need to be on >the order of what? Maybe 400 watts?

Regulation response would suffer, since each exciter would have to respond, then influence its generator to respond.

Best regulation would have a fast controller feeding the main generator field. The controller power source (exciter) would have to provide capacity for transient load conditions.

PS Exciter power is also related to load power factor, Google will find lots of info.

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Reply to
Jim Michaels
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|>What if the regulator controlled a small generator on the end of the shaft |>which was then used as the direct exciter power source. The first generator |>would not be directly voltage sensed, but instead, would vary in voltage |>to power the exciter of the 2nd generator, where the voltage sensing for |>the regulator would be present. Assuming only 20 kW was needed for the 2nd |>generator exciter, the 1st generator would probably only need to be rated |>to 24 kW or so. The exciter of the 1st generator would only need to be on |>the order of what? Maybe 400 watts? | | Regulation response would suffer, since each exciter would have to | respond, then influence its generator to respond.

But wouldn't the response factor be multiplied?

| Best regulation would have a fast controller feeding the main | generator field. The controller power source (exciter) would have | to provide capacity for transient load conditions.

What would be the slowdown from my example?

| PS Exciter power is also related to load power factor, Google will | find lots of info.

Unfortunately, Google seems to be quite filled with sites trying to sell generators, rather than sites specific to designs. Limiting the search to .edu sites gets lots of course syllabi, but relatively little content.

Reply to
phil-news-nospam

This is a good point. After all, one *could* build a generator with a very large number of turns in the field winding. Then only a small current would be needed to establish the same amount of magnetic field. But the field winding would be so inductive, it would take several seconds to respond to correct a voltage variation.

Or one could build a very large generator and use extremely oversized conductors in the field windings to reduce the I^2R losses in the field to a minimum (or design with superconductors). But such a design would probably cost more than the energy losses in conventional designs.

As with so many engineering decisions, it is full of compromises... sigh...

daestrom

Reply to
daestrom

the brushless systems use exactly the type described. the dc goes into the field of the first generator. this is the stator. the corresponding rotor output is rectified and fed into the main rotor winding. the regulation is read from the output of the main stator, this is the main oputput.

this is the system on my 25 kva 3 phase diesel unit. i have others similar.

the long time constant may be the problem on the really big systems. i can imagine inductance is the killer. sam

Reply to
sammmm

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