Generator Efficiency: Learn About RPM Impact

[apc3161]

Hey, I was wondering if someone could link me to a good source where I can read about electrical generator efficiencies and how they relate to RPM. I tried doing some derivations myself and came up with a somewhat believable result, but am not ready to trust myself as this is far outside my area of knowledge. Any help is appreciated, thanks.

 

[russ_watters]

There is no overall relationship between efficiency and rpm. There may be some for a specific generator, but it wouldn't be a readily mathematically model-able function.

 

[mheslep]

Well there are the friction losses - bearing and windage - which are a function of RPM, but they typically don't dominate in normal operation. But most generators are designed to run at a fixed speed. Drive them at a different RPM than for which they were rated and they produce nothing.

 

[vk6kro]

The main reason you would drive a generator at a particular speed is to get a particular frequency of AC out of the generator. If you want 60 Hz from a 4 pole generator, you would want to drive it at 1800 RPM.
It will certainly produce output at other frequencies if it is driven at a different RPM.

I searched Google for "Generator Efficiency Curves" and got a lot of information.
http://www.esat.kuleuven.be/electa/publications/fulltexts/pub_1602.pdf
This was one of them. Seems to have a lot of useful graphs in it.
See the last two pages.

But there was a lot more than that on Google.

 

[Bob S]

Here are six generators (300 megawatts each) that run synchronously at 112.5 RPM. This is a pumped -water storage facility where the generators pump water uphill at night, and run the water back downhill through the generators during the day. The total round-trip efficiency (for storing and generating energy) is about 70%.
http://www.consumersenergy.com/apps/pdf/LudingtonPumpedStorage.pdf

 

[vk6kro]

Hi BobS.

Looks like the real source of power is still coal, but they use the reservoir to even out the demand during the 24 hour day. Stunning project.

They say the TURBINES run at 112.5 RPM but I guess they must have 32 pole generators and actually run the generators at 112.5 RPM to get 60 Hz out? Couldn't find that in the brochure.

 

[Bob S]

Here is a detailed article on the Mt. Elbert 400 MW pumped water motor-generators (200 RPM, 100 MW each). I do not have access to the article.
http://ieeexplore.ieee.org/Xplore/l...&isnumber=&arnumber=1601670&authDecision=-203.
Here is another
http://ieeexplore.ieee.org/Xplore/l...00023143.pdf?arnumber=23143&authDecision=-203
Here is another free article with more detail. Their generator efficiencies are in the range of 98% with power factor = 1.0 (see page 11):
http://files.asme.org/ASMEORG/Communities/History/Landmarks/5567.pdf