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Some time ago, over some beers with a fellow EE who's much more experienced than me, we were discussing the unique challenges present in utility-scale power generators, like the ones in nuclear power plants. A single generator can be in excess of 1000MWe, and has all the more exotic stuff like hydrogen cooling, etc.

In passing, my friend told me

At least they don't have to spin at 3000 RPM, they have many poles on the stator to reduce the need for rotor speed...

Which I thought was a good solution to an engineering problem.

It seems, though, that they don't actually use it often.

When reading about the Chernobyl disaster, I found that the RBMK reactors, and indeed even the modern VVER varieties, use generators spun at 3000 RPM. Excerpt from Wikipedia:

The turbine and the generator rotors are mounted on the same shaft; the combined weight of the rotors is almost 200 t (220 short tons) and their nominal rotational speed is 3000 rpm.

Also, from the Chernobyl NPP page:

The Kharkiv turbine plant later developed a new version of the turbine, K-500-65/3000-2, in an attempt to reduce use of valuable metal. The Chernobyl plant was equipped with both types of turbines; block 4 had the newer ones. The newer turbines, however, turned out to be more sensitive to their operating parameters, and their bearings had frequent problems with vibrations.

Indeed, on the night of the Chernobyl disaster there actually were two tests being done about the same time: the famous turbine-rundown test we all know about, and a quite obscure vibration measurement test (it's not widely described, but search for "vibration" here if interested).

Another data point: the only time I went on an inside tour of a working NPP (a VVER design) I noticed that one of their instruments in the control room was sometimes dipping into the redline area, and when I asked, we were told it's the vibrations indicator.

So all in all it seems that vibrations could be quite the hassle, and one has to wonder why they don't utilize the half- or one-third-speed approach, which will allow for 1500 or 1000 RPM on 50Hz grids.

It's evidently done on 60 Hz grids, for example the Bruce NPP's generator runs at 1800 RPM, and GE offers both half-speed and full-speed generators (e.g. see here, pg. 22 and 23).

Question

So the technology to use lower generator speeds is there, but why is it only used for halving the speed, why not divide further? What are the downsides of lower speeds?

The question may be not appropriate for this StackExchange, as the reason may be purely materials/mechanical issue (less RPM needs higher torque, ...). I'm interested if there are EE reasons that make the higher RPM desirable.

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  • \$\begingroup\$ Why do you think it's bad to rotate at 3000 or 3600RPM? \$\endgroup\$ – Kevin White Nov 27 '19 at 21:46
  • \$\begingroup\$ Also Chernobyl isn't really a good reference point, I hear a rather unfortunate accident occurred while they were "optimising" something \$\endgroup\$ – JonRB Nov 27 '19 at 21:50
  • \$\begingroup\$ 1800 RPM = 30 Hz , 1500 RPM = 25 Hz \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 27 '19 at 21:50
  • \$\begingroup\$ The generators at large hydroelectric power stations operate at much lower revs. I toured the Glen Canyon dam in Arizona, U.S.A. one time, and for some reason, the number 150 rpm (24 pole pairs) sticks in my mind. One of the generators close to the viewing windows was somewhat opened up for maintenance or upgrade, and I'm thinking that 24 pole pairs would be consistent with what I remember seeing of its innards. \$\endgroup\$ – Solomon Slow Nov 27 '19 at 22:11
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Steam turbines are more efficient at higher speeds.

If we are talking about hydro turbines, this approach works, and at hydropower plant you can meet 48-pair poles turbine running at 62.5RPM.

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  • \$\begingroup\$ Surely it's the blade speed rather than the rotational speed that is important. A slower turbine would be larger diameter to get the same speed, that would make the entire machine larger and probably more expensive.. \$\endgroup\$ – Kevin White Nov 27 '19 at 22:05
  • \$\begingroup\$ @KevinWhite blade speed or energy applied to the blade - some turbines are getting 4m^3 a second... \$\endgroup\$ – Solar Mike Nov 28 '19 at 5:27

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