Generator size vs frequency question

[girts]

Hi folks , a simple question, is it true that generators are like transformers in regard to the frequency of the AC being generated versus the size of the device for given amount of power transfer?
We know that transformers can have smaller cores with fewer turns if the frequency of the AC is increased, like in smps. Does the same logic applies to generators?
Let's take for example a hydro power plant generator mounted on top of a turbine, the rotor field current is DC so the rotor has given amount of magnetic pole pairs that travel past a fixed stator poles into which AC is induced whose frequency is proportional to the rpm of the rotor, now let's forget that we need to maintain the grid frequency, if we would say double or quadruple the rpm of the rotor, how would that affect the output power for size of the generator?
So essentially what I want to understand is the relationship between the physical size and the frequency in generators and does it relates to the same law in transformers?

 

[anorlunda]

There is an additional factor; the number of pole pairs. To generate 60 hertz with one pole pair, the generator must spin at 3600 RPM. With 2 pole pairs, it is 1800 RPM, and so on.

Hydro generators have very many pole pairs to deliberately slow down the RPM because hydro turbines can't spin so fast. For example, 180 RPM needs 20 pole pairs. They have to be big in diameter to make room for all those separate poles.

Aircraft systems sometimes use 400 HZ power because transformers and generators are both less heavy. But aircraft don't have to ship the power long distances, so the increased reactance in the wires doesn't matter so much.

But maximum power generated is usually limited by max current, not RPM. Max voltage is usually limited by insulation considerations.

 

[girts]

Well yes , in a local hydro plant I know they have 68 pole pairs, the plant is rather large, 10 turbines in general and about 1000MW of capacity.
isn't the main reason why hydro generators have so much pole pairs the fact that as you said the turbine can't spin fast and in order to get the required 50hz frequency they have to use many poles otherwise the frequency would be very low?
I assume frequency is critical to the AC grid because few Hz +- and the generator either becomes a load on the grid or a generator for the grid?I am thinking about some generator concepts, let ,me ask , would a generator that has fewer turns and thicker wire be better than one with more turns and thinner wire, I guess I'm essentially asking is a generator better if it acts more like a voltage source than a current source or vice versa?
I assume it also depends on the application, although most of them use a step up transformer right after the generator.

I assume it would be very beneficial to have a generator whose rpm is not linked to its output frequency? although classical type generator with pole pairs cannot have such phenomenon I read there is a specific case of a generator that exhibits this type of behavior, namely the homopolar generator. Although due to its nuances it requires sliding contacts that are hard to make and is complicated to get more than few turns on it I guess.
Although if you said that max power is usually limited by current not RPM, then wouldn't the homopolar/faraday generator be able to produce more power for the same size as a typical generator given that it has few turns but they are made from thick copper material and due to its very low resistance can produce huge amount of current? I understand that with brushed and a static B field the output would be low voltage DC, so I'm not asking this from a practical viewpoint but from a theoretical one.

 

[anorlunda]

It is mandatory that steady state generator frequency is identical to grid frequency (50 HZ or 60 HZ).

Large generators don't transfer power current through slip rings. Synchronous generators send only exciter current to the rotor.

Except in mobile applications, the size and weight of generators are not very important. Most important are efficiency, maintenance, lifetime and cost.

You talk about wire. Large synchronous generators use huge bars, not wires.

Finding room for them and dealing with the magnetic forces that try to push them apart are major design issues.

 

[girts]

thanks anorlunda for answers, I was hoping to hear a bit more about the things I asked in my previous post.

by the way is that rotor from a generator used in a nuclear power plant? I assume so because the rotors seems to have only two poles and they run on high rpm because the steam turbine demands them?

Well yes I know that the stator is the part where the generated current goes towards the load. Even though for such large generators the power sent through slip rings to rotor excitation field seems also quite large.

What about liquid metal contacts, like mercury for example

 

[anorlunda]

Sorry, I have company today, so I don't have time for better answers.

There is also a new generation of electronically switched and controlled generators coming. They will be more flexible in many ways.

 

[girts]

Well that's fine I can understand that,
whenever you have time I would love to hear more about the generators you mentioned in your last post as well as some of the questions I raised in my previous posts.

thanks

 

[Dr.D]

by the way is that rotor from a generator used in a nuclear power plant? I assume so because the rotors seems to have only two poles and they run on high rpm because the steam turbine demands them?

A two pole generator indicates that either 50 Hz or 60 Hz power is being generated, and that implies a steam turbine speed of 3000 rpm or 3600 rpm. Where the steam comes from makes no difference at all. Steam generated in a nuke plant is much like steam in a coal or gas fired plant; it all expands through the turbine to drive the load.

 

[anorlunda]

Although if you said that max power is usually limited by current not RPM, then wouldn't the homopolar/faraday generator be able to produce more power for the same size as a typical generator given that it has few turns but they are made from thick copper material and due to its very low resistance can produce huge amount of current?

No. First of all, resistance plays very little role. It is inductance, not resistance that is important. You can make superconducting generators that aren't so very different.

Second, if you want to optimize the generator design, make the voltage as high as possible, Hold P constant, and think of P=VI; double V and you halve I. Voltage is limited by the insulation; too much V and it arcs and burns holes in the insulation. 25KV is a frequent number for generator stator voltage.

Fossil plants deliver superheated steam, and nuclear plants saturated steam. Because of that, the low pressure turbine blades need to be longer for the nuclear case. So long are the blades that centrifugal forces become too big. So, they add a second pair of poles and reduce the RPM to 1500 or 1800 RPM.

Cooling generators (because resistance is nonzero), and containing the huge forces that want to make them fly apart are much bigger challenges in large generator design than their electrical properties. If you are interested in better designs, it would be more fruitful to think about cooling and strength than turns and wire size.