Hello,

When using a cylindrical rotor of lets say 1 pole pair, to produce a 50hz frequency it needs to rotate at 3000rpm (50cycles per second). while if you're using a salient pole rotor with say 10 pole pairs, the rotation speed needs to be much less, at 300rpm.

Now I understand that if you increase the fuel flow rate you will get a larger current at the output, because it will be locked to the grid at 50hz. an increase in speed of the turbine, the torque generated will be converted to amps.

But which turbine is more suitable to run at 3000rpm and 300rpm? there must be something that works better at a higher/lower speed, as you can't have everything in life.

With regards to the generator, i know that it is mounted onto the stator, which is rotating to produce the required voltage at the output. but how do you get the DC current, since if it's rotating, you'll have AC current, you'll need rectification to get DC current no?

Cheers

 

LukeeVasallo,

I've typed almost this entire message twice, and both times it's been lost in the great bit bucket in the sky.... Let's hope the third time's a charm!

Generators are devices for converting torque into amperes--just like electric motors are devices for converting amperes into torque. Electricity is just the way the torque is transmitted from the device driving the generator (the prime move, or, as you've chosen: a turbine) to the electric motor driving a pump or blower or air conditioner compressor or lift. In an AC (Alternating Current) power system, the frequency of the alternating current is very important and no single synchronous generator can rotate faster or slower than all the other synchronous generators it is synchronized together with on the system (grid). That is, one generator can't run at a speed proportional to 50.3 Hz and another run at a speed proportional to 49.5 Hz--they all have to run at speeds proportional to the frequency of the grid, which is nominally 50.0 Hz for your example. As you've rightly noted, each generator has a synchronous speed that is a function of the number of poles of the generator.

Most prime movers, turbines in your post, are coupled to the synchronous generator rotors either directly or through reduction gears to change the prime mover speed into the synchronous speed required by the generator for the frequency of the system it is synchronized to. Different types of turbines operate more efficiently at different speeds, some speeds even much higher than the generator's synchronous speed (hence the need for the reduction gear). Many steam turbines and heavy duty gas turbines are designed to rotate at synchronous speed (that is, two-pole synchronous speed--3000 RPM in your example). Some smaller steam turbines and most aircraft-derivative gas turbines operate at much higher speeds than two-pole synchronous speed and they often require reduction gears. A two-pole generator is less expensive to build, so they are most often used.

Many hydro turbines operate most efficiently at much lower synchronous speeds, hence the need for multi-pair pole synchronous generators. So, the speed of the generator is really a function if the type of prime mover in most cases and the generator is chosen based on the prime mover (generally).

As for the speed of the prime mover when the energy flow-rate into thee prime mover is increased--the speed <b>DOESN'T</b> change when the generator and its prime mover are synchronized to the grid with other synchronous generators and their prime movers. There are very great magnetic forces at work in the generator between two magnetic fields (the rotor and the stator) that keep it spinning at synchronous speed (which is a function of the frequency of the grid it is synchronized to). Remember: No single generator can rotate faster or slower than its synchronous speed when synchronized to a grid--and since the prime movers are coupled to the generator rotors the speeds of the prime movers are also fixed by the frequency of the grid and the number of poles of the synchronous generator.

Increasing the energy flow-rate into a prime mover causes more current to flow in the synchronous generator's stator windings which means it is producing more electric power. But neither the speed of the generator nor the speed of the prime mover coupled to the generator rotor will change--it's fixed by the frequency of the grid with which it is synchronized. That's key to understanding AC power systems and prime mover governors and load control.

Your last question is a little more difficult. I think you're referring to the DC current that is applied to the synchronous generator rotor's windings--called excitation current. That can be done in many different ways. Some generators use an AC power source through a rectifier bridge to convert the AC to DC that is applied to the rotor windings via slip rings. Some generators have a small DC generator that is coupled to the generator rotor and spun by the prime mover to produce DC power that is then applied to the generator rotor windings, again, usually through slip rings. Some generators use a small AC generator coupled to the generator rotor to produce AC that is then applied to rectifier diodes mounted on the generator rotor to convert the AC into DC that is then applied to the generator rotor windings. There are many ways to do this, but commutator windings are not generally used these days, because of the maintenance requirements of brushes and commutator bars.

I hope this answers most of your questions!

 

LukeeVasallo,

You may be referring to a type of excitation system ("exciter") that's called a brushless excitation system. A small, stationary DC field, which is powered from the excitation control system ("AVR"), is used to create AC on a rotating armature inside the stationary DC field. The rotating AC armature is coupled to the synchronous generator rotor shaft and is driven by the turbine. The output of the rotating AC armature is fed to rectification diodes which convert the AC to DC, which is then applied to the synchronous generator rotor windings.

Again, there are MANY different ways to apply DC power to the rotor of a synchronous generator. Some older methods did, in fact, use DC generators with commutators, but that's not very common today in most modern generators. And, again, the reason is usually to reduce maintenance costs, as well as construction costs.

 

CSA,

Thank you for your detailed reply. Now it makes more sense, as of course the rotor type you choose will depend on what will be driving it.

With regards to the DC excitation system, with the diodes that will be brushless yes, as everything is mounted on the rotor.

What I don't understand is since the armature coil of a dc exciter, is rotating on the same shaft that is generating the rotating magnetic field, it couldn't be generating power are there wouldn't be flux linkage no? and the armature of the dc generator would also be 3 phases?

Again many thanks

 

LukeeVasallo,

The rotating portions of brushless exciters, and PMG exciters (yet another type of exciter used on smaller synchronous generators), are mounted on stub shaft that is coupled to the end of the generator rotor opposite the prime mover coupling. The exciter rotating portions are electrically and magnetically separate from the generator rotor windings--except when diodes are used to convert the AC from the brushless exciter into DC to apply to the generator rotor windings.

A synchronous generator used to produce electricity supplied to a grid or load(s) has AC flowing in the stationary windings (the stator windings), and that is sometimes called the armature. A brushless exciter is a synchronous AC generator with a stationary DC field and rotating AC armature. There doesn't need to be any brushes because the AC output is connected directly to the rotating diodes on the generator rotor shaft that convert it to DC and than apply it to the generator rotor windings.

I think the problem here is the use of the word 'armature' which I've always believed referred to the part of a rotating electric machine where AC flows or is produced. That can be stationary, as in AC electric generators and -motors, and rotating as in rotating DC motors and generators. Remember all that's required is relative motion--it doesn't matter if the armature is stationary or rotating as long as the conductors cut through or are cut by another magnetic field.

It can get confusing in the beginning, especially when one can't always walk out to the turbine deck and observe examples of what's being taught or discussed or studied.

As for the number of phases of the DC generator armature, that depends on the construction of the machine. I've seen single-phase and three-phase; three-phase produces a slightly smoother wave-form when rectified.

Hope this helps!