Dear ALL,

I have a small query regarding alternator as follows,

If the alternator Produce 50mw (rated load) with 2 poles in 50hz frequency at 3000rpm coupled with gas or steam turbine.

is it possible to change the 2 poles to 4 poles alternator (same rated as 2 poles load), for the same 50hz frequency with 1500rpm (rotational power) from the same turbine with reduction gear?

kindly reply...

 

Jameelrahman,

In theory it's possible to change the alternator RPM input required to produce the same frequency by changing the number of generator poles. However, the same turbine could not be used unless the reduction gear box was also changed. Alternator prime movers are all designed and built to produce torque at a particular speed. If the prime mover speed had to decrease by half to produce the same frequency at the generator terminals the ability of the prime mover to produce torque would likely be much less than half of its ability at the higher speed--especially for a turbine as opposed to a reciprocating engine's ability to produce power over a wider speed range.

If you're referring to a combustion (gas) turbine, it's the limitation of the axial compressor to move air at lower speeds that represents the inability of the unit to produce rated power at half the speed. Gas turbine axial compressors are extremely inefficient below design (rated) speed. It would be very necessary to change both the reduction gear ratio if the number of generator poles were changed.

 

Jameelrahman...

I'm pretty sure that CSA meant to say that regardless of rotor-type, i.e., salient-pole or cylindrical, it is not physically possible to change the number of poles a generator has.

Thus, the only possible approach would be to alter rotational velocity (rpm). But, that entails severe constraints! Some of which CSA has discussed.

Now, if you really meant replacing a 50Hz machine with a 60Hz machine, or vice-versa, there are several Control.Com threads that you may find helpful.

Regards,
Phil Corso

 

Phil Corso,

CSA said precisely what he meant to say, choosing not to delve into how difficult--or impossible, probably--to design and build a four-pole replacement rotor that could safely be used in a two-pole alternator at the same power as was previously being produced.

Frankly, the original post is really unclear. It wasn't clear if the original machine had an existing reduction gear or was directly coupled to the alternator and operated at 3000 RPM.

In re-re-re-re-reading for a second time, he may be asking if an entire four-pole alternator of the same rating could be substituted for a two-pole alternator and then, using a reduction gear could the same turbine be used to drive the four-pole alternator. If so, the answer would be be, in theory, yes, but the engineering and structural (foundation) changes required would be substantial and likely extremely costly. The addition of the reduction gear (if one was not previously present) would make the turbine-alternator set longer, and would require more oil piping and a larger L.O. heat exchanger and changes to the high voltage connections, more fire extinguishing capability, new load couplings, etc.

But, still, the original post is unclear and I said precisely what I meant to say, consciously choosing not to talk about the near-impossibility of replacing a two-pole rotor in an existing alternator with a four-pole rotor while maintaining the same power rating and frequency, concentrating, instead, on the difficulties of try to change the speed of the turbine to match the newly configured generator.

One never knows the reason for questions like this; many times it's just to test their understanding of principles not that this would ever be attempted unless there was a near unlimited source of money and and an extremely high payback for the re-configured turbine-alternator.

And, I don't believe that we will ever know the reason for the question or get any clarify--without doubts, those dreaded doubts.

Thanks, though for treading where I didn't want to go. Hopefully between the two answers jameelrahman will understand what is and isn't possible--but might be possible given unlimited funds and engineering resources and time. Perhaps a two-pole alternator was destroyed and a similarly rated four-pole alternator could be found and it was hoped it could be coupled through a reduction gear to keep the same turbine in service. Who knows; no matter how one tries to understand the original post it really is a very odd question without context unless it's entirely theoretical.