Have engine moment of inertia, have generator moment of inertia, what is the combined moment?

[tumbleweed1]

We're driving a gen through a gearbox. I have the value of the engine moment and the generator moment. How would I calculate the combined value of moment of inertia for the genset?

 

[Skogsgurra]

Transmission ratio squared.

If engine runs at 3600 and generator at 1800, then Itotal is Ieng/i^2 + Igen, or Ieng/4 + Igen. If you look at it from engine side. From generator side it is Igen + Ieng*4.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

Yup, absolutely correct as usual.

Out of curiousity, do you have any 3600rpm engines driving 1800rpm generators through gearbox where you are?

I figure you probably just didn't give a lot of thought to numbers used for an example, but it did pique my interest.


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(2B)+(2B)' ?

 

[ScottyUK]

ePete,

Very little runs at 60Hz in Europe.

 

[electricpete]

Ha, there is that. My point was gearbox to step a 2-pole speed down to 4-pole electrical generator speed (adds cost of gearbox and makes the generator more expensive to boot... no logical reason). But yeah, there's that 60/50 hz thing too ;-)

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(2B)+(2B)' ?

 

[ScottyUK]

Hi ePete,

The other possibility could be a large low speed engine and the gearbox is bringing the speed up to something more manageable and avoiding the use of a costly low speed generator. The nature of the question suggests that this isn't a small set and 3600 rpm is awfully fast for a large diesel engine. I have the feeling Gunnar picked the figures just to demonstrate a point, so we'll have to wait for the OP to add a bit more detail.

 

[tumbleweed1]

Ah interesting.

Our engine is 1500rpm gearbox to a gen spinning 1800rpm.

I received some info about the gearbox and coupling inertia values which is good. Would it be correct in thinking then:

Drivetrain is the following:
engine-coupling-gearbox-coupling-generator

where w is inertia constant...
[(Ew + Cw + Gw)*(1500/1800)^2] + Cw + Gw = Inertia seen at generator end

Correct?

 

[electricpete]

Yup, that is correct assuming your analysis requires adding them together as a single total effective inertia seen at the generator.
That type model is probably good enough for questions of how long does it take to accelerate from 0 to full speed (with some specified torque characteristic specified for engine and effective torque on generator side factor of 1800/1500 higher).

I would think that other types of dynamic analyses such of finding system eigenfrequencies may require to model as multiple inertia's connected by torsional springs. Obviously a lot more involved.

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(2B)+(2B)' ?

 

[Artisi]

Questions, what power rating for the diesel engine and why 1500rpm, unless it is a large unit, if not- 1500rpm is a waste of hardware.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[iop95]

It's not possible to speed-up engine to 1800rpm (+20%) and remove gearbox?

 

[tumbleweed1]

Artisi- not a diesel, its a nat gas. Likely 1500 because it was built/designed in europe meant for 50Hz system.

iop95- No. Engine is designed and built for most efficient operation at 1500rpm. Yes theres some drivetrain losses but such is life.