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

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

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

(OP)
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?

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

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

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

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.


=====================================
(2B)+(2B)' ?

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

ePete,

Very little runs at 60Hz in Europe. smile

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

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 winky smile

=====================================
(2B)+(2B)' ?

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

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. smile

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

(OP)
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?

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

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.

=====================================
(2B)+(2B)' ?

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

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.)

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

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

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

(OP)
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.

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