Is the Moment of Inertia of a compressor dependent on its speed? Why?

[JustSomeRoark]

Dear all,

When compressor Vendors state the moment of inertia of their machines/drivers they normally state the MOI and a reference speed. I have never fully understood why: I assumed that the moment of inertia of a rotating element is constant irrespective of its speed.

Aspentech HYSYS' manual for dynamic simulation states the following:

"If you have inertia for different parts of the train rotating at different speeds, you need to pro-rate these to a common speed basis using the formula

Ib = SUM(Ii * GRi^2)

where:

Ib = rotational inertia with respect to some base speed
Ii = rotational inertia of the ith component on the rotating shaft (with respect to the speed at this gear ratio)
GRi = gear ratio of the ith component defined at wi/wb (angular speed of ith component divided by base angular speed)"

Therefore, if the compressor has a MOI of 2 kgm2 @ 18000 rpm but I want to express its MOI at 1800 rpm then the following calculation is required:

MOI@1800rpm = MOI@18000rpm * (18000/1800)^2 = 2 * (10)^2 = 200 kgm2 @ 1800 rpm

Does this make sense to you? Why is the MOI dependent of a certain "reference speed"?

Regards.

 

[zeusfaber]

My guess is they're quoting a moment of inertia measured at the motor shaft, assuming that the compressor speed is set using pulley ratios (change those, and you change the moment of inertia presented to the motor).

A.

 

[1503-44]

https://engineering.stackexchange.com/questions/13173/inertia-referred-to-motor-shaft

 

[Gholam Baghery]

Hi
It seems very clear. if you consider the L (angular moment) equal to L=mvr which m s the mass and v is velocity and r is rotation radius you simply see all of them are constantly in a compressor except for velocity. The moment of inertia is I=L/ω and again the ω affects the velocity. So, all change in your moment of inertia of your compressor is just speed of rotation. It will be logical if the vendors tell you about this parameter for indicating the compressor moment of inertia.

 

[Strong]

My understanding is that MOI is not speed dependent per se, but if there are multiple rotating masses at different speeds because of gears/belt/chain speed change, then the composite MOI is based on torque (speed Ratio).

Walt

 

[desertfox]

Hi

Have a look at this site:-

https://isaacphysics.org/concepts/cp_moment_inertia

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein