Moment of Inertia?
Moment of Inertia?
(OP)
Hi
I have an 11kV, 50Hz, 4.818MVA, 16-pole hydro generator. Shaft height = 1m, width = 1.95m. Any idea what the moment of inertia in J in kg.m2 is please? or what is a typical value for this type of machine. Have tried the manufacturer but it's like trying to pull chicken teeth!
Thanks in advance.
I have an 11kV, 50Hz, 4.818MVA, 16-pole hydro generator. Shaft height = 1m, width = 1.95m. Any idea what the moment of inertia in J in kg.m2 is please? or what is a typical value for this type of machine. Have tried the manufacturer but it's like trying to pull chicken teeth!
Thanks in advance.





RE: Moment of Inertia?
RE: Moment of Inertia?
RE: Moment of Inertia?
J=GD^2=1/8*W*D^2 W=weight in kg ; D=rotor diameter [m]; J [kg.m^2]
If it is not known ,we may use the H constant formula:
J=2*H*Srated/(2*π*Frated/N)^2[kg.m^2]
Where H constant [2-4 Ws/VA]; Srated [4818000 VA]; Frated =50 Hz N=number of pole pairs[8]
In your case it could be 12500 to 25000 kg.m^2.
The above formula it is the same as per IEEE Std 399/1997 Brown Book 4.9.3.3.2 The H constant.
See[for instance]:
http://www.mathworks.com/help/physmod/sps/ref/mach...
file:///D:/D_My%20Documents/Engineering/Generators/Synchronous%20generator%20dynamics%20Lund%20Un.pdf
RE: Moment of Inertia?
Still working through your equations. First one no problem. Busy with 2nd one.
I did finally manage to get the rotor data from the OEM. The stated J = 2723 kg·m2 is substantially lower than the predictions in this post and I'm wandering why?
However, if I use the equation for J = m·r2/2 (identical to your first equation) = 3080kg·m2 as follows:
Add up the weights of the fan + shaft + magnetic wheel + rectifiers + exciter = 14582kg
Assume radius, r = 0.65m (approximately) to get
J = 14582*0.652/2 = 3080kg·m2 which is in the ballpark of 2723kg.m2
RE: Moment of Inertia?
Missing decimal place?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Moment of Inertia?
H = K/Sr = (5.48e-6·J·N2)/Sr =(5.48e-6·2723·3752)/4818000 = 0.436 J/VA.
where K = kinetic energy at rated frequency in Joules
Sr = rated power in VA
N = speed in rpm
Also a bit outside the range of 2-4 Ws/VA.
RE: Moment of Inertia?
Your 2nd formula, J = 2*H*Sr/(2*Π*f/N)2 is the same as mine. I used rpm instead of frequency and pole pairs. If H = 0.436, I get J = 2723 again using your formula.
Still a bit perturbed by the substantial discrepancy between the OEM values for J and the values suggested in this post.
RE: Moment of Inertia?
RE: Moment of Inertia?
Joe retires and the new guy, usually with an MBA or what ever throws "all that old junk" out.
I have seen this time and time again!
I have seen the inertia checked from a free run down. It was done graphically, but was 40 years ago, so sorry cannot remember the details. It was done at low speed, to minimize windage. I don't know the practicalities of doing it in your case.
RE: Moment of Inertia?
J=GD^2/4/g g=9.81 m/s^2
See[for instance]:
http://www.nissei-gtr.co.jp/english/gtr/pdf/mini_e...
RE: Moment of Inertia?
RE: Moment of Inertia?
Sorry, could not open the attachment. Needs some Adobe Acrobat Extended Asian Language font pack which I unsuccessfully tried to install. Maybe an alternative way to resend?
RE: Moment of Inertia?
When saving the file and attempting to read it, it does not open.
Capturing two screen shots and inserting them into a word document, I came up with this:
RE: Moment of Inertia?
7anoter4 - makes sense now.
Hoxton - I know what you mean. Thankfully, this is a new generator and the OEM is still around. Just not very good at communicating.
Regards.
RE: Moment of Inertia?
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(2B)+(2B)' ?
RE: Moment of Inertia?
Anyway, I offer the app note here. Comments (even corrections!) welcome.
Curt Wilson
Omron Delta Tau
RE: Moment of Inertia?
J = m·r2/2 is the formula for the inertia of a solid cylinder of mass m and radius r rotating on its axis. Units are kg·m2 (SI). I have seen this in more than one reputable text on the subject. However, I will also be first to admit that I have taken it on face value as I have never delved into deriving it from first principles.
Reading through the excellent text of Curt, also verifies this formula.
So I am still interested to know why you say there is no /2?
RE: Moment of Inertia?
Yours would apply for solid cylinder of radius R.
I don't know which context you were working with.
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(2B)+(2B)' ?
RE: Moment of Inertia?
RE: Moment of Inertia?
Are you able to share any “hint” as to what you’re actually attempting to solve?
Your original post was worded, “…Any idea what the moment of inertia in J in kg.m2 is please? or what is a typical value for this type of machine.”
For proprietary reasons, it may be out of place to ask what you are going to do with the answer when you finally narrow it down.
Yet, I’ll throw the above sentence in the recycle bin and ask, what are you going to do with the answer?
John
RE: Moment of Inertia?
LoM = ROCOF and vector shift. Client cannot afford any transient studies and has requested I "see what I can do". I have found some guidance regarding this from ENA Engineering Recommendation G59, Aug 2014. In particular section 13.7.2 shows how the df/dt can be calculated manually.
Hence my request for the J value. If you are interested I'm willing to share my findings.
RE: Moment of Inertia?
RE: Moment of Inertia?