whats the moment of inertia of a 900 KW motor ? 1

[bshamad]

while i was trying to simulate a 900 KW compressor motor using ETAP; i get stuck with the moment of inertia values for the motor, coupling, and compressor. does the starting torque of the motor give any indication about the motor moment of inertia?

cheers.

 

[electricpete]

This is data that normally would be obtained from OEM....data sheets... outline drawing.

You can also estimate WK^2 from dimensions and density... but this can be a little tricky when you have a part with ducts and holes in it.

NEMA MG-1 14.46 (Small and medium motors) has a formula for estimating motor rotor inertia for the purposes of dynamic braking calculations.
WK^2 =0.02*2^(poles/2)*hp^(1.35-0.05*poles/2)
HP in HP. WK^2 in ft^2-pounds

You motor would be a large motor so I'm not sure how applicable.

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

 

[niallnz]

Extrapolating a bit for the ABB HV/MV motor catalog (only goes up to 710kW for 2pole, inertia 15 kgm2) I would guess that a 900kW 2pole will be about 18 kgm2.

Cheers Niall

 

[LionelHutz]

Get it from the motor manufacturer. I've seen the inertia vary probably over 1000% between motors of similar kW ratings depending on the motor speed and the design of the motor. That is a lot of error if making a "best guess".

 

[lyen12]

Anyone knows what WK and GD stands for in WK^2 and GD^2?
I know that GD^2 = WK^2 * 4 (the former is in diameter, the latter is in radius).

 

[electricpete]

k stands for radius of gyration

W perversely stands for weight for unfathomable US/British historical reasons (see thread404-219055 )
... but I would just pretend W is mass in lbm and life is better.... since the quantity we're after should have units of mass times distance squared.

I try to stay away from GD^2. But if it has the relationship to WK^2 that you stated, then G would have to be mass and D would be twice the radius of gyration.

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

 

[bshamad]

thanks for all,I went through data sheets of this motor, what i could find is the starting torque (LR torque), can i guess the MOI from that?

 

[electricpete]

Not really - there is no connection between locked rotor torque and inertia that I can think of.

An indirect way - If you have access to accelerating time vs t curves for the motor by itself along with motor torque speed curve, you could compute inertia from that. Likewise if you have access to accelerating time vs t curves for motor driving pump, along with motor and pump torque/speed curves, you could find total inertia of pump and motor from that.

Even more indirect, I guess if you had the torque vs speed curve and thermal limit curve, you could calculate the maximum inertia that would allow the motor to safely accelerate itself under DOL start... would probably be much higher than actual inertia.

If you had detailed drawing of rotor, you could estimate it from that with a lot of work.

Of course direct way is preferable. Get it from OEM or their documentation. Do you have motor outline drawing... sometimes it is stated on there.


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

 

[bshamad]

oh.. your right! i could see this under the time curve of the motor (Wk2 = 135 Lb-Ft2, friction torque = 478 Lb.Ft)!
great ... now i can simulate starting this motor