Induction motor Inertia
Induction motor Inertia
(OP)
Hello,
We are currently simulating the motor starting of some large medium voltage motors.
I have some questions that I hope to receive your feedback to understand:
1) Manufacturers' catalogue state intertia. Is this value the inertia value for the motor without load, I mean, only the rotor without any mechanical load coupled?
2) Does this inertia value vary too much?
I mean, in one manufacturer it could be 100 and for the same motor characteristics, another manufacturer says 20.
3)If we do not have any data about the mechanical data, and we know it is only a fan or a pump, how can we model the load characteristic?
I thank in advance your answers/ideas.
We are currently simulating the motor starting of some large medium voltage motors.
I have some questions that I hope to receive your feedback to understand:
1) Manufacturers' catalogue state intertia. Is this value the inertia value for the motor without load, I mean, only the rotor without any mechanical load coupled?
2) Does this inertia value vary too much?
I mean, in one manufacturer it could be 100 and for the same motor characteristics, another manufacturer says 20.
3)If we do not have any data about the mechanical data, and we know it is only a fan or a pump, how can we model the load characteristic?
I thank in advance your answers/ideas.





RE: Induction motor Inertia
2. It should be similar for same hp and speed.
3. You can't. But consider typical variable torque loads such as fans and pumps will have similar speed-torque curves.
RE: Induction motor Inertia
RE: Induction motor Inertia
3)For a first order approximation, model the torque proportional to the square of speed ( or power as the cube of speed).
RE: Induction motor Inertia
I thank your answers!
Just one additional question:
We modeled a load using a square torque approximation and the motor starts faster with this load that without it.
Could anybody give us a clue or tell us why?
Best Regards
RE: Induction motor Inertia
RE: Induction motor Inertia
RE: Induction motor Inertia
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RE: Induction motor Inertia
GD^2 [metric] in to variants: in kgf.m^2 or kg.m^2 where kgf.m^2=gmetric*kg.m^2 [gmetric=9.81 m/sec^2 gravity acceleration]
Wk^2[British] also in to variants in lbf.ft^2 or lb.ft^2 where lbf.ft^2=gbritish*lb.ft^2 [gbritish=32.19 ft/sec^2 gravity acceleration]
I [or J] =GD^2/4/gmetric if GD^2 is expressed in kgf.m^2 or I= GD^2/4 if it is kg.m^2
In British units I=Wk^2[lb.ft^2] or Wk^2[lbf.ft^2]/gbritish.
Then 1kg.m^2[GD^2] corresponds to 5.933 lb.ft^2[Wk^2]
If one knows rotor Mass [kg or lb] and the rotor diameter [m or ft] then I=Mass*Dia^2/8 [kg.m^2 or lb.ft^2].
If one don't know rotor Mass [kg or lb] and the rotor diameter [m or ft] then [as a [very!] approximate mode of calculation] one could calculate as follows:
Regards