srinivasan
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- Bangalore,India
the formula to calculate the rotor motor inertia for 3 phase induction motor. in kg-m2
rotor motor inertia.
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petersonra
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- Northern illinois
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- Semi-retired engineer
There is no special way to calculate motor rotor inertia. It is calculated the same way you would calculate the inertia of anything else. To do so you need to know the shape of the thing and its mass. Most of the time you can get this information from the manufacturer fairly easily.
I assume you mean motor rotor inertia rather than rotor motor inertia?
As petersonra has stated, the rules are no different for this.
The difficulty here it that it is a complex assembly with different materials and usually with some holes.
Depends on how accurately you want it and for what purpose. For a quick and dirty approximation I'd take a solid steel cylinder the length and diameter of the rotor and the steel shaft, also as a cylinder and add the two together. This is likely to give you a worst case.
FWIW, solid cylinder inertia is
I = m*r2/2
m being mass.
m = pi(r2) * length * material density.
The density of steel is around 7,800kgm-3
I don't know what application you require this for.
In most that I've dealt with where inertia calculations are required, the load inertia has greatly exceeded motor inertia when referred to either load or motor relative speed.
srinivasan
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- Bangalore,India
my customer asking this information , and we are not a motor manufacture i can get simply from supplier. as a electrical person i need to know what is the thing. so only asked. any way thanks sir...
srinivasan
Member
- Location
- Bangalore,India
fd fan motor for incinerator package. my customer is asking this information.. better i can ask from supplier. but now i know something because of you. if any worst case i can apply this... thanks
YW.
We've put a number of variable speed drives on fans and, in my experience, the fan inertia far exceeds that of the motor.
kingpb
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- SE USA as far as you can go
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- Engineer, Registered
This data is typically provided by the supplier.
The formula -
H = 5.48E-9 * WR2 * RPM2 / MVA (When WR2 is in kg-m2)
[h=6]Total RPM
[/h] The total RPM is equal to the Motor RPM.
[h=6]Total WR2[/h] The total WR2 is calculated based on the Total RPM and Total H using the equation above.
[h=6]Total H[/h] This is the arithmetic sum of the Motor, Coupling and Load H in MW-sec/MVA.
The formula -
H = 5.48E-9 * WR2 * RPM2 / MVA (When WR2 is in kg-m2)
[h=6]Total RPM
[/h] The total RPM is equal to the Motor RPM.
[h=6]Total WR2[/h] The total WR2 is calculated based on the Total RPM and Total H using the equation above.
[h=6]Total H[/h] This is the arithmetic sum of the Motor, Coupling and Load H in MW-sec/MVA.
Inertia does not depend on rotational speed. See post #3.
FWIW, solid cylinder inertia is
I = m*r2/2
m being mass.
No rpm required.
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
You'll notice that the formula is for H when inertia and RPM are known, or to calculate inertia when H and RPM are known.
It does give the inertia when no other info is known.
Sorry if it was misleading, maybe should have clarified better. ETAP uses this info to do motor starting dynamic analysis.
You'll have to explain to me what H is a measure of.
I've had to do a lot of dynamic analysis to determine machine and drive ratings for various projects we have undertaken.
But H is new to me.
In simple terms what is it?
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
H is the inertia constant:
Where:
H = Inertia constant in MW-sec / MVA
J = Moment of inertia in kgm2 of the rotating mass
ω = nominal speed of rotation in rad/sec
MVA = MVA rating of the machine
Where:
H = Inertia constant in MW-sec / MVA
J = Moment of inertia in kgm2 of the rotating mass
ω = nominal speed of rotation in rad/sec
MVA = MVA rating of the machine
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OK.
1/2Jω2
gives the energy stored in the rotating mass or the energy required to get it to that rotational speed.
Incidentally, it's Joules rather than MW seconds. But that's just a multiplier.
So that gives you H as a measure of energy divided by MVA rating.
I don't know how you'd use that. You could have different machines with the same inertia and rated speed but different MVA ratings.
And the original question was about calculating inertia.
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