1. ## rotor motor inertia.

the formula to calculate the rotor motor inertia for 3 phase induction motor. in kg-m2

2. Senior Member
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Aug 2004
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11,170
Originally Posted by srinivasan
the formula to calculate the rotor motor inertia for 3 phase induction motor. in kg-m2
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.

3. Originally Posted by srinivasan
the formula to calculate the rotor motor inertia for 3 phase induction motor. in kg-m2
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.

4. Originally Posted by petersonra
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.
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...

5. Originally Posted by Besoeker
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*r2/2m being mass.m = pi(r2) * length * material density.The density of steel is around 7,800kgm-3I 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.
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

6. Originally Posted by srinivasan
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.

7. This data is typically provided by the supplier.

The formula -

H = 5.48E-9 * WR2 * RPM2 / MVA (When WR2 is in kg-m2)

Total RPM

The total RPM is equal to the Motor RPM.
Total WR2

The total WR2 is calculated based on the Total RPM and Total H using the equation above.
Total H

This is the arithmetic sum of the Motor, Coupling and Load H in MW-sec/MVA.

8. Originally Posted by kingpb
This data is typically provided by the supplier.

The formula -

H = 5.48E-9 * WR2 * RPM2 / MVA (When WR2 is in kg-m2)

Total RPM

The total RPM is equal to the Motor RPM.
Total WR2

The total WR2 is calculated based on the Total RPM and Total H using the equation above.
Total 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.

9. Originally Posted by Besoeker
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.
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.

10. Originally Posted by kingpb
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.
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?

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