load inertia is greater than NEMA MG 1 2

[ianertia]

Hi,

I am working with a spec that states "Load inertia within NEMA MG 1 (when load inertia is greater than NEMA MG 1, use API STD 541 motor)." I've looks through NEMA MG 1, and I'm having trouble understanding exactly what to look for. I've calculated that my Load Inertia is 246.03 lb-ft² for a particular 300HP 3600rpm motor, but do not know if that is considered greater than NEMA MG 1. Would someone please explain?

Thank you!
Ian

 

[jraef]

Likely this is referring to the info used in determining the starts per hour capability of the motor. That's where load inertia is mentioned. How that will relate to the rest of your spec is unclear from the lack of context.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

Howdy,
I find it s tad suspicious that your calculated load inertia is exactly what is permitted by NEMA MG-1 (Table 12-7). Is this a coincidence.?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[ianertia]

I used the equation they gave in MG 1. I had overlooked that chart. I guess what I am asking is how I would know if the motor load inertia is higher than that.

THanks!

 

[jraef]

That would come from the ME involved, or if there is no ME, the machine mfr. If that's not possible either, you can look at the machine itself and try to calculate it, but it's not easy if you don't know material weights and/or can't see all of the connected (mechanical) power transmission components.

But here's the process:
WK² = W (weight in lbs.) x K² (radius of gyration)² = lb. ft.² (moment of inertia)
W = weight of the load in pounds
K² (radius of gyration) = (radius of disc. in feet)² / 2

But if you have gears and such, you have to determine the total WK² for each one. Same for any belt drives / sheaves.

This is why it's better to get a value from someone who knows it, because in order to size the power train components, they had to know all of this too.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[GroovyGuy]

jraef is correct; your equipment supplier or OEM should be able to provide the inertia of the load.
Do you suspect there might be an issue with your load having high inertia?
What type of load is it (ie centrifugal fan, pump, compressor, conveyor)?
Is there a gear-box in the drive train?
Do you have the option for a VF Drive, or is DOL starter the preferred option?
GG
BTW a API 541 motor is lotsa more $$ than a standard NEMA MG-1.



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[ianertia]

Thank you all for explaining. I was actually initially confused as to what information I needed. Seems like it would be called Allowable Load Wk² for what the motor can handle. Anyway, I understand what I need now. Thanks again!