Motor inertia information 3

[c2sco]

Please can you point me to information on how to estimate the inertia of a motor from its power, speed, etc, and to tables of data for the same?
Thanks

 

[itsmoked]

Go back to your physics text books.
You will see from the physics books that inertia has no connection to power. You need the weight of the rotor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[c2sco]

Thanks, yes I know that. What I'm looking for is pragmatic guidance on approximate values given that I don't know the weight and dimensions of the rotor, just what it says on the nameplate. This is for input into dynamic modelling of pumping systems that are at risk from water hammer.
Stuart

 

[mohpower]

C2sco, i think you need to contact the manufacutrer to get you the data. you cannot find them in the nameplate of the motor. the mass of the rotor is not in the nameplate ...which software are you using for this study. can you send us what exactly what is required by the software?.
thank you

 

[c2sco]

Yes, perhaps I will. I'd hoped to find something general and approximate to save the trouble of finding out exactly what motor it is. I'm using software packages Flowmaster and HiTrans. They just need to know the total inertia of the pump to model the slowing down of the system after a pump trip. Other factors such as frictional torque are also needed and I have ways to estimate those.
Usually the motor inertia is the dominant factor in the total inertia (except for magnetic drive pumps) hence my interest. I came across some algorithms some years ago to estimate inertia from the motor speed and power, but I found them inaccurate compared to the manufacturer data I collected then. I'm doing a new study, and Google etc didn't turn up anything earlier today so I thought I'd try asking the expert!
Stuart

 

[sms]

For large motors inertia is usually listed on the machinery data sheet in the operation or maintenance manual. If you have a spare rotor you can also do a dimensional drawing based on measurement and then do a pretty accuarate calclation..But start with the data sheets.

-The future's so bright I gotta wear shades!

 

[Clyde38]

Calculate inertia from published data for the motor.
?m Mechanical Time Constant (seconds)
R Resistance (ohms)
KT Torque Constant (nM/A)
KE Back EMF Constant (v/r/s)

Inertia = (?m•KT•KE)/R Kg•m2

You can use other units of course, just be consistent.

 

[THooper]

A load which tends to cause the motor shaft to continue to rotate after the power has been removed (stored kinetic energy). If this continued rotation cannot be tolerarted, some mechanical or electrical braking means must be applied. This application may require a special motor due to the energy required to accelerate the inertia. Inertia is measured in either lb.ft.2 or oz.in.2

Inertia reflected to the shaft of the motor=
(Load RPM)squared/Motor RPM.

This info is on Reliance web site.

kind regards
Tim

 

[THooper]

http://www.reliance.com/mtr/aplel.htm

 

[DickDV]

I don't know what level of accuracy you need but, it seems to me that a reasonable estimate of rotor wk^2 could be based on the NEMA frame number.

If you've got a 326T frame motor, whether the motor is wound 30hp 6 pole or 40hp 4pole or 50hp 2 pole wouldn't change the rotor inertia much. Or would it?

 

[electricpete]

NEMA MG-1 has exactly what you're looking for in a section about dynamic braking. I'll see if I can track it down.

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[electricpete]

Here it is:

NEMA MG-1 2003
"14.46 ROTOR INERTIA FOR DYNAMIC BRAKING
The rotor inertia (Wk2) in lb-ft2 for the application of medium ac induction motors with dynamic braking
equipment may be estimated by the following formula:
WK^2=0.02*2^(poles/2)*hp^(1.35-0.05*poles/2)"
HP in HP. WK^2 in ft^2-pounds


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[jraef]

He beat me to it. A star for being quick at the reference.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[aolalde]

Good Electricpete, I have been looking for that formula for several years and it was there!!. you deserve another star.

 

[KLee777]

c2sco,

When modeling pump inertia for a transient hydraulic analysis, you should also take into account the inertia of the pump and entrained liquid, not just the motor.

AFT Impulse software provides the following reference to estimate the inertia of the pump impeller, entrained fluid, and motor rotational inertia:

Wylie, E.B., V.L. Streeter & L. Suo, Fluid Transients in Systems, Prentice Hall, Englewood Hills, New Jersey, 1993

KLee777

 

[c2sco]

Thanks - that's a text I don't have, I'll look it up.
Thanks to all - very helpful.
Stuart