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Acceleration Calculation


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#1 pens92

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Posted 28 March 2011 - 05:23 PM

So given a flywheel with an inertia 'I' I am trying to figure out what the inertia torque would be so I can properly size the AC motor. I have come across many topics and they all say to select an alpha, determine the torque (T = I x alpha), and compare this torque to motor torques with different gearboxes (neglecting friction for the moment). My problem is I can set alpha to whatever I want but in reality it will be dictated by the motor. Isn't alpha a dependent variable once you select a motor and if so how might one compute alpha?

#2 jOmega

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Posted 28 March 2011 - 10:17 PM

QUOTE (pens92 @ Mar 28 2011, 12:23 PM) <{POST_SNAPBACK}>
So given a flywheel with an inertia 'I' I am trying to figure out what the inertia torque would be so I can properly size the AC motor. I have come across many topics and they all say to select an alpha, determine the torque (T = I x alpha), and compare this torque to motor torques with different gearboxes (neglecting friction for the moment). My problem is I can set alpha to whatever I want but in reality it will be dictated by the motor. Isn't alpha a dependent variable once you select a motor and if so how might one compute alpha?



Pens92

Before sizing the motor.... some things must be made known; not the least of which is "What is Alpha: What does it represent" Remember, we all aren't privy to what you've been looking at.

Also, there are some questions that must be asked and answered before getting to an answer (result) ..

  1. Load Inertia: Direct Driven or geared to motor
  2. If geared, geared up or geared down ?
  3. What is the gear ratio ? (from motor to driven load)
  4. What is Inertia of driven load ? (and in what unit of measure is it stated ?)
  5. what is minimum speed -of driven load - (speed at which acceleration begins) ?
  6. What is final speed - of driven load - (speed at which acceleration ends) ?
  7. How do min & max load speed correlate to motor speed - i.e., min load speed = what motor speed and what max load speed = what max motor speed.....
  8. How much time is required to accelerate the driven load from minimum speed to maximum speed ?
From this information, the minimum HP (kW) of the motor can be calculated. I say minimum because you also need to take into account the Duty Cycle HP ..i.e., how often between acceleration / deceleration events ... and how long will it run at constant speed..... Duty Cycle can add heat to the motor which may require increasing the Power Rating of the motor to accommodate the additional heat.

Ok.. that should give you a header in the right direction .... and while you're gathering the answers to the above questions, take a look at ...

see: Constant Torque Acceleration Time of Fan

Some years back, I did a presentation on Torque - Acceleration - Time for a fan.

The analysis and graph are presented in the above link and therein you will see the relationship between inertia - torque -speed - time ...

I think you will find that it will be appropriate to your inquiry.

Look forward to further discussion on this with you.

Kind Regards...

#3 SCIM

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Posted 29 April 2011 - 12:50 AM

Pens92,

You will need to know what's your total load inertia and your load curve.

The final motor selection would be base on your load curve, your total load inertia and method of starting.
With those information, your motor supplier should also be able to provide you a selection.

Or you can select a motor kW rating and ask your motor supplier for the the maximum allowable load inertia which the motor can handle.

Motor manufacturers should be able to provide this information base on DOL starting.


I have done up a spreadsheet for approximate acceleration time calculations complete with curves.
This can also be used to determine if reduced voltage starting would start load etc.

The motor's torque speed curve is fixed and mainly set up for non stewed rotors bars. i.e larger ratings.

Details you will need to have in order to run the acceleration time cals is input of the following -

Motorís Full Load Speed
Motor's KW
Motorís Lock Rotor Torque
Motorís Breakdown Torque
Rotorís Inertia
Loadís Inertia

Select load curve to run -
Full Load kW at Full Load Speed for standard Centrifugal load curve and Constant Torque Load curve.
Or you can custom the load curve to suit.

PM me if you are interested to purchase this spreadsheet.




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