How to stop Inertial Rotation in Dynamic Simulation?

[Anonymous]

I set up a simulation of an Alternator (or generator) for a wind energy system which I summarized in the model presented.

The large circular piece would be the Alternator, with a mass of 12,300 kg, which is welded to a 100mm Shaft. Both are connected to a Base by a Revolution joint.

In the DS, I applied a torque of 10 N.mm for 15 seconds, using the Input Graph and leaving the Previous and Later Sectors "Free".

The simulation time was 180 seconds.

I expected that with the applied torque, the Alternator would accelerate for 15 seconds and then, with the end of the torque, it would lose speed constantly, as there would be no force acting and the rotation is very low (23 rpm).

But what happens is that after these 15 seconds he keeps his speed constant, forever, even with zero force and acceleration, as the report presents.

Should I apply any friction to the joint? If so, how can I calculate this friction?

Or should I parameterize any other options?

What is missing? 

@johnsonshiue 

[imajar]

Dynamic simulation does not make any assumptions about real world operating conditions.  It does only what you specify.  This includes friction, air drag, motor inefficiencies, among other things.  You must define all of that yourself.

In your case, I would expect the two biggest parasites to be air drag and bearing friction.  In both cases, you will need to do the research to find mathematically correct models that you can add to the simulation.  I honestly don't know if bearing friction is constant, speed varying, load varying, etc.  And the flow resistance is always challenging to model accurately (unless the flow stays laminar, then it might be reasonable).

It will also be important to know the accuracy of your parasitic loads.  If you only know them to within +/- 40%, then the time the rotor takes to slow down should also be given a similar tolerance.

[Anonymous]

Thank you!

I think I'm going to have to do that then. So if you can help me with some more information, it would be great!

- I intend to discard the air resistance for this model. I will focus only on friction. But, if I want to apply air resistance, where do I find the options to do this? At DS I didn't see anything about air resistance, would it be in another complement?

- Regarding friction, I can configure it in the joint properties, in the torque option, am I correct? There is a Dry Friction area with two fields, Coefficient and Radius. I read the help documentation, but it only says what it is, not how it is used. And it seemed to me concepts from Inventor himself. Could you tell me where is the documentation on how to use these fields?

[imajar · Accepted solution]

My knowledge is limited in dynamic simulation, so hopefully others will chime in as well.

There is no built in tool for air drag, you would have to do the research to determine the appropriate equation and input them yourself, probably as a torque dependent on rotational velocity.  Or possibly as an equation in the joint (also dependent on rotational velocity)

For the friction, yes you input that in the joint properties, joint torque.  You can specify simple dry friction (sorry, I dont know where the documentation is for that, but it's probably as simple as it sounds, just input the coefficient of friction and radius to the friction interface), or you can use the input grapher to create a more accurate drag model as a function of speed, or other parameters etc.  Again, you will need to judge which is the most appropriate one(s) to use depending on your bearing type and how accurate it needs to be.

[Anonymous · Accepted solution]

I think I realized using the Help Center to search and I should have done that before, right? Hehe. So I found some information that helped a lot. I'll give you the links, but I'll summarize what I found to make it easier:

- If I understand correctly, Dry Friction and Radius must be used when lubricating oil is not used. As the Alternator Shaft is lubricated, the Damping field must be used (it is next to the Torque value).

- To use Damping, you must search for the type of oil to be used, in the manufacturer's tables. My question lies in what value to put. Was it viscosity? So I did some tests here with the model I sent, based on a viscosity oil 40 (Castrol 10W-40, for example).

- In the 3 tests I increased the torque to 1200 Nmm for 15 initial seconds in a total of 30 seconds.

- Test 1 with Damping 0 and Dry Friction 0:                   2787 rpm - no speed drop after 15 seconds
- Test 2 with 0 Damping and Dry Friction 0.15 x 5mm: 2576 rpm - speed drop after 15 seconds
- Test 3 with 0.004 Damping and Dry Friction 0:          2688 rpm - speed drop after 15 seconds

- Note that the test values ​​make sense. The test with Damping was between the Dry Friction test and the one with nothing.

- The problem is the source of the data from these tests. Regarding Dry Friction, I used the Coefficient (0.15) that is in the document I found (link below) and the Radius is the radius of the model's axis. Regarding the cushioning ... I had to do several tests. I put 40 (of viscosity) and it gave a very low rpm value. I put 4, then 0.4 and also 0.04. Only 0.004 gave a result that made some sense. But with 0% certainty, hehe.

Then that's it. Thanks for the help. I'll take another look to see if I resolve these doubts. But I will leave the post open for a while to see if anyone is willing to bring some light to the end of the tunnel.

http://help.autodesk.com/view/INVNTOR/2020/PTB/?guid=GUID-5D4A103B-E6B0-4995-94E3-05702FF60AFF

http://help.autodesk.com/view/INVNTOR/2020/PTB/?guid=GUID-F1565A8B-EA71-4E2D-9A79-9CFFF808B4C5#GUID-...