The model appears useful but Iâ€™m a like a fish outta water trying to understand it.
Do I need to worry about imaginary currents? Are the values in the 9C model workable? What do I need to change to get the model to work properly?
Many thanks, KF
You are right about using the sin(angle) to determine the current in the phase. But you have to make sure that your Phase A (assuming that is the phase that starts at 0 degrees) is actually at a zero-crossing of the voltage (i.e a maximum point of the Flux Linkage).
I took the liberty of updating your model so that it will work, and attached the script to get torque as you rotate the rotor.
Here is what I did to the model.
1) I moved it to so that the center was at 0,0 so that the torque calculations work. To figure out how far to move it, I selected the 2 arcs that form the center hole, and then click the "scale" button, it automatically selects the center of the points selected as the default base point (I usually copy and paste those values into a text document then paste them back into FEMM after I select all the geometry and click the "Move" button, but in this case I suspect you actually drew your design in FEMM because the center was at 122 , 122 I think) . That scale to find the center trick comes in handy when you import a DXF and it puts the center at some randomish point
2) I made all the components of the rotor part of group 2 (I use the group tool, and circle select to select everything starting from 0,0, make it all group 2, then I do the same thing again but only drag the circle select out to the airgap between the stator and the rotor so that only the stator is selected then make that group 1 again)
3) I added air around the outside of the stator and put a boundry conditino on the outside of that. That way you can see what happens if you make your rotor back iron too thin. I made the center air aswell for the same reason as adding air around the outside
4) I made your blocks to "let triangle choose block size" because your meshing value of 1 was way too slow at solving and doesn't provide much more accurate information. As a rule, I always let the triangle choose block size, and if I find I want tigheter meshing in a particular spot, I will redifine a block around the part I really want to know the details, it is much .... much faster and provides the same results. (I was lazy and didn't take the time to do all the copper sections , but it is still much faster and good enough .. I'll let you fix the copper blocks). I also updated your stator to use M19 laminations rather than solid iron, it makes a small difference to the saturation point of the stator.
5) Set the problem frequency to 0 (the frequency has to be done manually along with the rotation of the rotor in the lua script), and set the currents in the circuits all to 0 aswell.
After that I ran my script to rotate the rotor 1/2 an electrical cycle counter clockwise to see how the EMF looked. It appers as though if you want to drive this as a motor counter clockwise (from the position that your rotor is in right now), you want your phases to sart off at
Because of the crazy tooth / pole combination it makes it hard to visually determine if the rotor is at the maximum flux linkage position for phase A, but from my quick simulation it seems good enough to use those as your torques. I made my simulation take larger steps because of the tight meshing makes things slow and I didn't want to wait an hour for the result so I am not that confident that the phase angles of A is exactly -180, from the visualy inspection I suspect it is actuall more like -190 degrees (i.e the rotor is a couple degrees counter clockwise past the maximum flux linkage position of phase A)
Anyways, from this simulation I suspect the Kv of the motor to be 17.1 RPM/V in Delta, or 9.84in Wye. Those seem low to me, so there might be some parameters wrong in the model. What is the real Kv of this motor?
I attached a .zip with the modified .FEM file and the 3 lua scripts I updated to work on the model. New flux goes thorugh and gets flux linkage as you rotate the rotor at no-load, so you can paste that into a spreadsheet and do back emf calculations and stuff. Running Torque is pretty much the same thing as new-flux but you set the RMS phase current you want (in Delta) and it extracts the torque as it goes through the rotation of the rotor, you can take screen shots of that (use the mo_savebitmap command I think) to see what happens to the flux density in the teeth as you increase the current. Variable torque lets you quickly figure out how current in the coils affects the torque. The settings I have in there show that even as you push 100A RMS per phase through this thing (100NM of torque) the Kt is still pretty linear wich is a good sign that the magnetic design can handle that much toruqe, To figure out if the electrical circuit can handle that much heat is the bigger question.
have fun with that.
The next things to do to improve your model woudl be to make the stator teeth body section uniform thickness rather than the stator slots uniform width. The way it is right now, the bottom of the slots pinch the flux in the tooth. Also the thickness of the tooth heads looks a little thin to me, but I haven't looked at a 9C stator so maybe that is how they are made.
*Edit changed the sign on the C phase angle so that it was correct, and removed the text about how to tell if the rotor is in maximum flux becase it doesn't apply to this crazy pole/slot combination.