kenkad said:
This is what I meant by vertical laminations, bent into a 'C', stacked, to produce a trapezoidal end. It seems to me that the flux path out the end is now distributed. I used different colors for the lam layers to better see the result. I have never seen a sim of this, but have always wondered why this is not done. My preference would have been to do this as a powdered core so that the center could also be trapezoidal, providing a better motor fill with a split copper strip winding like Shane did in one of his motor experiments. I hope this explains my question to everyone. As to why the more than one 3-phase group question. That is because starting torque requirements to get a vehicle moving are greater than running torque requirements at a speed, so I would op to turn off a 3-phase group to reduce motor power consumption since I do not believe the cogging would be noticeable at speed, momemtum effects.
There's a few separate issues we can break down. First, lamination direction: vertical/radial vs. horizontal/tangential. Eddy currents are due to the induced EMF, whose direction is given by v x B (vector cross product). For an axial-flux motor, this gives an induced EMF in the radial direction (v is tangential, B is axial, thus EMF is radial). Thus, eddy currents in the radial direction will be created in any conductive material present. By laminating the cores in a horizontal fashion, we keep the conductive distance in the radial direction short and thus limit the eddy currents which can flow.
Second, the shape of the teeth at the end of the cores. I agree with you that it would probably be beneficial to make the ends wider than the center - the proper tradeoff between iron and copper can be determined through simulation later. Looking in towards the center, this would give the cores a sort of dog-bone shape. You're also talking about making face of the tooth wedge-shaped, rather than rectangular (looking axially). I brought that up a little while ago as well. That would be relatively hard to do with this lamination direction, and I'm not certain it would really give significant benefit. On the other hand, leaving the ends rectangular will I think give the BEMF more of a sinusoidal shape, which might actually be a plus.
Third, Miles is correct that electrical steel is a better material than a powdered core. Harder to work with and more expensive, but better magnetic properties.
Lebowski said:
I looked at it from a different angle... In my opinion you need to convert
electrical power efficiently into mechanical power, independent of whether
you want to race uphill or just ride around on the flat. Core laminations help
with efficiency but only in a certain rpm range (too low rpm -> cogging,
too high rpm -> eddy current losses). No iron core gives a lower specific
torque but this can be solved by chosing a different gear ratio so I wouldn't
say this is a disadvantage of having no iron core.
For me in the end the efficiency with no core was good enough, the disadvantages
of the iron core outweighed its advantages.
Again, a couple of separate issues. Miles is correct that an iron core gives more torque. Specifically, because you can achieve higher flux densities, the motor has a higher power/torque
density (obviously you could make a large ironless motor with more torque than a small iron-core motor).
Second, efficiency. I disagree that cogging represents an efficiency loss. Cogging is both positive and negative torque, so the net energy contribution is basically zero. It increases the torque ripple, but since Miles is gearing his motor down I think torque ripple will be much less noticeable than with direct-drive (higher frequency, lower amplitude).
There's probably a study out there to prove this one way or the other, but I would tend to think that ironless is not inherently more efficient than an iron core. You do avoid most of the eddy losses, but in exchange you need more current to achieve the same torque with lower flux density, so the copper losses will be higher. I suspect that eddy vs copper losses would be a pretty even tradeoff in a motor with a high-quality core (thin lams of high-grade electrical steel). For our practical purposes, however, I think ironless is attractive because it's easier to build a good DIY motor this way even if there is a torque density tradeoff. You could also probably say that ironless has more advantages at high RPMs and iron core has more at low RPMs.