Flux and the Air Gap

Since I've started writing about induction motors, I've been searching the internet for references to back up what I've been saying. In all honest I must report that I've found very little. In particular, no one else seems to make the point that repulsion is the dominant mode, especially during start-up. So I'm kind of out on a limb here. I don't know if this is a good thing or a bad thing.

Today I'm going to go even farther out on a limb when I talk about the importance of the air gap in a motor. From what I've seen so far, people regard the air gap as a necessary evil. Without the air gap, the rotor and stator are stuck together. You need just enough air gap to let the rotor turn. If you increase it too much, you pay the price in excessive magnetising current.

I'm not entirely buying this argument. I have a problem with the idea of a very small air gap. Last time I showed a sketch of a motor running near full load condition. According to how I understand it, the currents and fields are distributed more or less like so: (Remember I have the stator windings on top, with the field sweeping from left to right:)

You can see I've got the rotor field flowing in a region where the field is pretty strong. You might also notice that I've drawn a pretty healthy air gap. I did this for a reason. Because something nasty happens if you try to close the air gap to a minimum:

If you compare the two pictures, you see that as the air gap closes, the magnetic field lines choose to skew around the slots with the rotor bars, making straight for the iron. Those field lines would rather be flowing through iron than air or copper. That's a problem, because the rotor bars only experience a force when the magnetic field passes right through them, not around them.

Isn't this just as much of a problem with the wider air gap? Not to the same extent. The magnetic field lines arrange themselves to provide the most favorable path. In the case of the wide air gap, they can shorten the air gap by perhaps 20% by choosing to avoid the copper bars; but this comes at the expense of a certain degree of overcrowding. So there's a compromise. By contrast, with the narrow air gap, the incentive to crowd is much greater. In anthropomorphic terms, the flux lines would much rather go from stator iron to rotor iron rather than tire themselves out going through all that air and copper. For the case of the wider air gap, it just doesn't make as much difference: the flux lines have already gone through so much air, a little more copper doesn't matter that much.

The wider air gap does not come without a price. The magnetisation current needed to set up the field is that much greater. In a transformer, the mangentisation current is 5% to 7% of the full load current, so we tend to ignore it. In a motor, because of the air gap, it is already much more to begin with. Increasing the air gap only makes it worse. So there must be a design comporomise. Nevertheless, I can't see how a wider air gap isn't desirable from the point of view of generating more torque. The more air gap, the bigger you can effectively make your rotor slots while still taking advantage of the available magnetic flux.

And that's how I see it. As I said, there is a dearth of confirmatory information on the internet, so I'm kind of out on a limb here. If someone wants to tell me I'm right or I'm wrong, fire away. I'd really like to know.