For a surface mounted PM, the air gap flux density should be as close as possible to the value of the max BH product (energy) of the PM material. Other choices could be taken in case of special working condition has to be considered (overloads, high temperature of magnets, etc...).
If the PM has a Br of 1.2T, you will never be able to have an airgap flux density of 1.2T because in the magnetic circuit, the airgap is a "load" and the magnet is able to achieve the Br only in "no load" condition. You have to consider the B-H curve of the magnet (2nd quadrant), calculate the max B*H point which is providing you the air gap flux density and the overall H*l "produced" by the magnet. Based on these two values you can design the air gap and the full magnetic circuit.
If you are ready to pay more and 1T is your only solution the reluctant circuit equation will give you the different thickness of material. But it will depend on where are the magnets. Inserted magnet topologies allow flux concentration. For surface magnet reluctant circuit give you an equation that clear give Br as a limit. (In that case it is basically a function of Br, magnet relative permeability and the ration between air gap and magnet size). You can reach 1.2T with good rare earth (NdFeB) grades. But be careful about saturation in the lamination.
We produce with V shape arrangement of the magnets where same polarity facing each other and pushing the magnetic flux to the air gap. As long as the stator tooth and the rotor parts do not saturate one can keep the air gap flux density as much as possible. We design up to 1000 KW low speed hydro generators with V shape arrangement more than 1 tesla and keep the tooth density just below 1.8 tesla. We make BLDC machines with Cobalt Iron Stamping where we keep the tooth flux density upto 2.2 Tesla in the tooth where the saturation flux density of Cobalt Iron is around 2.35 tesla. It goes with the design and application.
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