I am hoping the efficiency will be about 90%. It is not a bldc motor. It is a synchronous ac generator...
Thank you very much for your applies... Flux was right... Suppliers are not really helpfull. Some of them already told me that the minimum order for custom made magnets is 2000E...
I warmly encourage all efforts to understand and design machines, and improvements can be made, so I'm sure you will have success.
You may find that 90% efficiency is available to you in a wide variety of axial-flux generators, even mediocre ones. In the cases I've seen, however, this only happens at very light loads. You can load them significantly more than that and still safely shed the heat coming from I-squared-R loss and eddy currents. To illustrate what I mean, I can work backwards from your target efficiency:
Assume the input (driving) power is 1000 watts. You are getting 900 watts output AC electricity, and all of the losses sum to 100 watts.
Assume that 10 watts are the eddy current loss (in the ballpark for air-gap core, more if the stator has laminations)
This leaves you with losses of 90 watts that account for wire resistance. I will ignore any reactance because (I am told) this is small in the axial design. Looking at that 90 watts for a moment, some practical experience will tell you that it only takes a small surface area to dissipate that with manageable temperature rise. Electro-mechanical machines made of copper and steel can handle a lot more heat than that.
Next we can make an assumption about the electric load on the generator. The generator could be feeding the utility grid at 120VAC. If this is the case, then the output power of 900W gives:
900W / 1.73 / 120V = 4.3 Amperes per line (3-phase Star)
That same current is also causing heat in the wires due to resistance. If we only allow 90 watts of such heat, then:
90W / 4.3A / 4.3A = 4.8 Ohms per phase (star)
At this point you can branch out the design, selecting a range of cut-in speeds, diameters, magnet sizes, wire types, and so on. Eventually you find a set that satisfy your 90% efficiency requirement. What all of that is done, you will have an axial flux generator optimized for this point. Build it and put it on the table.
Then someone like me can come along, and drive it with 1200 watts of shaft power. Now that more power is going in, it will either run faster, or the load will keep the speed constant and more current will flow. Either way, more output power will be realized. At the same time, the increased output current will cause higher heat losses as I described above. It will run less efficiently. If it's 80%, then your output power is 960 watts (increased) and the heat loss is 240 watts (also increased).
Provided that the materials you used to build this alternator are not so flimsy and fragile that they cannot dissipate the extra heat, you will probably find that more and more power can be poured into the alternator. More and more power output can be realized
despite the lower efficiency. This doesn't compromise the goal - not at all! It increases the utility of your generator, in fact. Testing the unit will tell you what operating speeds and temperatures are safe, and you will rate the power accordingly.
The limiting factor in many electro-mechanical devices is their
ability to shed heat. Not an arbitrary efficiency goal.
I've personally bench tested several different types of generators, and each time I measured high efficiency ~80%+ at low-power ranges and low efficiency at high-power ranges. Running them very lightly - just above the cut-in speed, and the efficiency stays very high, but the generator realizes only a fraction of its potential.
Good luck, and I hope this was helpful.
