Best electrical machines have specific power near 6 kW/kg. Some small electrical machines have 10 kW/kg but its power is very low. Are these values the theoretical limit of conventional electric machines? Is it possible to increase these values without going to new materials? How do we achieve incredible values of 25, 30...50 kW/kg for future aircraft EM?
I think that a more significant figure of merit would be specific torque (Nm/kg) for a given cooling method (we should not compare an air cooled motor with a water/oil cooled high performance motor). The size of the machine (and hence mass) is more related to torque than power, because one could design a 100.000 rpm machine with a high specific power. Therefore, a low speed machine could never compete in terms of power density...
It would perhaps be worth looking at the company YASA Motors, which is really nice advanced new technology (high power and high torque density) which represents a clear step change for the industry as a whole.
It depends on the rotational speed, power, cooling system, etc. These guys claim that their motor can give up to 10kW/kg at 6000rpm: https://emrax.com/products/emrax-228
We have one in the lab but haven't testet it yet.
We are now testing a MW class generator (15000rpm) - with a power density about 16kW/kg with a potential grow to 20kW/kg.
The power density 8-10kW/kg should be feasible for propulsion motors with a power range 500-1000kW, and the speed 2000-3000rpm.
The value of specific power also depends on system requirements. For example for aviation generator we need to have peak power which is 2 times higher than nominal value. It causes weight increasing. If we will change system requirements we will have opportunity to increase specific power of electrical machines.
https://newatlas.com/automotive/equipmake-worlds-most-power-dense-electric-motor/
Here the fantastic characteristics of the motor are promised. But the motor has not yet been tested :)
I think that a more significant figure of merit would be specific torque (Nm/kg) for a given cooling method (we should not compare an air cooled motor with a water/oil cooled high performance motor). The size of the machine (and hence mass) is more related to torque than power, because one could design a 100.000 rpm machine with a high specific power. Therefore, a low speed machine could never compete in terms of power density...
You may be interested in reading our paper, 120kW peak power, 320Nm (for 5 minutes, 80kW rated power) with 25kg of active materials, air cooled. I suggest that electric machines need to be tailored on the specific application. Integration among mechanics, cooling system and power electronics is often the key to increase specific power and other performance. Conference Paper PM brushless motor for helicopters electric tail rotor drive system
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