why Commutator power capacity is limited
In short, the commutator suffers more than many others, under the law of diminishing returns.


As you increase voltage potentials across the commutator, you must increase the dielectric strength of the commutator. To increase dielectric strength, you have to make it much higher diameter (increasing friction losses &amp; damage), or seal it as a closed system, and fill with expensive dielectric materials.
As you increase current through the commutator, you have to increase the size, thickness &amp; weight of all current-carrying components (brushes, commutator plates). Also, the heavier components will require more spring-force to hold everything in place while running. The added friction heating, combined with the added ohmic heating, would require better heat dissipation, which costs/weighs more.


So, while it's pretty mich always possible to 'just make it bigger,' with commutators you'll probably end up deciding that the costs are too high to be 'worth it.'