Most of the PMDC motors that I see are designed for 24VDC. From what I read, a PMDC motor can be made to run at 180VDC(peak). What changes in construction of the motor are need to support higher input voltages?
PMDC motors are being researched and developed for higher voltages and power ratings. It appears that permanent magnets and their physical arrangement are the clue. Better magnetic materials are more expensive and still researched, which one would be the best.
The main difference in changing from a low voltage DCPM motor and higher voltage is the winding and the commutator. Higher voltages need more commutator bars. Lower voltages use less. For example, a 24 volt motor with 16 slots in the laminations would have a 16 bar comm. a 115 volt motor would have 32 bar comm.