The correct rotor size for torque production can easily be estimated with the tangential stress:
tngs=Pi/pi()^2/rpm/Dr^2/li where Dr is the rotor diameter li=the length of the rotor Pi=Er*Ia for DC and Si for
ac motor=internal power of the rotor. Si=3*Er*Ir*cos(Er,Ir)= aparent internal power.
Since Vr =pi()*Dr^2*li rotor volume then tngs=Pi/pi()/rpm/Vr.
If Vr_dc=Vr_ac and rpm_dc=rpm_ac then Pi_dc/Pi_ac=tngs_dc/tngs_ac.
According to -for instance-to:
"Design of Rotating Electrical Machines" by Juha Pyrhönen and other ch.6
see :
the tangential torque of a dc motor could be double as for an induction motor.
That means for the same volume of rotor and for the same rpm the output power of DC motor is double of an AC motor.
The torque is relative constant for a PM dc motor [something as for external supply of excitation of brushed dc motor] but
the induction motor torque depends on rotor type: squirrel cage or wound rotor and depends on rotor slot type-deep slot, double cage and so on. For a usual class B NEMA squirrel cage the starting torque could be 1.5 the rated but the rated is less 0.5 of a dc motor.
