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Torque is proportional to current through the torque constant, so current is torque divided by Kt. The supply voltage then splits between the resistive drop and the back-EMF, and whatever is left for back-EMF sets the maximum speed at that torque, since Ke in V per rad/s equals Kt numerically in SI units.
Motor current
I = T / Kt; available back-EMF = V - I R; speed = back-EMF / Ke
Because of energy conservation: mechanical power out equals electrical power in for the ideal machine, which forces N·m/A and V·s/rad to be the same number in SI units.
Winding temperature. Copper loss is I squared R, so the thermal path from winding to ambient sets the continuous rating.