Originally Posted by
Zetopan
As can be easily seen from the speed vs torque curves that I displayed above, a torque load change on the PMDC motor causes a proportional drop in RPM following that straight line (also commonly known as the load line). In contrast the same load change on an induction motor causes the rotor slip factor to change and the available torque can actually increase by a large amount, easily self regulating the speed to within a few percent. The AC induction motor performance plot shown assumes a constant frequency and constant AC voltage on the motor stator (of course neither of those are true due to the Tormach 3 phase inverter) . Note that the pullout torque limit is over 4x the continuous run load torque and the RPM drop there approaches 20%.
Compare the higher slope magnitude of the vertical torque vs RPM curve on the AC induction motor to the much lower slope magnitude of the PMDC motor curve.