110329-1105 EDT
What do you mean better torque?
What output speed torque requirements do you have?
A DC motor with fixed field excitation has a torque output approximately proportional to armature current. From a thermal standpoint the motor will have a maximum armature current specification based on internal temperature rise. Suppose this is N amperes. Then Torque = K*N essentially independent of speed, where K is a constant. Note: when an armature is rotating there is substantial self cooling. Thus, if no external cooling is provided, then you have to lower the maximum current at lower speed. So motors to be run at maximum torque over wide speed ranges need supplemental cooling. This is usually supplied by an external blower.
For short times you can greatly exceed the normal maximum torque rating of a DC motor by supplying large armature current.
AC induction motors have there own unique torque characteristics which are complex to describe. But will not provide the short time peak torque capability of the same physical size DC motor. Generally when operated from a VFD or Vector control will not have the low speed capability of the DC motor.
An AC synchronous motor when operated as a so called DC brushless motor has pretty much the same basic characteristics as the DC commutator type motor but no sliding electrical contacts. The rotating permanent magnets in a DC brushless are the equivalent of the fixed stationary field of the DC commutator motor. Basically it is a DC motor turned inside out and electronic commutation is employed instead of brushes and a mechanical commutator. The DC brushless has better thermal characteristics because the coils are on the outside instead of on the rotor.
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