what motor is best for constant speed operation?
DC motor is best for constant operation.
Series-wound DC-AC motors are notoriously short-lived. However with sufficiently robust bearings they can last much longer, the limiting factor being the brushes in frictional contact with the armature.
In continuous duty, series-wound motors are a maintenance nightmare although they are remarkably power efficient.
Different day same Rocmike posting pattern. Dave Palmer stops posting and it is followed by another Rocmike alias Bill with some Anonymous posts thrown in. We have around another 11 hours of Rocmike aliases posting the same thing over and over.