PM Slow Speed Synchronous Motors
is one kind of totally enclosed motor, which have high pole count motors that naturally turn at slower speeds(72 or 60RPM). They only need a resistor-capacitor(RC) network to operate from single-phase AC utility power. For loads that operate at 72RPM or slower, they are very cost effective and simple to use.
Other motor technologies(induction,DC,servo and step motors)either need gear reducers,or electronic drives to match the speed of SOK synchronous motors. The cost of just the gear reduction or the cost of the electronic drive will usually exceed the total cost of the SOK synchronous motor.
For even slower speeds planetary gear reducers are offered. SOK synchronous motors produce very slow speed with only modest gear reductions.