AC Synchronous Motors
From Hansen Corporation

Synchronous motors are inherently constant-speed motors and they operate in absolute synchronism with line frequency. Hansen offers both Hysteresis and Permanent Magnet synchronous motors (PM Synchronous Motors) to meet your exact needs. The hysteresis design, Hansen's famous Genuine Synchron®, is extremely quiet and smooth in operation and has very low cogging torque, ideal for back-driving. Because it is light in torque, it is always attached to a Hansen ultra precision gearbox. The permanent magnet motor design offers higher torque and is available with or without a gearbox. The high strength magnets create substantial cogging torque, making these synchronous motors ideal for applications where you need to hold position after removing power.