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.