A commercial GM cryocooler is employed to cool the rotor of the first Siemens 400 kW HTS machine. Excellent thermal connection between cold head and rotor is achieved using a thermosyphon. At the rotor’s inner surface the required cooling power is provided by evaporating fluid, that is recondensed at the coldhead. Our configuration allows an easy mechanical decoupling of the stationary cold head and the rotor, using a magnetic liquid rotary seal. In order to shorten cool‐down time, a precool to 70 K is done with a thermosyphon filling of nitrogen, while a motor operating temperature of 25 K is reached using neon. Temperature difference between the thermosyphon’s cold and warm ends is below 1 K for a heat transfer of 40 W. During operation, a temperature controller stabilizes condenser temperature and hence rotor temperature. The self‐regulating cooling system has been operated continuously and without problems since Spring 2001. The machine was also operated with newly developed pulse‐tube cryocoolers, th...