A complex motor (called the spinning head) is described which can drive 20lb rotors at speeds up to 60 000 r.p.m. and at the same time allow the phase relative to another unit or a standard to be controlled. The system has a natural phase stability of 0.5°, and with a simple servo system, phase control to 0.1° is possible.The unit was developed for neutron spectrometer work and its value in this field is discussed. A number of spinning heads are in use in several neutron spectrometers and have been shown to be very reliable in operation.