The invention relates to an electrical machine and to a method for the operation thereof, particularly as a drive motor for an electrical tool or as starter generator for a motor vehicle. The electrical machine includes a rotor excited by a permanent magnet and a stator carrying a multi-phase winding, and operates in a voltage-controlled, lower rotational speed range via a transformer on a DC voltage source. The electrical machine can also be operated in a higher rotational speed range by field weakening, and the structure of the machine can be changed by reducing the flux linkage between the rotor and the stator in order to weaken the field. Preferably, the change to the structure of the machine is carried out by turning off winding parts or by switching them between series and parallel connections.