Abstract: The invention relates to a device for measuring a rotor imbalance. Said device comprises a main shaft which is rotatably mounted in a machine housing and to which the rotor to be measured is fixed, a single-phase alternating-current motor for driving the main shaft, with a single-phase winding (4) and an auxiliary winding (5) which is offset in phase for driving the main shaft (1) up to measurement speed and a control device (17) that adjusts the current of the motor. Said control device (17) has an assembly (8, 6; 7; 20; 22, 23) which reduces the supply current delivered to the two windings (4, 5) of the motor (3; 21) once the measurement speed has been reached, in such a way that the motor creates a reduced torque which substantially compensates a speed drop produced by friction and drag.