A balancing device is provided that is able to balance a rotor of an electrical machine. The machine includes a mounted rotor in a central bore of a stator and coaxially thereto. Between the rotor and the stator there is an air gap. The rotor preferably comprises a plurality of axially extending ferromagnetic teeth and non-magnetic wedges being arranged alternately around the circumference of the surface of the rotor. The teeth and/or the wedges preferably include balancing openings for balancing weights. The balancing device includes a mounting device and/or a locking device and/or a confirmation device. The device has a radial height that is smaller than the minimal clearance of the air gap and an axial extension such that it can be inserted into the air gap. With such a device it is possible to balance a rotor in an efficient way, when it is located in the bore of the stator.