A stack of a rotor comprises a plurality of stack elements which include material of first magnetic conductance. The rotor includes sectorial sections distributed round a rotational axis of the rotor. Each of the stack elements has an aperture of second magnetic conductance in each of the sectorial sections. Outer ends of the apertures are located at a rim of the stack elements or at a bridge of the rim. The apertures form channels through the stack in a direction of a rotational axis of the rotor. At least two of the apertures of a common channel of the channels have the outer ends at different radial distances from the rotational axis of the rotor.