Basically an alternating carrier signal can be represented by superposition of two carrier signals rotating in a dq coordi- nate system in opposite directions (Fig. 2). The carrier signal is oriented to the estimated position of the anisotropy of the rotor by a tracking controller (similar to a PLL). As the theo- retical background of HF injection methods with alternating signals is presented in the literature sufficiently (e. g. [11], [12], [15]), this is not shown in detail here.