Automatically controlled polarization controllers (PC) are important devices for treating states of polarization (SOP) of the optical signals in fibers, which are the essential integral parts in an automatic compensator for polarization mode dispersion (PMD). Two types of commercial available PCs are taken as examples to show theoretically and experimentally that except for the reset-free problem it is necessary to adjust at least three instead of two waveplates in order to transform any input state into any other state covering the entire Poincaré sphere. Therefore we can achieve complete PMD compensation at least using three degrees of freedom instead of two in each stage PMD compensator.