A centrifuge rotor assembly includes a chamber assembly, a bearing support, and a counterweight that is substantially diametrically opposed to the bearing support with respect to the chamber assembly. The chamber assembly receives a processing chamber of a fluid circuit, while the bearing support receives a portion of an umbilicus that is fluidly connected to the processing chamber. The bearing support and counterweight are rotated about a central axis of the chamber assembly by an electric drive of the centrifuge rotor assembly, with the bearing support and counterweight remaining substantially diametrically opposed to each other with respect to the chamber assembly while being rotated about the central axis. The electric drive may rotate the bearing support and counterweight at a plurality of different speeds, with the counterweight automatically moving with respect to the bearing support between radially innermost and radially outermost positions to balance the centrifuge rotor assembly.
