Methods, apparatus, and systems are provided for regulating the operation of a drive system within a battery voltage and power range. The apparatus includes a PI regulator producing a signal for regulating a torque output of the drive system based on a DC voltage source limit and/or power limit, a converter normalizing the signal from the PI regulator to produce a new torque limit, and a switch selecting either the new torque limit or an available torque limit of the drive system. The PI regulator includes an integrator having an initial value based on the torque output of the drive system and a copper loss term.