A voltage command estimator is configured to estimate a minimum required variable DC bus voltage based on the first direct-axis current/voltage command, the first quadrature-axis current/voltage command, the second direct-axis current/voltage command, and the second quadrature-axis current/voltage command for a respective time interval. The voltage command estimator is configured to provide the estimated minimum required variable DC bus voltage to a voltage regulator to adjust the observed voltage level of the variable DC voltage bus to the estimated minimum required variable DC bus voltage to maintain the operation, as commanded by the voltage/current commands, of the first electric machine under the first variable load and the second electric machine under the second variable load at the time interval.