A system and method for operating a motor drive unit. The motor drive unit
is coupled to a motor driving a load and is configured to selectively
control the motor according to a torque control mode and a speed control
mode. The motor drive unit includes a circuit configured to generate an
operational condition signal indicating an operational condition of the
motor and a regulator configured to receive the operational condition
signal and generate a motor control signal. The motor drive unit also
includes a first controller configured to receive the motor control
signal and a reference control signal and select either the motor control
signal or the reference control signal to use as a control signal to
drive the motor according to either torque control or speed control. The
motor drive unit also includes a second controller configured to monitor
the operational condition signal and, upon detecting a preliminary
indication of a change in the load, bypass the first controller to drive
the motor according to the other of torque control or speed control.