A drive controller for a motor of a compressor, the drive controller comprising: a drive circuit configured to apply voltages to windings of the motor; a speed control module configured to control the drive circuit to rotate the motor at a requested speed; a speed determination module configured to generate the requested speed based on a speed demand from a system controller; and a lost rotor control module configured to identify a lost rotor condition in response to a comparison of a speed error with an adaptive threshold, wherein the speed error is based on a difference between the requested speed and an estimated speed of the motor; during a first system state, the adaptive threshold is equal to a first threshold, during a second system state, the adaptive threshold is equal to a second threshold; characterized in that: the lost rotor control module is configured to, in response to identifying the lost rotor condition, instruct the speed determination module to set the requested speed to an override speed that is lower than the speed demand; the drive controller further comprises an envelope determination module configured to determine a minimum operating speed based on (i) a refrigerant pressure at a condenser in fluid communication with the compressor and (ii) a refrigerant pressure at an evaporator in fluid communication with the compressor; the lost rotor control module is configured to decrease the override speed by a predetermined increment in response to each additional identification of the lost rotor condition, and the lost rotor control module is configured to disable the motor in response to the override speed decreasing below the minimum operating speed. a drive circuit configured to apply voltages to windings of the motor; a speed control module configured to control the drive circuit to rotate the motor at a requested speed; a speed determination module configured to generate the requested speed based on a speed demand from a system controller; and a lost rotor control module configured to identify a lost rotor condition in response to a comparison of a speed error with an adaptive threshold, wherein the speed error is based on a difference between the requested speed and an estimated speed of the motor; during a first system state, the adaptive threshold is equal to a first threshold, during a second system state, the adaptive threshold is equal to a second threshold; characterized in that: the lost rotor control module is configured to, in response to identifying the lost rotor condition, instruct the speed determination module to set the requested speed to an override speed that is lower than the speed demand; the drive controller further comprises an envelope determination module configured to determine a minimum operating speed based on (i) a refrigerant pressure at a condenser in fluid communication with the compressor and (ii) a refrigerant pressure at an evaporator in fluid communication with the compressor; the lost rotor control module is configured to decrease the override speed by a predetermined increment in response to each additional identification of the lost rotor condition, and the lost rotor control module is configured to disable the motor in response to the override speed decreasing below the minimum operating speed. the lost rotor control module is configured to, in response to identifying the lost rotor condition, instruct the speed determination module to set the requested speed to an override speed that is lower than the speed demand; the drive controller further comprises an envelope determination module configured to determine a minimum operating speed based on (i) a refrigerant pressure at a condenser in fluid communication with the compressor and (ii) a refrigerant pressure at an evaporator in fluid communication with the compressor; the lost rotor control module is configured to decrease the override speed by a predetermined increment in response to each additional identification of the lost rotor condition, and the lost rotor control module is configured to disable the motor in response to the override speed decreasing below the minimum operating speed.