Abstract: The failure of a motor to maintain a commanded speed is detected by the disclosed variable speed fan failure detector. The commanded speed varies as a function of temperature, such that at temperatures below a first predetermined temperature the motor runs at minimum speed and at temperatures above a second predetermined temperature it runs at maximum speed, with the speed varying substantially linearly between the first predetermined temperature and second predetermined temperature. The failure detector develops a first voltage proportional to motor speed, senses a temperature at a point where a predetermined temperature is to be maintained, develops a second voltage proportional to a desired speed based on the sensed temperature.