Ring motor drive mills are loaded with several hundred tons of ores. This enormous mass in rotation generates forces acting directly on the main bearings supporting the mill. Furthermore, the mill structure supporting the ring motor poles may change shape leading to modifications of the motor air gap which in turn may be sufficiently large to jeopardize the correct operation of the mill. In order to avoid serious problems of this nature, continuous monitoring of the air gap is therefore crucial during mill start-up and the milling process. Air gap sensors mounted at several locations around the stator and a specific monitoring unit continuously monitors the mill and provide a trip signal within milliseconds if the air gap reduction should exceed a critical value. For additional protection, a rotating air gap placed on the rotor can enhance the diagnostic capabilities of the monitoring system.