Enhanced Motor Protection With the Slip-Dependent Thermal Model: A Case Study

Protection of induction motors can be enhanced with today´s microprocessor-based protective relays and a slip- dependent thermal model. This paper briefly introduces the concept of the thermal model and explains how to apply a slip- dependent thermal model to better protect motors in retrofit applications, where very little data are available. In such cases, assumptions must be made to estimate safe locked-rotor times based on historical motor starting times and electromechanical relay settings. These assumptions are then checked by reviewing motor start report data collected on the initial starts of the motor. Thermal capacity measured during start is used to validate the improved protection and settings may then be revised without the risk of a trip on normal starts. The motor start reports from multiple motors will be presented, along with the protection settings produced in this manner. The paper describes how microprocessor-based protection is successfully provided for all conditions in this application, replacing the thermal overload protection, which had been blocked for two minutes during starting because of the difficulties in starting high-inertia loads.