Why Can't I Start My Motor? Demystifying Motor Relay Lockouts

As the driver of many vital processes, motors are by far the most proliferous heavy machinery found in industrial facilities. Protection & control is key to maximizing operational life and process availability. Motor protection relays often employ thermal modeling and start inhibits to estimate the motor's thermal state and block restart attempts to prevent overheating of the stator windings. Adequately and accurately setting a motor's thermal protection is both an art and science – one that can cause confusion for operators if not correctly executed. This paper reviews the fundamental concepts and configuration of motor thermal protection. Various factors that play into typical restart blocking logic will be discussed, accompanied by case studies illustrating the consequences of improper configuration. Methods of troubleshooting will be examined and suggestions for configuration with limited motor data will be provided to avoid inadvertent and unnecessary motor lockouts.