Keep on Running—Select Motor Relay Settings to Balance Protection and Operation

Electric motors are essential for industrial processes. Incorrect operation of motor protective relays could remove essential motors from service, resulting in economic loss due to process interruptions. Motor protection schemes should cause minimum process downtime while providing adequate protection. These schemes should allow operators to maximize process availability. Thermal overload protection is a critical part of any motor protection scheme. This paper presents methods to set the thermal overload trip and reset settings correctly and provides examples of their application to several real-world installations. It discusses how the thermal overload settings can be developed from the motor data sheets when available. It also discusses how the settings can be developed for retrofit applications where complete motor thermal information might not be available. Additionally, this paper discusses how these settings affect motor operation and provides case studies of real-world motors.