Why can't I start my motor: Lessons learned from bad motor protective settings

Motor management relays can enhance protection by locking out a motor after a trip or a stop until the motor has cooled enough to restart the motor. This feature enhances motor protection because it insures the start will not cause the motor temperature to exceed the thermal damage curve. As with any protective relay setting, correct settings are important, but with restart protection they can be particularly difficult due to the fact that they prevent restart of the process that is driven by the motor. Topics to be discussed in this paper include: motor hot-to-cold safe stall ratio, RTD biasing, learned motor starting data, motor thermal capacity starting margin, running cool time constant and stopped cool time constant. Each of these values will be discussed in detail and will be shown with a case study that unnecessarily prevented restart of a motor.