6. Motor protection Advanced external motor protection relays
Advanced external motor protection relays More advanced external motor protection systems can also protect against overvoltage, phase imbalance, too many starts/stops, vibrations, PT100 temperature monitoring of stator and bearings, insulation resistance and monitor ambient temperature. Further, advanced external motor protection systems are able to handle the signal from built-in thermal protection. Thermal protection device will be covered later on in this chapter. These external motor protection relays are designed to protect three-phase motors against conditions, which can damage them in the short or the long run. In addition to motor protection, the external protection relay has features that can protect the motor in different situations: • Give an alarm before damage results from a process malfunction • Diagnose problems after a fault • Allow verification of correct relay operation during routine maintenance • Monitor bearings for temperature and vibration It is possible to connect overload relays throughout an entire plant to a central control system and constantly monitor and make a fast fault diagnose. When an external protection relay in an overload relay is installed, the downtime decreases due to process problems. The explanation is that it is possible to detect the fault quickly and avoid that it causes any damages to the motor. For instance, the motor can be protected against: • Overload • Locked rotor • Stall / mechanical jam • Repeated starts • Open phase • Ground fault • Overtemperature (using PT100 or thermistors signal from the motor) • Undercurrent • Overload warning
Overload Short circuit Locked rotor Stall / mechanical jam Repeated starts Open phase / imbalance Ground fault Overtemperature Undercurrent Overload warning
Advanced motor protection relay
Grundfos Motor Book 119