Motor Protection Principles.pdf

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Overload Protection - Thermal Model A motor can run overloaded without a fault in motor or supply A primary motor protective element of the motor protection relay is the thermal overload element and this is accomplished through motor thermal image modeling. This model must account for thermal process in the motor while motor is starting, running at normal load, running overloaded and stopped. Algorithm of the thermal model integrates both stator and rotor heating into a single model. • Main Factors and Elements Comprising the Thermal Model are: • Overload Pickup Level • Overload Curve • Running & Stopped Cooling Time Constants • Hot/Cold Stall Time Ratio • RTD & Unbalance Biasing • Motor State Machine 8
Thermal Model - Motor States • Motor Stopped: Current < “0” threshold & contactor/breaker is open. • Motor Starting: Previous state is “Stopped” & Current > “0” threshold. Motor current must increase to the level higher than overload pickup within seconds. • Motor Running: Previous state is “Starting” or “Overloading” & Current drops below overload pickup level. • Motor Overloading: Previous state is “Running” & Current raises above overload pickup level. Thermal Capacity Used (TCU) begins to accumulate during overload. 9
Page 1 and 2: Motor Protection Principles Craig W
Page 3 and 4: Various Industry Motor Applications
Page 5 and 6: Thermal Stress Causes Motor Failure
Page 7: Motor Electrical Protection • The
Page 11 and 12: Thermal Overload Pickup • Set to
Page 13 and 14: Overload Curve Selection for Therma
Page 15 and 16: Hot/Cold Safe Stall Ratio HCR = LRT
Page 17 and 18: Overload Curve Selection A custom o
Page 19 and 20: Thermal Model - Current Unbalance B
Page 21 and 22: Thermal Model - RTD Bias • Accele
Page 23 and 24: Overvoltage Protection • The over
Page 25 and 26: Unbalance Protection • Indication
Page 27 and 28: Ground Fault Protection Zero Sequen
Page 29 and 30: Differential Protection • Differe
Page 31 and 32: Differential Protection Biased diff
Page 33 and 34: Stator RTD Protection • A simple
Page 35: Conclusions • Induction & synchro

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