Motor Protection Principles.pdf

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Thermal Model Behavior - Long Starts • Issue Duration of a high inertia load start is longer than the allowed motor safe stall time. • For these starts, thermal model must account for the current change during acceleration and also use the acceleration thermal limits for TCU calculations. • <strong>Motor</strong> thermal limit is growing along with motor rotation speed during acceleration. • Starting current is proportional to system voltage during motor acceleration, thus voltage could be a good indication of the current level corresponding to the locked rotor conditions. • Voltage dependant dynamic thermal limit curve is employed to enhance the thermal model algorithm. • <strong>Motor</strong> relay will shift acceleration thermal limit curve linearly and constantly based on measured line voltage during a motor start. 18
Thermal Model - Current Unbalance Bias Negative sequence currents (or unbalanced phase currents) will cause additional rotor heating that will be accounted for in Thermal Model. Positive Sequence • Main causes of current unbalance • Blown fuses • Loose connections • Stator turn-to-turn faults • System voltage distortion and unbalance • Faults Negative Sequence 19
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 and 8: Motor Electrical Protection • The
Page 9 and 10: Thermal Model - Motor States • Mo
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: Overload Curve Selection A custom o
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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