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Overcurrent Coordination Basics Motors
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Purpose
The purpose of this guide is to provide a basic overcurrent protection philosophy for motors.
MV Motor Overcurrent Protection
Time-current curve (TCC) landmarks (figure 1)
Full load amps located in the upper decade Starting time located in the middle decades
Locked rotor amps located in the lowest decade Motor starting curve located in all 5 decades
Stator damage curve located in the upper decade
Rotor damage curve located in the middle decades
TCC regions (figure 2)
Equipment operating region located at and to the left of the motor starting curve
Equipment damage region located to the right and above the damage curves Protective device operating region located between the equipment operating and
damage regions
Suggested overcurrent protection (figure 3)
Set protection above the motor starting curve
Set protection below the damage curves
Comments
If the motor is allowed to operate at locked rotor for a time at or above the locked rotor
limit curve, rotor damage will occur. If current penetrates the limits of the stator overload limit curve, insulation life is
reduced.
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Fig. 1 MV motor TCC landmarks
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Fig. 2 MV motor TCC region
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Fig. 3 MV motor overcurrent protection
LV Motor Overcurrent ProtectionTCC landmarks (figure 4)
Full load amps located in the upper decade
Starting time located in the middle decades Locked rotor amps located in the lowest decade Motor starting curve located in all 5 decades Rotor hot stall point located in the upper middle decades
TCC regions (figure 5)
Equipment operating region located at and to the left of the motor starting curve Equipment damage region located to the right and above the rotor hot stall point
Protective device operating region located between the equipment operating anddamage regions
Suggested overcurrent protection (figure 6)
Set protection above the motor starting curve Set protection below the hot stall point
Comments
If the motor is allowed to operate at locked rotor for a time at or above the hot stall point,rotor damage will occur.
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Fig. 4 LV motor TCC landmarks
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Fig. 5 LV motor TCC regions
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Fig. 6 LV motor overcurrent protection
References
Other Application Guides offered by SKM Systems Analysis at www.skm.com
Electrical Transmission and Distribution Reference Book, ABB Power T&D Company,Raleigh, North Carolina, 1997
Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004
The latest revision of:
IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrialand Commercial Power Systems (IEEE Buff Book)
IEEE Std 620, IEEE Guide for the Presentation of Thermal Limit Curve for Squirrel CageInduction Machines
IEEE Std C37.96, IEEE Guide for AC Motor Protection NEMA Std MG-1, Motors and Generators
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