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. 		 
   
  Fig. 1 MV motor TCC landmarks  
   
  Fig. 2 MV motor TCC region  
   
  Fig. 3 MV motor overcurrent protection  
     
  LV Motor Overcurrent Protection
TCC 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 and damage 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.  
   
  Fig. 4 LV motor TCC landmarks  
   
   Fig. 5 LV motor TCC regions  
   
  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 Industrial and Commercial Power Systems (IEEE Buff Book)
•   IEEE Std 620, IEEE Guide for the Presentation of Thermal Limit Curve for Squirrel Cage Induction Machines
•   IEEE Std C37.96, IEEE Guide for AC Motor Protection
•   NEMA Std MG-1, Motors and Generators 		 
     
     
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