7/27/2019 12. Overcurrent Coordination Basics Motors

1/7

Frequently Asked Questions Application Guides Power Systems Study Specs Tech Support

PTW Tutorial Videos

Overcurrent Coordination Basics Motors

The information presented in this application guide is for review, approval, interpretation andapplication by a registered professional engineer only. SKM disclaims any responsibility and

liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM

Systems Analysis Inc.

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.

7/27/2019 12. Overcurrent Coordination Basics Motors

2/7

Fig. 1 MV motor TCC landmarks

7/27/2019 12. Overcurrent Coordination Basics Motors

3/7

Fig. 2 MV motor TCC region

7/27/2019 12. Overcurrent Coordination Basics Motors

4/7

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.

7/27/2019 12. Overcurrent Coordination Basics Motors

5/7

Fig. 4 LV motor TCC landmarks

7/27/2019 12. Overcurrent Coordination Basics Motors

6/7

Fig. 5 LV motor TCC regions

7/27/2019 12. Overcurrent Coordination Basics Motors

7/7

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

back to Application guides

products support contact us distributors Copyright SKM 2010