Calculating Motor Start Time

Timothy L. O'Hearn, P.E.

Course Outline

In this course, a method is described to approximate the motor starting time using the characteristics provided by the manufacturer.  The motor torque versus speed curve, the load torque versus speed curve, and the current versus speed curve are utilized along with the rotating inertia to calculate the acceleration time for the motor.  The acceleration curve is plotted on a time versus current curve along with the motor’s thermal limits to select the proper over current protection.  A discussion of the results and the methodology for the calculation is included.  An example calculation is included for discussion purposes.

I. Introduction to calculating motor starting time
II. Method used to calculate motor starting time
III. Example motor characteristics and typical manufacturer supplied curves
IV.  An example motor starting calculation and a discussion of the results.
V. Course Summary.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

• Have an understanding the method used to calculate an approximate value for the motor’s starting time for full voltage across the line starting;
• Be capable of using the motor and load characteristics obtain from the equipment manufacturer to calculate the motor’s start time;
• Have an understanding of the effect of a voltage dip on the motor’s starting time; and
• Understand how to use this method to assist in the selection of the proper over current protection for the motor for starting conditions.

Intended Audience

The intended audience is professional engineers, project managers, and project engineers, interested in the methods used to calculate time it takes to start an electric motor.

Benefit to Attendees

A better understanding of the purpose for and the techniques involved in calculating motor start time. An example calculation is included to assist in understanding the numerical methods utilized.

Course Content

In this course, you are required to study Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-2 "Concepts of Alternating Current" Chapter 1:

Calculating Motor Start Time

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Course Summary

A method is described to approximate the motor starting time using the characteristics provided by the manufacturer.  The motor torque versus speed curve, the load torque versus speed curve, and the current versus speed curve are utilized along with the rotating inertia to calculate the acceleration time for the motor.  The acceleration curve is plotted on a time versus current curve along with the motor’s thermal limits to select the proper over current protection.

References

1.     NEMA MG1-1993.

2.       IEEE Std. 399 – 1990 “Power System Analysis”.

3.       IEEE Std. 620 IEEE “Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines”.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.