scope:

INTRODUCTION

General Format.

The application guide for relay protection of squirrel-cage and wound-rotor induction motors presents a review of the generally accepted forms of motor protection and summarizes the use of relays and devices, singly and in combination, to enable the reader to properly select the necessary equipment to obtain adequate motor protection. The Guide is prepared primarily for the protection of three-phase integral-horsepower motors and refers to single- or two-phase motors only in special instances and only where specifically indicated. It is especially useful where the motor installation is sufficiently important, due to either its size or the consequences of motor malfunction, to justify specific design of the protective system. It also provides the information necessary to judge whether there are safety or economic reasons for such a specific design.

This Guide is not a Standard and does not purport to detail the protective requirements of all motors in every situation. Its recommendation is of a general nature designed to cover the usual or typical motor installation. Sufficient background of material on objectives, application, and setting philosophy is presented, however, to enable the reader to evaluate the need for various forms of protection and to select and properly apply suitable protective equipment for most situations.

Section II, Motor Protection Requirements, presents a brief description of the damaging effect on a motor of abnormal voltage, current, temperature, and incorrect operating conditions or procedures. A clear understanding of the electric and mechanical response of a motor to these abnormalities will greatly assist the reader in evaluating the need for and the means of obtaining adequate motor protection for any specific situation.

Section III, Motor Protection Specifications, presents detailed recommendations in a series of tables and diagrams showing good engineering practice. The tables and diagrams are classified according to type of switching, nominal source voltage, and motor or circuit rating; they show the combination of devices normally applied for the associated protective function.

Section IV, Setting and Adjustment of Protection Devices, presents a discussion of the various factors which must be considered in determining the settings of each relay or device. Whenever it is applicable a discussion is given on the desirability of using a device to actuate an alarm or a trip.