Guide for AC Motor Protection
|Publication Date:||30 March 2000|
This application guide presents generally accepted methods of protection for ac motors. It identifies and summarizes the functions necessary for adequate protection of motors based on type, size, and application. This guide does not purport to detail the protective requirements of all motors in every situation. What it does provide is enough information and guidance for the user to implement adequate protection for particular applications.
The recommendations in this guide are based on typical types of installations. However, sufficient background information relating to protection requirements, applications, and setting philosophy is provided to enable the reader to determine required protective functions for motor installation. The protective functions discussed in this guide can also be implemented with multifunction, microprocessor-based
Relay protection of squirrel-cage, wound-rotor induction motors, and synchronous motors is presented herein, with a review of the generally accepted forms of motor protection. This guide also summarizes the uses of relays and devices, individually and in combination, so the user may select the necessary equipment to obtain adequate motor protection.
This guide is concerned primarily with the protection of three-phase, integral horsepower motors and adjustable-speed motors where specifically indicated. Its recommendations are of a general nature, designed to cover typical motor installations. Sufficient background information on objectives, application, and setting philosophy is presented.
The following outline describes the order in which information is presented in this guide:
a) Clause 2 lists applicable references.
b) Clause 3 contains a glossary of terminology.
c) Clause 4 gives a brief description of the design of induction and synchronous motors. It also discusses various types of switching and control devices used to interrupt low- and medium-voltage motors, and provides schematics that show how these devices are controlled and protected.
d) Clause 5 gives 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 electrical and mechanical response of the motor to these abnormalities will greatly assist the user in evaluating the need for, and the means of obtaining, adequate motor protection in any situation.
e) Clause 6 presents detailed recommendations for good engineering practice in a series of tables and diagrams. The tables and diagrams are classified according to type of switching, normal source voltage, and motor and circuit ratings; they show the combinations of devices normally applied for an associated protective function. See Table 7 for a complete listing of all device designations [both American National Standard Institute (ANSI) and National Electrical Manufacturer's Association (NEMA)] used in this guide. Also included in this clause is application guidance for protection of adjustable-speed drive applications.
f) Clause 7 includes a discussion of the various factors that must be considered in determining the setting of each relay, protective function, or device. Whenever it is applicable, information is provided on the desirability of using a device to actuate an alarm or a trip.
g) Clause 8 includes a discussion of the important aspects of multifunction, microprocessor-based